Pre-1980 Tephra-Fall Deposits Erupted 

From Mount St. Helens, Washington

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1563
Cover. Mount St. Helens from the northeast, in 1964, showing relatively smooth 

uneroded profile prior to 1980, the fan of debris that dammed preexisting valleys to form 

Spirit Lake (foreground), and the plug domes that studded its northern flank, from left to 

right, Dogs Head, Sugar Bowl, and Goat Rocks.
Pre-1980 Tephra-Fall Deposits Erupted 

From Mount St. Helens, Washington

By Donal R. Mullineaux

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1563

A study of the stratigraphy, age, distribution, and 

characteristics of more than 100 tephra-fall deposits 

erupted during the past 40,000 years

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1996
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

For sale by U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

Mullineaux, Donal Ray, 1925-

Pre-1980 tephra-fall deposits erupted from Mount St. Helens, Washington / by Donal R. 

Mullineaux.

p. cm. — (U.S. Geological Survey professional paper ; 1563)

Includes bibliographical references.

Supt.ofDocs.no.: I 19.16:1563

1. Volcanic ash, tuff, etc.—Washington (State)—St. Helens, Mount, 

Region. 2. Geology, Stratigraphic. I. Title. II. Series. 

QE461.M778 1996

552'.23'0979784—dc20 95-3747

CIP
CONTENTS

Abstract........................................................................................................................... 1

Introduction..................................................................................................................... 2

Terminology............................................................................................................ 4

Tephra............................................................................................................. 4

Sets, Layers, and Beds .................................................................................... 5

Color................................................................................................................ 5

Letter Designations......................................................................................... 5

Thickness................................................................................................................ 5

Deposits of Ash Clouds and Other Pyroclastic Flo wage Phenomena..................... 6

Oxidized Ash-Rich Deposits of Undetermined Origin........................................... 6

Direction and Distance From Mount St. Helens..................................................... 6

Ferromagnesian Minerals........................................................................................ 6

Chemical Analyses.................................................................................................. 7

Age.......................................................................................................................... 7

Previous Work......................................................................................................... 9

Overview of Tephra Sequence........................................................................................ 10

Distribution............................................................................................................. 10

Preservation............................................................................................................. 10

Location of Relatively Complete Tephra Sequences.............................................. 11

Description of Tephra Units............................................................................................ 15

Tephra Set C............................................................................................................ 15

Composition.................................................................................................... 15

Diagnostic Features......................................................................................... 15

Stratigraphic Relations.................................................................................... 15

Distribution and Thickness ............................................................................. 15

Origin.............................................................................................................. 16

Age.................................................................................................................. 17

Previous Designations and Correlative Units ................................................. 19

Description of Layers in Set C........................................................................ 19

Layer Cb.................................................................................................. 20

Layer Ct................................................................................................... 22

Layer Cw................................................................................................. 22

Layer Cm................................................................................................. 22

Layer Cy.................................................................................................. 22

LayerCs.................................................................................................. 23

Unnamed Ash Deposits Associated with Set C.............................................. 23

Tephra Set M........................................................................................................... 23

Composition.................................................................................................... 23

Diagnostic Features......................................................................................... 24

Stratigraphic Relations.................................................................................... 24

Distribution and Thickness ............................................................................. 24

Origin.............................................................................................................. 24

Age.................................................................................................................. 25

Previous Designations..................................................................................... 25

Description of Layers in Set M....................................................................... 25

LayerMg................................................................................................. 25

Layer Mo................................................................................................. 25

Layer Me................................................................................................. 26

Layer Mp................................................................................................. 27

in
IV CONTENTS

Layer Mm................................................................................................ 27

Layer Mt.................................................................................................. 28

Tephra Set K........................................................................................................... 28

Distribution and Thickness............................................................................. 29

Age.................................................................................................................. 30

TephraSetS............................................................................................................ 30

Composition.................................................................................................... 30

Diagnostic Features......................................................................................... 30

Stratigraphic Relations.................................................................................... 30

Distribution and Thickness............................................................................. 30

Age.................................................................................................................. 31

Description of Layers in Set S........................................................................ 32

LayerSb.................................................................................................. 32

LayerSw................................................................................................. 33

Layer Ss.................................................................................................. 34

LayerSg.................................................................................................. 34

Layer So.................................................................................................. 36

Tephra Set J............................................................................................................. 37

Composition.................................................................................................... 37

Diagnostic Features......................................................................................... 37

Stratigraphic Relations.................................................................................... 38

Distribution and Thickness............................................................................. 38

Age.................................................................................................................. 38

Description of Layers in Set J......................................................................... 39

Layer Js................................................................................................... 39

Layer Jy................................................................................................... 41

Layer Jb................................................................................................... 42

Layer Jg................................................................................................... 42

Tephra Set Y........................................................................................................... 43

Composition.................................................................................................... 44

Diagnostic Features......................................................................................... 44

Stratigraphic Relations.................................................................................... 45

Distribution and Thickness............................................................................. 45

Age.................................................................................................................. 45

Previous Names and Correlative Units........................................................... 46

Description of Layers...................................................................................... 46

Layer Yb................................................................................................. 47

Layer Yd................................................................................................. 50

Layer Yn................................................................................................. 50

Layer Ye.................................................................................................. 51

Layer Yf.................................................................................................. 51

Layer Yo................................................................................................. 51

Layer Yu................................................................................................. 52

Deposits of Probable Flowage Origin Associated with Set Y........................ 52

Ash Bed yc.............................................................................................. 52

Ash Bed ya.............................................................................................. 53

Ash Bed yp.............................................................................................. 53

AshBedpbp............................................................................................ 53

Tephra Set P............................................................................................................ 54

Composition.................................................................................................... 54

Diagnostic Features......................................................................................... 54

Stratigraphic Relations.................................................................................... 54

Distribution and Thickness............................................................................. 55

Origin.............................................................................................................. 56
CONTENTS V

Age............................................^ 56

Description of Layers in Set P........................................................................ 57

Layer Pm................................................................................................. 57

Layer Ps.................................................................................................. 57

Layer Pu.................................................................................................. 58

Layer Py.................................................................................................. 58

Ash of Probable Flowage Origin Associated with Set P................................ 58

Ash Bed pa.............................................................................................. 58

Ash Bed v ............................................................................................... 58

Tephra Set B ........................................................................................................... 59

Composition.................................................................................................... 59

Diagnostic Features........................................................................................ 60

Stratigraphic Relations.................................................................................... 60

Distribution and Thickness............................................................................. 60

Age.................................................................................................................. 60

Description of Layers in Set B........................................................................ 60

Layer Bh................................................................................................. 61

Layer Bo................................................................................................. 61

Dacitic Tephra........................................................................................ 62

LayerBd......................................................................................... 62

LayerBi.......................................................................................... 64

Layer Bu................................................................................................. 64

Ash Bed ba...................................................................................................... 64

Tephra Layer D....................................................................................................... 65

Diagnostic Features........................................................................................ 65

Stratigraphic Relations.................................................................................... 65

Distribution and Thickness............................................................................. 65

Origin.............................................................................................................. 66

Age.................................................................................................................. 66

Tephra Set W.......................................................................................................... 66

Composition.................................................................................................... 67

Diagnostic Features........................................................................................ 67

Stratigraphic Relations.................................................................................... 68

Distribution and Thickness............................................................................. 68

Age.................................................................................................................. 69

Description of Layers in Set W...................................................................... 69

Layer Wn................................................................................................ 71

Layer Wa................................................................................................ 72

Layer Wb................................................................................................ 72

Layer We................................................................................................ 72

LayerWd................................................................................................ 73

Reworked Pumice Lapilli Deposits................................................................ 74

Tephra Set X........................................................................................................... 74

Composition.................................................................................................... 74

Diagnostic Features........................................................................................ 75

Stratigraphic Relations.................................................................................... 75

Distribution and Thickness............................................................................. 75

Origin.............................................................................................................. 75

Age.................................................................................................................. 75

Description of Layers in Set X....................................................................... 75

Layer Xb................................................................................................. 75

Layer Xs.................................................................................................. 77

LayerXm................................................................................................ 78

LayerXh................................................................................................. 79

Ash Bed z........................................................................................................ 80
VI CONTENTS

Tephra Layer T ....................................................................................................... 80

Diagnostic Features......................................................................................... 81

Stratigraphic Relations.................................................................................... 81

Distribution and Thickness............................................................................. 81

Origin and Source........................................................................................... 81

Age.................................................................................................................. 81

Previous Designations and Correlative Units................................................. 82

19th Century Lithic Ash.......................................................................................... 82

Eruptive History.............................................................................................................. 83

Subdivisions of Eruptive History............................................................................ 83

Old Mount St. Helens ..................................................................................... 83

Modern Mount St. Helens............................................................................... 83

Eruptive Stages and Periods.................................................................................... 84

Ape Canyon Stage........................................................................................... 85

Ape Canyon-Cougar Interval......................................................................... 85

Cougar Stage................................................................................................... 85

Cougar-Swift Creek Interval.......................................................................... 86

Swift Creek Stage ........................................................................................... 86

Swift Creek-Spirit Lake Interval.................................................................... 87

Spirit Lake Stage............................................................................................. 87

Smith Creek Period................................................................................. 87

Smith Creek-Pine Creek Interval........................................................... 88

Pine Creek Period ................................................................................... 88

Pine Creek-Castle Creek Interval........................................................... 88

Castle Creek Period................................................................................. 88

Sugar Bowl Period.................................................................................. 89

Kalama Period......................................................................................... 89

Goat Rocks Period.................................................................................. 90

Hazards From Tephra Eruptions..................................................................................... 90

References Cited............................................................................................................. 92

Appendix—Chemical Analyses of Pumice from Described Tephra Deposits............... 97

MEASURED SECTIONS

Measured Section C-l, Road 25 Site ............................................................................. 21

Measured Section C-2, Muddy River Quarry Site......................................................... 21

Measured Section M-l, Muddy River Quarry Site........................................................ 27

Measured Section M-2................................................................................................... 27

Measured Section S-l..................................................................................................... 32

Measured Section J-l..................................................................................................... 41

Measured Section Y-l.................................................................................................... 47

Measured Section P-1..................................................................................................... 56

Measured Section B-1.................................................................................................... 62

Measured Section B-2.................................................................................................... 62

Measured Section W-l................................................................................................... 71

Measured Section X-l.................................................................................................... 77
	CONTENTS VII

	FIGURES

1. Photograph of Mount St. Helens from the northeast showing pre-1980 profile.......................................................... 3

2. Map showing location of Mount St. Helens ................................................................................................................ 4

3. Photograph of thick tephra sequence........................................................................................................................... 10

4. Diagram showing wind directions over Mount St. Helens.......................................................................................... 11

5. Map showing areas below 600 m altitude near Mount St. Helens............................................................................... 14

6. Photographs showing field characteristics of tephra sets............................................................................................. 16

7. Photograph of set C at Road 25 site............................................................................................................................. 18

8. Composite columnar section of set C .......................................................................................................................... 18

9. Map showing approximate thickness of tephra in set C.............................................................................................. 19

10. Columnar sections of set C .......................................................................................................................................... 19

11. Photograph of set C at Muddy River quarry site ......................................................................................................... 20

12. Photograph showing typical colors in set M................................................................................................................ 24

13. Composite columnar section of set M.......................................................................................................................... 24

14. Map showing approximate thickness of tephra in set M.............................................................................................. 25

15. Columnar sections of set M ......................................................................................................................................... 26

16. Photograph of upper part of set M showing layer Mt.................................................................................................. 28

17. Photograph of typical thin, disturbed tephras of set K................................................................................................. 29

18. Map showing location and thickness of tephra in set K............................................................................................... 29

19. Columnar sections of set K.......................................................................................................................................... 30

20. Photograph of set S ................................................................................................................._^ 31

21. Composite columnar section of set S........................................................................................................................... 31

22. Map showing location and thickness of selected outcrops of set S ............................................................................. 32

23. Columnar sections of set S........................................................................................................................................... 33

24. Photograph of set S .................................................................................................................._^ 34

25. Map showing thickness of layers in set S.................................................................................................................... 35

26. Photograph of layer Ss................................................................................................................................................. 36

27. Photograph of set J....................................................................................................................................................... 37

28. Columnar sections of set J ........................................................................................................................................... 37

29. Map showing thickness of tephra set J......................................................................................................................... 38

30. Columnar sections of set J ........................................................................................................................................... 39

31. Maps showing thickness of layers in set J................................................................................................................... 40

32. Photograph of layer Jy ................................................................................................................................................. 41

33. Photograph of layer Jg ................................................................................................................................................. 43

34. Photograph of set Y showing thick layer Yn............................................................................................................... 44

35. Photograph of set Y showing thick layer Ye ............................................................................................................... 44

36. Composite columnar section of set Y.......................................................................................................................... 45

37. Map showing thickness of set Y.................................................................................................................................. 45

38. Columnar sections of set Y.......................................................................................................................................... 46

39. Photographs of set Y.................................................................................................................................................... 48

40. Maps showing thickness of layers in set Y.................................................................................................................. 49

41. Photograph of layer Yd................................................................................................................................................ 50

42. Photograph of upper part of set Y................................................................................................................................ 52

43. Map showing thickness of ashbedpbp ....................................................................................................................... 53

44. Photographs of set P..................................................................................................................................................... 54

45. Composite columnar section of set P........................................................................................................................... 55

46. Map showing thickness of set P................................................................................................................................... 56

47. Columnar sections of set P........................................................................................................................................... 57

48. Photograph of set B...................................................................................................................................................... 59

49. Composite columnar section of set B .......................................................................................................................... 59

50. Thicknesses of set B..................................................................................................................................................... 60

51. Columnar sections of set B .......................................................................................................................................... 61

52. Maps showing thickness of layers in set B .................................................................................................................. 63
VIII CONTENTS

53. Photograph of set B showing layer Bi......................................................................................................................... 64

54. Map showing thickness of ash bed ba......................................................................................................................... 65

55. Map showing distribution and thickness of layer D.................................................................................................... 66

56. Photograph of set W.................................................................................................................................................... 67

57. Composite columnar section of set W......................................................................................................................... 67

58. Photograph of set W showing thick layer We............................................................................................................. 68

59. Map showing thickness of set W................................................................................................................................. 68

60. Columnar sections of set W........................................................................................................................................ 69

61. Maps showing thickness of layers in set W................................................................................................................ 70

62. Photograph of large clasts in layer Wa........................................................................................................................ 72

63. Photograph of clasts in layer Wb................................................................................................................................ 73

64. Photograph of set X..................................................................................................................................................... 74

65. Composite columnar section of set X ......................................................................................................................... 74

66. Map showing thickness of set X................................................................................................................................. 76

67. Photograph of layers in set X...................................................................................................................................... 77

68. Columnar sections of set X......................................................................................................................................... 78

69. Maps showing thickness of layers in set X and of ash bed z...................................................................................... 79

70. Photograph of layer T.................................................................................................................................................. 80

71. Map showing distribution and thickness of layer T.................................................................................................... 81

72. Photograph of ash bed erupted in 1842....................................................................................................................... 82

	TABLES

1. Grain-size classification for tephra fragments ............................................................................................................ 5

2. Radiocarbon dates pertaining to age of tephra layers at Mount St. Helens ................................................................ 8

3. Relation of principal tephra units to eruptive periods and stages at Mount St. Helens .............................................. 11

4. Principal tephra-fall units erupted from Mount St. Helens prior to 1980................................................................... 12

5. Representative ferromagnesian mineral contents of layers of set B ........................................................................... 60

6. Estimated relative proportions of ferromagnesian phenocrysts in layers of set X...................................................... 75

7. Classification of pre-1980 eruptive history, Mount St. Helens................................................................................... 84
PRE-1980 TEPHRA-FALL DEPOSITS ERUPTED 

FROM MOUNT ST. HELENS, WASHINGTON

By Donal R. Mullineaux

ABSTRACT

Eruptions of Mount St. Helens, a highly explosive and 

frequently active volcano in the Cascade Range have, within 

the past 40,000 years, produced more than 100 tephra deposits now recognizable as distinct strata. The volcano has also 

erupted abundant pyroclastic flows, surges, and ash clouds, 

as well as lava flows and domes. Tephra deposits and those 

other products record a complex eruptive history of Mount 

St. Helens and provide information about the hazard it poses 

to people and property. The tephra strata also serve as timestratigraphic marker beds that are widespread in Pacific 

Northwest States.

Mount St. Helens' eruptive history consists of a long 

silicic phase followed by a shorter, more complex episode 

that included mafic as well as silicic eruptions. Until about 

2,500 years ago, the volcano produced only dacite and silicic 

andesite. At that time, mafic andesite appeared, and since 

then eruptions of andesite have alternated irregularly with 

those of dacite and even basalt.

The volcano's eruptive record is divided into four 

named stages, Ape Canyon (oldest), Cougar, Swift Creek, 

and Spirit Lake (youngest). The Spirit Lake stage is subdivided into six named periods, Smith Creek (oldest), Pine 

Creek, Castle Creek, Sugar Bowl, Kalama, and Goat Rocks 

(youngest). The eruptive history of the Ape Canyon and 

Cougar stages is relatively obscure; they occurred before and 

during the last major glaciation, and large parts of their 

deposits have been eroded away or strongly disturbed. 

Deposits of the Swift Creek stage are better preserved but not 

as well as those of the Spirit Lake stage, which are almost as 

well preserved as the deposits of A.D. 1980.

Most of the tephra strata are classified into ten major 

groups called sets. Each set includes more than one named 

layer, and each layer represents a different eruptive event or 

group of events. In addition, three single tephra strata are 

described separately. At least one tephra set or separately 

described layer was erupted during each named eruptive 

stage and period.

Tephra sets are distinguished chiefly on the basis of evidence of elapsed time, ferromagnesian mineral composition,

and grain size. Most sets and many layers are characterized 

by distinctive combinations of ferromagnesian minerals. 

Many of those combinations can be recognized in assemblages of heavy-mineral particles obtained by crushing and 

washing small pumice fragments.

Most tephras from Mount St. Helens are east of a northsouth line through the volcano because of transport by prevailing westerly winds. Most tephras of the oldest two 

stages, Ape Canyon and Cougar, are preserved well enough 

to decipher their stratigraphic relations only at sites that are 

below about 600 m in altitude. Almost all such sites are east 

or southeast of the volcano.

Tephra set C was produced during the Ape Canyon 

stage, which began about 40,000 or perhaps even 50,000 

years ago and continued until about 36,000 years ago. The 

set contains at least two large-volume dacitic pumice layers 

and other layers of smaller volume. The voluminous layers 

consist chiefly of lapilli and small bombs near the volcano 

and initially must have formed recognizable strata far downwind. One of them, erupted near the end of Ape Canyon 

time, records one of the largest volume tephra eruptions 

known from Mount St. Helens and has been recognized as 

far away as Nevada.

Set M was erupted during the early part of the Cougar 

stage, which began about 20,500 years ago. The set is characterized by several moderate-volume dacitic layers of 

pumiceous and lithic lapilli and ash, none of which is more 

than a few tens of centimeters thick near the volcano or as 

voluminous as the major layers of set C. Nevertheless, one 

ash bed that probably represents part of this set has been recognized as far away as Nevada.

Tephra set K was produced during the latter part of the 

Cougar stage about 19,000 years ago; it consists of multiple 

thin beds of dacitic pumice and ash. Set K is small in volume, 

and no layers in it are separately described. It was not recognized beyond the immediate vicinity of the volcano.

Sets S and J were erupted between about 13,000 and 

10,500 years ago during the early and late parts of the Swift 

Creek stage, respectively. Both are characterized by a few 

large-volume dacitic pumice layers that consist chiefly of
PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

lapilli near the volcano. Layers of each set have been recognized hundreds of kilometers east of Mount St. Helens.

A dormant period of more than 6,000 years, between 

about 10,500 and 4,000 years ago, followed eruption of set J. 

It is the longest time span known for which no evidence has 

been found of any eruptive activity at the volcano.

The Smith Creek period began the Spirit Lake eruptive 

stage with eruption of the dacitic set Y. This period is characterized by abundant and varied tephras but relatively few 

pyroclastic flows. Set Y eruptions started shortly after 4,000 

years ago and continued until at least about 3,300 years ago. 

The tephra set consists chiefly of two voluminous, coarse 

pumice layers interbedded with many smaller layers. One of 

the coarse layers, layer Yn, is the largest volume Holocene 

tephra known from Mount St. Helens; it and the similar, but 

smaller volume layer Ye have been found several hundred 

kilometers downwind.

The dacitic set P was produced by multiple small eruptions during the Pine Creek period between about 3,000 and 

2,500 years ago. In contrast to Smith Creek time, the Pine 

Creek period is characterized by relatively few tephra layers 

but many pyroclastic flow deposits. Set P consequently 

includes only relatively small volume, fine-grained tephra 

layers. Ash beds that represent set P have been recognized 

several hundred kilometers downwind, but no specific layers 

of this set were traced farther downwind than a few tens of 

kilometers.

Tephra set B includes andesitic, dacitic, and basaltic 

tephra accompanied by abundant lava flows but relatively 

few pyroclastic flows; all were erupted during the Castle 

Creek period between 2,500 and 1,600 years ago. Set B contains several small- to moderate-volume layers that are 

somewhat thicker and coarser than those of set P near the 

volcano. None, however, is as voluminous as the major layers of set Y or has been recognized as far downwind as ash 

layers of set P. No individual layers of set B were recognized 

beyond a few tens of kilometers from the volcano. The set B 

tephras record repeated changes in composition of magma 

discharged. Initial layers of set B are andesitic, and they are 

overlain by dacitic and finally basaltic tephras.

About 1,200 years ago, a small-volume dacitic tephra 

called layer D was ejected during an eruptive episode that 

emplaced the Sugar Bowl dome on the north flank of the volcano.

Eruption of the dacitic set W began the Kalama period 

late in the 15th century, probably in A.D. 1480. The initial 

event produced the large-volume, pumiceous layer Wn, the 

second largest Holocene tephra from Mount St. Helens. 

Layer Wn is overlain by several smaller pumiceous tephras, 

including the moderate-volume layer We. Both layers Wn 

and We have been traced for hundreds- of kilometers downwind.

Tephra set X, erupted next during the early part of the 

16th century, records a change to a more mafic composition 

within the Kalama period. This tephra set contains numerous

fine-grained andesitic beds that are smaller in volume than 

set W deposits. Set X beds have been recognized only near 

the volcano.

The Goat Rocks period began about A.D. 1800 with the 

eruption of the pumiceous layer T, which records a return to 

dacitic magma. Layer T was the last voluminous tephra 

ejected by Mount St. Helens before 1980, although several 

small-volume eruptions of lithic ash occurred later, near the 

middle of the 19th century. Only one ash bed from those 

events, probably erupted in A.D. 1842, has been traced 

across multiple outcrops.

Mount St. Helens will surely erupt in the future. Its 

eruptive history as determined before 1980 strongly indicated future activity, and the 1980 eruptions erased any 

doubts. Tephra from future eruptions will, as in 1980, affect 

distant as well as nearby communities. Although no way to 

prevent such eruptions is known, recognition of their potential hazards and appropriate planning can significantly 

reduce damage.

INTRODUCTION

Mount St. Helens in Washington State (figs. 1, 2) has 

been one of the most frequently active and most explosive 

volcanoes in the Cascade Range. Within the past 40,000 

years it has erupted more than 100 tephra deposits now recognizable as distinct strata. Its repeated eruptions have also 

produced abundant fragmental flowage deposits, lava flows, 

and domes. The fragmental flowage deposits are described 

in a comprehensive report by Crandell (1987). The present 

report describes the other major category of explosively 

erupted deposits, variously called tephra-fall, air-fall, or 

pyroclastic-fall deposits, or simply tephra. As used here, the 

word tephra describes only fall deposits, thus excluding 

deposits that resulted from pyroclastic flows, pyroclastic 

surges, and their accompanying ash clouds.

Strata described in this report record only some of the 

eruptions that have produced tephra in the past. They record 

only those eruptions large enough to produce widespread 

strata that have been preserved well enough to be recognized 

and traced. Among the tephra deposits recognized, those of 

relatively small to moderate volume far outnumber those of 

larger volume. Many other eruptions, which left deposits too 

small or too disturbed to be recognized in this study, also 

must have occurred. Thus, more tephra-producing eruptions 

surely occurred than are recorded by the deposits described 

herein.

Both the distribution and physical character of the 

tephra strata demonstrate that they are fall deposits. The 

deposits originated from eruptions that projected volcanic 

detritus up into the air, where it commonly was carried laterally before falling back to the ground. The deposits relatively 

uniformly blanket diverse terrain and are present as remnants
INTRODUCTION

Figure 1. Mount St. Helens from the northeast in 1964 showing relatively smooth pre-1980 profile and fan of debris that dammed preexisting valleys to form Spirit Lake (in the foreground). Letters D, S, and G designate the Dogs Head, Sugar Bowl, and Goat Rocks domes.

on widely separated ridge crests; such distributions show 

that the deposits dropped down through the air. The strata 

also are either widespread, coarse, or thick enough to ensure 

that they are not local deposits reworked by wind from preexisting strata. Each stratum described was identified in multiple outcrops, each except the upper layers of set X contains 

fragments of lapilli size or larger, and each is, at least locally, 

more than 1 cm thick.

Almost all tephra strata described herein are separated 

from each other by ash of mostly undetermined origin. Many 

of those ash beds probably are also tephra, ejected during 

early or late stages of eruptions that produced the coarser 

layers or during separate minor eruptive pulses. They probably also include some ash of pyroclastic-surge, ash-cloud, 

and eolian origin. A few ash beds are distinctive and widespread enough to be useful for correlation and are described 

as part of the stratigraphic sequence (see table 4). Tephra 

strata are also commonly interbedded with coarser pyroclastic-flow, mudflow, and colluvial deposits.

Recognition of the tephra sequence is based mainly on 

observed stratigraphic superposition. Unless otherwise 

noted, the relative stratigraphic position of each tephra unit 

was established by overlapping field relations. If the stratigraphic relation of two strata was not observed, the best estimate of that relation is stated and the absence of overlap 

noted.

These deposits provide important information about 

kinds, frequency, and magnitude of eruptions and about 

probable effects. Their thicknesses and distributions indicate 

that similar eruptions in the future would endanger life and 

property over large areas. An assessment of those potential 

hazards and hazards from other eruptive products was a principal goal of this study and that of Crandell (1987). These 

two studies describe data that were the basis of a pre-1980 

assessment of volcanic hazards at Mount St. Helens (Crandell and Mullineaux, 1978).

The tephras also provide information about compositional variations in magmas under Mount St. Helens. The
PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Figure 2. Location of Mount St. Helens relative to some nearby 

volcanoes in the Cascade Range of southern Washington and northern Oregon. Modified from Doukas (1990).

tephra sequence records both gradual and abrupt changes of 

mineralogical and chemical composition within and between 

successive eruptive periods. It records, for example, the 

appearance of mafic andesite that marks the beginning of the 

modern Mount St. Helens. Other changes that followed, 

including numerous variations in composition among andesite, dacite, and basalt, are also recorded.

Many tephra deposits are so widespread that they are 

especially useful as time-stratigraphic marker beds that 

allow correlation and dating of associated deposits from one 

location to another. Near Mount St. Helens, many distinctive 

tephra units can be identified in wide arcs around the volcano, allowing volcanic and other kinds of flowage deposits 

to be correlated from one valley to another. In addition, 

many individual layers are distinctive enough to be used as 

markers hundreds of kilometers from the volcano. The voluminous layer Yn in set Y, for example, forms a single, thick 

stratum recognizable to the north-northeast past Mount 

Rainier National Park (Crandell and others, 1962;

Mullineaux, 1974) and into Canada (Fulton and Armstrong, 

1965; Westgate and others, 1970). Similarly, layers Wn and 

We of set W extend northeast and east of the volcano across 

Washington State into Canada and Idaho, respectively 

(Smith and others, 1977). Because they are dated, the tephras 

provide ages as well as traceable time horizons for a variety 

of geologic and other scientific studies.

The value of several tephras as markers is enhanced by 

the fact that they contain distinctive mineral suites. In the 

Pacific Northwest, cummingtonite has been found in ferromagnesian mineral suites of volcanic ejecta from only 

Mount St. Helens and thus is virtually diagnostic of a Mount 

St. Helens origin. Some Mount St. Helens tephras contain 

other uncommon components, such as iron-rich hypersthene 

phenocrysts whose refractive index is high enough to discriminate them from hypersthene in other tephras.

TERMINOLOGY

TEPHRA

The term "tephra" refers to particles that were erupted 

into the air and then fell back to the ground or to deposits of 

those particles. The term was introduced by Thorarinsson 

(1944, 1954) to describe volcanic ash and coarser detritus 

that were projected through the air; deposits of pyroclastic 

flows were not included. Later, Thorarinsson (1974) 

expanded the term to include pyroclastic-flow materials. As 

such, tephra would describe virtually all pyroclastic material 

and be almost synonomous with pyroclastic and pyroclastic 

deposit. In this report, the term "tephra" is used in the earlier, 

more restricted sense in order to differentiate the deposits 

thus described from deposits of pyroclastic flowage events. 

As used here, "tephra" includes the materials and deposits 

resulting from events described as tephra fall, air fall, and 

pyroclastic fall but not deposits resulting from flowage 

events.

Most particles in Mount St. Helens tephra deposits were 

transported by wind. Some large, ballistic fragments, however, were little affected by wind and fell very near the 

source vent. In contrast, many wind-carried deposits form 

elongate, lobate blankets far downwind from the volcano 

that become progressively thinner and their particles finer 

with increasing distance. They also become thinner and finer 

from their long axes toward the sides of the lobes.

Tephra is classified chiefly by clast size, shape, vesicularity, and composition. Particles whose intermediate axes 

measure 2 mm or less are described as ash (table 1). Fine and 

coarse ash particles are smaller and larger than 1/16 mm 

across, respectively. Lapilli have intermediate axes from 2 to 

64 mm in length, and blocks and bombs are more than 64 

mm wide. Bombs have shapes or textures such as vesicularity that indicate they were liquid or plastic when erupted.
INTRODUCTION

Table 1. Grain-size classification for tephra fragments.

Grain size 

(millimeters)

A4 . __

2

1/16

Tephra fragments

Blocks and bombs

Lapilli

Ash, coarse

Ash, fine

Blocks generally are more angular and were solid when 

erupted.

Tephra clasts from Mount St. Helens are composed 

mostly of vesicular glass; pale clasts are called pumice and 

darker clasts scoria. Pumice and scoria clasts record eruption 

of new magma; that is, they represent rock material that was 

molten when erupted and that expanded into a froth before 

solidifying. The terms "pumice" and "scoria" are used both 

for highly vesicular particles and for some particles that are 

only moderately vesicular and have specific gravities greater 

than 1.

All tephra deposits from Mount St. Helens contain 

some lithic fragments that are dense or only slightly vesicular rock composed of glass and minerals. Tephras commonly 

also contain discrete mineral crystals and fragments of such 

crystals.

Compositions of Mount St. Helens tephras range from 

dacitic to basaltic. Compositional terms used herein are 

chiefly field names, based on color and on ferromagnesian 

mineral suites. If chemical compositions are known, names 

are as recommended by Le Bas and others (1986). Most 

pumice is dacitic or andesitic, and all the scoria is andesitic 

or basaltic.

SETS, LAYERS, AND BEDS

The tephra sequence is subdivided into sets, layers, and 

beds, based chiefly on evidence of elapsed time, differences 

in composition, and differences in grain size. Evidence of 

elapsed time between deposition of strata generally consists 

of oxidation or other soil development or deposition of intervening strata. Compositional differences may be recognizable in the field by differences in color and ferromagnesian 

phenocryst suites. Mineral content is especially useful; criteria such as grain size, thickness, vesicularity, color, and evidence of elapsed time can vary significantly from one 

outcrop to another, but ferromagnesian mineral suites are 

almost constant for specific strata.

Tephra sets are groups of strata that are similar in age 

and can be separated from older and younger deposits by 

composition or evidence of elapsed time. Each set records an 

episode of more than one eruption. A tephra layer consists of

one or more beds that can be traced as a stratigraphic unit, 

even though each bed might not be individually traceable. 

Units described as beds are strata that were observed locally 

but not traced laterally. Layers and beds record different 

eruptions within episodes represented by sets.

COLOR

Colors are described in word terms from the RockColor Chart distributed by the National Research Council 

(Goddard and others, 1948) or the Munsell Soil Color Charts 

(Munsell Color Company, Inc., 1954). Names alone are used 

rather than names and (or) the more specific color notations. 

Use of names alone describes a general rather than specific 

color, which is appropriate for these tephra deposits. The 

color of the tephras varies greatly according to local conditions, especially in the important hues of yellow and red, 

even with the depth one excavates into a cutbank, whereas 

the relative hues and strength of colors tend to be more consistent. Thus, pumice layers of set S are consistently lighter 

in color and less reddish than similar layers in set J, both 

where the tephras are strongly colored and where they are 

pale.

LETTER DESIGNATIONS

Letters used to name tephra strata are derived mostly 

from words used for field descriptions and do not follow an 

alphabetical or other regular pattern. Layer We, for example, 

is white and extends east of the volcano. Many of these designations have been defined in various earlier reports of D.R. 

Crandell, J.H. Hyde, and D.R. Mullineaux.

THICKNESS

The tephra strata show strong variation in thickness 

over distances of less than a meter as well as over longer distances (see fig. 3). Some of that variation developed as the 

tephra fell and represents primary differences from the average thickness of tephra that fell. Such differences can result 

from a variety of factors including downslope sloughing during fall, being carried down by rain, shielding of ground next 

to tree trunks, excess accumulation between trees by shedding of detritus from tree limbs (Waitt and Dzurisin, 1981, p. 

607), and flocculation of particles (Sorem, 1982) into larger 

(and thus faster falling) clasts (Carey and Sigurdsson, 1982).

Secondary variations in thickness developed after 

tephra falls have ended result chiefly from thinning or 

removal on the one hand and excess accumulation on the 

other. Erosion by water and ice, creep, and more rapid landsliding cause major variations. A long list of other factors are 

known to cause differences that are lesser but distinct.
PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Animal burrowing and tree-root throw are especially notable 

near Mount St. Helens.

As expected, the greatest variations in thickness are 

associated with slopes. Slopes commonly are stripped or 

thinned of a deposit. Coarse particles, especially pumice, 

tend to collect and form thick deposits near their bases, 

whereas fine particles commonly are blown up slopes that 

are steep and bare and form anomalously thick deposits near 

their tops.

Except in measured sections, thicknesses presented in 

this report represent my estimate of average thickness for 

the deposit, determined where possible from multiple outcrops. Their main purpose is to facilitate correlation and 

identification.

ferromagnesian mineral compositions similar to those of the 

tephra below the ash-rich deposit, whereas others have compositions similar to those of the tephra above. It is likely that 

lapilli from the tephras have mixed both upward and downward into the intervening ash-rich deposit.

These ash-rich deposits probably resulted from a variety of small-magnitude events including tephra eruptions, 

flowage phenomena, and eolian reworking of older deposits. 

Eruptions that contributed to the deposits probably were of 

low volume and low frequency. Consequently, individual 

deposits were thin, and considerable time ensued between 

their emplacement. During that time, near-surface turnover 

disrupted most evidence of stratification before overlying 

deposits were laid down, resulting in the generally massive 

character of the deposits.

DEPOSITS OF ASH CLOUDS AND OTHER 

PYROCLASTIC FLOWAGE PHENOMENA

Deposits of ash clouds derived from pyroclastic flows 

(Crandell and Mullineaux, 1973) are interbedded with tephra 

in almost every set recognized at Mount St. Helens. In addition, tephra strata are interbedded with deposits that originated from pyroclastic flows and other events called 

pyroclastic surges, base surges, and pyroclastic density 

flows. In general, those deposits are more variable in thickness and grain size than the tephra strata and less traceable. 

In this report, ash deposits that appear to have been formed 

by any kind of pyroclastic fragment and gas flow, whether 

directly from the flow itself or from elutriate clouds rising 

above it, are described as either ash-cloud deposits or ash of 

flowage origin.

OXIDIZED ASH-RICH DEPOSITS OF 

UNDETERMINED ORIGIN

Poorly sorted, massive to faintly bedded, ash-rich 

deposits that commonly are 50-200 cm thick are present 

between all the tephra sets of Pleistocene age. These deposits contain abundant volcanic glass particles and ferromagnesian minerals and scattered pumice lapilli. They are finer 

grained, more coherent, and more oxidized than most of the 

recognized tephras; many form protruding ledges in outcrop. These deposits commonly are widespread enough to 

serve as stratigraphic markers between the Pleistocene 

tephra sets.

In most places, the oxidized ash-rich deposits are 

mostly massive and have been strongly disturbed by nearsurface turnover within the soil zone. Locally, faint bedding 

shows a sequence of thin depositional units. In some deposits 

between tephras having different ferromagnesian mineral 

composition, scattered lapilli were sampled to compare with 

lapilli in the tephras. Some of these lapilli have

DIRECTION AND DISTANCE 

FROM MOUNT ST. HELENS

Direction and distance reported herein are measured 

from a presumed central vent of Mount St. Helens, unless 

otherwise stated. Because of the volcano's size, most tephra 

that is within about 4 km of that center is on its flanks. Most 

deposits, however, were carried much farther downwind. 

Wind-carried tephra is distributed in all directions from 

Mount St. Helens, although most was blown into the easterly 

quadrant from northeast to southeast of the volcano center.

FERROMAGNESIAN MINERALS

Ferromagnesian phenocrysts are prominent constituents in Mount St. Helens rocks. They are especially useful 

for identification and correlation of tephra units, chiefly 

because the kinds and abundances of ferromagnesian phenocrysts can differ markedly from one tephra unit to another. 

Commonly, the ferromagnesian mineral assemblage alone 

allows reliable identification of a set or even a layer. Within 

some sets, however, identification of specific layers or beds 

by mineral content is not feasible because the mineral suites 

are too similar among their strata. Nevertheless, mineral 

suites used in conjunction with other evidence such as stratigraphic and textural characteristics are adequate to identify 

and correlate most tephra strata. Ferromagnesian minerals 

were used rather than other phenocrysts because they are 

more readily separated and identified and because ferromagnesian mineral assemblages vary enough to be effective as 

criteria for identification.

Ferromagnesian mineral suites in Mount St. Helens 

tephras consist almost entirely of two or more of the following: cummingtonite, hornblende, hypersthene, augite, olivine, and biotite. In most dacitic tephras the suites consist 

mostly of a pair of minerals, either cummingtonite and hornblende or hypersthene and hornblende. The distinction
INTRODUCTION 7

between cummingtonite and hypersthene is critical and reliably discriminates between many otherwise similar tephras. 

Augite, biotite, and even olivine are present but less abundant in dacitic tephras. Augite and olivine, however, are predominant in the more mafic tephras.

In this investigation, ferromagnesian mineral phenocrysts were studied as crushed fragments rather than in thin 

section. The fragment-study method allows rapid preparation of samples and determination of mineral identities. 

Moreover, the great number of fragments made available by 

crushing facilitates estimation of relative mineral abundances as well as discovery of minerals that are present in 

small amounts. Crushing of about 1 cm3 of Mount St. Helens 

pumice, for example, typically produces several thousand 

fragments of ferromagnesian minerals suitable for study.

Most ferromagnesian mineral samples were prepared 

by crushing lumps of pumice, scoria, or denser rock and separating heavy- from light-mineral fractions. In the field, this 

was done by crushing the lumps with a small mortar and pestle, then washing out the light fraction with water in the mortar. Although the resulting heavy fraction contains much 

glass and plagioclase, the ferromagnesian minerals can be 

concentrated enough to identify most of them. In the laboratory, samples were crushed, screened to obtain a desired size 

fraction (usually 1/16-1/8 mm), and separated in a heavy liquid. A brief rinsing of ferromagnesian mineral samples in 

hydrofluoric acid removed adhering glass and facilitated 

identifications.

Mineral suites were examined first with a binocular 

microscope, with which thousands of grains can be viewed 

in a single field. Next, several representatives of each mineral species that could be segregated by color, shape, and 

cleavage were picked up individually on spindle-stage needles and examined in index oils using a spindle stage (Wilcox, 1959a; Bloss, 1981) and a petrographic microscope. 

Identification of each ferromagnesian mineral species noted 

for each layer or bed was confirmed using the petrographic 

microscope. With practice, most mineral suites from Mount 

St. Helens tephras can be recognized using a binocular 

microscope. In addition, the suites typical of many important 

tephra units can be recognized in the field using a hand lens.

Ferromagnesian minerals present consistently in carefully cleaned samples of a pumiceous tephra are regarded as 

primary phenocrysts representative of the parent magma. 

Minerals that were found only sporadically are regarded as 

possible accidental minerals. Such minerals are common but 

generally make up only a small proportion of any given 

sample.

Although the different proportions of minerals from 

one tephra unit to another are useful for identification, those 

proportions can vary widely for different samples of the 

same unit. Estimates of hypersthene:hornblende ratios for 

various samples of layer Wn in this study, for example, range 

from about 10:1 to about 2:1. Similarly, modal abundances 

for layer Wn determined from counts of more than 300

grains show a range of hypersthene:hornblende ratios of 

about from 10:1 to 3:1 (Smith and Leeman, 1982).

CHEMICAL ANALYSES

Chemical analyses were used sparingly during this 

study. At least one sample from most of the described tephra 

units was obtained (appendix), but results are cited only 

infrequently for describing tephras, magmas, or chemical 

differences. Data shown in the appendix are as reported by 

laboratories. Silica percentages reported in the body of the 

text are derived from recalculation of laboratory analyses to 

100 percent, free of H2O.

AGE

Tephra ages are based chiefly on numerical dates 

obtained from radiocarbon samples (table 2). All reported 

ages older than about 600 years are based on radiocarbon and 

are stated in radiocarbon years rather than calendar years. 

Some ages younger than 600 years are based on radiocarbon, 

but most are derived from growth-ring counts of trees. Carbonaceous material suitable for dating is common in deposits 

from Mount St. Helens; preservation of much of it has been 

enhanced by charring from hot tephra or pyroclastic flows.

Radiocarbon ages provide the chronologic framework 

for this tephra study. These ages are approximate, however, 

and should not be used for precise dating or detailed correlation. Radiocarbon ages have wide ranges of uncertainty, 

even though they may be reported precisely in some reports 

to even a specific year. Multiple samples and analyses 

obtained during this study suggest that the ages older than 

about 30,000 years from Mount St. Helens deposits have 

ranges of uncertainty of as much as thousands of years. Similar multiple analyses of samples younger than about 4,000 

years indicate much smaller uncertainties but still measured 

in hundreds of years. Ages derived from growth-ring counts 

of trees, in contrast, are relatively precise and can provide 

even specific-year dates if trees were killed or severely damaged by tephra falls.

Uncertainties relative to radiocarbon ages result from 

many factors, mainly those associated with difficulties in 

measuring the amount of radiocarbon remaining in a sample, 

variations in amount of radiocarbon in the atmosphere in the 

past, the fact that usually only limiting ages can be determined, and contamination by older or younger organic 

matter.

Realistic measurement uncertainties commonly are 

greater than reported, in part because uncertainties due to 

counting variation typically are reported to one standard 

deviation, to allow results to be compared from one laboratory to another. Radiocarbon ages obtained during this study 

were calculated to one standard deviation using the Libby
PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Table 2. Radiocarbon dates pertaining to age of tephra layers at Mount St. Helens.

Laboratory 

number

W-2993

W-5343

W-3138

W-2990

W-2527

W-2924

W-2925

W-2923

W-2541

W-2829

W-2675

W-3262

W-2549

W-3144

W-1752

W-3911

W-2677

W-1751

W-2587

W-2702

W-2991

W-3257

W-5722

W-5731

W-3548

W-5718

W-5724

W-5719

W-2868

W-2870

W-2866

W-2832

W-2441

W-3133

W-3141

W-3136

W-2413

W-^531

W-2540

W-2976

W-2661

W-3259

Sample description

Wood, from Sugar Bowl deposit

Wood, from Sugar Bowl deposit

Wood, from below Sugar Bowl deposit

Wood, above layer Bu

Charcoal, between layers Bu and Bo

Peat, above layer Bi

Peat, below layer Bi

Charcoal, above layer Bh

Charcoal, within set P

Charred wood, between sets Y and P

Charred wood, between sets Y and P

Wood, from Lahar

Charred wood, within set Y

Charred wood, within set Y

Charred wood, beneath layer Yn

Charcoal, below set Y

Charcoal, below set Y

Charcoal, beneath set Y

Charcoal, within set J

Charcoal, within set J

Charcoal, within set J

Wood, above set J

Peat, above Jb, Conboy bog

Peat, above Jg, Fargher Lake

Wood, below Jg, South Fork Toutle

Peat, above Jb, Conboy

Peat, below Jg, Fargher Lake

Peat, below Jy(?), Fargher Lake

Charcoal, between S and J, Rd 140

Charcoal, between S and J, Rd 140

Charcoal, between S and J, Rd 140

Charcoal, below set J

Charcoal, below set J

Peat, above upper set S

Charcoal, beneath part of set S

Peat, beneath upper set S

Charcoal, above set K

Charcoal, above set K

Charcoal, below sets M and K

Charcoal, within set C

Charcoal, within set C

Charcoal, within(?) set C

Laboratory age

1, 150160

1, 2001200

1,410±70

1,620±50

1,740±70

1,780±60

1,850±60

2,200±60

2,670±70

2,930±60

2,960±50

3,280±90

3,350±50

3,380±60

3,510±80

3,850±70

3,900±50

4,680±80

8,300±90

8,430±100

8,900±70

9,170±100

9,260±300

10,580±250

10,710±150

10,740±250

10,980±250

11,580±250

12,110±110

11,550±230

11,900±190

11,700±90

11,880±110

12,120±100

12,910±160

13,650±120

18,560±180

19,160±250

20,350±350

36,000±2,500

37,600± 1,300

>42,000

Reference

Crandell and others (1981).

Crandell and Hoblitt (1986).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

This study.

This study.

Crandell and others (1981).

This study.

This study.

This study.

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (1981).

Crandell and others (.1981).

Crandell and others (1981).

half-life (5,568±30 yr) and are referenced to A.D. 1950 

(Meyer Rubin, U.S. Geological Survey, written commun., 

1986). The ages are presented here as reported by the U.S. 

Geological Survey Radiocarbon Laboratory, Reston, Virginia. They were not recalculated to account for past changes 

in atmospheric radiocarbon contents or for recalculations of

the radiocarbon half-life. They can be adjusted to estimates 

of calendar-year dates by using data from the 1993 calibration volume of "Radiocarbon" (v. 35, no. 1).

In some earlier reports on Mount St. Helens tephras, we 

reported uncertainties larger than one standard deviation in 

order to allow for sample characteristics that suggested
INTRODUCTION

probable errors greater than those introduced by counting 

variation alone. Although those samples have not been since 

more accurately dated, some uncertainties reported herein 

are lower. My experience with many samples taken from 

stratigraphically controlled deposits at Mount St. Helens 

suggests that an uncertainty larger than that reported at one 

standard deviation, perhaps by as much as a factor of two, 

should be considered likely when attempting to correlate 

ages or deposits.

The amount of radiocarbon in the atmosphere has varied enough in the past that radiocarbon content can be equal 

in samples of different calendar ages (Suess, 1970). Thus, 

even if the amount of radiocarbon left in some samples could 

be measured exactly, it would not necessarily identify the 

specific age of the samples.

Radiocarbon samples also generally provide only limiting dates for tephra deposits because the organic material 

formed either before or after the short time during which the 

tephra fell. Although some tephras enclosed in peat have 

been dated approximately by combining samples of peat 

from just below and just above the tephra to obtain a single 

sample for dating, the date relies on the assumption that ages 

of the peat sampled below and above differ equally from the 

age of the tephra.

Contamination can result in anomalously old or young 

ages, from incorporation of old, nonradioactive carbon or 

from incorporation of younger carbon into organic material 

long after formation of the latter. Anomolously young ages 

were obtained for several samples during this investigation. 

The problem was especially noticeable for charcoal samples 

taken from old, coarse pumice layers that had been exposed 

at the surface for long periods of time. The charcoal may 

have absorbed younger organic material after deposition, 

perhaps from humic compounds in water that infiltrated the 

pumice.

Thus, although radiocarbon dates provide the ages for 

most of the deposits described, those ages are considered to 

be approximate. For comparisons and correlations, they 

commonly are rounded to the nearest 100,500, or even 1,000 

years.

PREVIOUS WORK

Studies whose results were used extensively during this 

study start with that of Verhoogen (1937). Verhoogen's 

report on Mount St. Helens noted the youth of the modern 

volcano and the presence of an "old Mount St. Helens series" 

older than the modern cone. He also described both a young 

widespread pumice and an older yellowish pumice on the 

flanks of Mount St. Helens.

Lawrence (1938, 1939, 1941, 1954) described young 

pumice deposits at Mount St. Helens and used tree-ring 

counts to assign ages to some of them. He noted that a thick 

pumice deposit buried the roots of many trees near Spirit

Lake. Examination of their growth rings gave an age of 

about A.D. 1800 for the pumice (Lawrence, 1939, 1954). 

That pumice is designated layer T (Mullineaux, 1964) in the 

sequence described here.

Carithers (1946) described the Mount St. Helens pumice deposits and their economic importance. He described 

distributions, thicknesses, and volumes for a thick, coarse 

"older" yellowish pumice and for a thick, white "younger" 

pumice. In addition, he reported a scattering of even younger 

gray pumice lumps at the ground surface. The yellowish and 

white layers of Carithers consist chiefly of layers described 

here as Yn and Wn, respectively, and the younger, gray 

lumps are part of layer T of this report.

D.R. Crandell and others recognized several widespread tephra marker beds in Mount Rainier National Park 

during studies of the origin of the Osceola Mudflow (Crandell and Waldron, 1956) and of surficial geology (Crandell, 

1969) and glaciation (Crandell and Miller, 1974) in the park. 

They suspected that three of those marker beds, then termed 

layers O, Y and W, originated at one or more volcanoes other 

than Mount Rainier, and they determined that two of them 

were relatively thick in the southern part of the park. R.D. 

Miller and I traced those two marker beds southward and 

found that they originated at Mount St. Helens. Additional 

studies at Mount St. Helens showed that a younger thick and 

coarse tephra (layer T) could also be distinguished (Mullineaux, 1964). Hyde (1975) discovered several older tephra 

beds on the south side of the volcano, and further work led 

to the classification of a tephra sequence that spans the 

known history of the volcano (Mullineaux and others, 1972, 

1975, 1978; Mullineaux, 1986).

C.A. Hopson (1971), during geologic mapping at 

Mount St. Helens in the late 1960's, noted that the distribution of layer T on the north flank of the volcano suggested 

that the layer originated at a vent on that flank rather than at 

the summit of the volcano.

Acknowledgments.—This study was carried out as part 

of investigations with D.R. Crandell of eruptive products, 

histories, and potential volcanic hazards of Cascade Range 

volcanoes. It was conducted jointly with a study of flowage 

deposits of Mount St. Helens by Crandell, who made countless important contributions to this work. It also benefitted 

significantly from concurrent studies at Mount St. Helens by 

J.H. Hyde and R.P. Hoblitt of the U.S. Geological Survey 

and by C.A. Hopson of the University of California at Santa 

Barbara and from other contributions by U.S. Geological 

Survey colleagues R.E. Wilcox, R.D. Miller, and C.D. 

Miller. Laboratory identification of most of the ferromagnesian minerals used to classify tephra units depended on 

application of Wilcox's development of a spindle stage and 

focal masking methods (Wilcox, 1959a, 1962, 1983). In 

addition, Wilcox and H.A. Powers in 1963 first identified 

cummingtonite as phenocrysts in the Mount St. Helens 

tephras (Wilcox, 1965). That discovery proved extremely 

important for studies of the Mount St. Helens products and
10 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Figure 3. Thick tephra sequence about 6 km northeast of the center of Mount St. Helens along 

USFS Road 99. Outcrop is about 6 m high. All tephra deposits exposed here were erupted during the 

past 13,000 years. Layers Yn, Wn, and T are the most conspicuous tephras northeast of the volcano. 

Layers Yn and Wn are also two of the largest volume eruptions known of the past 13,000 years. Layer 

Yn thins noticeably from the middle of the outcrop to the left; only a small part of the difference evident is from scraping of the outcrop. Layer Wn, in contrast, thickens from the middle of the outcrop 

to the left. Photograph taken in 1965.

for recognition of downwind marker beds. Valuable assistance provided by many members of the Gifford Pinchot 

National Forest, U.S. Forest Service, greatly facilitated the 

study. J. Archuleta ably assisted me during the field season 

of 1972.

Radiocarbon dates obtained for this study and those 

reported in Crandell and others (1981j were analyzed in the 

U.S. Geological Survey Radiocarbon Laboratory in Reston, 

Virginia, under the supervision of Meyer Rubin.

OVERVIEW OF TEPHRA SEQUENCE

The abundant tephra deposits produced by Mount St. 

Helens during the past 40,000 years are present in thick 

sequences around the volcano (fig. 3) and record many different eruptive episodes. Those episodes are grouped into 

stages and periods, each of which includes multiple events 

and is separated from others by hundreds or thousands of 

years of either dormancy or relative quiescence (tables 3,4). 

The tephra strata are interbedded with abundant coarse flowage deposits and ash deposits probably derived from flowage 

events as well as with lava flows. Compositionally, the volcano erupted only dacite and silicic andesite throughout most 

of its history. The appearance of mafic andesite and basalt

about 2,500 years ago marks increasing compositional complexity and the beginning of the modern volcano.

The prominent tephra strata are subdivided into ten sets 

and two single layers (table 3); the sets include more than 

100 depositional units (table 4). One or more tephra sets are 

recognized in each stage and each period of the volcano's 

eruptive history. In addition, a single thin ash bed probably 

erupted during the mid-19th century is described separately.

DISTRIBUTION

Most tephra deposits from Mount St. Helens lie in the 

180° sector east of a north-south line through the volcano 

because the prevailing winds blow toward that sector. Wind 

records suggest that almost 90 percent of the tephra should 

be expected in that 180° sector and more than 50 percent in 

the 90° sector from northeast to southeast of the volcano 

(fig-4).

PRESERVATION

Preservation of these tephras varies greatly, chiefly 

with age and altitude. Older tephras are more weathered than 

young ones, regardless of altitude, because of the longer time
OVERVIEW OF TEPHRA SEQUENCE 11

Table 3. Relation of principal tephra units to eruptive periods 

and stages at Mount St. Helens. NNW NNE

Tephra unit

Layer T

SetX 

SetW

Layer D

SetB

SetP

SetY

SetJ 

SetS

SetK 

SetM

SetC

Eruptive period

Goat Rocks

Kalama 

Kalama

Sugar Bowl

Castle Creek

Pine Creek

Smith Creek

Eruptive stage

Spirit Lake

Spirit Lake 

Spirit Lake

Spirit Lake

Spirit Lake

Spirit Lake

Spirit Lake

Swift Creek 

Swift Creek

Cougar 

Cougar

Ape Canyon

exposed to alteration. In addition, they have been more disrupted by physical processes such as creep, rapid landsliding, frost heave, tree-root throw, and animal burrowing that 

mix, turn over, or simply remove parts of the sequence.

The three tephra sets erupted before 18,000 years ago 

exhibit the strongest disturbance, as a result of long exposure 

to weathering and the intensity of cold-climate processes 

active during the maximum of the last major (Fraser) glaciation between about 25,000 and 15,000 years ago. The next 

two younger sets, erupted from about 13,000 to 10,500 years 

ago, are less disturbed, yet show some effects of late-glacial 

rigorous climate. Tephras formed during the past 4,000 years 

are remarkably well preserved; part of that good preservation 

results from the frequency of eruptions during that time. 

Most of these tephra strata were buried soon after deposition, 

and thus the effects of surface weathering and other, mostly 

physical, disturbances were minimized. Although innumerable rills and streams have sliced down into the deposits of the 

past 4,000 years, nearly undisturbed stacks of those strata are 

preserved between the eroded gullies.

The pre-1980 tephra strata less than 4,000 years old are 

almost as well preserved as those erupted in 1980. Comparison of the 1980 tephras with earlier tephras of the past 4,000 

years suggests that virtually any stratum prominent enough 

to be identified in the 1980 sequence would, if duplicated in 

the older deposits, be identifiable in the older sequence.

Disturbance of tephra of all ages has been greater at 

higher altitudes, chiefly because of the colder climate. Some 

deposits at relatively high altitudes were simply removed by 

glaciers during the Fraser glaciation. Moreover, processes 

such as creep, landsliding, and frost heave were increasingly

WNW

ENE

WSW ESE

SW SE

SSW SSE

Figure 4. Approximate percentage of time that wind blows in various directions over Mount St. Helens. Percentages are rounded averages of frequencies determined at various altitudes between 3,000 

and 16,000 m at Salem, Oreg., and Quillayute, Wash. (Winds Aloft 

Summary of the Air Weather Service, U.S. Air Force, available 

from the National Climatic Center, Asheville, N.C.). Modified from 

Crandell and Mullineaux (1978).

severe with altitude. Tephra sets C, M, and K, all of which 

were deposited before or during the maximum of the Fraser 

glaciation, generally are well enough preserved for interpretation of eruptive events only at altitudes lower than about 

600 m around Mount St. Helens (fig. 5). Tephras of sets S 

and J, which were laid down during the waning stages of the 

Fraser glaciation, commonly are well preserved at altitudes 

of about 1,000 m or less and are sparsely preserved up to an 

altitude of about 1,250 m.

LOCATION OF RELATIVELY COMPLETE 

TEPHRA SEQUENCES

Because of the westerly winds and increased disturbance with altitude, areas that are both in the quadrant from 

northeast to southeast of the volcano and at low altitude are 

most likely to display relatively complete tephra sequences 

(fig. 5). The most complete sequence at a single site that I 

found is east-southeast of the volcano at the confluence of 

Smith Creek and Muddy River; all major tephra units except 

layer T are well represented there. The sequence crops out 

above a rock quarry. It is referred to as the Muddy River 

quarry site and has been used frequently as a reference section. Unfortunately, U.S. Forest Service (USFS) Road N92 

leading to the site was destroyed in 1980. In 1989, access
Table 4. Principal tephra-fall units erupted from Mount St. Helens prior to 1980.

[Tephra units are listed in order of increasing age. Other ash includes units of ash-cloud, multiple, and uncertain origins; unit designations are all lowercase. Color i

shades of gray or brown. Vesicularity is described as vesicular (v) or nonvesicular (nv), in order of abundance. Ferromagnesian minerals are listed in approximate o

marked decrease in abundance: hy, hypersthene; hb, hornblende; cm, cummingtonite; ag, augite; ol olivine. Distribution on flanks of volcano is given in a clockwis

carried beyond volcano, if known. Thickness is representative thickness at 8-10 km downwind from volcano or at distance noted in parentheses. Grain size is domi

lobe at 8-10 km downwind from volcano or at distance noted in parentheses in "thickness" column. A.D. dates are from dendrochronology; see text for references. 

uncorrected; only mean values are reported; laboratory identification numbers and probable errors are given in Crandell and others (1981); laboratory numbers are al

Eruptive

period or Tephra unit Other

stage Set

Goat Rocks

X

Kalama

W

Sugar Bowl

Castle Creek B

Pine Creek P

Smith Creek Y

Layer ash

T

z

Xh

Xm

Xs

Xb

Wd

We

Wb

Wa

Wn

D

Bu

Bi

Bd

bab

bag

Bo

Bh

V

Py

Pu

pa

Ps

Pm

pbp

Yu

Yo

yp

Yf

ya

yc

Ye

Yn

Yd

Yb

Color

Light

Med

Med

Dark

Med

Dark

Light

Light

Light

Light

Lieht

Light

Dark

Light

Light

Med

Dark

Dark

Dark

Med

Med

Med

Med

Med

Med

Med

Med

Med

Med

Med

Med

Med

Light

Light

Med

Light

Number of

stratigraphic

units (minimum)

1

Many

Many

Many

Many

1

1

1

2

2

1

1

2

1

1

Many

Many

3

2

Many

2

3

Many

2

1

Many

6

1

1

5

1

Many

1

1

3

2

Vesicularity

of abundant

particles

V

nv

v,nv

v,nv

nv,v

v,nv

v,nv

V

v,nv

nv,v

V

V

V

V

nv,v

nv

nv

V

V

nv,v

nv,v

nv,v

nv,v

nv,v

nv,v

V

nv,v

nv

nv

V

V

nv,v

v,nv

Composition Distribution

Ferromagnesian On volcano

minerals

hy,hb/ag

hy,ag/hb

hy,ol,ag

ol,ag/hy

ol,ag,hy/hb

ol,ag,hv/hb

hy/hb

hy/hb

hy/hb

hy/hb

hv/hb

hy,hb

ol

hy/ag

hy/ag/hb

ol,ag,hy

hv.ag

hy,hb

hy,hb

hy,hb

hv.hb

cm,hb

cm,hb

cm,hb

cm,hb

cm,hb

cm,hb

cm,hb/bt

Rock type

Dacite

Dacite

Andesite

Andesite

Andesite

Andesite

Dacite

Dacite

Dacite

Dacite

Dacite

D

Basalt

Dacite

Dacite

Andesite

Mafic andesite

Mafic andesite

Dacite

Dacite

Dacite

Dacite

Dacite

Dacite

Dacite

Dacite

Dacite

flanks

NE

N-SE

NE

N-NW

N-S

NE-E

N-E

NE-S

NE

NE

NE

NE

NW-SW

NE-SE

NW-N

NW-SW

NW-SW

N-W

NW-S

NE-S

N-S

NE-SE

NE-S

NE-SE

NE-S

NE-S

NE-SE

NE-E

NE-S

NE-E

NE-E

NE-E

NE-S • N-SE

NE-SE

N-SE

Downwind

direction

NE

ENE

E

NE

NE

E

NE

NE

NE

NE

E

E

E

ESE

E

E

E

NE

E

E

E

NE

NE

E

E

NNE

E

N, E

Thick

(centim

50

7

4

9

12

6 (

3

30

5

5

180

1(?

8

10

4

10

8

13

6

15

5

15

6

17

8

20

20

5

2

16

15

40

30

200

40

30
Jg2

J Jb 

Jy

Js

Med

Light 

Light 

Light

1

1 

3 

Many

V

V 

V

v,nv

hb/hy/ag

hy,hb 

hy,hb 

hv.hb

Andesite

Andesite 

Dacite 

Dacite

SW-NW

E-SE 

NE-S 

E-SE

WSW 40

SE 30 

E 30 

E 5-1

Swift Creek Ash-rich fine-grained sediments of undetermined origin and pyroclastic-flow deposits

So 

Sg 

S Ss 

Sw 

Sb

Light 

Light 

Light 

Light 

Med

1 

1 

1 

2 

Many

V 

V 

V

v,nv 

v,nv

cm,hb/hy 

cm,hb/hy 

cm,hb/hy 

cm,hb 

cm,hb/hy

Dacite 

Dacite 

Dacite 

Dacite 

Dacite

NE-SE 

NE-SE 

E-SE 

NE-SE 

NE-SE

ENE 20 

ENE 35 

SE 5 

5

ion

Ash-rich fine-grained sediments of undetermined origin and pyroclastic-flow deposits

K Med Many v,nv cm,hb E-S 40

Ash-rich fine-grained sediments of undetermined origin and pyroclastic-flow deposits

Cougar Mt 

Mm 

Mp 

M Me 

Mo 

Mg 

ms

Med 

Light 

Light 

Light 

Med 

Med 

Med

3 

1 

1 

1 

1 

1 

Many

v,nv

V 

V 

V 

V

nv,v 

v,nv

hb,hy/cm 

hb,hy/cm 

hb,hy,cm 

hb,cm/hy 

cm,hb/ol,hy 

cm,hb 

cm,hb

E 

E-S 

E-SE 

E-S 

E-SE 

E-SE 

E-SE

6 

10(1

5(1

25(1

4(1

4(1

20(1

At least three obscure, unnamed groups of cummingtonite-bearing tephra, pyroclastic flow deposits, and other fine-grained ash-rich sediments

Cs2 

Cy

Ape Canyon C Cm 

Cw 

Ct

Cb

Light 

Light

Light 

Light 

Light

Med

1 

1

3 

1 

2

Many

V 

V

V 

V

v,nv 

nv,v

cm,hb/bt 

cm,hb/bt

cm,hb/bt 

cm,hb/bt 

cm,hb/bt/hy

hb/cm/bt

Dacite 

Dacite

Dacite 

Dacite 

Dacite

Dacite

SE-SW 

E-SE

E-SE 

E-SE 

SE

E-S

SSE 80 (

60 (

25 (

40(1

5(1

100

Radiocarbon sample taken from pyroclastic flow compositionally similar to tephra; stratigraphic relations approximate. 

Postulated stratigraphic position; relation to stratigraphic units shown as underlying not observed. 

3Radiocarbon sample taken from soil profile developed in upper part of layer Cb after deposition of that layer.
14 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

7'30' 122°00'

R4E

Base from U.S. Geological Survey 

Mount St. Helens and vicinity, 

Washington-Oregon, 1-: 100,000

R5E R6E 

10 KILOMETERS

R7E

CONTOUR INTERVAL 50 METERS

Figure 5. Approximate area below 600 m altitude (between heavy lines) in valleys immediately east and south of Mount St. Helens, and 

several sites (circles) at which relatively well preserved tephra sequences older than the Fraser glaciation are exposed. Base from U.S. Geological Survey Mount St. Helens and vicinity, Washington-Oregon, scale 1:100,000.
DESCRIPTION OF TEPHRA UNITS 15

was possible from USFS Road 83 by crossing Smith Creek 

on foot upstream from its junction with Muddy River and 

traversing southward along the east valley wall. Several 

other nearby outcrops in the Smith Creek-Muddy River valley display relatively complete tephra sequences but were 

inaccessible or almost so by road in 1989.

DESCRIPTION OF TEPHRA UNITS

The following sections are written for reference rather 

than for continuous reading. Brief descriptions of these 

tephras have already been published (Mullineaux, 1986). In 

the following sections, each set is first described as a whole, 

including its composition, diagnostic features, stratigraphic 

relations, distribution and thickness, age, and origin, if 

appropriate. Tephra sets are described as overall units 

because many have distinctive characteristics (fig. 6). Sets 

are especially important as markers for stratigraphic correlation and age determination around the volcano because they 

are present in much wider arcs around it than are individual 

layers.

After the summary for each set, I describe the same 

characteristics for each layer of each set except set K. 

Although this arrangement results in some repetition, it is 

used to provide pertinent information about each set and 

each layer in the description of that unit rather than repeatedly referring to other places in the text where such information can be found.

TEPHRA SET C

Tephra set C consists chiefly of an upper part that contains three coarse, white or pale-yellowish-brown pumice 

lapilli layers that are interbedded with ash overlying a basal 

layer consisting of moderately vesicular to dense ash (figs. 7, 

8). At 10-15 km from the volcano, the pumice layers consist 

of lapilli and bombs several tens of centimeters thick, and the 

basal ash layer commonly is 100 cm thick. Except for the 

basal ash, ash-size tephra units are thin; probable thin tephra 

beds lie between pumice lapilli layers of the upper part, 

interbedded with thick ash of probable flowage origin. Set C 

was identified from east of the volcano clockwise around to 

southwest.

Set C overall is noticeably oxidized and generally 

brown in color. Pumice fragments are weathered strongly 

enough to be soft and easily crushed by hand. Many denser 

fragments are also weathered enough to be soft.

COMPOSITION

Tephra set C consists chiefly of pumiceous to slightly 

vesicular, glassy tephra, but all strata contain some dense 

fragments. Lapilli layers consist mostly of pumice, whereas

ash layers contain more particles that are moderately vesicular to dense. Ferromagnesian mineral suites from all strata 

examined are mostly cummingtonite and hornblende and 

lesser amounts of biotite; biotite crystals commonly are relatively large and exhibit embayed margins. Scattered brown 

hornblende phenocrysts are common in pumice at the top of 

set C but are sparse in pumice from lower in the set. Small 

amounts of hypersthene are common. A few chemical analyses indicate a range of about 62-64 percent SiO2 for the 

pumice in set C.

DIAGNOSTIC FEATURES

Set C strata can usually be distinguished by soft, weathered clasts, biotite content, and stratigraphic position at the 

base of the tephra sequence. In most, but not all strata, biotite 

is abundant enough to distinguish set C from overlying sets 

M and K. Throughout its known range, set C as a whole is 

generally at least twice as thick as set M or set K.

STRATIGRAPHIC RELATIONS

No tephra from Mount St. Helens was identified below 

set C. Set C generally lies on a brown, clayey soil developed 

in colluvium or glacial drift. It is overlain by 100-200 cm of 

an oxidized, massive to faintly bedded ash-rich deposit of 

undetermined origin. In most outcrops that deposit clearly 

separates set C from set M or any other younger tephra.

DISTRIBUTION AND THICKNESS

Set C was recognized from east of the volcano around 

to the southwest (fig. 9), generally at altitudes of less than 

about 600 m. This distribution probably reflects limited preservation rather than initial extent. Tephra that was deposited 

at higher altitudes has been either eroded away or so modified that it was not identified.

Toward the east and south-southeast, layers of set C 

have been identified in eastern Washington and Nevada as 

far as 300 and 700 km, respectively, from the volcano 

(Foley, 1982; Davis, 1985; Busacca and others, 1992). 

Toward the southwest, however, the set was identified only 

as far as about 40 km from Mount St. Helens.

Set C is one of the thickest tephra sets at Mount St. 

Helens. Both east and southeast of the volcano, relatively 

complete sequences of the set are about 200 cm thick (fig. 

9). In many outcrops, set C appears even thicker because 

interbedded ash deposits of other origins are 100-200 cm 

thick. The set is thinner in most outcrops toward the south 

(fig. 9) but is locally very thick on the southern flank of the 

volcano.
16 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

ORIGIN

The major pumice lapilli layers in set C are clearly 

tephra deposits that record voluminous plinian-type eruptions. Some interbedded, locally thick ash beds exhibit 

crossbedding and other characteristics that indicate they

are not fall deposits, and these beds are not further 

described here. Still other ash beds that are thin and lie 

between layers Cm and Cy probably are tephra strata but 

could not be traced well enough to identify them confidently as such, and they are not separately identified or 

described.
DESCRIPTION OF TEPHRA UNITS 17

Figure 6 (above and facing page). Field characteristics of tephra units, Mount St. Helens. A, Road 

cutbank about 10 km northeast of the volcano. Coarse grain size and white color of the lower unit 

shown identify tephra set W, which here consists almost entirely of layer Wn. Tephra set W is overlain 

by the dark, fine-grained set X and, in turn, by the light-gray to brown, coarse-grained layer T. The 

brown color of layer T results from iron staining in a surficial soil zone. Dark-gray ejecta from the 

1980 eruptions lie at the top of the outcrop. Fine-grained set X here forms a slight ledge in this outcrop. The ledge causes small particles of layer T and the 1980 deposits to cover the base of layer T, 

which is not finer in its lower part as the photograph might suggest. Small pick shown for scale. Photograph taken in 1982. B, Outcrop above quarry, at 9 km bearing 055° from the volcano. Tephra set S 

overlies glacial drift; tephra set S consists of coarse, pale layers Sg and So overlying ash-size deposits. 

Set J is made up here of one lapilli bed and is separated from set S by a thick, oxidized ash-rich deposit. Set Y consists of lapilli and bomb layers Yb and Yn interbedded with ash. Set P is characterized 

by pale, thin layers; a flowage deposit is interbedded with the tephra here. Set B is relatively dark and 

also is characterized by thin layers. Set W, about 100 cm thick, consists chiefly of lapilli and bombs 

of layer Wn at this site. Photograph taken in 1970. C, Road cutbank about 14 km northeast of the volcano. Roadcut exposures over a wide area northeast of Mount St. Helens show distinct pale bands of 

pumice lapilli separated by darker bands of chiefly ash. Here, pale layer T lapilli are underlain by the 

dark band of set X, which separates the layer from the coarse, pale set W (mostly layer Wn). Tephra 

set W is about 125 cm thick here. Sets B and P and the upper part of set Y form a dark ash stratum 

below set W. In a fresher exposure, layer Yn in the lower part of set Y would be seen as another stratum of pale lapilli (see fig. 34). Photograph taken in 1983.

AGE

Radiocarbon samples from set C tephras provide 

ages between 40,000 and 35,000 years. Ages of about 

37,600 and 36,000 years (W-2661 and W-2976, respectively; table 2) were obtained from carbonized wood 

taken from the set, but neither sample is from its base; 

both are younger than the basal tephra. In addition, a 

sample from a lahar of Ape Canyon age provided an age 

of approximately 36,000 years (Crandell and others, 

1981). These dates have large uncertainties, however, 

and none provides an age for the start of the tephra 

sequence.

Some other evidence suggests that the Ape Canyon 

stage and eruption of set C started before 40,000 years 

ago. One radiocarbon age of more than 42,000 years 

(W-3259; table 2) was obtained from charcoal in a 

pumiceous pyroclastic flow of the Ape Canyon stage 

(Crandell, 1987). Moreover, in eastern Washington, two 

cummingtonite-bearing ash beds older than one identified as layer Cw have been suggested to be possible correlatives of layer Cb or an older tephra not found at 

Mount St. Helens (Busacca and others, 1992).

In addition, the carbon that provided the oldest radiocarbon age from the tephra sequence, from the oxidation
18 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Figure 7. Tephra set C along USFS Road 25 about 17 km southeast of Mount St. Helens (measured section C-l). The oldest tephra 

recognized here consists of about 100 cm of layer Cb, which overlies 50-100 cm of brown colluvium, which in turn overlies bedrock. 

Layer Cb is overlain successively by (1) pale layer Cw, (2) dark ash, 

and (3) pale layer Cm. About 200 cm of ash that includes thin tephra 

beds among thicker ash-cloud deposits separates layer Cm from the 

coarse layer Cy at the top of the set. Shovel handle is about 50 cm 

long. Photograph taken in 1986.

profile in layer Cb (fig. 8), may have been laid down 

after most of that oxidation had occurred. The oxidation 

profile itself may have required thousands of years to 

form.

At present, the pumice lapilli layers of set C, all of 

which are above the basal layer Cb, are regarded to be 

between about 40,000 and 35,000 years old. Layer Cb, 

however, may be as old as about 50,000 years (Crandell, 

1987; Busacca and others, 1992).

EXPLANATION 

Lapilli or bombs 

Ash

Position of dated 14C sample 

Oxidation zone

CENTIMETERS 1—50

— 0

Layer

Cy

Ash

Ash

Cm

Cw

Ct

Cb

Figure 8. Composite columnar section of tephra set C, Mount St. 

Helens.
DESCRIPTION OF TEPHRA UNITS 19

46° 

15'

122°15'

46°

00'

MOUNTS! 

HELENS A

40

,250

122°00'

200

,195

.165

,215

.195 .40

.110

.70

.20

,150

10 KILOMETERS

PREVIOUS DESIGNATIONS AND CORRELATIVE UNITS

Deposits of set C were initially included in the 

unnamed, oldest tephra set of Mullineaux and others (1972).

DESCRIPTION OF LAYERS IN SET C

Figures 7, 8, 10, and 11 and measured sections C-l and 

C-2 illustrate the stratigraphy of set C. Except for layer Cs, 

all strata described in the set are represented at measured sections C-l and (or) C-2. Correlation of specific layers in the 

set from one site to another is problematic. Outcrops are 

sparse, and many layers are obviously partly eroded such 

that thicknesses and grain-size characteristics cannot be 

readily compared from one outcrop to another. Evidence 

from thick layers such as Wn and We in the young and wellpreserved set W show that coarse and thick pumice layers in 

the same apparent stratigraphic position in outcrops only a

Figure 9. Approximate thickness (in centimeters) of tephra in set 

C, Mount St. Helens. Most sequences are truncated by erosion and 

thus incomplete, and ash deposits of probable flowage origin are interbedded with tephra at most sites where set C has been recognized.

Figure 10. Columnar sections of tephra set C from east to southsouthwest of Mount St. Helens and diagram of their locations relative to the central vent of the volcano.

Site

Distance from vent 

(in kilometers)

Cb? ; >.•;

EXPLANATION 

Lapilli or bombs 

Ash

Position of dated

14C sample 

Oxidation zone

Road 81 

17

1 V .4 ^

v a <i

<J „ * *

' ft a «

\

\ 

\ 

\

Swift 

13

^^M'"'o"i''V'*

7^i : <\\ V " * <V; 

[y« v

a < ? *

,V-'-

V-Y

t i n < 

< » p

>%v; i * * » *"*

Cb? 

-— Base of ex

Road 90 

16

>%«

""'aC V t,

0 <7 *

a ' a o

ti" < * 

*\\<

» ^<r"

^"i"? 

fe^'4'"

r*^,\

^ 7

posure

Cs

/ / / / / /

oRoad 81

o Muddy River 

Quarry

0 Road 25

CENTIMETERS

I— 100

\ \

\ opine

o Swift \

o Road 90

\

& <

V " ^

r. " ^

Muddy

River

Quarry

11
20 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Figure 11. Lower part of tephra set C at the Muddy River quarry site, 11 km east of Mount St. 

Helens (measured section C-2). Scraped part of outcrop shows about 100 cm of layer Cb over colluvium that lies on bedrock; a brown oxidation profile extends 30-40 cm down into layer Cb from its 

top. Layer Cb is overlain by (1) gray ash and lapilli of layer Ct, (2) yellow-brown pumice of layers 

Cw and Cm, and (3) gray-brown ash that lies between layers Cm and Cy (layer Cy was not dug out 

here). Shovel is approximately 100 cm long. Photograph taken in 1979.

few kilometers apart are not necessarily parts of the same 

shower bed. Thus, even the major layers in set C, except 

layer Cb, are only provisionally correlated from place to 

place around the volcano.

The relation of layer Cs to layer Cy has not been 

resolved. The two may be parts of the same deposit. Layer 

Cs is described separately because it is so coarse and thick 

south of the volcano and it is present so far downwind to the 

south-southeast.

Oxidation profiles within set C record pauses during 

accumulation of the set (fig. 8). The most oxidized profile is 

in the upper part of the basal layer Cb, and as many as three 

other oxidation profiles are present between layers Cm and 

Cy. Field examination of these profiles at the Muddy River 

quarry by P.W. Birkeland of the University of Colorado suggested that they might contain significant amounts of 

pedogenic clay; however, laboratory analyses show that 

most of the fine material is silt rather than clay and that recognizable soil development consists chiefly of oxidation 

(P.W. Birkeland, written commun., 1973).

LAYER Cb

Layer Cb consists of several tens of planar, thin- to 

thick-bedded strata of moderately vesicular to dense ash that 

includes scattered lapilli and small bombs. These strata vary

considerably in grain size and sorting, and some are graded. 

Their ferromagnesian mineral suites consist of mostly cummingtonite and hornblende and lesser amounts of biotite, but 

biotite was not found in all samples of the layer. Clinopyroxene was found in some samples, and several percent of 

hypersthene was found in one sample. Overall, pyroxene 

was sparse or absent in most samples examined.

In many outcrops, the entire layer is weathered to 

brown, and bedding is obscure. Locally, the lower part is 

gray and less weathered and shows distinct bedding. The 

oxidation profile (fig. 11) at the top of layer Cb is as thick as 

70 cm and commonly contains small carbonaceous fragments.

Layer Cb is recognized by its grain size, multiple thin 

beds, thick oxidation profile, and basal stratigraphic position. It is the oldest eruptive deposit from Mount St. Helens 

found in stratigraphic successions and commonly lies on 

weathered colluvium or glacial drift. It is overlain locally by 

thin layer Ct but generally by other layers of set C.

Layer Cb was found from east of the volcano clockwise 

around to south, chiefly at altitudes of less than 600 m. It 

commonly is about 100 cm thick 10-15 km east and southeast of the volcano and apparently thins rapidly from the 

southeast around to the south and with increasing distance 

from the volcano.

This layer probably originated from repeated smallvolume, explosive eruptions of partly degassed magma. In
DESCRIPTION OF TEPHRA UNITS 21

MEASURED SECTION C-l, 

ROAD 25 SITE

East side of USFS Road 25, SE '/4SW lk sec. 36, T. 8 N., R. 6 E., 2.2 km along road 

from bridge across Muddy River. Tephras exposed above rock face in road cut (fig. 7).

Thickness 

(centimeters)

16. Ash; overlies set C; contains three oxidation profiles.......... 150-200

Set C:

14. Layer Cy. Pumice lapilli and ash, light-yellow-brown;

grades finer from bottom to top; contains lapilli as much

as 5 cm across in lower part; upper contact wavy,

gradational, and probably eroded; pumice lapilli

commonly weathered, soft, and sheared off parallel to

outcrop face........................................................................ 35-50

13. Ash, coarse and fine, faintly bedded, locally crossbedded, 

pale-brownish-gray; upper 25-30 cm weakly oxidized; 

contains accretionary lapilli and thin lenses of small 

pumice lapilli; carbonaceous material common at base..... 60-70

12. Ash, fine, bedded, locally crossbedded, brown to

grayish-brown, slightly oxidized in upper 10-15 cm ........ 15-25

11. Ash and small pumice lapilli, thin-bedded; relatively

continuous but thickness varies from 2 to 12 cm at this

outcrop; possibly tephra..................................................... 7

10. Ash, fine, laminated; contains thin lenses of pumice; locally

fills channels ...................................................................... 15-40

9. Small pumice lapilli in ash, well-sorted; probably tephra...... 1-2

8. Ash, bedded, brownish-gray; contains at least five couplets

of fine and coarse ash ........................................................ 7

7. Ash and small lapilli, faintly planar bedded; bedding locally 

contorted; contains thin lenses rich in carbonaceous 

material; upper half slightly oxidized................................. 40-80

6. Layer Cm. Pumice lapilli and ash, brownish-gray to brown; 

crudely bedded or zoned, relatively fine in upper 15 cm; 

coarser zones near base and middle; contains pumice 

lapilli as much as 3 cm across, and abundant dark-stained 

lithic lapilli near middle of unit.......................................... 45

5. Ash, gray to brownish-gray, pumiceous.................................. 10

4. Layer Cw. Yellow-gray pumice lapilli as much as 4 cm 

across and dark-stained lithic fragments in ash; slightly 

finer in upper 10-15 cm...................................................... 25

3. Ash, fine, gray, sticky; at base of layer Cw.............................. 2

2. Layer Cb. Ash, planar-bedded to massive; near base coarse 

brown ash beds alternate with finer grayer beds; 

remainder mostly disturbed and oxidized but locally 

contains lenses of gray, bedded ash; carbonaceous 

material common near top ................................................. 70-100

Below set C: 

1. Colluvium, oxidized, brown; overlies bedrock. 70

places the deposits are similar in grain size and thickness to 

those resulting from pyroclastic flowage events, but crossbedding or other evidence of flowage was not seen.

No carbonaceous material that might provide a date for 

the onset of eruptions of set C was found associated with 

layer Cb. The oldest dated carbon recovered anywhere from 

the Mount St. Helens tephra sequence is from the oxidized 

zone in the upper part of layer Cb (measured section C-2); it

MEASURED SECTION C-2, 

MUDDY RIVER QUARRY SITE

East bank of Muddy River at its confluence with Smith Creek, near the center of 

NW '/4 sec. 15, T. 8 N., R. 6 E. Tephras crop out above the rock face of quarry 

along an abandoned section of former USFS Forest Road N92 (fig. 11). Most tephras vary substantially in thickness across the outcrop. Thicknesses reported are estimates of average thickness or ranges observed at this outcrop.

Thickness 

(centimeters)

10. Ash, oxidized; overlies set C.................................................... 150-200

SetC:

9. Layer Cy. Pumice lapilli and ash; upper 15-20 cm relatively 

fine grained, yellow brown; basal 5-10 cm partly bleached 

to pale gray; charcoal common at base (source of 

radiocarbon sample W-2976 (table 2), dated as 

36,000±2,500 yr B.P.) ......................................................... 60

8. Ash; contains scattered moderately vesicular lapilli; gray at 

base, grades upward to brown in oxidation profile in 

uppermost 30-40 cm .......................................................... 90

7. Ash, lithic and pumiceous, brown to gray; oxidation profile

visible locally in uppermost 10-15 cm ............................... 25

6. Layer Cm. Small pumice lapilli and ash, yellowish brown:

Small lapilli in ash .............................................................. 20

Ash....................................................................................... 1

Small lapilli in ash; many fragments stained dark .............. 5

Ash....................................................................................... 1

Small lapilli in ash; crudely bedded ................................... 10

5. Ash; below lowest lapilli of layer Cm....................................... 2

4. Layer Cw. Chiefly pumice lapilli and small bombs, massive, 

pale-gray to yellowish-brown; pumice in basal 10 cm is 

bleached and clay rich......................................................... 40

3. Layer Ct. Small pumice lapilli and ash

Ash, fine to coarse, gray....................................................... 6

Small lapilli and ash; two beds evident locally.................... 2-5

Ash, fine, gray ..................................................................... 2

2. Layer Cb. Moderately to nonvesicular ash containing

scattered lapilli; massive to planar bedded; brownish gray

at base, upper part brown in oxidation profile; oxidation

profile extends down 30-40 cm from top and contains

scattered charcoal (source of radiocarbon sample W-2661

[table 2], dated at 37,600±1,300 years B.P.) ....................... 100

Below set C 

1. Colluvium, brown, strongly oxidized; overlies bedrock. 75-100

was dated as approximately 37,600 years old (W-2661; table 

2). That carbon probably had been mixed downward into the 

oxidized part of the layer by turnover within a soil zone. If 

so, the carbonaceous material is younger than the topmost 

bed of layer Cb and younger than part of the oxidation. Neither the time that elapsed between deposition of the top of 

layer Cb and formation of that carbonaceous material nor the 

time represented by the oxidation is known.

Ash from Mount St. Helens in eastern Washington, possibly correlative with layer Cb, has been estimated to be as 

old as about 50,000 years (Busacca and others, 1992).
22 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

LAYER Ct

Layer Ct consists of as many as three thin, poorly 

defined beds of small lapilli enclosed in ash. At measured 

section C-2, two zones of lapilli are separated by and 

enclosed in ash. More than one eruption may have formed 

this layer, but the multiple lapilli beds might represent only 

changing conditions during a single eruption. Curnrningtonite and hornblende make up most of the ferromagnesian 

mineral suite of this layer. No diagnostic features were recognized with which to differentiate layer Ct from other pumice layers in set C; it is identified only as a lapilli-bearing 

layer that directly overlies layer Cb.

Layer Ct directly overlies the oxidized top of layer Cb 

and underlies the coarser layer Cw. It was recognized only 

toward the southeast, within 15 km of the volcano.

LAYER Cw

Layer Cw is a single thick, relatively massive deposit 

that contains pumice lapilli and bombs. It typically is pale 

brownish gray and locally has a bleached, white zone of soft, 

clayey pumice at its base. Its ferromagnesian mineral suite 

consists chiefly of cummingtonite, green to olive hornblende, and biotite; brownish hornblende is sparse.

The layer is generally coarser grained than layers 

immediately below and above it. It is the coarsest layer in the 

set except for layers Cy and Cs at the top. Its identification is 

based chiefly on its grain size and thickness and its stratigraphic position in the set. In most outcrops, its clayey basal 

zone and massive character distinguish it from layer Cm, and 

the paucity of brown hornblende in its ferromagnesian suite 

generally distinguishes it from layer Cy at the top of the set.

In some places, layer Cw overlies layer Ct; elsewhere, 

it lies directly on layer Cb. Where it has been recognized, it 

is overlain by layer Cm.

The layer is well exposed east and southeast of the volcano, where it is about 40 and 25 cm thick at 11 and 17 km 

distance, respectively. It is similarly thick at several other 

sites within about 3 km of those locations but was not identified farther around the volcano. A single pumice lapilli 

layer in the lower part of set C on the south side of the volcano might be correlative with layer Cw, but that possibility 

could not be confirmed. Because it is thick and coarse 

grained, layer Cw should make a recognizable marker bed 

downwind. About 300 km to the east, a layer identified as 

Cw has been found in at least four different sites (Busacca 

and others, 1992).

LAYER Cm

Layer Cm consists of pumice and lithic lapilli and ash 

in a deposit that typically is crudely stratified. At the Muddy 

River quarry, three beds or zones of lapilli are interbedded

with ash (measured section C-2). The lower bed consists 

dominantly of crudely bedded pumiceous ash and lapilli. 

Lapilli in the middle bed are smaller and consist of about 

equal parts of pumice and lithic fragments, many of which 

are stained black. The upper bed consists of ash and lapilli 

that contain abundant lithic fragments. The ferromagnesian 

mineral suite of layer Cm is dominated by cummingtonite 

and green to olive hornblende; it also contains biotite and 

sparse brownish hornblende.

The stratification, stained lithic fragments, and lack of 

a zone of soft, weathered pumice at its base best distinguish 

layer Cm from layer Cw. Those same features, as well as its 

paucity of brownish hornblende and its stratigraphic position, distinguish it from layer Cy.

Layer Cm directly overlies layer Cw. It is overlain by 

ash beds that contain as many as three oxidation profiles and 

separate it from the pumice layer Cy at the top of the set 

(fig. 7).

Layer Cm was identified from approximately east to 

southeast of the volcano, where it is as thick as 30 cm at a 

distance of almost 20 km. On the south flank of the volcano, 

only a single pumice lapilli layer was seen in the lower part 

of set C, and whether it is layer Cw, Cm, or yet another layer 

is not known.

The grain-size changes that produce the stratification in 

layer Cm could have resulted from multiple small eruptions 

or perhaps only from fluctuations in strength of a single 

eruption, such as occurred at Mount St. Helens on May 18, 

1980.

LAYER Cy

Layer Cy is a single, normally graded pumice deposit 

that is the thickest layer east of the volcano, where it forms 

the top of set C (fig. 8). At the Muddy River quarry 11 km 

east-southeast of the volcano, the layer consists of 60 cm of 

mostly pumice lapilli and small bombs. It also contains as 

much as about 10 percent lithic fragments. Mineralogically, 

it is similar to other layers in the set except that it contains 

abundant brown hornblende, which is sparse in ferromagnesian mineral suites from layers in the lower part of set C. 

Pumice lumps in layer Cy typically are soft and readily broken by hand.

This layer is identified east and southeast of the volcano 

by its grain size, vertical grading, thickness, and stratigraphic position at the top of the set. The abundance of 

brown hornblende distinguishes it from pumice layers lower 

in the set but not from layer Cs, which also contains abundant brown hornblende.

Layer Cy overlies ash deposits that are in part of flowage origin and commonly are 100-200 cm thick 10-15 km 

east and southeast of the volcano. At the Muddy River 

quarry, relatively abundant carbonaceous material is present 

at the base of layer Cy. The layer is overlain by an oxidized
DESCRIPTION OF TEPHRA UNITS 23

ash-rich deposit of uncertain origin that commonly is a meter 

or more thick and contains multiple oxidation profiles.

Layer Cy was recognized in and near the Smith 

Creek-Muddy River valley east and southeast of the volcano, mostly in outcrops within a few kilometers of the 

Muddy River quarry. At the Muddy River quarry the layer is 

about 60 cm thick, and at the Road 25 site 6 km southeast of 

the quarry the layer is 30-50 cm thick. The grain size and 

thickness of layer Cy indicate that it should form a marker 

bed far downwind, but the direction of the downwind lobe is 

not known.

The carbonaceous material at the base of layer Cy indicates that the pumice was hot and thick enough to char vegetation sufficiently to preserve it. The age of that carbon, 

which has been dated at about 36,000 years (measured section C-l) (W-2976; table 2), probably is close to the age of 

the overlying pumice.

LAYER Cs

Layer Cs is a thick, pumice lapilli and bomb deposit at 

the top of set C south-southeast and south of the volcano. It 

is the thickest known Pleistocene tephra erupted from Mount 

St. Helens and perhaps the most voluminous. Similar to layer 

Cy, it consists mostly of pumice and is normally graded, and 

the two layers have similar ferromagnesian mineral suites. 

Layer Cs is weathered enough that most of its pumice lumps 

are soft and can be easily cut through.

Layer Cs lies at the top of set C and is similar to layer 

Cy, and the two may be parts of the same deposit. My 

attempts to prove or disprove that correlation were unsuccessful; no outcrops showing overlap of the two layers were 

discovered. The two are described separately because their 

known thickness characteristics do not match that expected 

for a single shower bed. From sites south of the volcano, 

layer Cs thins northeastward toward outcrops of layer Cy. 

Similarly, from sites southeast of the volcano, layer Cy thins 

southwestward toward outcrops in which layer Cs is thick 

and coarse.

Layer Cs is commonly underlain by 100-200 cm of ash 

that is in part of flowage origin and contains thin oxidation 

profiles. It is overlain by an oxidized ash-rich deposit that is 

similarly thick and also contains oxidation profiles.

Layer Cs is 75-100 cm thick and contains lapilli and 

small bombs at sites between 10 and 20 km distance southsoutheast of the volcano. At 100 km south-southeast of 

Mount St. Helens it is 40 cm thick and includes small lapilli 

in ash (Crandell, 1980, p. 73). Farther to the south-southeast, 

a cummingtonite- and biotite-bearing ash bed in southern 

Oregon (Davis, 1985) and Nevada (Davis, 1978) probably is 

correlative with layer Cs.

Its stratigraphic position similar to that of layer Cy suggests that layer Cs is about 36,000 years old. That approximate age is supported by a radiocarbon sample from 25 cm

above a probably correlative ash bed in Nevada that has been 

dated at 33,65011,720 years B.P. (Davis, 1978).

UNNAMED ASH DEPOSITS ASSOCIATED WITH SET C

Several ash strata of undetermined origin, from a few 

centimeters to more than 100 cm in thickness, lie between 

the coarse pumice layers in the lower and upper parts of set 

C (figs. 7,8; measured sections C-l, C-2). Ash and scattered 

lapilli in these strata range from nonvesicular to highly 

vesicular, and ferromagnesian mineral suites are similar to 

those of the pumice in described tephra layers of the set. 

Although bedding is obscure in most outcrops, many of these 

deposits probably were distinctly bedded when fresh. 

Locally consistent thickness and grain size suggest that some 

of these thin deposits record tephra eruptions of small volume. In contrast, other strata show crossbedding and strong 

lateral variation in thickness and grain size and contain 

accretionary lapilli, all of which suggest pyroclastic flowage 

phenomena.

TEPHRA SET M

Set M, which is about half the thickness of set C, is 

made up of several yellowish-brown to gray pumice and 

lithic lapilli layers, from a few centimeters to about 15 cm in 

thickness, that are interbedded with purplish-gray to brown 

ash beds (figs. 12,13). As a whole, the set typically is recognizable as a sequence of thin lapilli layers interbedded with 

the purplish-hued ash. Pumice lumps in set M commonly are 

weathered enough to be soft and easily broken by hand, 

though they are less weathered than pumice in set C. No oxidation profiles were identified within set M. Except near its 

base, set M is not commonly interbedded with ash deposits 

of flowage origin.

COMPOSITION

All named tephra layers in set M contain abundant lithic 

and pumice fragments. Ferromagnesian minerals from the 

lower part of the set are chiefly cummingtonite and hornblende; hypersthene is sparse. Upward within the set, however, hypersthene increasingly replaces cummingtonite, and 

ferromagnesian mineral suites of upper layers consist almost 

entirely of hypersthene and hornblende. Some layers contain 

small amounts of biotite, and one, layer Mo, contains a small 

amount of olivine, even though the layer is dacitic in composition.

Ash between the pumice lapilli layers consists of pumice and mineral grains, glass shards, and a variety of lithic 

particles, including many fresh, gray rock fragments similar 

in mineral composition to the pumice.
24

**LLL •-£*&••* "'^ -

ia» Vfc r " ">;>

PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

EXPLANATION

Lapilli or bombs 

Ash

CENTIMETERS 

|—20

Mt

Figure 12. Typical colors of lapilli layers and interbedded ash in 

tephra set M 9 km east of Mount St. Helens. Yellowish-brown pumiceous layers Mm and Mp in the upper part of the set are separated 

by brown ash. Tephra layers Me, Mo, and Mg lower in the set are 

separated by purplish-gray to purplish-brown ash. Ice axe is shown 

for scale. Photograph taken in 1971.

DIAGNOSTIC FEATURES

Multiple thin lapilli layers, purplish-gray interbedded 

ash, and a thickness of about half that of underlying set C 

usually identify set M in the field. In places, the interbedded 

purplish-gray ash distinguishes set M from all other sets. In 

addition, the progressive upward substitution of hypersthene 

for cummingtonite in ferromagnesian mineral suites identifies the set wherever enough layers are present. The presence 

of olivine in a cummingtonite-hornblende ferromagnesian 

mineral suite in one layer also identifies the set as M.

STRATIGRAPHIC RELATIONS

Set M is separated from set C by 100-200 cm of a grayish-brown ash-rich deposit that contains multiple oxidation

Figure 13. Composite columnar section of tephra set M, 

Mount St. Helens.

profiles. The tephra set is overlain by another similar ashrich deposit, also commonly 100-200 cm thick, that separates it from tephra set K.

DISTRIBUTION AND THICKNESS

Set M has been identified from east of the volcano 

clockwise around to south-southwest (fig. 14). The most 

complete stratigraphic sequences are 9-15 km east and 

southeast of the volcano. There, the set commonly is 50-100 

cm thick and includes all the tephra layers described. Set M 

tephra was surely deposited more widely but elsewhere has 

been mostly eroded away or incorporated into colluvial and 

glacial deposits.

I did not trace set M tephras beyond about 20 km 

downwind from the volcano. Cummingtonite-bearing ash 

of approximately the age of set M has been identified about 

500 km to the northeast in Canada (Westgate and Fulton, 

1975) and about 700 km to the south-southeast in Nevada 

(Davis, 1985).

ORIGIN

Tephra layers of set M probably resulted from explosive eruptions of small to moderate volume. Abundant fresh 

lithic fragments similar in composition to the pumice suggest 

that domes formed during intervals between explosive eruptions. Faintly bedded, lenticular ash strata interbedded with 

the lower part of the set (ash unit ms, fig. 13) probably 

include some beds generated by flowage events, but such 

deposits are less abundant than those associated with set C.
DESCRIPTION OF TEPHRA UNITS 25

122°15' 122°00'

46° 

15'

46° 

00'

-40

MOUNTS! 

HELENS A .75

,30 

-45

30

.40

.30

10 KILOMETERS

Figure 14. Approximate thickness (in centimeters) of tephra in set 

M, Mount St. Helens. Ash deposits of probable flowage origin are 

commonly interbedded with tephra in the lower part of the set.

AGE

Radiocarbon dates of about 20,350±350 (W-2540) and 

19,1601250 (W-4531) (table 2) years B.P. were obtained 

from pyroclastic-flow deposits that underlie and overlie, 

respectively, sequences of set M and set K tephras south and 

southeast of the volcano. Those sequences, however, are 

incomplete. No carbonaceous material for dating was recovered from the more complete series of layers in the set 

exposed to the east. No weathering profiles were noted 

within set M, and the set probably was erupted within a relatively short time between about 20,000 and 19,000 years 

ago.

PREVIOUS DESIGNATIONS

Set M was included as part of the oldest, unnamed set 

of Mullineaux and others (1972).

DESCRIPTION OF LAYERS IN SET M

Six widespread lapilli and ash layers are described as 

tephras in this set (fig. 15). Ash deposits of uncertain origin 

near the base of set M include as many as three thin beds that 

may also be tephra layers, as well as at least three lenticular

deposits of probable flowage origin. These deposits were not 

traceable laterally and are not described separately. The layers described as tephra, in contrast, could be traced for at 

least a few kilometers around the volcano. All the layers 

described as tephra in set M are exposed at the Muddy River 

quarry site (measured section M-l) and along a former road 

across the river to the west (measured section M-2).

East of the volcano, as many as three thin beds of pumiceous lapilli in ash are locally distinguishable immediately 

above layer Mm (fig. 15). These beds, layer Mt, differ mineralogically from other tephras in set M by their scarcity or 

absence of cummingtonite. They are described as part of set 

M because they overlie earlier tephras of the set with no recognizable break and because they continue the trend of 

decreasing amounts of cummingtonite upward in set M.

LAYER Mg

The oldest tephra described in this set, layer Mg, is in 

most places a single layer as thick as 10 cm of pumiceous ash 

and lapilli that contains abundant gray lithic fragments (fig. 

12). Ferromagnesian mineral suites from its pumice consist 

almost entirely of cummingtonite and olive hornblende; 

small amounts of hypersthene and biotite were found in 

some samples. Southeast of the volcano the layer locally 

contains two lapilli beds, which suggests that the layer consists of more than one overlapping shower bed.

The layer typically is only a few centimeters thick and 

lacks distinctive characteristics. It is recognized chiefly as 

the lapilli-bearing layer immediately under the distinctive 

layer Mo.

Layer Mg generally overlies pumiceous ash beds of 

probable flowage origin that are approximately the same age 

as set M tephra, which in turn overlie an older, oxidized, ashrich deposit. Layer Mg is overlain by layer Mo and separated 

from it by purplish-gray ash.

Layer Mg was identified only from east of the volcano 

clockwise around to the southeast; its maximum known 

thickness is about 10 cm. The grain size and thickness differences observed do not define a single axis of its distribution 

lobe.

LAYER Mo

Layer Mo is a single bed, a few centimeters thick, of 

pumiceous and lithic ash and small lapilli. Cummingtonite 

and hornblende make up most of the ferromagnesian mineral 

suites from the pumice, but small amounts of olivine are consistently present. Olivine was not found in any other bed of 

the set, and it makes layer Mo the most identifiable stratum 

of set M. Lithic fragments are about equal in abundance to, 

or even more abundant than, pumice clasts in the layer.
26 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Site

Distance from vent 

(in kilometers)

Section 

29

16

Muddy 

River 

Quarry

11

Mm?

EXPLANATION

Lapilli or bombs 

Ash

Position of dated 

14C sample

Mount St 

Helens vent

Section 

29

o Road 90

Figure 15. Columnar sections of tephra set M from approximately east to south of Mount St. Helens and diagram of their locations relative 

to the central vent of the volcano.

Although the layer is widespread (fig. 15), no evidence of 

more than one bed in layer Mo was seen.

The layer was identified at numerous sites from east of 

the volcano around to the southeast (fig. 15). The maximum 

thickness known is about 5 cm. Toward the southeast, it was 

identified to a distance of about 20 km, but the direction of 

the axis of its lobe is not known. Because of its olivine content, the layer should form a useful stratigraphic marker for 

at least several tens of kilometers from the volcano.

LAYER Me

In most outcrops, layer Me (fig. 12) is the most conspicuous tephra in set M. It generally is the thickest layer in the 

set, and even where not thickest it usually is the coarsest.

Close to the volcano it contains abundant lapilli and even 

small bombs and consists of roughly equal proportions of 

pumice and lithic fragments. Ferromagnesian mineral suites 

from its pumice are dominated by cummingtonite and hornblende but consistently include a few percent hypersthene.

The layer is best recognized by its coarse grain size and 

thickness. In addition, it generally contains more hypersthene than older deposits in the set and more cummingtonite 

than overlying strata. Layer Me overlies layer Mo and underlies layer Mp and is separated from both by purplish-gray to 

reddish-brown ash.

The layer is readily traced from east to southeast of the 

volcano and apparently is one of the two layers of set M on 

the south flank of the volcano. At the Muddy River quarry, 

it consists of only a few centimeters of ash and lapilli (fig. 

15). From there, it thickens and coarsens toward the
DESCRIPTION OF TEPHRA UNITS 27

MEASURED SECTION M-l, 

MUDDY RIVER QUARRY SITE

East bank of Muddy River at its confluence with Smith Creek, near center of NW1/! 

sec. 15, T. 8 N., R. 6 E. Tephras crop out above the rock face of quarry along an 

abandoned section of former USFS Forest Road N92. Most tephra units vary substantially in thickness across the outcrop. Thicknesses reported are estimates of 

either average thickness or ranges in this outcrop.

Thickness 

(centimeters)

20. Ash-rich deposit, oxidized; separates sets M and K.

SetM:

19. Layer Mt. Three beds of pumice lapilli in ash

Lapilli as much as 3 cm across in brown ash.....................

Ash, reddish-brown to gray...............................................

Small pumice lapilli in dark-gray ash ...............................

Ash; dark reddish brown ash overlies dark-gray ash ........

Ash, coarse, yellowish-gray; contains scattered lapilli .....

18. Ash, dark-gray........................................................................

17. Layer Mm. Pumice, yellow, and gray lithic lapilli................

16. Ash, brown..............................................................................

15. Layer Mp. Pumice, yellowish-gray; contains gray lithic

lapilli..................................................................................

14. Ash, reddish-gray to brown....................................................

13. Ash, dark-gray........................................................................

12. Ash, reddish-brown.................................................................

11. Layer Me, Pumice, yellow to gray; contains lithic lapilli;

coarsest tephra in layer M at this site ................................

10. Ash, bedded, purplish-brown overlying gray .........................

9. Layer Mo. Pumice lapilli, small, yellowish-gray, and

abundant gray lithic lapilli.................................................

8. Ash, purplish-gray...................................................................

7. Layer Mg. Pumice lapilli, small, yellow-gray, and abundant 

gray lithic lapilli................................................................

6. Ash, reddish- to grayish-brown, faintly bedded; contains

lenses of pumiceous and lithic granules ...........................

5. Pumiceous ash and small lapilli, yellowish-gray, faintly

bedded ...............................................................................

4. Ash, grayish-brown, faintly bedded; contains lenses of

pumice...............................................................................

3. Ash, coarse and fine, laminated, reddish-gray ........................

Base of set M

100

1-3 

2 

1 

3 

3 

1 

7 

1

4

2

1

3

2-5 

6

2

3

15

15

25 

1-2

southeast side of the volcano, then thins farther toward the 

west (fig. 15). It is as thick as 25 cm 15 km southeast of the 

volcano along the apparent downwind axis of its lobe. This 

layer should be identifiable much farther downwind, but it 

was not traced away from the volcano.

LAYER Mp

Layer Mp (fig. 12) is a single bed of pumice lapilli and 

ash as thick as 10 cm, and, similar to most other tephras in 

set M, it contains abundant gray lithic fragments. Its ferromagnesian mineral suite is dominated by hornblende and 

contains approximately equal proportions of cummingtonite 

and hypersthene.

MEASURED SECTION M-2

Abandoned logging road 130, SWASW'A sec. 9, T. 8 N., R. 6 E., on north valley 

wall of creek tributary to Smith Creek. Site of figure 12.

Thickness 

(centimeters)

15. Ash-rich deposit, oxidized; between tephra sets M and K.......... 150

SetM:

14. Layer Mt. Ash and small lapilli, pumiceous, grayish-yellow.... 4

13. Ash, gray..................................................................................... 1

12. Layer Mm. Lapilli and ash, pumiceous and lithic..................... 10-15

11. Ash, brown to grayish-brown...................................................... 3

10. Layer Mp. Lapilli and ash, lithic and pumiceous ...................... 5-10

9. Ash, purplish-gray, scattered pumice lapilli in middle................ 10-15

8. Layer Me. Lapilli, relatively coarse, pumiceous and lithic........ 5

7. Ash, purplish-gray; scattered small lapilli................................... 10

6. Layer Mo. Lapilli and ash, lithic and pumiceous....................... 4

5. Ash, purplish-gray....................................................................... 5-10

4. Layer Mg. Lapilli and ash, lithic and pumiceous; in lenses....... 3

3. Ash, bedded, purplish-gray, brown, and gray.............................. 20

2. Ash, coarse; scattered lapilli........................................................ 30

1. Ash, brown to gray; lenses of lapilli............................................ 15-20

Base of set M

The abundance of hypersthene probably distinguishes 

layer Mp from older layers in set M. Cummingtonite likely 

is more abundant than in overlying layers, although the proportions may not differ enough for reliable identification. 

Layer Mp generally is notably finer grained than the underlying layer Me and both thinner and finer grained than the 

overlying layer Mm.

Layer Mp is underlain by layer Me, separated from it by 

purplish gray-brown ash. It is overlain by thin brown ash that 

lacks the purplish hue and by layer Mm.

Layer Mp was identified from approximately east to 

southeast of the volcano. In most outcrops it is only a few 

centimeters thick, and the maximum thickness measured, 

about 9 km directly to the east of the volcano, is only about 

10cm.

LAYER Mm

Layer Mm is a single pumiceous lapilli and ash bed as 

thick as 15 cm (fig. 12); it is the thickest layer of set M east 

of the volcano. Gray lithic fragments are abundant in layer 

Mm but probably less so than in older layers of the set. Ferromagnesian minerals in its pumice are hypersthene, hornblende, and small amounts of cummingtonite. Sparse biotite 

and oxyhornblende are common but were not found consistently in pumice samples.

Layer Mm apparently is the only thick and coarse bed 

in set M whose ferromagnesian mineral suite is dominated 

by hypersthene and hornblende as well as by small but consistent amounts of cummingtonite. It is similar in color to
28 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Figure 16. Upper part of tephra set M showing layer Mt, Mount St. Helens. Pocketknife is at the 

top of the upper lapilli-bearing bed of the layer. Photograph taken in 1979.

layer Mp but thicker and coarser. Those two tephras and the 

intervening brown ash form a brown triplet that contrasts 

noticeably with the underlying reddish-gray beds in set M.

Layer Mm is underlain and overlain by layers Mp and 

Mt, respectively, separated only by thin ash.

Layer Mm was identified from east to southeast of 

Mount St. Helens. In addition, it may be the upper of the two 

set M layers that crop out south of the volcano (fig. 15). At 

the Muddy River quarry and nearby sites east-southeast of 

the volcano, it is 5-10 cm thick and consists mostly of lapilli. 

It is somewhat thicker toward the southeast, and its maximum known thickness is about 15 cm at 17 km in that direction. The layer should be recognizable much farther 

southeast but was not traced for more than about 20 km from 

the volcano.

LAYER Mt

Layer Mt consists of at least three thin beds of pumiceous small lapilli and ash separated by thicker deposits of 

ash (figs. 15,16). Pumice in each bed is characterized by ferromagnesian mineral suites of hypersthene and hornblende 

in which cummingtonite is sparse and in some samples 

absent.

The layer is recognized as thin, lapilli-bearing beds 

above layer Mm and by its ferromagnesian mineral suite. 

The beds are the only known tephras older than set J that 

contain little or no cummingtonite.

The lowest lapilli bed in the layer is underlain by brown 

ash that separates it from the coarser and thicker layer Mm. 

The upper bed is overlain by the oxidized, ash-rich deposit 

that separates set M from set K.

Layer Mt was identified at only a few outcrops east of 

the volcano. Its maximum known thickness, including the 

ash that separates the lapilli-bearing beds, is 12 cm.

TEPHRA SET K

Tephra set K consists of a series of at least 10 thin beds 

of pumiceous and lithic ash and small lapilli. It commonly 

forms a relatively obscure gray, stratified band between 

thick, oxidized ash-rich deposits east and south of the volcano (fig. 17). No large lapilli or bombs were found in the 

set. All its tephra beds are fine grained as well as thin, compared with tephras in the underlying and overlying sets of 

Pleistocene age. No distinctive beds were recognized within 

set K, and no layers or beds are described individually.
DESCRIPTION OF TEPHRA UNITS 29

*$^>fefc#- ^^£tv^- 

$£?%&&'£&£ ..;': J^^fc:. *

ttJSb- ^^^s^^ -•' iv-" * r**.^*«:; "- ,

• - a**?<*"" '^-•f<^Wr ' V^ fc -7>?".v- *^? ^-r-*-'-* "^

122°15' 122°00'

Figure 17. Thin tephras of set K, Mount St. Helens. Dips are primary. Pods of brown ashy material in the part of the set that is gray 

and bedded probably are animal-burrow backfills. Pocketknife 

shown for scale. Photograph taken in 1979.

A tephra sequence that includes two layers of lapilli, 

each 30-40 cm thick, interbedded with several tens of thinner ash beds was observed between sets M and S in a single, 

disturbed outcrop 8 km northeast of the volcano. Pumice in 

that sequence is mineralogically similar to that in set K, but 

the relation of those strata to the fine-grained set K east and 

south of the volcano is not known.

Generally, set K strata are partly oxidized and brownish 

gray, but pumice lapilli are not weathered enough to be 

noticeably soft. At least two thin, oxidized zones within the 

set suggest short time breaks during its deposition. At most 

outcrops, the tephras are only patchily preserved (fig. 17); 

burrowing animals and other near-surface disturbances have 

almost everywhere strongly affected these thin deposits.

Set K consists chiefly of finely vesicular pumice and 

abundant lithic fragments. Ferromagnesian mineral suites in 

pumice of the set consist mostly of cummingtonite and hornblende. Small amounts of biotite were found, but not consistently, and are not diagnostic of the set. Because its 

composition is not distinctive, set K is identified only by its

46° 

15'

46° 

00'

-30

MOUNTS! 

HELENS A

\

.20

,50

,10

10 KILOMETERS

_J

Figure 18. Location and thickness (in centimeters) of tephra in set 

K. At most outcrops, set K has been truncated by erosion, and its primary thickness is not known.

stratigraphic position, characteristic thin beds, and fine grain 

size.

Set K is both underlain and overlain near the volcano by 

100-200 cm of oxidized, ash-rich deposits that generally are 

as thick or thicker than set K itself. Commonly, both the base 

and top of the set are so disturbed that its stratigraphic 

boundaries are uncertain (fig. 17).

DISTRIBUTION AND THICKNESS

Set K is present fairly consistently from east of the volcano clockwise to the south, below altitudes of 600 m. In 

areas 10-15 km east and southeast of the volcano, it commonly is 30-50 cm thick (figs, 18, 19). Sixteen kilometers 

south of the volcano, it is almost entirely ash and is about 10 

cm thick. The deposits of set K that are preserved probably 

do not represent the primary thickness of the set because 

unknown thicknesses of the tephra have been mixed into 

underlying and overlying deposits. Set K was traced only as 

far as 15-20 km east and south of the volcano.

Pumice lapilli layers and interbedded ash deposits 8 km 

northeast of the volcano, which might be correlative with set 

K, are as thick as 200 cm. The two coarse, thick pumice layers that were observed there probably extend much farther to 

the northeast but were not traced or specifically identified 

elsewhere.
30 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Section

Site 29 

Distance from vent 

(in kilometers) 16

Pine 

14.5

Mount St

Helens vent 

o -_

j\ ~— — -____ Muddy 

\ ~~"~~o River 

Quarry

Vine

EXPLANATION 

Lapilli or bombs 

Ash

Section 

29

Position of dated 

14C sample

Pyroclastic-flow 

deposit

CENTIMETERS i—20

Muddy

River

Quarry

11

i^ir- "veS^

Figure 19. Columnar sections of tephra set K from approximately east to south of Mount St. Helens and diagram of their locations 

relative to the central vent of the volcano.

AGE

Set K apparently is between about 20,000 and 19,000 

years old, based on carbonized wood recovered from pyroclastic-flow deposits (table 2). South of Mount St. Helens, 

sets K and M both overlie a pyroclastic-flow deposit dated at 

20,350±350 years old (W-2540). Southeast and south of the 

volcano, respectively, carbonized wood samples from pyroclastic-flow deposits that overlie set K have been dated as 

19,160±250 (W-4531) and 18,560±180 (W-2413) years 

old.

TEPHRA SET S

Set S is characterized by two conspicuous yellow to 

brown pumice lapilli and bomb layers that overlie yellowish-gray, finer layers of chiefly ash (figs. 20, 21). In the 

field, that series of beds of those colors and grain sizes is 

almost diagnostic of set S. The two coarse layers at its 

top dominate the set east of the volcano, but the underlying finer strata are more widespread and locally are 

thicker. Pumice in the set is somewhat weathered, in many 

places enough that pumice lumps are soft and readily broken or cut. Near the volcano, tephra in the lower and middle parts of the set typically is interbedded with deposits 

of ash-cloud or other flowage origin.

In many outcrops near Mount St. Helens, set S is the 

oldest readily recognizable tephra set because older sets have 

been removed or disturbed beyond recognition.

COMPOSITION

Dacitic pumice makes up most of set S, especially in the 

coarser strata. Ferromagnesian mineral suites consist chiefly 

of cummingtonite and olive to brownish hornblende. Brown

hornblende makes up as much as half of the hornblende fraction and probably is more abundant than in sets K and M. 

Hypersthene is sparse in layer Sw, in the middle of the set, 

but makes up several percent of the ferromagnesian mineral 

fraction in most other layers. Chemical analyses of pumice 

from the base, middle, and top of set S show SiC»2 values of 

about 62-64 percent.

DIAGNOSTIC FEATURES

Near Mount St. Helens, set S is recognized by its succession of yellow pumiceous lapilli layers overlying yellowish-gray ash, by its stratigraphic position below the darker 

brown set J, and by its ferromagnesian mineral composition. 

In outcrop, pumice layers of set S are thicker and more yellow in color than layers of sets M and K and are lighter in 

color than layers of set J. Abundance of brown hornblende 

helps to separate set S from older cummingtonite-bearing 

pumices that also contain hypersthene.

STRATIGRAPHIC RELATIONS

On uplands near the volcano, set S is usually both 

underlain and overlain by oxidized, ash-rich deposits that 

commonly are as thick as 100 cm. Layer So at the top of the 

set commonly has been noticeably weathered and disturbed 

in an oxidized zone. On and near the valley floors, pyroclastic-flow deposits almost everywhere overlie set S and separate it from set J.

DISTRIBUTION AND THICKNESS

Set S was recognized all around the volcano except 

directly to the northwest (figs. 22, 23), and it is thickest,
DESCRIPTION OF TEPHRA UNITS 31

Figure 20. Tephra set S about 7 km east of Mount St. Helens and 100 m above floor of Smith Creek 

valley. Thin lenses of grayish-brown ash (at tip of shovel) are the lowest beds of set S here; the overlying ash contains lenses of probable flowage origin. Layer Sw consists of multiple thin beds of 

brownish-gray ash containing white lapilli; it is overlain by crossbedded ash of probable flowage origin. Layer Ss here is a thin, brown ash that is barely visible under layer Sg. The reddish-brown upper 

part of layer So was weathered when that layer was exposed at the surface for more than a thousand 

years before being buried; pumice lapilli in the upper part of set S are soft enough to have been cut 

through during scraping of the outcrop. Shovel blade is about 25 cm long. Photograph taken in 1979.

EXPLANATION 

Lapilli 

Ash

CENTIMETERS 1—20

So

!— 0

Sb

Figure 21. Composite columnar section of tephra 

set S, Mount St. Helens.

coarsest, and most complete toward the east-northeast and 

east. Toward the east, it is commonly 50-70 cm thick at 

distances of 10-15 km from the volcano. At 10-15 km 

north and south of the volcano, where only one or two

lapilli-bearing beds are present, the set as a whole generally 

is only 10-20 cm thick. Toward the southwest and west, it 

is even thinner, and only one lapilli-bearing layer was 

found (fig. 22). Downwind, as many as three beds of set S 

have been identified at multiple sites in eastern Washington 

as far as about 300 km from the volcano (Foley, 1976, 

1982; Moody, 1977, 1978; Mullineaux and others, 1978; 

Waitt, 1980; Stradling and Kiver, 1986; Busacca and 

others, 1992).

AGE

Set S apparently was erupted between about 13,600 

and 12,500 years ago, probably close to about 13,000 years 

ago. About 50 km northeast of Mount St. Helens, peat from 

below and above set S tephra in a bog has been dated at 

about 13,650 (W-3136) and 12,120 (W-3133) years B.P., 

respectively (table 2) (Mullineaux and others, 1975; Crandell and others, 1981). On the south flank of the volcano, a 

date of about 12,910 years B.P. (W-3141; table 2) was 

obtained for charred wood from a pyroclastic-flow deposit 

that underlies one layer of set S tephra there and is compositionally similar to the tephra (fig. 23) (Mullineaux and 

others, 1975).
32 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

122°15' 122°00'

46° 

15'

46° 

00'

70

MOUNT ST 

HELENS A

JO

60 .50

.60 

.70 »70 -50

40 "50

.10

10 KILOMETERS 

I

Figure 22. Approximate thickness (in centimeters) of tephra set S 

at selected locations, Mount St. Helens. Thicknesses reported include only thicknesses of deposits interpreted as tephra; most stratigraphic sections also include interbedded deposits of flowage 

origin.

Dates of approximately 13,800 and 13,600 years B.P. 

for peat above beds regarded as set S about 100 km north of 

Mount St. Helens (Davis and others, 1982; Porter and others, 

1983) conflict slightly with the dates described preceding. 

The discrepancy may result merely from imprecision of 

radiocarbon dating. No radiocarbon date is available, however, for the base of set S at Mount St. Helens, and eruptions 

that produced the set may have started before 13,600 years 

ago.

MEASURED SECTION S-l

Road cut along pre-1980 logging road 140 on west wall of Smith Creek valley north 

of Ape Canyon Creek, about 100 m above valley floor; 7 km east of Mount St. Helens. 

See figure 20.

Thickness 

(centimeters)

17. Pyroclastic-flow deposit, lithic.................................................. 60-100

Set S and unidentified deposits of similar age:

16. Pumiceous ash and small lapilli; locally crudely layered; 

contains small amounts of organic material; may be 

reworked from layer So........................................................ 2-5

15. Layer So. Chiefly brown pumice lapilli; scattered small 

bombs and lithic fragments; upper half more oxidized 

than lower half..................................................................... 25

14. Ash, pumiceous, brown............................................................. 1-2

13. Layer Sg. Pumice lapilli, yellow-brown, normally graded, 

and small bombs; contains small proportion of lithic 

fragments............................................................................. 35-40

12. Ash, fine, grayish-brown.....................................................—... 2

11. Layer Ss. Ash, pumiceous, brown; contains scattered small

to large pumice lapilli ......................................................... 3

10. Ash, fine, brown to gray: contains lenses of coarse ash and

small lapilli; crossbedded, probably of flowage origin........ 20-30

9. Layer Sw:

Ash, coarse; contains small, white pumice lapilli................ 2-3

Ash, fine, gray to grayish-brown ......................................... 1

Ash, coarse, gray; contains small, white pumice lapilli....... 2

8. Ash, grayish brown; coarser and darker near base; contains

scattered pumice lapilli........................................................ 2-4

7. Ash, fine, brown to gray, laminated; streaked with iron

oxide............................................................................... 2-A

6. Ash, coarse, and small yellowish-brown pumice lapilli;

lenticular ............................................................................. 3

5. Ash, fine, gray to brown............................................................ 2-4

4. Ash, lithic, coarse, dark-gray; lenticular.................................... 1-4

3. Ash, gray-brown to brown, locally wavy bedded—................... 15-20

2. Ash, coarse to fine, gray, bedded; lenticular.............................. 0-5

Below deposits of set S age:

1. Ash-rich deposit, oxidized, brown; contains some pumice 

lapilli ............................................................................. 50

eruptions, or they may represent only staining by oxidized 

material deposited from ground water.

DESCRIPTION OF LAYERS IN SET S

All the described layers in set S crop out directly east of 

the volcano, although the tephra strata in the lower part of the 

set there are thin and interbedded with thicker ash beds of 

probable flowage origin (figs. 20, 23; measured section 

S-l). Set S tephras also crop out on the north, west, and 

south sides of the volcano, but their relation to layers 

described east of the volcano is not known, and they were not 

identified specifically or described separately.

Two thin oxidization zones in the lower part of set S 

were found at a few outcrops. These may represent surface 

oxidation and record brief weathering periods between

LAYER Sb

The oldest identified tephra in this set, layer Sb, is 

defined to include several lapilli-bearing tephra beds below 

the distinctive gray layer Sw. Locally, layer Sb consists of at 

least four thin, yellowish-brown beds of small pumice lapilli 

in ash. Layer Sb is relatively poorly defined; its beds are less 

continuous and more disturbed than other tephras of the set 

and commonly are intimately associated with ash of flowage 

origin. Consequently, it is commonly unclear which strata 

below layer Sw are tephra deposits and which are not. Ferromagnesian mineral suites from pumice in layer Sb consist 

almost entirely of cummingtonite and hornblende, but the 

suite in one bed contains a few percent hypersthene.
DESCRIPTION OF TEPHRA UNITS 33

Site

Distance from vent 

(in kilometers)

EXPLANATION 

Lapilli or bombs 

Ash

Position of dated 

14C sample

Pyroclastic-flow 

deposit

Zone of oxidation

CENTIMETERS I—20

Goat 

9

Swift 

12

Road 380 

8

Muddy River 

Quarry

11

Road 140 

7

^ 

1 4

<3

0 * C 

A B

"

\u\

Goat

"— 0

QCold

\ 

\

\

\

\ /

Mount St \ x/ 

Helens vent,o ^:^_

\

100-150- 

cm

\

-o Road 140

— — — o Muddy River 

Quarry

"° R0ad 380

So

Sg

Sw 

Sb

Sw?

So

Sg

\\ 

\

Sw\ 

Sb(?) \

N92

7

Cold 

10

Base of depositsof set S age

O Swift

4

V

t>

""TS

"_

3

( (

i * *

% * '

& o ...,t — _

c, -a *

f«V£

c 4 a

o „ c>

^ilv^

^ t> ' *

So

Sg

Sw

Figure 23. Columnar sections of tephra set S and diagram of their locations relative to the central vent of the volcano.

Layer Sb is distinguished from other layers of set S 

chiefly by stratigraphic position and from overlying layer Sw 

chiefly by color. East of the volcano, layer Sb overlies either 

the oxidized ash-rich deposit that separates it from the underlying set K or ash of probable flowage origin. Throughout its 

known distribution, it is overlain by layer Sw and other 

younger beds in set S.

Layer Sb was identified from approximately northeast 

to southeast of the volcano. It is thickest toward the eastnortheast; the cumulative maximum known thickness there 

of the multiple lapilli-bearing beds assigned to the layer is 

only about 10 cm at a distance of 10-15 km. At 15-20 km 

east-northeast of the volcano, in the Clearwater Creek valley 

(fig. 23), four lapilli-bearing beds assigned to the layer have 

a cumulative thickness of 5-10 cm. At 11 km east of the volcano, two beds, each about 1 cm thick, are separated by several centimeters of ash, possibly of flowage origin. Although

the cumulative thickness for the tephra beds of layer Sb is 

nowhere more than about 10 cm, the layer commonly is 

associated with ash of probable flowage origin as thick as 

100 cm.

LAYER Sw

Layer Sw consists of at least three thin beds of small 

pumiceous and lithic lapilli in ash that are characterized by a 

pale-gray color that contrasts with the brown colors of other 

layers of set S (fig. 24). Pumice in layer Sw probably is less 

vesicular than that in other set S layers. Ferromagnesian mineral suites consist mostly of cummingtonite and hornblende; 

minor amounts of hypersthene and oxyhornblende were 

found in some samples. Layer Sw is inconspicuous, and its 

identification depends chiefly on its pale-gray color and thin 

beds and the denser character of its pumice lapilli.
34 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Figure 24. Tephra set S at the Muddy River quarry site about 11 

km east of Mount St. Helens. The gray color of the coarse ash and 

lapilli of layer Sw contrasts with the yellow and brown colors of other parts of the set. Only a few centimeters of the lower part of layer 

So is present above layer Sg; the remainder of layer So has been 

mixed into an overlying oxidized, ash-rich deposit by soil-zone processes. Pocketknife shown for scale. Photograph taken in 1979.

Locally, layer Sw directly overlies layer Sb, but more 

commonly it lies on lenticular ash deposits of flowage origin. 

In some places, it is directly overlain by layer Ss. More commonly, however, it is overlain by ash deposits of flowage origin or by layer Sg.

Layer Sw was identified from northeast to southeast of 

the volcano at distances of as much as 16 km. Its maximum 

known thickness, however, is only slightly more than 5 cm. 

Both its broad distribution relative to its limited thickness 

and its multiple beds suggest that it consists of multiple small 

shower beds that were carried in different directions.

LAYER Ss

Layer Ss is a single, highly pumiceous brown stratum of 

well-sorted lapilli and coarse ash (fig. 26). Although its maximum known thickness is less than 15 cm, it is the thickest 

tephra shower bed in set S below the upper two layers. Its 

ferromagnesian minerals are mostly cummingtonite and 

hornblende, but it also contains several percent hypersthene.

Layer Ss is identified by its brown color, highly pumiceous character, thickness, and stratigraphic position 

between the usually distinctive layers Sw and Sg. Southeast

of the volcano, it is coarser and thicker than the underlying 

layer Sw. It is much thinner and finer than the overlying 

layer Sg everywhere except toward the southeast, where 

locally the two are similar in grain size and thickness 

(fig. 26).

Although layer Ss directly overlies layer Sw in some 

outcrops, it more commonly is separated from the latter by 

ash of flowage origin (fig. 20). It generally directly underlies 

layer Sg, separated only by thin ash.

The layer was identified from east to southeast of the 

volcano. Toward the east, it is thin or missing from many 

outcrops (figs. 20, 24). It is thickest toward the southeast, 

where it is almost 10 cm thick at a distance of 10 km and contains abundant small lapilli (fig. 26). It was not recognized 

only slightly farther west on the south-southeast flank of the 

volcano, which suggests that westerly winds during its eruption produced a sharp western margin of its lobe.

LAYER Sg

Layer Sg is a single, normally graded, grayish-yellow to 

brown deposit of pumice lapilli and bombs as thick as 50 cm 

(fig. 20). It is the most voluminous single tephra unit of set 

S. Most clasts in this layer are pumice, but lithic fragments 

are common. Ferromagnesian mineral suites consist of 

chiefly cummingtonite and hornblende and a few percent 

hypersthene; layer Sg thus is mineralogically similar to pumice of most other layers in set S. The SiC>2 content of a single 

sample analyzed from this layer is between 63 and 64 percent.

Layer Sg is distinguished by its color, grain size and 

thickness, and graded character. It is similar to the overlying 

layer So but is thicker and more strongly graded. Moreover, 

pumiceous glass from layer Sg has slightly lower calcium 

and iron contents relative to potassium than does pumice 

from layer So. The difference probably is sufficient to distinguish between the layers using chemical analyses (Mullineaux and others, 1978; Sarna-Wojcicki and others, 1983).

Layer Sg overlies layer Ss southeast of the volcano and 

either layer Ss or Sw toward the east and northeast (figs 20, 

23). It is overlain east and northeast of the volcano by layer 

So. Where layer So is absent toward the south-southeast, 

layer Sg commonly forms the top of the set and is overlain 

by a thick, oxidized, ash-rich deposit that separates set S 

from set J.

Layer Sg crops out from northeast to southeast of the 

volcano and is the thickest layer in set S in almost all such 

outcrops. It is thickest toward the east or east-northeast. Its 

lobe probably is broad and has an axis that trends east-northeast. In that direction, the layer commonly is thick as 50 cm 

at distances of 10-15 km (fig. 26) and consists mostly of 

lapilli and a few bombs. Farther to the north, it apparently 

thins, but most outcrops are at higher altitudes and are 

disturbed. It thins progressively southward from east of the
122°151

DESCRIPTION OF TEPHRA UNITS

122°00' 122°151

35

122°00'

46° 

15'

46° 

00'

MOUNT ST 

HELENS A

MOUNT ST 

HELENS A

46° 

15'

46°

B

.35

,15 

.15

.15

D

10 20 KILOMETERS 

I

Figure 25. Measured thickness (in centimeters) of layers in tephra set S, Mount St. Helens. A, Layer So. No measurement reported for 

layer So is likely to represent a primary thickness because of truncation of the top of that layer.fi, Layer Sg. C, Layer Ss. D, Layer Sw.
36 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Figure 26. Layer Ss about 9 km southeast of the volcano. Here it is coarser than and about as thick 

as layer Sg. Only irregular lenses of lapilli remain of layer So, which has been mostly mixed into an 

overlying ash-rich deposit of undetermined origin. Pocketknife shown for scale. Photograph taken in 

1979.

volcano and is only about as thick as layer Ss on the the 

southeast flank.

Layer Sg was traced for about 40 km northeast of 

Mount St. Helens and has been identified far downwind in 

eastern Washington (Mullineaux and others, 1978; Busacca 

and others, 1992), where it forms a marker ash bed 1-2 cm 

thick.

LAYER So

Layer So is a single yellow-brown layer of pumice 

lapilli and a few bombs that is as thick as 30 cm and forms 

the top of set S (figs. 20, 21). Except for layer Sg, it is the 

thickest layer in the set. It contains only small amounts of 

lithic material, and its ferromagnesian mineral suite consists 

of cummingtonite and hornblende and a few percent hypersthene. Chemical analysis of one pumice sample from layer 

So shows a SiC>2 content of between 62 and 63 percent.

The upper part of this layer typically is oxidized and 

disturbed by soil development (figs. 24, 26), a result of nearsurface exposure for 1,000-2,000 years before being covered by set J tephras.

Layer So is identified chiefly by its coarse, thick character and stratigraphic position above layer Sg. East of the 

volcano, layer So differs from layer Sg by its darker color 

and less distinct grading. Where only one of these layers is 

seen, identification can be based on distribution or chemical 

analysis. Slightly higher amounts of calcium and iron relative to potassium in glass from layer So compared with those 

in layer Sg distinguishes between these two layers (Mullineaux and others, 1978; Sarna-Wojcicki and others, 1983).

Wherever it has been found near the volcano, layer So 

overlies layer Sg. The weathered, disturbed top of layer So 

typically is overlain by a thick, oxidized, ash-rich deposit or 

by pyroclastic-flow deposits.

Layer So was identified from northeast to southeast of 

the volcano. Part of the layer probably fell farther to the 

north but was not preserved well enough to be identified. It 

is thickest northeast and east of the volcano, where it is as 

thick as 30 cm at distances of 5-10 km and as thick as 20 cm 

at 15-20 km (fig. 25); however, because the upper part of the 

layer is eroded and weathered in all the outcrops observed, 

these thicknesses probably are less than primary thicknesses. 

Layer So was traced to about 40 km northeast of Mount St. 

Helens and has been identified in eastern Washington about
DESCRIPTION OF TEPHRA UNITS 37

wsw ESE

Figure 27. Coarse pumice layers of tephra set J about 8 km east of 

the central vent of Mount St. Helens. Most of layer Jb here has been 

disturbed by near-surface processes. Locally, the deposit of colluvium and windblown material above layer Jb contains identifiable 

lenses of ash from Mount Mazama. Pocketknife shown for scale. 

Photograph taken in 1983.

200-300 km downwind (Mullineaux and others, 1978; 

Busacca and others, 1992).

TEPHRA SET J

Set J consists mostly of multiple coarse pumice layers 

that together form a thick, brown deposit characterized by 

lapilli and bombs (figs. 27, 28) both east and west of the volcano. Toward the east, two pumiceous lapilli and bomb layers overlie thin ash; to the west, the set consists of one 

coarse, thick layer. Lithic material is sparse, and no layers of 

lithic fragments were found.

A relatively strong brown color is characteristic of the 

set as a whole, and its upper 20^40 cm generally has been 

strongly mixed by soil-zone processes. Associated ash of 

flowage origin is not common and was not found interbedded with the major tephra layers of set J.

EXPLANATION

Lapilli or bombs 

Ash

CENTIMETERS 

|—20

Jg

— o

* »*

< fc * p

Jb

Jy

Jyo

Js

Figure 28. Columnar sections of tephra set J about 10 km eastsoutheast and west-southwest of the central vent of the volcano.

COMPOSITION

Pumice makes up most of set J; it ranges in composition 

from dacite in the lower part of the set to andesite in the 

upper. Pumice from the lower two layers Js and Jy contains 

only hypersthene and hornblende as abundant ferromagnesian minerals and has a SiC>2 content between 62 and 64 percent. The next younger layer, Jb, contains the same 

ferromagnesian minerals, but its SiC>2 content is lower, 

between 55 and 60 percent. Layer Jg at the top of the set also 

has a SiC>2 content between 55 and 60 percent, but its ferromagnesian mineral suite consists of mostly hornblende, 

lesser amounts of hypersthene, and traces of augite.

DIAGNOSTIC FEATURES

Set J is the only coarse, thick, and strongly oxidized 

tephra set at Mount St. Helens that has a ferromagnesian 

mineral suite dominated by hypersthene and hornblende. 

These features readily distinguish it from all other tephra sets
38 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

of the volcano. In the field, set J can commonly be recognized simply by its coarse grain size and color, which is 

darker than the yellow to pale-brown colors typical of the 

underlying and overlying tephra sets S and Y.

STRATIGRAPHIC RELATIONS

Set J usually is separated from the underlying set S by 

an oxidized ash-rich deposit commonly 100 cm thick or by 

pyroclastic-flow deposits. Close to valley floors, pyroclastic-flow deposits generally directly underlie set J. At one 

site, 9 km directly east of the volcano, thin remnants of three 

pyroclastic-flow deposits, each of which contains a thin oxidation profile at its top, were observed between sets S and J 

about 50 m above the valley floor (Crandell, 1987, p. 33-34).

Near the volcano, set J generally is directly overlain by 

set Y, but the coarse pumice at the top of set J typically was 

strongly mixed by soil-zone processes during the period of 

6,000-7,000 years that it was exposed at the surface before 

deposition of set Y. Locally near Mount St. Helens, Mazama 

ash from Crater Lake, Oregon, can be identified between sets 

J and Y. More commonly, however, volcanic ash or other 

fine material that fell on the coarse upper part of set J before 

set Y was erupted has simply filtered down into the coarse 

pumice deposit. In contrast, where set J is finer at distances 

of more than about 10 km to the west and 20 km to the east 

of Mount St. Helens, Mazama ash commonly can be recognized between sets J and Y.

DISTRIBUTION AND THICKNESS

Set J has been identified in all directions from the volcano except to the north. From the northeast to southeast, it 

typically consists of 50-100 cm of lapilli and small bombs at 

distances of 5-15 km from the volcano. It is much thicker 

locally, especially to the southeast, but may have been overthickened by slumping or creep. The set thins from the 

southeast around to the south, where it is 25-50 cm thick at 

comparable distances (fig. 29). It crops out only sparsely 

north-northeast of the volcano and has been strongly disturbed wherever found in that direction. Farther downwind 

toward the east, it is as thick as 30 cm at a distance of 30 km 

and as thick as 1 cm at 200 km, and it has been identified as 

far as about 700 km from the volcano in Montana (Carrara 

and others, 1984, 1986).

Toward the west, set J consists of only one layer. 

Locally it is as thick as 250 cm at a distance of 5 km, but 

those thick deposits exhibit crude layering and may have 

been overthickened by slumping. The layer generally is 

10-30 cm thick at a distance of about 10 km through an arc 

from southwest to west (fig. 29).

No deposits of set J were recognized north of the volcano, even at relatively low altitudes at a distance of 10 km

122°15' 122°00'

46° 

00'

30

10 KILOMETERS

Figure 29. Approximate thickness (in centimeters) of tephra set 

J, Mount St. Helens.

where tephras of both sets S and Y are preserved; apparently 

little or no tephra of set J was carried in that direction.

AGE

Set J probably was erupted between 12,000 and 10,500 

years ago, although some evidence suggests that its eruption 

could have begun several hundred years earlier. An older 

limiting age of less than 12,000 and perhaps 11,500 years 

ago is indicated by several radiocarbon samples from near 

the volcano (table 2). Samples from just below the base of 

the set there have been dated between approximately 11,900 

and 11,550 years B.P. (Crandell and others, 1981) (W-2866, 

W-2441, W-2832, W-2870; table 2); thus, multiple samples 

indicate that the eruptions began after 12,000 years ago and 

perhaps as recently as about 11,500 years ago.

Samples from lacustrine deposits downwind suggest, 

however, that set J eruptions might have begun as early as 

about 12,500 years ago (Carrara and Trimble, 1992; P.E. 

Carrara, written commun., 1993). Similar discrepancies, in 

which lacustrine samples indicate an age older than terrestrial samples, were noted in Canada by Mathewes and Westgate (1980) and in Alaska by Riehle and others (1992), who 

emphasized the uncertainty in comparing dates of terrestrial 

and lacustrine samples.

The younger limiting date of about 10,500 years is 

based chiefly on samples from peat bogs that are several tens 

of kilometers from Mount St. Helens (table 2). Peat from a 

site 70 km to the east-southeast that was dated at about 

10,740 years B.P. (W-5718) came from above layer Jb, the
DESCRIPTION OF TEPHRA UNITS 39

Site 5700 

Distance from vent 

(in kilometers) 8

11 830

10

CE NTIMETERS -20

— 0

' V 4 " -3 "_ 

& & ' t> '

« t> *" 4

' f > ^ ^

& V ' V

1 a * " c

>: IvJ9 /

/ X

^x

» A t> 

^ fc

/^ /<

k ^ <1 ' |

Base notexposed

Jy

(undiff)

—--~-~

— p— a— T—

[> < V

< •? 6 

4 4 '

^ » •" v &

^<3 ^ * "

v 4 > 

' a t> * 

^a s? <3 

^ ^ ^

\\

\*

\\

\ 

\

EXPLANATION

14 > " ''< Lapilli or bombs

Ash

•

{^

Position 14Csa 

Oxidized

of dated 

mple 

zone

£><)!>!> 

4 V > V

4 v < ft

=7 4 & A 

i « < t>

a 7 fr A •

1 « £> &

" 4 *" p

4 v l> V

<3 V 4 *

<3 4 V ». 

> t, > v

Jb

Section 

o 19

Mount St 

Helens vent

Section

-033

5700o———————?v^E^-—————— o917n

917s

Figure 30. Columnar sections of tephra set J from northeast to west of Mount St. Helens and diagram of their locations relative to the central vent of the volcano.

youngest layer of the set that extends in that direction. At 45 

km south-south west of Mount St. Helens, peat from above 

layer Jg, believed to be the youngest layer of set J, was dated 

at approximately 10,580 years (W-5731).

When first described (Mullineaux and others, 1972, 

1975), the younger part of set J was thought to be between 

9,000 and 8,000 years old. Three radiocarbon samples of 

charcoal from coarse pumice deposits of set J east of the volcano were dated as approximately 8,300 (W-2587), 8,430 

(W-2702), and 8,900 (W-2991) years B.P. (table 2). Recent 

dates from above set J that are older than 10,000 years indicate that the dates younger than 9,000 years are not valid. 

Resampling of material similar to that dated as 9,000-8,000 

years old suggests that the anomalous dates may have 

resulted from contamination. Apparently the charcoal fragments that gave those dates were contaminated by younger 

humic or other carbonaceous material during the long period 

that the coarse pumice was exposed at the surface, and the 

dates should be disregarded.

DESCRIPTION OF LAYERS IN SET J

Set J includes only a few traceable strata, and its 

sequence is not complicated by ash deposits of flowage

origin. Four layers are recognized in the set: a thin, basal ash 

deposit and three coarser, thicker strata, two of which extend 

toward the east and one toward the west (fig. 30). The layer 

that extends toward the west is the thickest tephra deposit 

known on the flanks of Mount St. Helens. Ash containing 

carbonaceous fragments at one site between two beds or 

zones of the layer that extends to the west suggests that a 

pause may have occurred during eruption of layer Jg, but no 

such zones were found in that layer in other outcrops nearby.

LAYER Js

Several thin, compact, grayish-brown, poorly sorted 

pumiceous ash beds containing scattered small pumice 

lapilli crop out widely but discontinuously at the base of the 

set and are grouped together as layer Js. No individual beds 

in the layer were traced from one outcrop to another. Typically, the layer is faintly planar bedded; no crossbedding was 

seen. Ferromagnesian mineral suites from pumice consist 

essentially of hypersthene and hornblende, and a single sample analyzed chemically has a SiO2 content of about 62 percent. Layer Js differs from other layers in set J by its fine 

grain size, lesser thickness, and greater compactness.
40 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

122°15' 122°00' 122° 15' 122°00'

46° 

15'

46°

00'

B

MOUNT ST 

HELENS A

MOUNTS! 

HELENS A ,15

.10 .10 10

•10

MOUNTST 

HELENS A

D

5

i I

10 20 KILOMETERS

Figure 31 (above and facing column). Thickness (in centimeters) of layers in tephra set J, Mount St. Helens. A, Layer Js. B, Layer 

Jyo. C, Bed Jyn. D, Bed Jb. £, Layer Jg.

On uplands, layer Js commonly is separated from set S 

by a thick, fine-grained, oxidized ash-rich deposit that probably originated in part from flowage phenomena. Near and 

on valley floors, pyroclastic-flow deposits almost everywhere separate layer Js from set S. The coarser layer Jy overlies layer Js throughout the known extent of the finer layer. 

Layer Js was identified only east and southeast of the 

volcano (fig. 31). Its maximum thickness is about 5 cm at
DESCRIPTION OF TEPHRA UNITS 41

122°15' 122°00'

46° 

15'

46° 

00'

MOUNTS! 

HELENS

10 KILOMETERS

MEASURED SECTION J-l

West side of Smith Creek valley, about 50 m northwest of site of bridge that crossed 

Smith Creek in 1983, in NE'ASE'A sec. 5, T. 8 N., R. 6 E. See figure 27.

Thickness 

(centimeters)

5. SetY. Ash and lapilli of layer Yb; charcoal at base................... 5-10

4. Colluvium, ashy; contains lenses of ash from Mount

Mazama ............................................................................... 3-6

SetJ: 

3. Layer Jb. Pumice lapilli and ash, brown; openwork in lower

10 cm, mixed and ash rich in upper 15 cm .......................... 20-30

2. Layer Jy:

Bed Jyn. Pumice lapilli and bombs in sparse ash;

openwork; fragments coated with gray ash.......................... 25-35

BedJyo. Pumice lapilli, brown; a few bombs; openwork... 5-10 

1. Ashy sand and pumice pebbles; probable reworked material

from pyroclastic-flow deposits and layer Js ........................ >30

Base not exposed

distances of 5-8 km toward the east and southeast; the farthest from the volcano that it was identified is 13 km toward 

the east-southeast. The layer was identified only where it is 

overlain by the coarse layer Jy, and its possible extent farther 

around the volcano is not known.

The planar bedding and thin, widespread character of 

layer Js suggests that it originated as a fall deposit, but it may 

have been deposited in part by ash clouds from flowage 

events.

Figure 32. Layer Jy between layers Js and Jb 9 km southeast of 

the volcano along USFS Road N83 where it crosses the Muddy River. Bed Jyn, about 8 cm thick, overlies bed Jyo, which is about 30 

cm thick. Layer Js here overlies a pyroclastic-flow or lahar deposit. 

Photograph taken in 1983.

LAYER Jy

Layer Jy consists of two similar, thick, well-sorted beds 

of grayish-yellow to brown pumice lapilli and bombs, designated Jyo and Jyn (figs. 27, 32). Bed Jyo is mostly south and 

east of the volcano, and the overlying bed Jyn extends from 

southeast to northeast. The two beds overlap on the east and 

southeast, but that overlap is evident only in well-preserved 

stratigraphic sequences. Elsewhere, the layer apparently 

consists of only a single bed. Ferromagnesian mineral suites 

in both beds consist almost entirely of hypersthene and hornblende. Whole-rock samples of pumice from layer Jy contain 

about 63 percent SiC>2.

Along their respective distribution axes, beds Jyo 

and Jyn are similar in thickness, grain size, and color; 

where they overlap, they are not separated by a distinct 

ash stratum. In some outcrops showing overlap, they can 

be distinguished by differences in texture or in the color of 

interstitial ash. Ash in bed Jyn typically is more reddish
42 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

than the yellowish-brown color typical of bed Jyo. Even in 

the zone of overlap, however, layer Jy could not be subdivided reliably in many outcrops.

Layer Jy is easily distinguished from layer Js by its 

coarser grain size and greater thickness. Where layer Jy and 

the overlying layer Jb are present together, layer Jy commonly is slightly coarser, thicker, and lighter brown. In most 

outcrops south and southeast of the volcano, however, the 

two were not different enough to be separated in this study.

Layer Jy overlies layer Js or forms the base of the set, 

in which case it lies on either an oxidized, ash-rich deposit or 

on pyroclastic-flow deposits that separate it from set S. 

Layer Jy is overlain by identifiable layer Jb in some outcrops 

toward the east-southeast and by set Y in other directions.

Layer Jy forms a broad lobe from northeast to south of 

the volcano. The older bed Jyo probably makes up the entire 

layer on the south and is also thick toward the southeast (fig. 

32). Bed Jyn makes up much of the layer on the east (fig. 27) 

and probably most or all of it on the northeast. The thickness 

of the two beds together in the zone of overlap at about 10 

km from the volcano is approximately the same as the thickness of beds Jyn and Jyo along their respective axes.

East-southeast of the volcano, at a distance of 10 km, 

bed Jyo is 30-40 cm thick and contains abundant lapilli and 

small bombs as much as 10 cm across. It thins westward 

across the south flank of the volcano and does not form a 

well-defined stratum farther to the west, on the southwest 

flank of the volcano. The northern margin of its lobe is relatively abrupt; 13-14 km east-southeast of the volcano, bed 

Jyo thins northward from 30 to 15 cm over a distance of 

about 2 km and was not identified only 1 km farther to the 

north (figs. 30,31).

Bed Jyn is thickest northeast of the volcano. At about 9 

km in that direction, it is 60-70 cm thick (figs. 30, 31). To 

the north-northeast, it is much disturbed and poorly preserved in outcrops close to the volcano, and it is uncertain 

whether or not it is thicker or coarser in that direction. From 

the northeast, the layer thins clockwise around the volcano 

(figs. 27, 31) and was identified only around to about southeast of the volcano.

In many outcrops on steep slopes, layer Jy is anomalously thick both northeast and south of the volcano. These 

deposits may have been overthickened by slumping as they 

fell or by later reworking, but they could be a result of unidentified tephra lobes that extend in those directions.

Layer Jy forms an important marker bed, and its downwind component probably is one of the ash beds correlated 

with set J in eastern Washington about 200 km from the volcano (Moody, 1978) and the set J ash bed identified in Montana about 600 km from the volcano (Carrara and others, 

1986).

Beds Jyo and Jyn may represent two stages of one eruption or two distinctly different events. No evidence was seen 

to indicate how much time might have elapsed between two 

such stages or eruptions. The overlapping relation of two

well-defined beds probably precludes origin from a shift in 

wind direction during a single continuous eruption.

LAYER Jb

Layer Jb is a single bed, at least 30 cm thick, of brown 

pumice lapilli and small bombs. It overlies layer Jy (figs. 27, 

30,31) and forms the top of set J east and southeast of Mount 

St. Helens. Similar to older layers in the set, layer Jb contains 

only hypersthene and hornblende in its ferromagnesian mineral suite. Where it has been identified overlying layer Jy, 

layer Jb commonly is more reddish brown and locally is finer 

grained; however, no field or mineralogical criteria are 

known that reliably distinguish between the two. Where 

stratigraphic relations do not distinguish between them, 

chemical analyses may. Whole-rock and glass analyses of 

pumice from layer Jb indicate that it contains less silica and 

more iron than pumice from the older layers of set J (appendix) (J.A. Westgate, written commun., 1972).

Throughout its known extent, layer Jb directly overlies layer Jy, and near the volcano layer Jb is overlain by 

set Y. Locally, Mazama ash is preserved between layer Jb 

and set Y.

Layer Jb is present from east to southeast of the volcano 

and was not identified north of that trend. It also was not 

identified west of a line leading south-southeast, although it 

might be hidden there in the disturbed zone at the top of the 

set. At 10 km to the east-southeast, it is as thick as 30 cm and 

contains lapilli as much as 5 cm across (figs. 29-31). At 20 

km in that direction, it consists of small lapilli and ash about 

10 cm thick. At that site, it is coarser than layer Jy, even 

though layer Jy is coarser at that distance in other directions, 

because the locality is along the downwind axis of layer Jb 

but between the downwind axes of beds Jyo and Jyn.

LAYER Jg

Layer Jg is a thick, conspicuous deposit of well-sorted, 

gray-brown pumice lapilli and bombs on the west and southwest flanks of the volcano. Its pumice generally is darker, 

especially on freshly broken surfaces, than pumice in other 

layers of set J. Crude bedding of layer Jg in many places (fig. 

33) may record multiple eruption pulses, more than one 

shower bed, or simply downslope movement caused by a 

rapid rate of fall. The ferromagnesian mineral suite of layer 

Jg consists chiefly of hornblende that is darker and browner 

than hornblende in other layers of the set; the suite also contains hypersthene in lesser amounts and sparse augite. A single chemical analysis of pumice from layer Jg shows a SiC>2 

content of between 57 and 58 percent.

The upper part of layer Jg typically is oxidized and 

mixed by soil-zone processes to depths of a few tens of
DESCRIPTION OF TEPHRA UNITS 43

Figure 33. Crude bedding in layer Jg, about 5 km southwest of the center of Mount St. Helens. 

Small pack at base of outcrop is about 50 cm high.

centimeters. Overall, the entire layer is darker in outcrop 

than are other pumice units in set J.

Layer Jg is readily identified on the west side of the volcano because it is the only thick deposit of pumice lapilli 

there. Downwind, it can be distinguished from other pumiceous tephras by its ferromagnesian mineral suite.

Because other layers of set J are absent west of the volcano, layer Jg generally lies directly on set S tephra or on ash 

deposits of undetermined origin. Layer Jg generally is overlain by set Y tephra or by ash-cloud deposits associated with 

that set. Locally, Mazama ash is preserved between layer Jg 

and set Y.

Stratigraphic evidence that layer Jg is younger than 

other layers of set J was found at only one site, in a peat bog 

45 km south-southwest of the volcano. There, a thin ash bed 

whose ferromagnesian mineral suite identifies it as layer Jg 

overlies peat that in turn overlies an ash bed whose ferromagnesian suite is similar to that of set J layers east of the 

volcano. That relation indicates that layer Jg is younger than 

at least part of the set J deposits east of the volcano. It is also 

likely that it is younger than all the set J layers east of the volcano. A single change in ferromagnesian mineral composition from that of the layers east of the volcano to that of layer 

Jg is more likely than two changes that include a reversal, 

from that of layer Jy to that of layer Jg and then back to that 

of layer Jb.

Layer Jg probably overlaps layer Jy on the south-southwest flank of Mount St. Helens. Both layers are thin there, 

however, and are so strongly disturbed that their stratigraphic relation could not be confirmed.

Layer Jg forms a short, broad lobe toward the west and 

southwest. Its lobe axis near the volcano is along a bearing 

of about 250° (fig. 31). At 5 km from the center of Mount St. 

Helens, layer Jg is as thick as 250 cm and consists almost 

entirely of bombs and lapilli. These great thicknesses may 

have resulted, however, from slumping and accumulation in 

swales during rapid deposition. Layer Jg decreases more rapidly in thickness and grain size away from the volcano than 

do tephras carried to the east. At 10 km, layer Jg is only about 

30 cm thick, and its thickness decreases progressively to 

about 10 cm at 15 km and 1 cm at 25 km.

Layer Jg is a useful marker bed for a few tens of kilometers downwind because its composition makes it readily 

identifiable. Its range probably is limited compared with layers carried eastward, however, and its downwind extent is 

not known.

The unusually great thickness of layer Jg, and the rapid 

decrease of thickness with distance, suggests either that its 

eruptive column rose to a relatively low altitude or that eruption occurred during a time of weak winds.

TEPHRA SET Y

Set Y consists of three thick layers of pale-brown to yellowish-gray, well-sorted pumice lapilli and bombs and interbedded ash that are overlain by a series of thin ash deposits. 

Near the volcano, the set typically is 100-200 cm thick and 

comprises many thin ash strata enclosing one or more looser, 

thick layers of coarse pumice. Toward the northeast, the set 

consists mostly of a single layer (Yn) of lapilli and bombs
44 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Figure 34. Tephra set Y 15 km northeast of Mount St. Helens. 

Layer Yn, the oldest layer identifiable here, is almost 100 cm thick. 

The overlying strata of the upper part of set Y, ash bed pbp, and set 

P are each 20-25 cm thick. Tape bands on pickhandle are 10 cm 

wide. Photograph taken in 1989.

Figure 35. Tephra set Y about 8 km east of Mount St. Helens 

(measured section Y-l). Layer Ye, about 45 cm thick here, is both 

underlain and overlain by ash bed yc of flowage origin. Photograph 

taken in 1983.

and numerous thin ash beds (fig. 34). Toward the east and 

southeast (fig. 35), it also consists of a single lapilli layer 

(Ye) among beds of chiefly ash and thins from east around 

to the southeast.

A weakly oxidized zone that typically contains carbonaceous material marks the top of the lower part of the set (fig. 

36). Radiocarbon dates indicate that the oxidized zone 

formed in no more than a few hundred years.

Tephra layers in this set are interbedded with lenticular, 

locally thick deposits of pumiceous and lithic ash probably 

deposited by pyroclastic flows, surges, and ash clouds.

COMPOSITION

Highly vesicular pumice from bomb to ash size makes 

up most of the thickest layers Yn and Ye; smaller deposits

contain more lithic particles. Ferromagnesian mineral suites 

in pumice from all layers consist almost entirely of cummingtonite and hornblende; however, small amounts of 

biotite are common in the basal layer. Sparse hypersthene, 

which may not be primary, was found in samples from a few 

stratigraphic horizons.

Four of five samples of set Y pumice analyzed chemically contain between 65 and 66 percent SiC^; the fifth, 

from the uppermost part of the set, contains about 69 percent SiO2-

DIAGNOSTIC FEATURES

Set Y is distinguished from all other tephra deposits 

from Mount St. Helens younger than set S by abundant 

cummingtonite, and it is less weathered than older
DESCRIPTION OF TEPHRA UNITS 45

EXPLANATION

Lapilli or bombs 

Ash

Position of dated

14C sample 

Oxidized zone

CENTIMETERS 

^20

Uppers

Middle<

Lower <

d " " <J

" 'v • •. '' "\"a 

•J v v 4 ^ A 

? p <1 f ^

V <7 & b 4

a j> a

' ' A " 0

p A a > 

w „ t> " i

v „ 7 * & • 

* <,* > 4 v

0 ^ <J

* & 0

•*?•.£/:•£•££;'•:

>^ f t> 4 7

o" " f & A ^

1 9 *> <7 V > ^ < 

& V A k £ 4 

4 <7 t> p <] «

' A * P V <? '

a < J7 /r p v< <? > *

t, A 7 W d 1 

< ' " ^ "

•>* V^V4

» v p " t>" ; * A

' fc 7 A t> *

V *% '/»

, ^< 0 * " *

a & ^ a « < 

A 7 »• fr & 

& c> <1 <i 4 o 

*>***«*

V ^ « ,, 

« <,^ P4 on-vr^ <1 <3 ^ V 4 „

* > ^' " i I"

0 « ^ A

4 < v " « a '

>;::;^<

'" " ' A. A

^v^ » 6 6 « 

^ b !> * "a 7

< v . p * 

<l > * " 

„. 6 7 [7 •» V

4 * 7 1 <

Yu

Yo

YP

Yf

ya

yc

Ye

yc

Yn

Yd

Yb

122°15' 122°00'

Figure 36. Composite columnar section of tephra set Y, 

Mount St. Helens.

cummingtonite-rich deposits. It is lighter colored than the 

underlying set J and much thicker and coarser than the 

overlying set P. Its color and typical assemblage of coarse 

pumice layers interbedded with ash beds generally are 

adequate to identify the set in the field.

STRATIGRAPHIC RELATIONS

Set Y typically directly overlies the weathered upper 

part of set J, although colluvium and Mazama ash are present 

locally between those two sets. Near the volcano, set Y is 

overlain by set P. The thin-bedded upper part of set Y is similar in grain size and thickness to set P, but the two are separated by the ash bed pbp.

46° 

00'

.20

• 10

,15

10 KILOMETERS 

_J

Figure 37. Approximate thickness (in centimeters) of tephra set 

Y, Mount St. Helens. Thickness reported is cumulative thickness of 

strata interpreted as tephra; most stratigraphic sequences also 

include interbedded deposits of probable flowage origin.

DISTRIBUTION AND THICKNESS

Set Y is conspicuous from the north clockwise around 

to the south flank of the volcano. From north to east around 

the volcano, at distances of about 10 km, the set as a whole 

is 50-150 cm thick (fig. 37); it commonly has a thin, coarse 

layer at its base and a thicker coarse layer higher in the set. 

It is thickest toward the north-northeast along the path of 

layer Yn, where the set is more than 100 cm thick at 20 km 

and 10-20 cm at 100 km. Both of the two thickest layers and 

the largest number of identified strata are present toward the 

east-northeast, although the set is only about 100 cm thick at 

10 km in that direction. Toward the east along the axis of 

layer Ye, the set is about 50 cm thick at 10 km distance and 

about 10 cm thick at 100 km. Deposits of set Y are present 

west of Mount St. Helens, but no specific layers were identified. Tephra of the set that was carried westward is mostly 

obscured by ash of flowage origin that dominates the stratigraphic interval of set Y there.

AGE

Set Y was erupted between about 4,000 and 3,000 years 

ago. The older limiting date is provided by radiocarbon dates
46 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Site

Distance from vent 

(in kilometers)

EXPLANATION 

Lapilli 

Ash 

Oxidized zone

CENTIMETERS 

i-50

Number 11 Road 83 Road 140

TopYn

oCold Base ofsetY

pbp

yp

yp

Ye

\

F-'-o

\ '

Mount St^ 

Helens vent/

^ONumber 19

-ORoad 140

- 0 Road 83

Yc

N 

Yu

>

\

\

\ 

\ 

\

umber 1 

10

iz^ki^

^ "* "»'

r^t!

* A V * ' *::-

»^A b ' 

->„*

N""' 

0 7^4 

" ' ^

&

r ><l *•*

. >4 a <

.-;»

P

9 

pbp

Y° /

vf /

J

V/ - 

/

Vn /X

X/ / 

Yd/ Yb

V

/

Cold 

8

^ fl > ,

* <7 V f> 

V 4 4 <

<7 < 4 O

/:«:

tf 0 * 0

::<:

^ 6 *

"-Y t'3 b *

« d >

& V »A

r^T:

> " "*„»'

g _ N ^ o

F.I. 

3

Yn

& ^ & ^

rO^ „

Mud 

flow

o Number 11

Figure 38. Columnar sections of tephra set Y from northwest to south of Mount St. Helens and diagram of their locations relative to the 

central vent of the volcano.

of approximately 3,900 (W-2677) and 3,850 (W-3911) years 

B.P. (table 2) obtained from charred plant material at the base 

of the set. Charcoal samples from a pyroclastic-flow deposit, 

an ash-cloud deposit, and a lahar, all of which are interbedded 

with tephra near the top of the set, were dated at about 3,380 

((W-3144) (table 2), 3,350 (W-2549), and 3,280 (W-3262) 

years B.P., respectively (Crandell and others, 1981). The oldest limiting date above the entire set is provided by charred 

wood fragments from the base of the overlying set P that were 

dated as about 2,960 (W-2675) years B.P. (Crandell and others, 1981) (table 2). Enough time passed for vegetation to 

become established between eruption of sets Y and P, but no 

evidence is available to indicate whether the interval between 

the eruptions was only a few years or was the few hundred 

years allowed by the radiocarbon dates.

In Canada, one ash layer identified as tephra of set Y 

has been reported as 4,300 or 4,400 years old. That tephra is 

mineralogically and chemically similar to layer Yn; its age 

was based on an estimate of the sedimentation rate of peat 

between radiocarbon ages of about 3,800 and 6,500 years 

(Westgate, 1975, 1977). A review of evidence near the volcano, including a search for a part of set Y older than that 

now recognized, failed to support the existence of a Yn-like 

layer below the base of set Y at the volcano. An alternate

explanation for the postulated 4,300-year-old bed is that it is 

layer Yn itself. A tephra layer identified as layer Yn and 

dated at about 3,500 years B.P. has been found nearby 

(Westgate and others, 1970), but nowhere have two Yn-like 

layers been found superposed. Moreover, the mineralogical 

and chemical compositions of the postulated older Yn-like 

bed and layer Yn itself are reported to be too similar to distinguish between them (Westgate, 1977). Thus, only the estimated rate of sedimentation indicates the presence of two 

similar but separate beds, and, alternately, there may be only 

one bed. Here, the postulated 4,300-year-old bed is regarded 

as layer Yn.

PREVIOUS NAMES AND CORRELATIVE UNITS

Set Y of this report is a major component of the "older 

pumice" described by Carithers (1946).

DESCRIPTION OF LAYERS

Set Y can usefully be subdivided into lower, middle, 

and upper parts, which are present in large areas east of the 

volcano (figs. 36, 38). The lower part consists of layers Yb 

and Yd. It is topped by a thin oxidation zone and
DESCRIPTION OF TEPHRA UNITS 47

MEASURED SECTION Y-l

West valley wall of Smith Creek, at west end of bridge that crossed Smith Creek in 

1983, in NE'ASE'A sec. 5, T. 8 N., R. 6 E. See figure 35.

Thickness 

(centimeters)

14. Ash bed pbp

SetY:

13. LayerYu:

Ash, fine and coarse, lithic-rich; locally contains granules

and small lapilli ................................................................ 1-2

Ash and small lapilli; most lapilli are lithic........................... 0.5-1

Ash, fine and coarse, gray, lithic; coarsest in middle ............ 2

Ash and small lapilli, pumice and lithic, bedded .................. 1-2

Ash, fine to coarse, lithic and some pumice, gray ................ 2

12. Layer Yo. Lapilli, small, lithic, gray, angular, in sparse ash...... 3

11. Ash bed yp.................................................................................. 0.5-1

10. Lapilli (pumice and lithic) in ash; clasts as large as

7 by 5 by 3 cm; probable flowage deposit ............................ 4

9. Layer Yf:

Ash, fine, compact ................................................................ 3

Lapilli, small, mostly lithic, in coarse ash............................. 4

8. Ash bed yc. Fine, compact; grayish-brown ash overlies

pinkish-brown ash ................................................................. 5

7. Layer Ye:

Ash and small pumice lapilli, brownish-gray, compact........ 2-5

Pumice lapilli and small bombs ............................................ 32

Small lapilli and ash ............................................................. 3

Pumice lapilli and small bombs ............................................ 6

6. Ash bed yc. Ash, grayish-brown, compact................................. 7

5. Ash, gray, fine and coarse; small lapilli; bedding contorted;

probable flowage deposit....................................................... 35

4. Layer Yn. Pumice lapilli, small, well-sorted.............................. 10

3. Layer Yd

Ash, fine; upper 2 cm bleached gray,; carbonaceous material

at top ................................................................................. 25

Ash, coarse, and lithic; some pumice lapilli ......................... 2

Ash, interbedded fine and coarse........................................... 4

Ash and small lapilli, mostly lithic; scattered pumice .......... 2

Ash, fine, brown .................................................................... 1

2. Layer Yb. Ash and lapilli, crudely bedded, lithic (most

abundant) and pumice (largest) ............................................ 5

Below set Y: 

1. Colluvium, brown; consists mostly of set J material.................. >10

carbonaceous material that represent a dormant period long 

enough for plants to have become established. The two 

thickest layers Yn and Ye and interbedded ash make up a relatively coarse middle part of the set. The upper part, in contrast, consists of many thinner and finer layers of ash and 

small lapilli.

LAYER Yb

Layer Yb consists of several poorly defined beds of 

pale-yellow to brownish-gray pumiceous lapilli and

bombs at the base of the set (fig. 39). The number and distribution of individual beds are not known. The layer is 

thickest toward the north and northeast; toward the northeast, four beds or coarse zones are present 5 km from the 

volcano. Most pumice in this layer is denser than that in 

other major layers of set Y; only a small proportion of 

pumice clasts is highly vesicular. Cummingtonite and 

hornblende make up almost its entire ferromagnesian mineral suite, but small amounts of biotite are common. A 

single, whole-rock chemical analysis of layer Yb shows 

65-66 percent SiO2-

Layer Yb is identifiable chiefly as the basal coarse 

pumiceous unit of set Y. Its ferromagnesian mineral content distinguishes it from the underlying set J, but without 

stratigraphic control it is difficult to distinguish layer Yb 

from other strata in set Y. Relatively dense pumice and 

biotite content are the most useful criteria with which to 

make that distinction. In addition, electron-probe analyses 

of ash from Canada (Westgate, 1977) and from a site 50 

km southeast of Mount St. Helens (Mack, 1980) suggest 

that chemical differences might distinguish layer Yb from 

other layers of set Y; however, samples from stratigraphically confirmed layers at the volcano were not analyzed 

to test that possibility.

Typically, layer Yb lies directly on the weathered 

upper part of set J, although in a few places it lies on colluvium or on Mazama ash. It is overlain throughout its 

known extent by younger layers of set Y.

Layer Yb was identified from slightly west of north 

of the volcano clockwise around to slightly south of east. 

At 10 km to the north, it consists of lapilli and ash 20-25 

cm thick. At 5 km due northeast, it consists of as many as 

four coarse zones or beds of lapilli in a 35-cm-thick 

deposit (fig. 39A). East of the volcano, it is much thinner, 

consisting of scattered lapilli and even a few bombs in a 

few centimeters of ash at distances of 10-15 km (figs. 

39B, 40).

Farther downwind, ash beds that probably are parts of 

layer Yb are present 50 km north of Mount St. Helens at 

Davis Lake (Barnosky, 1981), 80 km to the northeast in 

Mount Rainier National Park (Mullineaux, 1974), and 50 

km to the east (Mack, 1980). At one time, only one pumice bed below layer Yn was thought to be extensive (Mullineaux and others, 1975), but these widespread 

occurrences show that multiple beds are present and that 

they extend far downwind in very different directions.

Several radiocarbon samples about 3,900 years old 

were recovered from immediately below layer Yb 

(W-3911 and W-2677; table 2), and layer Yb underlies 

radiocarbon dated at about 3,500 years old (W-1752; table 

2). The time required to develop an incipient soil that is 

above layer Yb but below the 3,500-year-old samples suggests that the age of layer Yb is close to 3,900 years.
48 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

''r". >7i*>;^f^'^£*£ v* •

i^SF ^SSKSS? ^yfeaesLl-?^^;-

n :^f' - - ,

\v *r"-^»i' - .^••--..*r ^X"u^ " *

B

Figure 39. Tephra set Y, Mount St. Helens. A, Multiple coarse 

beds in layer Yb about 5km northeast of the volcano. Shovel lies on 

oxidized top of set J. B, 11 km due east of the volcano. Pick head 

lies on oxidized top of set J. Layer Yb consists of scattered lapilli 

and a few bombs in ash overlain by a few centimeters of coarse and 

fine beds of layer Yd and finer ash of undetermined origin. Layer 

Yn consists of only about 1 cm of pale-brown ash. Knife is at top of 

setY.
DESCRIPTION OF TEPHRA UNITS 49

46° 

15'

46° 

15'

46° 

00'

122°15' 122°00' 122°15' 122°00'

.10

MOUNT ST 

HELENS A

•25

,30

A

MOUNT ST 

HELENS A

B

10 KILOMETERS

-20

,15 .10

J5

Figure 40. Thickness (in centimeters) of layers in tephra set Y, 

Mount St. Helens. A, Layer Ye. B, Layer Yn. C, Layer Yb.
50 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Figure 41. Alternating coarse and fine beds in layer Yd 7 km 

northeast of Mount St. Helens. All the beds shown are pans of layer 

Yd. Shovel head, about 25 cm long, is shown for scale. Photograph 

taken in 1979.

LAYER Yd

Layer Yd is composed of at least 14 beds of moderately 

vesicular to nonvesicular, well-sorted lapilli and ash (fig. 

41). Near the volcano, beds of lapilli alternate with coarse 

ash in a sequence of highly similar strata. The layer is coarse 

and thick only close to the volcano, but thin ash of the layer 

is widespread east of the cone. Curnmingtonite and hornblende are the only abundant ferromagnesian minerals in the 

layer.

A thin soil profile, consisting of scattered carbonaceous 

fragments in a bleached zone that overlies a weakly oxidized 

zone, has been developed in the upper part of the layer.

Layer Yd is recognized chiefly by its multiple similar 

beds, relatively dense yet vesicular fragments, thin soil at the 

top, and stratigraphic position between layers Yb and Yn.

The thickest and coarsest part of layer Yd extends eastnortheast. It contains abundant lapilli and is as thick as 60 cm 

5-6 km from the volcano and as thick as 40 cm at 9 km. 

Toward the east, it is about 20 cm thick but consists almost 

entirely of ash. Ash of layer Yd probably also extends to the 

southeast but was not differentiated from ash of layer Yb. 

Ash in the stratigraphic position of the two layers is about 20 

cm thick 8 km southeast of the volcano. Layer Yd is probably more widespread than is now known, but it was not 

traced elsewhere around the volcano or farther downwind.

Similar to layer Yb, layer Yd is younger than 3,900 

years and older than 3,500 years, and its age also probably is 

close to the 3,900-year date.

LAYER Yn

Layer Yn (figs. 34, 36) is a deposit of mostly lapilli and 

bombs of highly vesicular, pale-yellow or brown pumice that 

is 100-200 cm thick at the northeast base of the volcano. It 

is the coarsest, thickest, and most voluminous single pumice 

layer in set Y and one of the most voluminous ever erupted 

by Mount St. Helens. Its pumice contains only cummingtonite and hornblende as ferromagnesian phenocrysts, and hornblende overgrowths on cummingtonite phenocrysts are 

common. Chemically, the pumice is similar to that of layers 

Yb and Ye; some data (Westgate, 1977) suggest, however, 

that glass in layer Yn is slightly more silicic than glass in Yb.

Along the north-northeast-trending axis of its lobe, 

layer Yn is readily identified because it is the thickest and 

coarsest pumice of any age. Its pumice generally is firmer 

than that in older coarse layers, and its yellow color distinguishes it from younger coarse pumice units. Its ferromagnesian mineral suite readily distinguishes it from most layers in 

set S by the absence of hypersthene and from all other 

tephras younger than set S by the presence of cummingtonite.

Near the volcano, layer Yn overlies an incipient soil at 

the top of layer Yd. It is overlain by younger layers of set Y, 

although at the east and northeast base of the cone, ash-cloud 

deposits are between it and the younger deposits.

On the volcano, layer Yn generally is buried by younger 

deposits. Beyond the volcano, it forms a thick lobe that 

extends north-northeast (fig. 40). Along its lobe axis, the 

layer consists of as much as 200 cm of lapilli and bombs at a 

distance of 5 km, and it is about 100 cm thick at 15-20 km. 

Its thickness decreases to about 20 cm at 100 km and 5 cm at 

250-300 km (Mullineaux, 1977). North of the volcano, layer 

Yn consists of lapilli and bombs but generally is only 20-30 

cm thick at 10-15 km.

Layer Yn thins abruptly along the western margin of its 

lobe but thins and fines gradually along the eastern edge to a 

obscure deposit of thin ash. At 10-15 km directly east of the 

volcano, 1-2 cm of ash makes up the entire layer.

Layer Yn is one of the most important marker beds of 

Holocene age from Mount St. Helens. In Mount Rainier 

National Park, it is slightly more than 30 cm thick at a distance of 80 km (Mullineaux, 1974), and it can be identified 

as far as 900-1,000 km from Mount St. Helens (Westgate 

and others, 1970).

The layer is about 3,500 or 3,400 radiocarbon years old. 

A review of radiocarbon dates available in 1973 (Mullineaux, 1974) suggests that the age to the closest hundred 

years is 3,400 years. A recent review (Vogel and others, 

1990) proposes a more precise age of 3,360±65 years B.P.
DESCRIPTION OF TEPHRA UNITS 51

Because of the imprecision of radiocarbon dates, either 

3,400 radiocarbon years or the frequently used date of about 

3,500 radiocarbon years is reasonable for the age of layer 

Yn.

Layer Yn of this report was first identified at Mount 

Rainier and described as layer Y (Crandell and others, 1962). 

Layer Yn in Mount Rainier National Park was described as 

"the major pumice and ash layer from Mount Rainier" by 

Hopson and others (1962) and Fiske and others (1963).

LAYER Ye

Layer Ye (figs. 35, 36) is a pale-yellow-brown pumice 

lapilli and small bomb deposit that is as thick as 50 cm at the 

east base of the volcano. It is very similar to layer Yn but less 

voluminous and is present only east and southeast of the volcano. Although in most outcrops layer Ye appears to be a 

single shower bed, it locally exhibits two coarse zones (measured section Y-l). The two coarse zones were seen only on 

slopes and may represent minor slumping during accumulation rather than different eruptive pulses.

Layer Ye consists chiefly of highly vesicular pumice 

and contains only scattered lithic fragments. The ferromagnesian mineral suite consists almost entirely of cummingtonite and hornblende. Whole-rock chemical analyses show that 

layer Ye pumice is similar to pumice of layers Yb and Yn; 

however, some analyses (Westgate, 1977) suggest that differences in silica content of glass from the layers may be 

enough to distinguish each from the others.

Layer Ye is distinguished from layers Yb and Yn 

chiefly by stratigraphic position and distribution. It differs 

from pre-Y cummingtonite-bearing pumice deposits east of 

the volcano by its firmer pumice and absence of hypersthene 

and from coarse units in other Holocene tephra sets by its 

abundance of cummingtonite.

Close to the volcano, layer Ye overlies layer Yn or 

other layers of set Y, separated by ash deposits of probable 

ash-cloud origin. It is overlain successively by more ash of 

probable ash-cloud origin and thin tephras of the upper part 

of set Y.

Layer Ye was identified from about northeast of the 

volcano around to the south. It is thickest and coarsest 

toward the east (fig. 40). At a distance of 8-10 km in that 

direction, it consists mostly of lapilli and is as thick as 40 cm. 

In contrast, it is only 1-2 cm thick at 8 km directly toward 

the northeast and only 5-10 cm thick at 8 km directly toward 

the southeast.

Layer Ye forms a readily identified marker bed downwind; it is the main Holocene cummingtonite-bearing tephra 

east and east-southeast of the volcano. Layer Ye presumably 

is the "Y-like" cummingtonite-bearing ash bed reported to 

be 2-7 cm thick at a distance of 400 km in northeastern Oregon (Norgren and others, 1970; Borchardt and others, 1972).

LAYER Yf

At least five separate thin beds of ash and small lithic 

and pumice lapilli are grouped as a layer termed Yf (fig. 42). 

This group has been recognized separately only close to the 

volcano toward the northeast and east, mainly in the Smith 

Creek valley. Locally, bedding is obscure and some beds are 

lenticular. Ferromagnesian minerals from all pumice lapilli 

sampled consist almost entirely of cummingtonite and hornblende. Deposits of layer Yf appear virtually unweathered; a 

thin, oxidized zone near the base of the deposits probably is 

a result of deposition from ground water rather than surface 

oxidation.

Layer Yf is similar to other deposits in the upper part of 

set Y and is identified primarily as a sequence of thin, fine 

beds in its stratigraphic position; it is the lowest group of thin 

tephra beds above layer Ye. It is separated from layer Ye in 

some places by the ash bed yc and in other places by ash bed 

ya or its associated pyroclastic-flow deposit. Layer Yf is 

overlain by the distinctive ash bed yp.

Layer Yf was identified from northeast to east of the 

volcano, where it is 15-20 cm thick at distances of 8-10 km. 

The stratigraphic interval that contains the layer, however, is 

locally as much as twice that thickness because of interbedded ash of probable flowage origin. Although layer Yf surely 

extends beyond the sites at which it was identified, no 

attempt was made to trace it.

LAYER Yo

Layer Yo is a single, well-sorted bed as thick as 6 cm of 

gray ash and small lapilli (fig. 42) northeast and east of the 

volcano. It consists mostly of fresh lithic fragments and contains small amounts of moderately vesicular pumice. Cummingtonite and hornblende are the only abundant 

ferromagnesian minerals in the layer, but sparse hypersthene 

is common.

Layer Yo is thin, but better sorted, more friable, and 

more dominantly lithic than other described layers in the 

upper part of set Y. It is approximately in the middle of the 

upper part of set Y, immediately above the thin, pinkishbrown ash yp and below several other thin beds in the uppermost part of the set.

Layer Yo was identified mainly in the Smith Creek valley from about northeast to east of the volcano. At 8-10 km 

from the volcano in those directions, it commonly consists of 

ash and small lapilli as large as 1 cm across, and it is 2-6 cm 

thick.

This layer is described as tephra because of its continuity and gradual changes of thickness and grain size over a 

distance of several kilometers. Locally, it is similar to the 

stratigraphically lower ash-cloud bed ya, but it is more consistent in thickness and texture than bed ya and lacks the 

charred wood that is typical of bed ya.
52 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Figure 42. Layers in upper part of tephra set Y, Mount St. Helens, 8 km northeast of the center of 

the volcano. Pocketknife shown for scale. Photograph taken in 1977.

LAYER Yu

Layer Yu consists of as many as six poorly stratified, poorly defined thin beds of coarse ash and small 

lapilli (fig. 42) and makes up the uppermost part of set Y 

northeast and east of the volcano. These strata are planar 

bedded and lack crossbedding. Layer Yu consists mostly 

of somewhat vesicular to lithic fragments but contains 

some highly vesicular pumice. Ferromagnesian mineral 

suites in the various beds consist chiefly of cummingtonite and hornblende and typically include small amounts of 

hypersthene and oxyhornblende.

No diagnostic features were identified for this layer 

nor were any of its beds traced individually. It is defined 

as the lapilli-bearing tephra deposits of set Y that overlie 

layer Yo. Layer Yu is overlain directly by the ash bed 

pbp and in turn by tephra of set P.

Layer Yu is relatively consistent in grain size and 

thickness for a distance of several kilometers from northeast to east of the volcano but was noted separately from 

other parts of set Y only in that sector. At 9 km northeast 

of the volcano, it is about 15 cm thick and contains at 

least six distinguishable beds. At 8 km to the east, the 

layer is 5-10 cm thick and contains at least four identifiable beds.

DEPOSITS OF PROBABLE FLOWAGE ORIGIN 

ASSOCIATED WITH SET Y

Abundant ash deposits, produced at least in part by 

flowage phenomena, are interbedded with and immediately 

above set Y. Four such ash deposits that are distinctive, 

locally thick, or areally extensive are described herein 

because they are useful stratigraphic markers.

ASH BED yc

Ash bed yc consists of fine to coarse, brown to gray ash 

that locally is thick and probably consists of several depositional units. It lies directly on tephra layer Yn and is both 

below and above layer Ye (figs. 35, 36). It consists chiefly of 

glass, and ferromagnesian mineral suites are composed of 

mostly cummingtonite and hornblende and lesser amounts of 

hypersthene and oxyhornblende. Ash bed yc is recognized 

by its grain size, variable thickness, and stratigraphic position relative to layers Yn and Ye.

Ash bed yc was identified only in and near the Smith 

Creek valley northeast and east of the volcano. The thickest 

known deposit is 8 km east-northeast of the volcano, where 

it is as thick as 40 cm. It thins rapidly away from that area. 

Only about 3 km from that site, at both 9 km to the northeast
DESCRIPTION OF TEPHRA UNITS 53

of the volcano and 8 km to the east, the ash is only about 20 

cm thick.

ASH BED ya

Ash bed ya consists of a single, friable stratum, from a 

few centimeters to about 30 cm in thickness, of angular lithic 

fragments of moderate- to dark-gray ash and small lapilli. It 

is characterized by abundant charred conifer needles and 

small wood fragments. Hornblende and probable altered 

cummingtonite make up most of its ferromagnesian mineral 

suite. Its color, friable character, lithic content, and abundant 

charred plant fragments distinguish this layer from other 

strata associated with set Y.

Close to the volcano on the northeast and east, ash bed 

ya lies above ash bed yc (fig. 36). Ash bed ya is as thick as 

30 cm but is less than 10 cm thick over most of the area 

where it was found. It was identified as far as 10 km from the 

volcano toward both the northeast and east.

Before 1980, a lithic pyroclastic-flow deposit that 

stratigraphically correlates with bed ya was well exposed 

along Smith Creek about 8 km northeast of Mount St. Helens 

"(Crandell, 1987). That pyroclastic flow consisted of gray 

lithic fragments similar to those in ash bed ya, contained 

abundant charred logs, and was overlain by the same beds of 

set Y as overlie ash bed ya. Ash bed ya is interpreted to be 

the deposit of an ash cloud associated with that pyroclastic 

flow.

122°15' 122°00'

46° 

15'

46° 

00'

15-20

30

25 50

30

MOUNTS! 

HELENS A

50

45

20

16

20

10

10 KILOMETERS

Figure 43. Thickness (in centimeters) of ash bed pbp, Mount St. 

Helens

ASH BED yp

A thin, pinkish-brown bed of fine ash (figs. 36, 42) that 

is widespread northeast of the volcano is designated ash bed 

yp. Although widespread, it is nowhere known to be either 

coarse grained or more than about 2 cm thick. It consists 

chiefly of glass particles, but no pumice lapilli have been discovered in it, even close to the volcano. Cummingtonite and 

hornblende are its predominant ferromagnesian minerals, 

and oxyhornblende is abundant.

Although thin, ash bed yp is distinctive and can be recognized by its color, thinness, and stratigraphic position in 

approximately the middle of the upper part of set Y (figs. 36, 

42). The bed might be fine-grained tephra but is more likely 

ash that drifted downwind in one or more ash clouds generated by pyroclastic flows. Close to and northeast and east of 

the volcano, ash bed yp is an excellent stratigraphic marker 

in the upper part of set Y.

ASH BED pbp

A faintly bedded ash deposit that separates set Y 

from set P is termed ash bed pbp. This ash is widespread 

and is as thick as 50 cm. At one site 8 km northeast of

the volcano, it includes more than 10 recognizable depositional units of fine vitric and lithic ash. The deposit is 

mostly brown but typically is multicolored and commonly 

is reddish brown in both its lower and upper parts.

This ash deposit records a change in mineralogical 

character of material erupted from the volcano. At its 

base, ferromagnesian mineral suites are dominated by 

cummingtonite and hornblende and thus are similar to 

those in the upper part of set Y. Cummingtonite is less 

abundant higher in ash bed pbp, and at the top of the bed 

hypersthene is more abundant than cummingtonite. Oxyhornblende is common, especially in the upper and lower 

parts.

Ash bed pbp was found from northeast to south of 

the volcano. At 7-10 krn to the northeast and east, it commonly is 30-50 cm thick (fig. 43). At that distance southeast of the volcano it commonly is only 10-20 cm thick.

No individual lamina within ash bed pbp have been 

traced laterally, and how widespread any specific one 

might be is not known. Features diagnostic of either tephra 

or ash-cloud deposits were not seen, but both could be 

represented in this ash bed. The multiple beds and wide 

distribution of ash bed pbp suggest that it consists of many 

overlapping depositional units.
54 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Figure 44 (above and facing page). Tephra set P. A, About 8 km northeast of Mount St. Helens 

where layer Pm is relatively prominent and layer Py obscure (P-la). B, About 8 km east-southeast of 

the volcano where layer Pm is obscure and layer Py prominent (P-lb). See also figure 53.

TEPHRA SET P

Set P consists of a series of thin layers of gray dacitic 

ash and small lapilli that overlies a single basal layer of 

lapilli and bombs in ash (figs. 44, 45). All the layers contain 

abundant lithic fragments, and, overall, lithic fragments are 

more abundant than pumice clasts. Near the volcano, set P 

typically comprises a group of thin, light- and dark-gray 

strata sandwiched between finer, reddish-brown and purplish-brown ash beds. The set as a whole is similar in thickness and grain size to the group of thin beds that makes up 

the upper part of set Y but is separated from the latter by the 

relatively thick ash bed pbp. Set P contains no layers as thick 

or as coarse as the major layers in the older dacitic sets S, J, 

and Y or in the younger set W.

Set P appears unweathered and is grayer in outcrops 

than underlying and overlying tephra sets. No soil profiles 

were identified within or at the top of the set.

Pyroclastic-flow, surge, and ash-cloud deposits are 

conspicuously interbedded with tephra in set P, from nearly 

its base to its top (Crandell and Mullineaux, 1973).

ing one from each named layer, show a range of SiC>2 from 

about 62.5 to 64.5 percent.

DIAGNOSTIC FEATURES

In the field, set P is recognizable as a sequence of thin, 

gray, fine-grained beds overlying a coarse, poorly sorted 

basal layer. Except for the basal layer, the set is similar in 

grain size and bed thickness to the upper part of the underlying set Y. Ferromagnesian mineral content, however, readily 

distinguishes set P from set Y and from other older tephras 

except the much coarser and strongly oxidized set J.

Set P is readily differentiated from the overlying scoriaceous set B by its color, ferromagnesian mineral content, 

and chemical composition. It is distinguished from younger 

dacitic pumice in set W by its high proportion of lithic fragments, absence of thick, coarse beds, stratigraphic relation to 

set B, and ferromagnesian minerals. Set P pumice has lower 

ratios of hypersthene to hornblende than set W pumice, and 

the hypersthene of set P has lower refractive indexes.

COMPOSITION

Set P consists chiefly of fresh lithic material and pumice; lithic material predominates in all but the topmost layer 

Py. Both pumice and fresh lithic fragments contain only 

hypersthene and hornblende as abundant ferromagnesian 

minerals. Chemical analyses of five pumice samples, includSTRATIGRAPHIC RELATIONS

Set P overlies set Y all around the volcano and is separated from it in many places by ash bed pbp. Carbonaceous 

material, which is common at the base of set P, indicates that 

vegetation was established after deposition of set Y but 

before eruption of set P.
DESCRIPTION OF TEPHRA

Pi >- .^*vH%3 ^L ^•^'SSfc-l^- , _ 1 4* ^ ...»

1^4*' .'":^f" -.v-'-^-* ^^"^*r ^Hy" r ^"» v v PU ^ ^iC"

R(.-*..^%?'. r ^\^fe>.^^^^-^^^'^.- , ai^S^?^:^ v-,*i>^-:- * \V^t^^t/; ^ -^*~~^~^

55

Set P is overlain by set B near the volcano and is separated from it by the purplish-brown ash bed v. No evidence 

of a time lapse between deposition of set P and the overlying 

ash was observed.

DISTRIBUTION AND THICKNESS

Set P tephra containing lapilli crops out from northeast to south of the volcano, and compositionally similar 

ash was observed in all other directions. The maximum 

known thickness of set P tephra is about 50 cm, at the 

northeast base of the mountain 5 km from its center (fig. 

46). The actual thickness of tephra probably is closer to 

100 cm, but ash-size tephra near the volcano commonly 

cannot be distinguished satisfactorily from ash-cloud 

deposits. At many sites the stratigraphic interval containing set P tephra is 200-300 cm vertically. The excess 

thickness results from interbedded deposits of flowage 

origin.

At 8 km northeast of the volcano set P is as thick 

as 40 cm, and at 8 km to the southeast it is as thick as 

30 cm. At that distance to the south and west, it

EXPLANATION 

Lapilli 

Ash

Position of dated 

14C sample

CENTIMETERS 

1—20

a cA*'

PV

Pu

pa

Ps

Pm

Figure 45. Composite columnar section of tephra set P, 

Mount St. Helens.

probably consists of only a few centimeters of ash. 

Although some beds of ash on the south and west flanks 

may be tephra, most ash there probably was derived 

from flowage events.
56 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

122°15-

46° 

15'

46° 

OO1

,10

10 KILOMETERS

Figure 46. Thickness (in centimeters) of tephra set P, Mount St. 

Helens. Thickness is cumulative thickness of strata interpreted as 

tephra; most stratigraphic sections also include interbedded deposits of flowage origin.

Downwind, tephra of set P has been found 80-100 km 

to the northeast in Mount Rainier National Park (Mullineaux, 1974) and about 50 km to the east at Mount Adams 

(J.W. Vallance, 1982, written commun.). In addition, set P 

ash has been tentatively identified (Westgate, 1977) about 

400 km to the north-northeast in Canada. These downwind 

beds have not been correlated with specific layers at Mount 

St. Helens.

ORIGIN

Set P tephras probably originated in volumetrically 

small explosive eruptions of molten and partly solidified 

and degassed magma. This activity was accompanied by 

the generation of numerous lithic and pumiceous pyroclastic flows and growth of domes (Crandell and Mullineaux, 

1973; Crandell, 1987). The first eruption was highly 

explosive and opened the vent for the many pumiceous 

eruptions that followed. The abundance of fresh lithic particles in pumiceous tephras suggests that domes formed 

between those eruptions.

MEASURED SECTION P-l

Along south side of West Smith Creek valley, near center of sec. 30, T. 9 N., R. 6 E. 

See figure 44A.

Thickness 

(centimeters)

9. Ash bed v. Pinkish-brown fine ash................................................. 15

SetP:

8. Layer Py

Ash, fine, yellowish-gray ...................................................... 1-2

Ash, fine, gray ...................................................................... 0.5-1

Ash, yellow-gray; contains small pumice and lithic lapilli as

much as 3 cm across ......................................................... 2

7. Ash, grayish-brown..................................................................... 2

6. Layer Pu:

Ash, coarse, gray................................................................... 1

Lapilli, small, lithic and pumice, and ash; about one-third of

lapilli are pumice............................................................... 6

Ash, coarse, lithic; in three beds or zones, each 1-2 cm

thick, in fine ash ................................................................ 6-8

5. Bed pa:

Ash, fine, pinkish-brown ...................................................... 1

Ash, fine, gray ....................................................................... 1

Ash, fine, brown (iron stained) ............................................. 0.5

Ash, fine, pinkish-brown ...................................................... 0.5

Ash, coarse and fine, grayish-brown ..................................... 4-8

Ash, fine, pale-brown, wavy bedded .................................... 1-2

Ash, gray, wavy bedded ........................................................ 1-2

Ash, fine, pale-brown ........................................................... 6-8

4. Layer Ps:

Lapilli, lithic and pumice, in ash .......................................... 2

Ash....................;.....................................................-........-.... 0.5

Lapilli, smaller thaaabove, in ash ........................................ 3

Ash......................................................................................... 0.5

Lapilli, as in next higher bed, in ash ..................................... 3

3. Ash, gray and brown................................................................... 10

2. Layer Pm. Lapilli and a few small bombs; generally in ash

matrix; locally openwork, sparse ash matrix......................... 7-10

1. Ash bed pbp................................................................................ 50

AGE

Set P was erupted between 3,000 and probably about 

2,500 radiocarbon years ago. The eruptions began shortly 

after 3,000 years ago; two radiocarbon dates from the base of 

layer Pm (W-2675 and W-2829; table 2) and two from pyroclastic flows erupted early in Pine Creek time (Crandell and 

others, 1981) are between 3,000 and 2,900 years B.P.

The time at which set P eruptions ended is less well 

defined. No closely limiting ages have been obtained from 

tephra immediately above the set; thus the younger age limit 

for set P is inferred from ages of associated pyroclastic 

flows, which continued at least until about 2,500 years ago 

(Crandell, 1987) (table 7, later). Radiocarbon dates of about 

2,200 years B.P. from pyroclastic-flow deposits well above 

the base of deposits of Castle Creek time (Crandell, 1987) 

(table 7, later) suggest that eruptions of set P ended well 

before 2,200 years B.P.
DESCRIPTION OF TEPHRA UNITS 57

Site 055 

Distance from vent 

(in kilometers) 7

085 100

11

115

10

EXPLANATION 

Lapilli 

Ash

CENTIMETERS [—20

,055

Mount Sto.-————° 085 

Helens vent ^^ "——___

10 4 

««**

V <j ^ - 

\V 1 '

'Tl" *

•'.'.'•'• '•'•V. : '-'-:

*v>^

1 "t> <f 1

»«/<•/

7 y ^ ^ ° t>

Py

Pu

"~"~— — - __^

pa

\ 

\

\

PsX

Prri^\

—-— . ___^

& £> 

**/ *

' 1 " A 4

><l o >

^^Sv

"•'•"/• v/'^%'-";

< %* « '

V " & »

* A . 1>

1 a <

Figure 47. Columnar sections of tephra set P, from northeast to southeast of Mount St. Helens and diagram of their locations relative to 

the central vent of the volcano.

DESCRIPTION OF LAYERS IN SET P

The four named layers in set P are illustrated by figure 

47 and measured section P-l. Of the four, only the basal 

layer Pm is distinctive, in that it contains bombs and blocks. 

Each of the other three described layers consists of multiple 

beds. Layer Pm and overlying layer Ps in the lower part of 

the set are separated from layers in the upper part of the set 

by relatively thick, finer ash. No weathering profiles are 

known that record time periods between eruptions of various 

parts of set P.

as much as about 15 cm across. Many of these large clasts 

have diameters greater than the 5-10-cm-thickness of ash 

matrix in the layer and project above that matrix. Clasts that 

are large relative to layer thickness characterize this stratum 

east of the volcano to a distance of 10 km or more.

Its limited thickness, wide distribution, presence of 

fragments that are large compared with layer thickness, and 

abundant lithic clasts suggest that layer Pm originated from 

a short-lived but highly explosive eruption. That eruption 

ejected some gas-charged magma but also picked up many 

fragments of previously solidified magma.

LAYER Pm

Layer Pm is a single, thin, widespread deposit that 

includes lapilli and bomb-size fragments of both lithic and 

pumiceous rocks in a sparse ash matrix (fig. 44). It is distinctly coarser than other tephras of set P and is the only 

tephra in the set that contains bomb-size fragments. Hypersthene and hornblende are the only ferromagnesian phenocrysts found consistently in either pumice or fresh lithic 

fragments from the layer.

Layer Pm overlies set Y and is separated from it by ash 

bed pbp. The layer is overlain everywhere by other tephra 

layers of set P or by coeval ash of other origins.

At the base of the volcano, layer Pm forms a continuous 

bed from at least the northeast clockwise around to the south. 

On the northeast, it contains pumice bombs and lithic blocks

LAYER Ps

Two or more thin, well-sorted, gray, salt-and-pepper 

ash beds characterized by abundant small lithic and pumice 

lapilli are designated layer Ps (figs. 44,45). Lithic fragments 

are more abundant than pumice in this layer. Layer Ps can be 

described as either stratified or vertically zoned; vertical 

grain-size changes generally are gradual rather than sharp. 

East of the volcano, at least two such lapilli-bearing beds can 

be readily recognized even though they do not have welldefined boundaries. Lithic and pumice fragments in the layer 

are mineralogically and chemically similar to those in other 

layers in set P, and the layer is identified chiefly by its stratigraphic position in the lower part of set P.

Layer Ps overlies layer Pm, separated only by ash of 

undetermined origin. Near the volcano, this ash interbed is
58 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

locally as thick as either of the two named tephra layers. 

Layer Ps is overlain by relatively thick, multiple beds of fine 

ash (bed pa) that separate it from layer Pu.

Layer Ps was recognized from northeast to southeast of 

the volcano. Throughout that arc, at distances of 5-10 km, it 

commonly consists of 10-20 cm of small lapilli and ash (fig. 

47). It was not traced beyond about 10 km from the volcano, 

and its overall extent is unknown. It probably is one of the 

set P tephras identified at Mount Adams about 50 km downwind.

Layer Ps probably resulted from intermittent eruptions 

of fresh magma through domes composed of similar but 

solidified magma that was erupted earlier in Pine Creek time.

LAYER Pu

Layer Pu, similar to layer Ps, consists of multiple, thin, 

gray, salt-and-pepper beds of lithic and pumiceous lapilli in 

ash. No bombs or blocks were seen in it, and lithic material 

is more abundant than pumice. Layer Pu includes at least 

four lapilli-bearing beds or zones. The beds differ in ratios of 

lithic to pumice clasts, but no clasts are known to be mineralogically different from others in the layer. The layer is 

identified chiefly on the basis of its stratigraphic position in 

the upper part of the set above ash bed pa.

Layer Pu lies directly on ash bed pa. Where that ash bed 

was not identified, the layer was not distinguished from the 

older layer Ps. Layer Pu generally is overlain by layer Py, 

separated only by thin ash.

Layer Pu was recognized from northeast to southeast of 

the volcano. Along that arc, at distances of 5-10 km, it generally is 10-20 cm thick. It is thickest toward the northeast. 

At 8 km in that direction, it is 15-20 cm thick and contains 

at least four relatively well defined lapilli-bearing beds; 

pumice lapilli are as much as 3 cm across.

No specific bed in the layer was traced beyond about 15 

km from the volcano, and downwind extents are not well 

known. One or more of the beds at a distance of 80-100 km 

at Mount Rainier (Mullineaux, 1974), however, probably is 

part of layer Pu.

LAYER Py

Layer Py consists of two and locally three thin beds of 

pumiceous and lithic lapilli associated with fine, grayish-yellow ash (fig. 44). It is the youngest layer in set P as recognized and forms the top of the set northeast and east of the 

volcano. To the northeast, as many as three beds or zones are 

identifiable within layer Py. Layer Py typically contains a 

higher proportion of pumice than other layers of set P and 

generally can be recognized by its abundant pumice and 

associated yellow ash. Its mineralogical and chemical

composition, however, is the same as that of other layers in 

the set.

Throughout its extent, layer Py overlies layer Pu, separated from the latter by thin ash. Near the volcano, layer Py 

is generally overlain by the purplish-brown ash bed v, followed by scoria of set B.

Layer Py was found from the north flank of the volcano 

clockwise around to the south. Its maximum known thickness is about 20 cm, but it is only a few centimeters thick in 

most outcrops. The largest lapilli are about 6 cm across, but 

in most places the largest is only 1-2 cm. The layer was not 

traced downwind but probably is one of the set P strata identified 80-100 km downwind at Mount Rainier (Mullineaux, 

1974).

Its stratification, relatively consistent grain size and 

thickness, and wide distribution indicate that layer Py is a 

tephra deposit. The associated yellowish-gray ash may be 

related, however, to pumiceous pyroclastic flows that 

formed near the end of the Pine Creek period (Crandell, 

1987).

ASH OF PROBABLE FLOWAGE ORIGIN 

ASSOCIATED WITH SET P

ASH BED pa

Yellowish-gray to pinkish-brown fine ash, commonly 

mottled with orange-brown iron oxide stain, lies between 

layers Ps and Pu in the middle of set P (fig. 44). The ash commonly is faintly stratified and finer grained than the underlying and overlying tephra layers and almost everywhere 

includes a thin, iron-stained bed. Ash bed pa is the principal 

criterion used to separate layers Ps and Pu.

Locally, ash in the stratigraphic position of ash bed pa 

is clearly of flowage origin. For example, a thick ash deposit 

between layers Ps and Pu on the east and south flanks of the 

volcano (Crandell and Mullineaux, 1973; Crandell, 1987, p. 

46) thickens toward pyroclastic-flow deposits in the adjacent 

valleys and probably was deposited by ash clouds generated 

by the pyroclastic flows. In addition, pyroclastic-flow deposits themselves are conspicuously interbedded with set P 

tephra in places on the south flank of the volcano and also, 

before 1980, on the northeast flank.

ASH BED v

A widespread stratum of purplish-gray to pinkishbrown ash called ash bed v directly overlies set P east of the 

volcano (fig. 44) and separates it from the overlying set B. 

No material coarser than ash was observed in this bed. 

Locally, the ash bed is faintly stratified and is as thick as 50 

cm. Its ferromagnesian mineral suite is dominated by
DESCRIPTION OF TEPHRA UNITS 59

Figure 48. Tephra set B 8.5 km east-southeast of Mount St. 

Helens at a road cut along USFS Road N83,0.5 km northeast of the 

Muddy River (measured section B-l). Tape bands on pickhandle 

are 10 cm wide. Photograph taken in 1989.

hypersthene and hornblende, much of which has oxidized, 

hematitic rims.

Ash bed v is easily identified because of its stratigraphic position between the compositionally different sets 

P and B. No evidence of change from a composition similar 

to that of set P to that of set B was found in the few samples 

of ash bed v examined.

The ash bed probably originated from a variety of eruptive events, perhaps including small tephra eruptions and 

pyroclastic flows that produced ash clouds. Oxidized coatings on mineral rims suggest that many eruptions originated 

within or disrupted preexisting domes of Pine Creek time.

TEPHRA SET B

Set B is a conspicuous dark-brown stratigraphic unit 

that consists chiefly of several layers of scoriaceous

EXPLANATION 

Lapilli 

Ash 

Oxidation zone

CENTIMETERS 1—20

— 0

•^ p ^) <1 ^) a 

' ? v \ « \ *

'**{*?,

« v *** ,

; i * % ^ -

* „ fc ' a < ,

* A c. * *

A <J 0 4 t> f.

" f "••- „• -

„** *J- *

0 a * ^ <J P

'^"^"fC'.V 

y»- V; 'y"f*'*»\

» £><= ***

g" t>""p fl"V'i"i

Bu

Bi

ba

Bd

ba

Bo

Rh

Figure 49. Composite columnar section of tephra set 

B, Mount St. Helens.

lapilli and ash and a few bombs (fig. 48). The set crops 

out all around the volcano. With its dark color and mafic 

composition, it contrasts with all older tephras and all but 

one younger tephra. Between the mafic layers, the set 

includes two small dacitic tephras, one of moderately 

dense pumice and a second of highly vesicular pumice 

(figs. 48, 49).

This set is readily identified because of its overall 

dark color and its multiple beds of scoriaceous lapilli; only 

the younger set X contains scoria, and it has only one 

lapilli layer that is thin. Set B is relatively strongly oxidized and appears weathered overall, but a weathering 

profile was recognized only at the top of the set.

Set B is associated with ubiquitous thin mafic ash 

beds that are below, between, and above the named 

tephra layers. Many of those beds surely are tephra, 

ejected during early and late stages of eruptions of 

described layers, but they are not known to be distinctive 

and are not described separately.

COMPOSITION

Set B consists mostly of andesite but also contains 

basalt and dacite. Moderately to highly vesicular clasts of 

scoria and pumice dominate the set; lithic clasts are not 

abundant. Olivine is abundant in all scoria strata except the 

basal layer, which contains sparse olivine and abundant 

hypersthene and augite (table 5). Ferromagnesian minerals 

in the dacitic layers are mostly hypersthene and lesser augite and hornblende (table 5). Chemical compositions range 

from about 64 percent SiC>2 in the dacite layers to about 47 

percent SiC>2 in the basalt at the top of the set.
60 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Table 5. Representative ferromagnesian mineral contents of

layers of set B.

[Leaders (--) indicate not observed]

122°15' 122°00'

Layer

Bu

Bi

Bd

Bo

Bh

Olivine

Abundant

—

—

Abundant

Trace

Hypersthene

—

Abundant

Abundant

Common

Abundant

Augite

—

Minor

Minor

Common

Abundant

Hornblende

—

Trace

Minor

—

-

DIAGNOSTIC FEATURES

Its strong brown color and scoria content distinguish set 

B from other stratigraphic units in almost all outcrops. Presence of scoria alone distinguishes it from older tephras and 

from all younger tephras except set X. Multiple coarse scoria 

layers differentiate it from set X on all sides of the volcano 

except the south and west. Even in the absence of scoria 

lapilli, the scarcity of olivine at the base of set B and the presence of olivine without pyroxene in the upper part separate 

set B strata from those of set X.

STRATIGRAPHIC RELATIONS

Set B overlies set P and is separated from it east of the 

volcano by ash bed v. On the west and northwest sides of the 

volcano, set B commonly overlies dacitic ash that is mineralogically similar to that of set P but which may be of flowage origin. Set W tephra generally overlies set B east of a 

north-south line through the volcano except toward the 

northeast, where Sugar Bowl deposits locally intervene 

between the two.

No evidence was found of a period of time between 

eruption of sets P and B long enough for a soil profile to 

form. In contrast, the upper part of set B typically has been 

modified into a relatively conspicuous soil that represents 

hundreds of years of exposure before burial by younger 

deposits.

DISTRIBUTION AND THICKNESS

Set B crops out on all sides of the volcano. It is thickest 

from the northeast to southeast, where it contains abundant 

lapilli and is 3(M-0 cm thick at distances of 8-10 km (fig. 

50). In that sector, the set as a whole is about as thick as the 

underlying set P. Toward the south, set B is about 20-30 cm 

thick at 6 km; farther to the west it thins and is only 15-20 

cm thick at the southwest base of the volcano. From southwest of the volcano clockwise around to the north, the set 

typically contains only one layer of scoria lapilli but is as 

thick as 20 cm.

Downwind, set B can be readily traced to distances of 

about 30 km toward the northeast, east and southeast. Thin

46° 

00'

MOUNT ST 

20 A HELENS

10 KILOMETERS

Figure 50. Thickness (in centimeters) of tephra set B, Mount St. 

Helens. Thickness is cumulative thickness of strata interpreted as tephra; many stratigraphic sections also include interbedded deposits 

of flowage origin.

ash of set B also has been identified as far as 50 km to the 

east near Mount Adams.

AGE

Set B was erupted between about 2,500 and 1,600 

radiocarbon years B.P. No radiocarbon material has been 

found at the base of the set. The older limiting age is judged 

from radiocarbon ages in deposits of the next older Pine 

Creek period, which are as young as about 2,500 years. The 

oldest age from within set B is about 2,200 years (W-2923; 

table 2), from dacitic pyroclastic-flow deposits that overlie 

the two lower layers of the set.

A single radiocarbon date of about 1,620 years 

(W-2990; table 2) from above the upper layer of set B provides the younger age limit for the set.

DESCRIPTION OF LAYERS IN SET B

Stratigraphic positions and representative thicknesses 

of the named layers in set B are given by figure 51 and measured section B-l. No outcrop exhibits all the layers
DESCRIPTION OF TEPHRA UNITS 61

Site

Distance from vent 

(in kilometers)

EXPLANATION

Lapilli or bombs 

Ash 

Oxidation zone

CENTIMETERS 1—20

^0

0 045

Mount St / 

Helens vent o- —— —— ° °85

-0110

0145

0

' $

:i

V ' 

<S 4

A <

a A 

» 

a •

A

45 

6

« ?:.

***

t>

^

/

Bu xx

X

Bi /-^

ba

// Bo

~— — _____

Bh

^.-^^

085 

6

t y ») *

' 0 •« 5-

& <* v i

fr £> .'.:-.v

A* & _,,

V>r.?.i

\

___^, — -

^-

110

9

145

6

Bu

ba

Bo

YT

Bh

Base of set B

Figure 51. Columnar sections of tephra set B from northeast to south of Mount St. Helens and a diagram of their locations relative to the 

central vent of the volcano.

described. All but layer Bd do crop out, however, from 

northeast to the southeast near the volcano; layer Bd was 

found only toward the north. The dacitic layer Bi conspicuously separates the lower and upper scorias of the set east of 

the volcano.

Deposits of probable pyroclastic-flow, pyroclasticsurge, and ash-cloud origin are commonly interbedded with 

set B tephras, especially on the east and southeast sides of the 

volcano. Dacitic pyroclastic-flow deposits also lie between 

lower and upper parts of the set on the northwest flank of the 

volcano.

LAYER Bh

Layer Bh typically consists of a single bed of brown 

scoria lapilli but locally toward the north and east includes 

two beds separated by ash. Where two beds were observed, 

the upper bed is thicker and commonly is normally graded. 

Layer Bh contains little other than scoria and is relatively 

well sorted; close to the volcano, open spaces between lapilli 

are noticeable. Ferromagnesian phenocrysts from the scoria 

are chiefly hypersthene and augite (table 5). Olivine is sparse 

and was not found in all samples examined. A single sample 

analyzed from the upper part of the layer contains about 58 

percent SiC>2.

This layer is locally coarser and better sorted than the 

others in set B, but its identification depends almost completely on its stratigraphic position and on its ferromagnesian 

mineral suite of hypersthene and augite, which is unlike that 

in any other scoria at Mount St. Helens.

Layer Bh overlies set P throughout its known distribution. East of the volcano, it is separated from set P by the

purplish-brown ash bed v. Layer Bh is overlain in most 

places by layer Bo scorias but toward the north and northwest is locally overlain by the younger layer Bd or layer Bu.

Layer Bh was identified from northwest of the volcano 

clockwise around to the south. It is thickest and coarsest 

toward the north and east where it is as thick as 20 cm at a 

distance of 6 km. In other directions from the northwest 

around to the southeast, it commonly is only 5-10 cm thick 

at distances of 6-8 km (figs. 50-52). Lapilli are common at 

those distances, but bombs are rare. The layer was not traced 

more than about 15 km downwind but surely extends at least 

a few tens of kilometers farther.

LAYER Bo

Layer Bo is a relatively thick stratum consisting of several beds of brown scoria lapilli and ash (fig. 48). In the field, 

it is strongly similar in appearance to layer Bh. From the 

northeast to southeast flank of the volcano, layer Bo commonly has three or four discernible beds or zones, each of 

which contains highly vesicular scoria lapilli and is separated from the others by thin, dark ash (fig. 48). Although as 

many as four stratigraphic beds or zones can be recognized, 

their vertical boundaries are poorly defined, and efforts to 

trace them as discrete units beyond a few tens of meters were 

not successful.

Olivine is a major ferromagnesian constituent in all 

beds in layer Bo (table 5). In addition, pyroxene was present 

in samples from all beds of the layer tested but one. Both 

augite and hypersthene are common, although their proportions vary widely. A chemical analysis determined that the
62 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

MEASURED SECTION B-l

Along USFS Road 83 0.5 km northeast of Muddy River, NW!/4 NE l/4 sec. 20, T. 8 N., 

R. 6E. See figure 48.

Thickness 

(centimeters)

5. Layer Bu. Lapilli and ash; dark, mostly brown; dark gray in 

middle; chiefly ash at bottom and top; lapilli as much as 

3 cm across ........................................................................... 10

4. Layer Bi. Ash, light-yellow-gray; lapilli as much as about

5 cm across........................................................................... 2-5

3. Ash bed ba:

Ash, mottled brownish-gray;, iron oxide stain at base ........ 9

Ash, dark-gray; subhorizontal iron oxide streaks ............... 5

2. Layer Bo:

Lapilli, scoria; brown with gray interiors; openwork; clasts

as much as 2 cm across ................................................... 5

Ash; in irregular, lenticular bed ........................................... 1

Lapilli, scoria, small; in coarse ash ..................................... 4

Ash, dark-brownish-gray, fine.............................................. 1

Lapilli, scoria, small; in coarse ash ..................................... 3

Ash, grayish-brown, fine...................................................... 5

1. Layer Bh:

Scoria, lapilli, and coarse ash, brown; clasts as large as

2 cm ................................................................................. 4

Ash, dark-gray ..................................................................... 2

MEASURED SECTION B-2

South valley wall of Ape Canyon Creek, about 50 m west of junction with Smith Creek 

valley wall, SW'ANE'/t sec. 5, T. 8 N., R. 6 E.

Thickness 

(centimeters)

SiC>2 content of a single fragment from layer Bo is about 55 

percent.

Abundant olivine in layer Bo distinguishes it from the 

underlying layer Bh, and abundant pyroxene distinguishes it 

from the upper scoria layer Bu. The ferromagnesian mineral 

suite also distinguishes layer Bo from all other tephras 

known at Mount St. Helens except those in set X. Olivine in 

layer Bo has generally higher refractive indexes than olivine 

in layers of set X, but stratigraphic relations may be required 

to satisfactorily distinguish between them.

Layer Bo overlies layer Bh and is separated from it only 

by thin ash. Layer Bo is overlain by ash bed ba, followed by 

younger layers of set B.

Coarse, lapilli-bearing layer Bo was identified on all 

sides of the volcano except the northwest. It is thickest 

toward the southeast, where it is about 15 cm thick at 8-10 

km from the volcano (fig. 52). On the northeast and east, it 

commonly is 5-10 cm thick at 8-10 km. It was traced for 

only about 20 km from the volcano but presumably extends 

much farther.

The several beds or zones in layer Bo, their varying 

content of ferromagnesian minerals, and the distribution of 

the layer around the volcano suggest that the layer resulted 

from multiple eruptions. During various eruptions, wind 

directions were different, and the tephra may even have 

come from more than one vent. Even over distances of a few 

tens of meters, the number of recognizable beds in layer Bo 

changes markedly, suggesting that wind or snow melt may 

have reworked these beds during or shortly after their deposition.

13. Colluvium................................................................................. 5-20

Set B. DACITIC TEPHRA

12. Ash, fine, dark-gray, lenticular................................................. 3

11. Layer Bu. Small scoria lapilli as much as 4 cm across in Two dacitic tephra layers, Bd and Bi, are recognized in

sparse ash ............................................................................ 10 the middle of set B. In addition, pinkish-gray to brown ash

10. Ash, brown, fine........................................................................ 3 containing dacitic material is almost ubiquitous around the

9. Layer Bi. Ash and pumice lapilli as much as about 3 cm , . . . . . . - , . ,,, ,. , ,

across; lenticular, generally 5-10 cm thick ........................ 8 V°lcan° m Stratigraphic position of the middle of the Set and

8. Ash, brown, lenticular............................................................... 1-3 probably includes material derived from flowage events.

7. Lahar, bouldery......................................................................... 35

6. Pyroclastic-flow deposit; bouldery lithic ash and lapilli........... 15

5. Ash bed ba:

A u u c i LAYERBd Ash, brown, fine .................................................................. 2

Ash, dark-gray, coarse to fine ............................................. 10

4. Layer Bo: Layer Bd is a single, thin bed of relatively dense pumice

Scoria, lapilli in ash, brown ................................................ 3 of small lapilli and ash size that was found only on the north

Ash, dark-gray .................................................................... 1 an(j north-northwest sides of the volcano. Its ferromagnesian

Scoria, lapilli in ash, brown lenticular ................................ 3 ^^ ^ ^^ Qf mostl h mhene md small

3. Ash, brown................................................................................ 1 J Jr . .

2 Layer Bh- amounts of augite and hornblende. One chemical analysis or

Scoria, lapilli and ash, dark-brown ..................................... 1 layer Bd pumice indicates a SiO2 content of about 64 perAsh, gray, fine ..................................................................... 1 cent.

cona, api i an as ... ............................................. Layer Bd is recognized by the relatively dense character

Ash, dark-gray; lenticular, 1-5 cm thick ............................ 5 J ° J

Base of set B °f its constituent pumice, by its distribution, and by its strati- 

	graphic position between the lower scorias of the set and 

1. Ash bed v................................................................................... 11 layer Bu.
122°15'

DESCRIPTION OF TEPHRA UNITS

122°00' 122°15'

63

122°00'

46° 

15'

46° 

00'

MOUNTS! 

HELENS A

MOUNTS! 

HELENS A

46° 

15'

46° 

00'

MOUNT ST 

HELENS A

B

o

MOUNT STA 

HELENS

10

D

20 KILOMETERS

Figure 52. Thickness (in centimeters) of layers in set B, Mount St. Helens. A, Layer Bu. B, Layer Bi. C, Layer Bo. D, Layer Bh

The maximum known thickness of layer Bd is 5-10 cm 

on the pre-1980 north-northwest flank of the volcano. Layer 

Bd was identified in the South Cold water Creek valley about 

10 km north of the volcano, but its distribution farther to the 

north and in other directions is not known.

Layer Bd is similar in ferromagnesian mineral content 

and stratigraphic position to nearby dacitic pyroclastic-flow 

deposits that are dated at 2,200-2,000 years old (Crandell, 

1987). The tephra layer probably is a product of small explosive events during dacitic eruptions that produced the
64 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Figure 53. Tephra set B at a location where layer Bi is conspicuous, about 11 km east of Mount St. Helens. Tape bands on pickhandle are 10 cm wide.

pyroclastic-flow deposits and is approximately the same age 

as those flowage deposits.

LAYER Bi

Layer Bi is a single, well-sorted bed of highly vesicular 

white to yellow pumiceous ash, lapilli, and a few small 

bombs (fig. 53). It crops out only from northeast to southeast 

of the volcano. Its ferromagnesian minerals are chiefly 

hypersthene and lesser augite (table 5). This suite differentiates layer Bi from all other recognized tephras at Mount St. 

Helens. A single chemical analysis shows about 64 percent 

SiO2.

Layer Bi is usually easily recognized because it is the 

only dacitic tephra that lies between set B scorias east of the 

volcano. Its identity can be easily confirmed by examination 

of its ferromagnesian minerals. The layer lies directly above 

the upper part of ash bed ba and is overlain by the distinctive

layer Bu; no evidence of a time break at either horizon was 

seen.

In the sector from northeast to southeast of the volcano, 

layer Bi contains abundant lapilli and is 10-15 cm thick at 

distances of 5-10 km. Downwind to the east, over a short 

distance, it diminishes in thickness to about 1 cm at 20 km.

Radiocarbon ages of samples below and above layer Bi, 

from a peat bog 20 km east of the volcano, indicate that the 

layer is approximately 1,800 years old (W-2924, W-2925; 

table 2).

LAYER Bu

Layer Bu is a widespread but thin layer of scoriaceous 

lapilli and a few bombs in ash (figs. 49, 51). Locally it 

includes two discernible lapilli beds or zones, but in most 

outcrops only one could be distinguished. Scoria of layer Bu 

is highly vesicular and contains only olivine as an abundant 

ferromagnesian mineral. It is the most mafic of Mount St. 

Helens tephras in that it contains only about 47 percent SiC>2.

A soil profile that is virtually ubiquitous in the top of set 

B is developed mostly in layer Bu. As a result, the upper part 

or even all of the layer generally is part of a mixed soil zone 

that locally contains carbonaceous particles.

The presence of olivine and the almost total absence of 

pyroxene separate layer Bu from all other tephras known at 

Mount St. Helens except for one scoria bed in layer Bo.

Layer Bu forms the top of set B. From north of the volcano clockwise around to southeast, layer Bu overlies the 

dacitic layers Bd or Bi and elsewhere overlies ash bed ba, 

layer Bh, or layer Bo. It is overlain by layer D along a narrow 

zone toward the northeast and elsewhere by tephras of set W 

or set X.

Layer Bu was found on all sides of the volcano except 

the west. It commonly is at least 5 cm thick at distances of 

5-10 km in most directions. The layer is thickest toward the 

southeast, however, where it is as thick as 15 cm at a distance 

of 9 km. It was traced only a few tens of kilometers from the 

volcano.

The distribution of layer Bu suggests that it was produced by repeated eruptions, during which winds blew in 

almost all directions. An age of between 1,800 and 1,650 

years old is indicated by dated radiocarbon samples from 

below and above the layer (W-2527, W-2924, W-2990; 

table 2).

ASH BED ba

A multibedded deposit of gray to brown ash is immediately above the lower lapilli-bearing scoria layers of set B. 

The ash deposit consists of a lower, dark-brownish-gray unit 

(bed bag) that grades upward into an upper, lighter colored 

grayish-brown zone (bed bab) (figs. 48, 51). The lower unit 

generally is coarser and better sorted than the upper. Each
DESCRIPTION OF TEPHRA UNITS 65

122°15' 122°00'

6 11

20/22 15/1 ° 10/5

MOUNTS! 

HELENS A

2/1

10/7

6/8

8/9

10 KILOMETERS

Figure 54. Thickness (in centimeters) of ash bed ba. Two numbers separated by a slash represent thickness of the upper (bab) and 

lower part (bag) of the ash deposit, respectively.

part of the ash deposit is visibly stratified in some places, 

which suggests that each consists of many depositional units.

Ash bed ba varies strongly in composition of rock particles and ferromagnesian minerals. The lower unit consists 

mostly of material similar to that in the underlying layer Bo. 

The upper unit contains both andesitic and dacitic material. 

The color difference apparently reflects accurately the compositional difference.

Ash bed ba is readily recognized in the middle of set B 

from northwest of the volcano clockwise around to the 

southwest (fig. 54). It is thickest from the northeast to the 

southeast, where it commonly is as thick as 15 cm at a distance of 10km.

This ash bed may have originated from more than one 

kind of event, including small tephra eruptions. No tephra, 

however, was specifically identified within it. Lenticular 

crossbedding in the ash in some places suggests that the 

deposit resulted at least in part from flowage events.

TEPHRA LAYER D

About 1,200 years ago, explosions at the site of Sugar 

Bowl dome on the northeast flank of Mount St. Helens produced a single layer of tephra as well as pyroclastic flows 

and lahars (Crandell and Mullineaux, 1978; Mullineaux and 

Crandell, 1981; Crandell andHoblitt, 1986; Crandell, 1987).

The tephra consists of gray, dacitic, moderately vesicular 

blocks, bombs, lapilli, and ash. Most large clasts are close to 

the vent and were thrown along ballistic trajectories. Ash, 

however, was carried at least 30 km downwind by air currents (Crandell and Hoblitt, 1986). The layer is strongly lenticular and where thick forms a conspicuous coarse stratum 

on and near the volcano. Over most of the areal extent of the 

ash, however, it is thin and obscure.

Layer D is unstratified but moderately well sorted. 

Most fragments are vesicular, but vesicles are small and the 

fragments are relatively dense. Carbonized wood fragments 

are common; twigs that were converted to charcoal in place 

with needles still attached were recovered from the deposit. 

In most places layer D has a thin oxidized soil zone in its 

upper part, and a thin, discontinuous mat of carbonized vegetation is common at its top.

Hypersthene and hornblende were the only ferromagnesian phenocrysts identified in most samples of layer D 

dacite. Scattered fragments of nonvesicular dacite in the 

deposit contain cummingtonite, hornblende, and biotite, and 

these fragments may have been derived from preexisting 

deposits of Ape Canyon age (Crandell and Hoblitt, 1986).

Two samples of dacite associated with layer D contain 

about 69 percent SiC>2, the highest silica values of any Mount 

St. Helens tephra analyzed.

DIAGNOSTIC FEATURES

Layer D is easily distinguished from the underlying set 

B by color, dacitic composition, and ferromagnesian minerals. It is very similar to the overlying layer Wn but is thinner, 

more lenticular, and darker gray and is characterized by 

denser pumice. The thin soil zone in the upper part of layer 

D and the carbonaceous material at its top also help to distinguish layer D from layer Wn.

STRATIGRAPHIC RELATIONS

Over its known range, layer D overlies brown scoria of 

set B. It is overlain everywhere by the compositionally similar layer Wn. The contact between the two is obscure but is 

marked by the thin soil and carbon-rich zone at the top of 

layer D.

DISTRIBUTION AND THICKNESS

Layer D is limited to the northeast flank of the volcano 

and downwind toward the northeast (Crandell and Hoblitt, 

1986) (fig. 55). Its maximum known thickness is about 40 

cm only 3 km from the vent, where it includes bombs as large 

as 50 cm across. Downwind to the northeast, scattered lapilli 

1 cm or more across are present in ash for at least 25 km (fig. 

55) (Crandell and Hoblitt, 1986). Layer D clasts commonly 

have diameters larger than the layer is thick, and they project
66 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

122°0

Independence Pass

Spirit Lake 

Lodge

EXPLANATION

• -r-E Outcrop of air-fall deposit—Upper number 

is thickness (centimeter); lower number is 

maximum diameter (centimeter) of largest 

fragment 

° Site at which air-fall deposit was not

recognized 

— — — — Probable minimum extent of air-fall deposit

Sugar Bowl 

Dome

A Mount St Helens

10 KILOMETERS

Figure 55. Inferred distribution of tephra layer D of Sugar Bowl age. Modified from Crandell (1987, p. 64).

above the top of the associated ash matrix. This characteristic is especially conspicuous where the layer is thin near its 

margins; along these margins, the layer commonly appears 

as a line of small clasts rather than as a continuous stratum.

ORIGIN

Layer D formed by at least two explosive eruptions at 

the site of Sugar Bowl dome (Crandell and Hoblitt, 1986, p. 

32). Its grain size and distribution indicate that it consists 

partly of fragments carried along ballistic trajectories and 

partly of particles carried downwind by air currents.

AGE

Layer D overlies set B tephra that probably is as young 

as about 1,700 years old and underlies set W tephra erupted 

in A.D. 1480. Its postulated age of about 1,200 years is based

on one charcoal sample from the tephra deposit that has a 

radiocarbon age of about 1,150 years (W-2993; table 2) and 

on a second sample from an associated pyroclastic-flow 

deposit that has a radiocarbon age of about 1,200 years 

(W-5343; table 2).

TEPHRA SET W

Set W consists of two thick, white layers of pumice 

lapilli and bombs and at least three thinner layers (figs. 56, 

57). Because the two thick layers extend in different directions, generally only one is conspicuous at any single site 

(figs. 3, 58). The set forms a conspicuous, light-colored 

stratigraphic unit between the darker sets B and X. Pumice 

makes up most of set W; lithic fragments are common but 

subordinate in both major layers. Strata of the set appear 

almost unweathered.
DESCRIPTION OF TEPHRA UNITS 67

Figure 56. Tephra set W about 10 km east-northeast of volcano in a direction along which layers 

Wn and We overlap, along USFS Road 94 in the north-central part of sec. 28, T. 9 N., R. 6 E. Tape 

bands on pickhandle are 10 cm wide. Photograph taken in 1989.

EXPLANATION

Lapilli or bombs 

Ash

CENTIMETERS 1—20

Set 

X

Set

W

Set 

B

!*««*>,

'"*"<*•'

0 fl > V < O

^"AY^

;9 «'V*v <v

tV%v ^k

1 t> b t.^ &

^^^>:

^—

Wd

We

Wb

Wa

Wn

Figure 57.

St. Helens.

Composite columnar section of tephra set W, Mount

Lapilli and bombs in set W generally are only in the sector east of a north-south line through the volcano. Elsewhere, 

the tephra consists of ash. Toward the west, ash beds of the 

same age locally contain lapilli, but they are probably chiefly 

deposits of ash clouds and pyroclastic flows. Because pyroclastic-flow deposits are virtually limited to the western side 

of the volcano and identified tephra layers confined to the

east, the stratigraphic relations of the two kinds of deposits 

are not well known.

COMPOSITION

Highly vesicular pumice makes up most of the set. 

Lithic fragments generally make up about 5-20 percent of 

fragments in the two large-volume layers, but they are about 

equal in abundance with pumice in smaller volume layers. 

Ferromagnesian minerals in pumice from all layers are 

almost entirely hypersthene and hornblende; only a few fragments of other ferromagnesian minerals were found.

Hypersthene in set W has a higher refractive index than 

hypersthene in other Mount St. Helens dacitic pumice. In set 

W, the lowest refractive indexes measured are distinctly 

greater than 1.70 and near 1.71. In contrast, the lowest 

indexes of hypersthene from other pumice deposits from 

Mount St. Helens are less than 1.70.

Seven samples from set W show a range of SiC>2 of 

about 66-67 percent. Thus, set W pumice is higher in silica 

than all other Mount St. Helens deposits except pumice from 

the Sugar Bowl dome.

DIAGNOSTIC FEATURES

Northeast, east, and southeast of the volcano, set W is 

easily recognized because it contains a thick pumice layer 

that is coarse, almost white, and virtually unweathered. Its
PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

122°15' 122°00'

Figure 58. Tephra set W about 8 km east of Mount St. Helens. At 

this location the only conspicuous pumice layer is We. Photograph 

taken in 1983.

color readily distinguishes it from the underlying, dark set B, 

and it is much coarser than the next older set P. In some 

places, set W is similar in thickness and grain size to set Y, 

but it is easily distinguished from set Y and from other older 

pumiceous sets except set J by its ferromagnesian mineral 

suite.

The thick layers of set W are similar to the younger 

layer T but differ in ferromagnesian mineral composition. 

Moreover, the higher refractive index of hypersthene can be 

used to distinguish set W pumice from all other hypersthenehornblende pumices from Mount St. Helens.

STRATIGRAPHIC RELATIONS

Near the volcano, set W overlies a soil zone developed 

mostly in set B; carbonized wood is widespread at the top of 

the soil zone. Farther downwind, set W generally overlies a 

soil zone developed in older Mount St. Helens tephras or in 

deposits of other origins.

Set W is overlain by set X, and no physical evidence of 

a time lapse was seen. In many places lapilli of the overlying 

layer Xb fell alongside scattered lapilli of the topmost layer

46° 

15'

46° 

00'

0 5 10 KILOMETERS

I i i i , I______I

Figure 59. Thickness (in centimeters) of tephra set W, Mount St. 

Helens.

of set W, forming an apparent stratigraphic unit that includes 

clasts of the two different sets.

DISTRIBUTION AND THICKNESS

Recognized tephra deposits of set W crop out from 

approximately north of the volcano clockwise around to 

slightly west of south. Because its thickness and grain size 

are determined mostly by the thicknesses of layers Wn and 

We, set W is coarsest and thickest primarily toward the 

northeast and secondarily toward the east. At the northeast 

base of the volcano, it is as thick as 200 cm and consists 

mostly of lapilli and bombs. At 20 km toward the northeast, 

it is about 100 cm thick (fig. 59). Toward the'east and southeast, the set also consists chiefly of one lapilli layer between 

ash beds (figs. 48, 58) and is about 20 cm thick at 10 km. 

Toward the south, the set consists of lapilli and ash 5-10 cm 

thick at a distance of 10 km.

No tephra layers were identified among the ash deposits 

in the stratigraphic position of set W west of the volcano. 

Much and probably most of the ash of that age there is likely 

of flowage origin.
DESCRIPTION OF TEPHRA UNITS 69

Site 92G 

Distance from vent 

(in kilometers) 7

150-151 140

EXPLANATION

Lapilli or bombs 

Ash

CENTIMETERS i—20

Wd 

We

Wb 

Wa

"—0

,92G

Wn

0150-151

N92

Mount St~~~ 

Helens vent

a 4 " .

I> A > V _

L^lsJU

' t> ^1 S7

N " v a

" < <)

a * <7 -9

^ ft « -

£> V 4 ^

4" ' * T * '

0 4 <5 

»l*?\

*, ' * <

/

///

/

x/X

"^\

s\\\ \

\\ \ \ \ \

\

\

/

3 ,, \ »

> „ 0 V 

-W 4 > A

4 ^ t> 1

'£;'.'^ !•*-.:

^•'v-:-i''i::

q "q'a" '<

^ a & &

* V 4 V

4 v & t

<-:- : i^''':<'

• •» i ' >

X~^

/

/

/ y 

/ /

7 / 

/ 

/ 

/ 

/ 

/ 

/

153 

10

7 & & >

^77

< * 4 f

" C> A C

6 

C

p

3

v_ a

•3 > V t>

C

N92 

9

N83 

9

We *v

1? V

V

7 '

<7 a

a

a "

5 V <7 *

v t> v

^ il

I < „

e> u- o

<3 •<

•<!

<1 0

c> •fl

- — Base not exposed

Figure 60. Columnar sections of set W from northeast to southeast of Mount St. Helens and diagram of their locations relative to the central 

vent of the volcano.

Mostly because of layers Wn and We, set W is an excellent stratigraphic marker. Layer Wn has been recognized 

toward the northeast across Washington and into Canada for 

more than 400 km (Westgate and Fulton, 1975; Smith and 

others, 1977). Layer We has been recognized across southern Washington and into Idaho (Smith and others, 1977), 

also at a distance of more than 400 km. Although the ferromagnesian mineral content and general chemical composition of set W are not unusual for Cascade tephras, the 

refractive index of constituent hypersthene, chemical composition of glass, and closely known age of these layers 

make them especially useful time-stratigraphic marker beds.

AGE

Set W was erupted about 500 years ago, as first determined by tree-ring studies on glacial moraines in Mount 

Rainier National Park (Crandell, 1969). During the 1980's, 

growth-ring studies of trees showed that layers Wn and We 

probably were erupted in A.D. 1480 and 1482, respectively 

(Yamaguchi, 1983).

All of set W was erupted within a few tens of years, perhaps within just a decade. The closest limiting date for the

end of the set is from the age of a tree rooted in the overlying 

set X, which provides a 30-year maximum time for eruption 

of set W (Yamaguchi, 1993). Allowance for time between 

the first eruptions of set X and establishment of the tree suggests that set W might well have been erupted in less than 10 

years.

DESCRIPTION OF LAYERS IN SET W

Five layers in set W, each traceable for at least several 

kilometers northeast of the volcano, are separately described 

(figs. 56, 60; measured section W-l). Two other lapilli beds 

that may represent additional eruptions are present locally at 

the northeast base of the volcano, but they could not be 

traced and are not separately described.

Flowage deposits of the same age as set W are abundant 

on the west and southwest flanks of the volcano; they are 

mostly west of the recognized limits of the named tephra layers. Thin ash probably deposited by ash clouds also is 

present northeast of the volcano, both as a bed between the 

two uppermost layers of the set and as matrix ash in those 

layers.
70 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

122°15' 122°00' 122°15'

46° 

15'

46° 

00'

MOUNTS! 

HELENS A

46° 

15'

46° 

00'

B

MOUNTS! _-^»2 

HELENS A

D

20 KILOMETERS

122°00'

Figure 61 (above and facing column). Thickness (in centimeters) of layers in tephra set W, Mount St. Helens. A, Layer Wd. B, Layer 

We. C, Layer Wb. D, Layer Wa, E, Layer Wn.
DESCRIPTION OF TEPHRA UNITS 71

122°15' 122°00'

46° 

15'

'40

60g

MOUNT ST± 

HELENS

10 KILOMETERS

MEASURED SECTION W-l

Near nose of east-trending spur between forks of Smith Creek, 0.65 km 

southeast of northwest corner of sec. 30, T. 9 N., R. 6 E. See figure 63.

Thickness 

(centimeters)

10. Layer Xb. Scoria lapilli in ash. 1

SetW: 

9. Layer Wd. Pumice and lithic lapilli, in about equal

proportion, in predominantly pinkish brown ash ................. 3

8. Ash, pinkish-brown.............................................................—— 1

7. Layer We. Lapilli and small bombs, chiefly pumice, in sparse

ash; bombs as large as 9 by 8 by 5 cm, largest at top of

layer; small lithic lapilli concentrated near base ................. 8

6. Ash, brownish-gray.............................................................—— <1

5. Layer Wb. Pumice and lithic lapilli in ash; pumice lapilli

larger, as much as about 4 cm across; openwork texture ..... 5

4. Ash, brownish-gray...........................................................—.-... 0.5

3. Layer Wa. Lithic and pumice lapilli and small bombs in ash;

lithic clasts dominant but pumice clasts are larger, as

much as 6 cm across; coarsest in middle of layer;

openwork texture ................................................................. 5

2. Ash, brownish-gray................................................-....—........... 1

1. Layer Wn. Lapilli and bombs, mostly pumice; lithic clasts

about 5-10 percent; locally crudely layered; openwork;

bombs as large as 12 by 10 by 9 cm .................................... >60

Base covered

LAYER Wn

The coarse, thick layer Wn forms the base of set W 

in a broad band that trends northeast from the volcano. It 

is a single, well-sorted deposit of highly vesicular, almost 

unweathered, yellowish-white pumice containing scattered lithic fragments. Locally, layer Wn shows crude bedding but generally is not stratified or noticeably graded. 

Hypersthene and hornblende make up its ferromagnesian 

mineral suite; measured ratios range from about 10:1 to 

2:1 (Smith and Leeman, 1982; Mullineaux, 1986). Generally, those ratios are higher than hypersthene-hornblende 

ratios for other tephras that contain the same ferromagnesian minerals. Whole-rock SiC>2 contents measured for 

pumice from layer Wn range from about 66.5 to 68.5 

percent.

The unweathered nature of pumice in layer Wn and 

its thickness, coarse grain size, and stratigraphic position 

near the surface generally distinguish the layer from all 

tephra deposits except layer T. Layer Wn can be distinguished from layer T locally by greater thickness and 

grain size and generally by stratigraphic position and ferromagnesian mineral content.

In most places, the white pumice of layer Wn lies on a 

brown soil developed in set B. Locally, toward the

northeast, thin deposits of the Sugar Bowl eruptions intervene between set B and layer Wn. Layer Wn is overlain by 

younger tephra layers of set W everywhere except toward 

the north, where it is overlain directly by set X.

Layer Wn forms a thick lobe that bears about 035° 

from the volcano (fig. 61). On the volcano flank, layer Wn 

is as thick as 200 cm and contains bombs as much as 50 

cm in length and 30 cm across. At 20 km, it consists of 

as much as 100 cm of lapilli and a few bombs in ash. The 

layer decreases to 5-10 cm of mostly coarse ash at about 

100 km from the volcano and to about 1 cm of finer ash at 

400^50 km near where it crosses the border between 

northeastern Washington and Canada (Smith and others, 

1977, p. 215).

Layer Wn is a readily identified and closely dated 

marker bed. Tree-ring studies indicate that the eruption 

occurred after the growing season of A.D. 1479 and 

before the growing season of A.D. 1480 (Yamaguchi, 

1985). For simplicity, A.D. 1480 has been used as the date 

of eruption (Yamaguchi, 1985). Although a recent study 

of ash tentatively correlated with layer Wn on the basis of 

depth proposes an eruption date of A.D. 1479 (Fiacco and 

others, 1993), the date of A.D. 1480 is used herein.

Layer Wn of this report was initially called layer W 

at Mount Rainier (Crandell and others, 1962). It also 

makes up the bulk of the pumice described by Carithers 

(1946) as "the younger pumice sheet."
72 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Figure 62. Tephra set W about 11 km northeast of Mount St. 

Helens, where layer Wa contains large lapilli but layer Wb does not. 

Tape bands on pickhandle are 10 cm wide. Photograph taken in 

1989.

LAYER Wa

Layer Wa is a single thin bed of lapilli, ash, and a 

few bombs (fig. 62). Abundant fresh, gray lithic fragments make up about half or slightly more of the deposit. 

The layer typically contains clasts that are larger in diameter than the layer is thick (fig. 62). In composition, it is 

similar to other layers in set W. Identification of layer Wa 

is based mostly on its stratigraphic position, its high proportion of lithic fragments, and the presence of clasts that 

are large relative to the layer thickness.

Layer Wa is everywhere underlain by layer Wn, separated only by thin ash. It is separated from overlying 

layer Wb by similar thin ash.

Layer Wa was identified only northeast and east of 

the volcano. Its known maximum thickness is 5-10 cm at 

distances of 5-10 km (fig. 61); it was not identified 

beyond about 12 km. Because of its similarity to the more 

extensive layer Wn, it probably is not an important 

marker bed farther downwind.

The abundance of fresh lithic fragments in layer Wa 

suggests that a plug or dome of the same composition as set 

W was present before eruption of that tephra. This dome was 

fragmented in part by an explosive eruption of gas-charged 

magma to produce the mix of lithic and pumice clasts in 

layer Wa.

LAYER Wb

Layer Wb is another single thin deposit of lapilli, ash, 

and a few bombs (fig. 63) on and near the northeast flank of 

the volcano. Locally, it contains at least two coarse zones or 

beds. Layer Wb contains many fresh gray lithic fragments, 

although probably in less abundance than in layer Wa. Ferromagnesian minerals in layer Wb are almost all hypersthene 

and hornblende, similar to other tephra layers in set W. Thus, 

this layer is identified chiefly on the basis of grain size and 

stratigraphic position.

Layer Wb is separated from the underlying layer Wa, 

and also from the overlying layer We, only by thin ash.

Throughout its known extent, layer Wb is similar in 

thickness to layer Wa (fig. 61). At 5-10 km from its source, 

its maximum thickness is only 5—10 cm, and it was traced 

only to about 12 km from the volcano. Its grain size is somewhat smaller than that of layer Wa, and it generally does not 

contain bombs as large as those in layer Wa.

Similar to layer Wa, layer Wb probably originated from 

an explosive eruption that produced fresh magma, as well as 

fragments from a preexisting dome.

LAYER We

Layer We forms a well-defined single stratum of highly 

vesicular white pumice lapilli (fig. 58) east of the volcano. It 

contains many fresh and a few weathered lithic fragments; 

fresh lithic fragments are common and locally make up as 

much as about 30 percent of the layer. Both crude stratification and grading are present but not conspicuous. Hypersthene and hornblende make up almost its entire 

ferromagnesian mineral suite, similar to other layers of set 

W, and the silica content of its pumice is also similar. Microprobe analyses of glass show, however, minor differences in 

proportions of potassium, calcium, and iron in pumice from 

layers Wn and We and have been used to distinguish 

between the two (Smith and others, 1977).

East of the volcano, layer We is relatively easy to differentiate from other tephra layers of set W on the basis of 

grain size and thickness. To the east-northeast where its lobe 

overlaps that of layer Wn, it is identified by its stratigraphic 

position.

Toward the east and south, layer We overlies ash of set 

W that lies in turn on set B. It is overlain by younger ash of 

set W and in turn by set X. Farther north, layer We typically
DESCRIPTION OF TEPHRA UNITS 73

Figure 63. Tephra set W about 8 km northeast of the center of Mount St. Helens. Both layers Wa 

and Wb contain large lapilli. Photograph taken in 1983.

overlies layer Wb, separated only by ash, and is overlain by 

layer Wd or set X. Locally, pinkish-brown ash, perhaps of 

ash-cloud origin, is present in the upper part of layer We and 

between it and layer Wd.

Layer We forms a relatively broad lobe from northeast 

of the volcano around to the south. It is thickest toward the 

east, where it is as thick as 60 cm and consists mostly of 

lapilli at 5 km from the vent. Downwind to the east, it is 

20-30 cm thick at a distance of 10 km and contains lapilli as 

much as 5 cm across and is 10-20 cm thick at 20 km and contains lapilli as much as 2 cm across.

Layer We forms another important marker bed for 

young stratigraphic sequences across southern Washington 

into Idaho. Smith and others (1977) reported that layer We 

in Idaho about 450 km from the volcano is "an undisturbed 

1 cm layer in peat."

Tree-ring studies indicate that eruption of layer We 

occurred late in A.D. 1481 or in A.D. 1482 (Yamaguchi, 

1982), two years after eruption of layer Wn. For simplicity, 

the year A.D. 1482 is used for the eruption date of layer We.

LAYER Wd

Layer Wd is a widespread, thin stratum of pumice and 

lithic lapilli, bombs, and ash (fig. 56). It probably consists of

two or more overlapping thin shower beds north and east of 

the volcano. It is similar in mineral content and chemical 

composition to other layers in set W. Coarse clasts in layer 

Wd commonly are embedded in pinkish-brown ash and also 

are locally underlain and overlain by such ash. Similar to 

layer Wa, many of the clasts of layer Wd are larger in diameter than the layer is thick, and they extend above the level 

of matrix ash.

Layer Wd is identified by its stratigraphic position at 

the top of set W and by its coarse grain size relative to layer 

thickness.

Throughout its extent, layer Wd is underlain by older 

layers of set W and overlain by set X. The upper contact is 

poorly defined. In places layer Wd does not form a continuous deposit and is not overlain by an ash bed that clearly separates it from the overlying layer Xb. In some of those 

places, scoria clasts of layer Xb have dropped down between 

scattered lapilli of layer Wd and form a commingled deposit 

that looks much like a single stratigraphic unit.

Layer Wd was identified from the north-northeast of 

Mount St. Helens clockwise around to the east (fig. 61); it 

was not traced beyond about 15 km from the volcano. It is 

thickest to the northeast, where it is about 5 cm thick at 5-10 

km distance. From there, it thins southward, then thickens 

again directly east of the volcano. A second, thinner bed
t&#V&i**£8!& fxpuw

l"^3^?1fe*-«^l I***"*!. .

lW?T'.HfS&«5Zr' i.-«^«hr-:^* i*r \v.v::::::-v LayerT

Set 

x

Set

W

Xh 

Xm 

Xs 

Xb

Figure 64. Dark, fine-grained tephra set X between the coarse, 

pumiceous set W and layer T, about 5 km northeast of the center of 

Mount St. Helens. Pocketknife shown for scale. Photograph taken in 

1983.

extending toward the east suggests that the deposit was 

erupted in at least two different pulses and carried in slightly 

different directions.

The layer probably resulted from more than one explosive, short-lived eruption that ejected lapilli and bombs as 

well as ash. Pinkish-brown ash in the layer may have been 

derived from ash clouds associated with pyroclastic flows 

generated by the same eruptions.

REWORKED PUMICE LAPILLI DEPOSITS

Thick deposits of pumice similar in grain size and 

thickness to layer Wn but stratigraphically higher crop 

out on the volcano's north flank and at several sites in 

one area about 12 km north of the mountain. These deposits probably originated from erosion and subsequent redeposition of lapilli of layer Wn. They form well-sorted

Figure 65. Composite columnar section of tephra set X, Mount 

St. Helens, showing approximate thicknesses 5 km northeast of volcano center.

deposits that are similar to the primary tephra layer. 

Rounding of some constituent lapilli suggests reworking, 

as does a limited lateral extent for a deposit of its thickness.

TEPHRA SET X

Set X consists chiefly of multiple layers of scoriaceous ash that form a dark, fine-grained stratigraphic unit 

(fig. 64) that crops out all around the volcano. It is conspicuously darker and finer grained than either the underlying set W and or the overlying layer T (fig. 64). Of the 

four layers identified in the set (fig. 65), only the basal 

layer contains lapilli or bombs. The set is unweathered 

except for a slight oxidation zone at its top.

Set X tephras commonly are interbedded with pyroclastic-flow deposits and other deposits of flowage origin 

southwest of the volcano, and specific tephra layers were 

not identified there.

COMPOSITION

Most clasts in set X are vesicular, but denser fragments are also abundant. Although the set is mostly andesite or basaltic andesite, dacite is present as clots and 

bands in the scoria (Pallister and Hoblitt, 1985; Pallister 

and others, 1992, p. 134). Ferromagnesian mineral suites 

of all layers contain olivine, hypersthene, and augite in 

various proportions (table 6). Some hypersthene crystals in 

the lower part of the set have relatively high refractive 

indexes similar to those in set W; the few hypersthenes 

examined from the upper part of the set do not. Wholerock samples from each of three layers contain between 

about 55 and 60 percent SiO2-
DESCRIPTION OF TEPHRA UNITS 75

Table 6. Estimated relative proportions of ferromagnesian 

phenocrysts in layers of set X.

Layer Relative proportions (most to least abundant)

Xh 

Xm 

Xs 

Xb

Hypersthene, olivine, augite. 

Olivine, augite, hypersthene. 

Olivine, augite, hypersthene, hornblende. 

Olivine, augite, hypersthene, hornblende.

DIAGNOSTIC FEATURES

Except for set B, set X is readily distinguished from all 

other Mount St. Helens tephras by its dark color, constituent 

scoria, and abundance of olivine. It generally can be distinguished from scoriaceous layers of set B by its finer grain 

size, absence of strong oxidation, and stratigraphic position. 

In addition, the ferromagnesian mineral suites of set X differ 

from those in all layers in set B except layer Bo; distinction 

from layer Bo can be made on the basis of stratigraphic relations.

STRATIGRAPHIC RELATIONS

Set X overlies set W without any evident erosional or 

weathering break; in some outcrops scattered lapilli at the 

base of set X lie alongside lapilli of set W in the same stratigraphic zone. Generally, there is no erosion or other evidence of a time break between the top of set X and the 

overlying ash bed z. In places the contact of set X with ash 

bed z is gradational and in other places interfingered.

DISTRIBUTION AND THICKNESS

Set X is present all around the volcano (fig. 66); it is 

thickest and includes the most identified layers north-northeast of the volcano. At 5 km in that direction, it is about 50 

cm thick, but only the basal layer Xb contains clasts larger 

than ash size. At 20 km toward the northeast, the set is 5-10 

cm thick and consists almost entirely of ash. In other directions, the set is thinner and also consists entirely or almost 

entirely of ash.

Although thin, the set is an excellent marker near the 

volcano because it is readily identifiable by its color and ferromagnesian minerals and it extends in all directions. It is 

also useful as a marker for at least several tens of kilometers 

beyond the volcano. At present, it has been traced farthest to 

the east, to a maximum distance of about 50 km.

ORIGIN

Grain size and distribution of set X suggests that its 

deposition began with two or more brief eruptions as winds

blew from the southwest and west. Those eruptions were followed by less vigorous but repeated eruptions that produced 

finer but more voluminous deposits; wind directions during 

these latter eruptions varied enough to spread ash on all sides 

of the volcano.

The presence in set X of dacitic inclusions in mafic 

fragments, banded clasts, and hypersthene phenocrysts similar to those in set W suggest that mixing of the two magmas 

was an important cause of the eruptions that produced the set 

(Pallister and Hoblitt, 1985; Pallister and others, 1992). An 

increase in proportion of mafic components from the early to 

mid-set X eruptions is shown by the compositions of layers 

Xb, Xs, and Xm.

AGE

Set X probably began to accumulate within 30 years 

after set W eruptions ceased. Tree-ring studies (Yamaguchi, 

1993) indicate that a tree underlain by at least part of set X 

began to grow by A.D. 1510.

The time at which set X eruptions ended is not closely 

known. The closest upper limiting date of A.D. 1647 

(Yamaguchi and Hoblitt, 1986) suggests only that the set 

accumulated in less than about 150 years.

DESCRIPTION OF LAYERS IN SET X

Set X consists of three prominent and one obscure layer 

(figs. 64,67,68; measured section X-l). A coarse, thin basal 

stratum that contains lapilli and bombs (layer Xb) is overlain 

by a conspicuous, bedded ash deposit (layer Xs), a massive 

darker ash (layer Xm), and a similar but lighter colored ash 

(layer Xh) (fig. 64). No evidence of time lapses between the 

deposition of any of those layers was seen. All layers of the 

set are identifiable just northeast and east of the volcano. 

Only layer Xm, however, was identified toward the west.

LAYER Xb

Layer Xb is a thin layer of lapilli and bombs in 

sparse ash at the base of set X (figs. 64, 67). In many 

places its largest fragments extend above the surface of 

the ash matrix. Locally the layer consists of more than one 

bed, and elsewhere it consists of only scattered bombs 

and lapilli rather than a continuous stratum.

Most large clasts in the layer are vesicular, scoriaceous andesite, but some contain inclusions of dacite that 

are mineralogically similar to pumice in set W. The ferromagnesian minerals in layer Xb are hypersthene, olivine, 

augite, and hornblende (table 6). A few hypersthene phenocrysts examined had lowest refractive indexes well 

above 1.70 and thus are similar to hypersthene phenocrysts in set W.
76

46° 

3D 1

PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

122°15' 122°00' 121°45'

15'

46° 

00'

Elk Rock

10

Cougar

North woods 0

I ! I

10 KILOMETERS

Figure 66. Thickness (in centimeters, large dots) of tephra set X at selected locations, Mount St. Helens. Isopachs are smoothed from 

results of study by M.R. Couchman and R.P. Hoblitt (written commun., 1993); small dots are sites of their data points.

Layer Xb is readily distinguished from underlying 

and overlying dacitic layers by dark color and mafic 

composition and from set B layers by stratigraphic position and ferromagnesian minerals. It differs from overlying layer Xs by presence of lapilli and bombs.

Layer Xb throughout its extent overlies layer Wd or 

other layers of set W and is overlain by younger ash of set X. 

Lapilli of layer Xb lie alongside clasts of layer Wd in the 

same stratigraphic zone in many places. Lapilli and bombs of 

layer Xb probably fell down between scattered clasts of layer 

Wd, resulting in a stratigraphic unit that includes elements of 

both tephras.

Layer Xb was identified from approximately northeast to east of the volcano. Its maximum known thickness

and its coarsest part lie almost directly to the northeast 

(fig. 69). At 5 km, it is 5-10 cm thick but contains bombs 

as much as 10 cm across. Farther northeast, the layer 

thins to 4 cm at about 10 km and to 2 cm at about 25 km. 

East of the volcano, the layer contains lapilli as far from 

the volcano as 10 km but is only about 2 cm thick at that 

distance.

Layer Xb probably originated from at least two lowvolume eruptions during times when winds blew from the 

southwest and west. Comparison of its thickness with that 

of deposits resulting from minor Mount St. Helens eruptions after May 18, 1980, suggests that the eruptions that 

produced layer Xb were short, perhaps lasting only an 

hour or less.
DESCRIPTION OF TEPHRA UNITS 77

Figure 67. Layering in tephra set X 9 km east of the volcano center, Mount St. Helens. Striped layer 

Xs contrasts with dark layer Xm but differs from underlying layer Xb chiefly in grain size.

MEASURED SECTION X-l

Road cut at west end of the bridge that crossed Smith Creek in L983, NE'/4 SEV4 sec. 

5. T. 8 N., R. 6 E. See figure 67.

Thickness 

(centimeters)

8. Flowage deposit; lahar(?).......................................................... 75-100

7. Ash bed z. Fine, pale pinkish to grayish brown; contact with

set X locally distorted and interfingered ............................. 2

SetX: 

6. Layer Xh. Ash, fine, brownish-gray, gradational, obscure;

locally distorted contacts at base and top ............................ 4

5. Layer Xm. Ash, fine to coarse, moderate- to dark-gray,

relatively friable; contact with underlying layer Xs sharp

to gradational, locally distorted ...........................................

4. Layer Xs. Alternating beds of coarse and fine ash, pale- to

moderate-grayish-brown .....................................................

3. Layer Xb. Chiefly ash, brownish-gray; small scoriaceous and

dense lapilli as much as about 2 cm across .........................

SetW: 

2. Ash, fine; slightly pinkish gray cast...................................

1. Layer Wd. Dacitic pumiceous and lithic lapilli and a few 

small bombs in sparse ash ............................................

9

12 

1-2

2

2

LAYER Xs

Layer Xs is a series of at least 10 alternating beds of 

pale, coarse ash and dark, finer ash (figs. 64, 67). Changes in 

grain size from bed to bed are abrupt, and the fine-grained 

beds are more coherent. The differences in darkness and 

coherence commonly give the layer a banded appearance 

(figs. 64, 67).

Mafic scoria and lithic particles make up the layer, and 

its ferromagnesian minerals are olivine, augite, hypersthene, 

and hornblende (table 6). A few hypersthene phenocrysts, 

similar to those in layer Xb, have lowest refractive indexes 

well above 1.70. The SiC>2 content of one sample of layer Xs 

is about 60 percent.

Layer Xs is distinguished from the underlying layer Xb 

by its absence of lapilli and by its multiple contrasting beds. 

Its alternating fine and coarse beds and its hornblende content distinguish it from the overlying layer Xm.

Layer Xs overlies layer Xb on the northeast and east 

sides of the mountain; where layer Xb is not present, layer 

Xs lies directly on set W. Throughout its known extent, it is 

overlain by layer Xm.

Layer Xs was identified from north of the volcano 

clockwise around to the south. Toward the northeast, it is as
78 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Site Goat 

Distance from vent 

(in kilometers) 8

EXPLANATION 

X, undifferentiated

South 

Fork Outlet 100

5

Bridge 

9

Shoe 

5

818A 

5

CENTIMETERS I—20

^0

-—————________ 0 Bridge

Xh?

&

Xm

Xs

\

\

\

\ 

\

\

Xm

Xs

Goat0 ' VShoe

0818A

Figure 68. Columnar sections of tephra set X and diagram of their locations relative to Mount St. Helens.

thick as 25 cm at a distance of 5 km (figs. 68,69) and toward 

both the north and south is 5-10 cm thick at 5-10 km.

Its abrupt grain-size alternations and its wide distribution around the volcano suggest that layer Xs was erupted 

during many events, during times when winds blew from 

many different directions.

LAYER Xm

Layer Xm consists of well-sorted, friable, massive to 

laminated ash that is noticeably darker than underlying layer 

Xs (fig. 67). Its ferromagnesian mineral suite is similar to 

that of layer Xs except that hornblende is sparse or absent in 

layer Xm (table 6). In addition, hypersthene phenocrysts 

examined from layer Xm all had lowest refractive indexes of 

less than 1.70. A sample from the middle of the layer contains about 55 percent SiC>2, notably less than that of samples 

from the older layers of set X.

Layer Xm differs from the underlying layer Xs by its 

darker color, more uniform grain size, and ferromagnesian

mineral suite. It differs from the overlying layer Xh by its 

darker color and its proportions of ferromagnesian minerals.

Layer Xm overlies layer Xs with no erosional or weathering break, though the contact zone locally is distorted. It 

apparently grades upward into layer Xh with no well-defined 

contact. Northeast of the volcano, if layer Xh is not present, 

layer Xm is overlain by ash bed z or layer T. In other directions, layer Xm is overlain only by thin 19th century ash and 

forest duff or by deposits of the 1980 eruptions.

Layer Xm crops out all around Mount St. Helens. It is 

the most widespread layer known in set X and is the only 

layer of the set identified on the west side of the volcano. 

Toward the northeast, it is as thick as 25 cm at a distance of 

5 km and about 10 cm thick at 15 km. Toward the east and 

south, it commonly is 5-15 cm thick at a distance of 5 km. 

Beyond about 15 km from the volcano, it was not identified 

separately from other beds in the set.

Layer Xm probably was produced by a series of similar 

eruptions or, possibly, by a single, prolonged eruption. The 

thickness of the layer all around the volcano seems to require 

many eruptive events during times of very different wind 

directions.
DESCRIPTION OF TEPHRA UNITS 79

46° 

15'

46°

46° 

15'

46° 

00'

A

B

122°15 122°00' 122°15- 122°00'

MOUNTS! A 

HELENS

• 3

,10

10

D

• 5

MOUNTS! 

HELENS

20 KILOMETERS

Figure 69. Thickness (in centimeters) of layers in tephra set X, Mount St. Helens, and of the overlying ash bed z. A, Ash bed z. B, Layer 

Xm. C, Layer Xs. D, Layer Xb.

LAYER Xh possibility was not tested by chemical analysis. Olivine is

less abundant in layer Xh than in layer Xm and hypersthene 

Layer Xh is an inconspicuous deposit of ash that is sim- more abundant.

ilar to but lighter in color than underlying layer Xm. Layer In the field, layer Xh is identified chiefly on the basis of 

Xh may be slightly less mafic than layer Xm, but that color; it is paler than layer Xm but darker than the overlying
80 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Figure 70. Tephra layer T at the surface on the northeast flank of Mount St. Helens before 1980, 

overlying ash bed z and sets X and W. A brown, oxidized soil zone has developed in ash bed z and 

the upper part of set X. Ice axe shown for scale. Photograph taken in 1974.

ash bed z. It is also distinguished from layer Xm by its lower 

proportion of olivine and higher proportion of hypersthene 

and from ash bed z by abundant olivine.

Layer Xh overlies layer Xm with an apparently gradational contact. It is overlain by ash bed z, and no evidence of 

an erosional or weathering break was seen.

Layer Xh was recognized in only a few outcrops northeast and east of the volcano. Its maximum known thickness 

is about 10 cm at about 5 km from the volcano center.

This deposit probably formed by eruptions that were 

similar to but minor compared with those that produced layer 

Xm.

ASH BED z

A thin, slightly pinkish brown deposit of fine ash, ash 

bed z, overlies set X from north of Mount St. Helens clockwise around to the southeast (figs. 64, 67, 69). It consists 

chiefly of small lithic particles and crystal fragments; no 

pumice or clasts of lapilli size were found. Its ferromagnesian mineral suite consists chiefly of hypersthene, augite, 

and hornblende.

Ash bed z is distinguished from underlying ash in set X 

by color and by its ferromagnesian mineral suite. It differs 

from the overlying layer T most obviously by grain size but

also by lack of pumice. It generally is slightly oxidized in at 

least its upper part and commonly throughout the bed.

Ash bed z is thin everywhere it is known; its maximum 

thickness is about 15 cm (fig. 69). Its composition is similar 

to that of the pre-1980 summit dome, and it presumably was 

associated with emplacement of that dome. The deposit may 

be in part tephra, but more likely it consists chiefly of ashcloud deposits derived from lithic pyroclastic flows shed 

from the summit dome (Crandell, 1987).

TEPHRA LAYER T

Layer T is a single, thick deposit of pumice lapilli, 

bombs, and ash. It forms a conspicuous light-gray stratum 

near the ground surface along a narrow lobe leading northeast from Mount St. Helens (figs. 70, 71).-It was not found 

on other sides of the volcano.

The layer is unweathered, except for slight oxidation in 

its upper part. No pyroclastic-flow, surge, or ash-cloud 

deposits are associated with this tephra.

Layer T is well sorted and unstratified, and the pumice 

is highly vesicular. Its clasts are larger relative to the layer 

thickness than clasts in most other voluminous pumice layers. Ferromagnesian minerals in this layer are hypersthene,
DESCRIPTION OF TEPHRA UNITS 81

122°15' 122°00'

46° 

15'

46° 

00'

10 KILOMETERS

Figure 71. Thickness (in centimeters) of tephra layer T, Mount St. 

Helens. + indicates approximate position of probable source vent on 

north flank of volcano.

hornblende, and small amounts of augite. Hypersthene is 

usually slightly more abundant than hornblende. A few samples from both the lower and upper parts of the layer suggest 

that augite is less abundant in the lower part. According to 

Smith and others (1977, p. 210-211), extremely thin glassvesicle walls and subspherical voids characterize the pumice. Whole-rock analysis of three samples of the pumice 

indicates a SiC»2 content of 63-64 percent; samples from the 

bottom and top of the layer differ in SiC»2 content by less than 

0.1 percent.

DIAGNOSTIC FEATURES

Along its lobe, layer T can generally be identified as the 

uppermost coarse, thick, light-gray pumice layer at or near 

the surface that is older than the deposits of 1980. It is much 

coarser and lighter in color than any part of the underlying 

set X. It is similar to the older layer Wn that also extends to 

the northeast but is distinguished from it by stratigraphic 

position, larger grain size relative to layer thickness, and 

presence of augite. At more than about 20 km downwind, the 

large clast size relative to thickness of layer T is conspicuous.

Close to and northeast of the volcano, layer T is coarser 

and thicker than the A.D. 1980 tephras that overlie it. The

axis of the layer T lobe is north of that for the 1980 deposits, 

and layer T is virtually absent east of the volcano, where the 

1980 tephras are thickest (Sarna-Wojcicki and others, 1981; 

Waitt and Dzurisin, 1981). The augite content of layer T 

pumice, though small, is higher than that in the A.D. 1980 

pumice.

STRATIGRAPHIC RELATIONS

Near the volcano, layer T overlies a weak soil developed in ash bed z and the underlying set X. It is overlain only 

by forest duff and products of the A.D. 1980 eruptions.

DISTRIBUTION AND THICKNESS

Layer T is limited to the northeast side of the volcano 

and downwind toward the northeast (fig. 72). Before 1980, 

the apparent primary thickness of the layer exposed on the 

northeast flank of the volcano was as much as 215 cm. 

Bombs as much as 50 cm across, many with reddish-gray 

cores, were common. Locally, thicknesses of as much as 400 

cm could be measured on the northeast flank, but they may 

have represented downslope mass movement of the pumice 

during or after accumulation.

Downwind, layer T decreases rapidly in thickness and 

grain size. At 10 km from the volcano it consists of about 50 

cm of mostly lapilli (fig. 71). At 50 km it consists of 5 cm of 

small lapilli in ash and at 100 km of only 1-2 cm of ash. The 

layer has been recognized in northeast Washington and 

northern Idaho at distances of nearly 500 km (Smith and others, 1968; Okazaki and others, 1972).

ORIGIN AND SOURCE

The rapid decrease of thickness and grain size downwind suggests that layer T originated either from an eruption 

column that was relatively low or during winds of low or 

moderate velocity. Carey and others (1989) proposed a peak 

height of only about 16 km for the eruption column.

C.A. Hopson (oral commun., 1970) inferred from its 

distribution on the north flank of the volcano that layer T was 

erupted from a vent on the north flank rather than from the 

summit. Before 1980, a marked decrease in layer T thickness 

could be observed on the north flank from timberline 

upslope toward the summit.

AGE

Lawrence (1938, 1939, 1954) determined from growth 

rings in trees near Spirit Lake that layer T was erupted within 

a few years of A.D. 1800. Yamaguchi (1982) confirmed an 

A.D. 1800 date and suggested that the eruption occurred 

between the growth periods of A.D. 1799 and 1800.
82 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Figure 72. 1842(?) ash (at left of pocketknife), typically obscure and enclosed in forest duff, about 

7 km southeast of the center of Mount St. Helens. The thin edge of a gray pyroclastic-flow deposit, 

dark set X, and layer We are visible below the 1842(?) ash.

PREVIOUS DESIGNATIONS AND CORRELATIVE UNITS

Layer T is the unit designated as the "1802 lapilli 

deposit" by Lawrence (1939, p. 51). Okazaki and others 

(1972) have correlated the unit with the Blackfoot ash in 

northeastern Washington and northwestern Idaho.

19th CENTURY LITHIC ASH

A thin ash bed was found in the pre-1980 forest duff at 

several places around Mount St. Helens. On and near the 

southeast flank of the volcano (fig. 72), a lithic ash probably 

erupted in A.D. 1842 could be recognized in multiple outcrops over an area of a few square kilometers. It comprises a 

single, thin, gray bed of fine to coarse ash. The bed consists 

of nonvesicular to slightly vesicular glass, dense rock particles, and fragments of mineral crystals, predominantly 

hypersthene, hornblende, and augite. No pumice was found 

in the deposits. The bed was as thick as 3 cm and enclosed in 

organic forest litter; it was recognized only where it forms a 

distinct bed within that litter. No diagnostic characteristics 

other than its stratigraphic position are known.

The 1842(?) ash probably was produced by one or more 

small phreatic eruptions that ejected only particles of preexisting rock. Many such eruptions were reported from A.D. 

1831 through 1857 (Holmes, 1955; Hoblitt and others, 

1980). Although the deposit might include ash from any 

reported or unreported eruption, it probably resulted chiefly 

from the event of November 22 or 23,1842. That eruption is 

the only one for which a distinct bed has been described that 

reached beyond the volcano, and the bed extended southeastward. That eruption reportedly laid down a little more than a 

centimeter ("a half inch") of ash at The Dalles, Oregon, 

about 100 km southeast of Mount St. Helens (Fremont, 

1845).

A single, thin bed of ash younger than layer T was 

observed in several outcrops on the north side of the volcano 

during this as well as other investigations (Okazaki and others, 1972, p. 85). A sample from one of those outcrops is 

similar in grain size and composition to the 1842(?) ash; 

however, because the 1842(?) ash presumbly was carried 

southeastward, the ash bed on the north side probably 

resulted from one or more other 19th century eruptions.

Before 1980, thin deposits of ash-size material were 

visible elsewhere in forest duff around the volcano. Some of
ERUPTIVE HISTORY 83

these deposits probably originated from other eruptions in 

the 19th century that were reported to have deposited visible 

amounts of ash on the flanks of the volcano (Gairdner, 1836; 

Fremont, 1845; Holmes, 1955); however, others probably 

resulted from wind reworking and redepositing ash-size 

material.

identifiable tephra strata, numerous pyroclastic-flow and hot 

lahar deposits, several domes, and lava flows that flowed 

down all sides of the volcano. In contrast, during that same 

4,000-year period Mount Rainier produced only a few tephra 

deposits (Mullineaux, 1974) and relatively few deposits of 

pyroclastic flows, hot lahars, or lava flows (Crandell, 1971).

ERUPTIVE HISTORY

Mount St. Helens is much younger and has been more 

explosively active recently than other major Cascade Range 

volcanoes. In addition, its sequence of eruptive products 

records more changes in chemical and mineral composition 

than is typical of the other volcanoes. These fluctuations 

reflect many differences in physical or chemical conditions, 

or both, in the source magma or magmas.

Much of the known eruptive record of Mount St. Helens 

has been determined from study of fragmental deposits, 

including study of flowage deposits (Crandell, 1987) and 

tephras described herein. Fragmental deposits are particularly useful for study of eruptive histories because they form 

series of strata that are highly susceptible to erosion, which 

exposes their stratigraphic sequences. Fragmental deposits 

are also widespread, and they commonly char, bury, and preserve carbonaceous material, which provides radiocarbon 

ages for the eruptive events.

Even the complex eruptive history recognized for 

Mount St. Helens does not include all of its past eruptive 

events. For example, several indistinct, poorly preserved 

strata of unknown origin between tephra sets C and M suggest additional, unidentified eruptions. Moreover, comparison of small observed eruptions in October 1980 with the 

resulting obscure deposits indicates that comparable eruptions in the past might not have been detected during this 

study of tephra.

The youth of Mount St. Helens is demonstrated by the 

recency of both its oldest deposits and its visible cone. The 

oldest known deposits are only about 40,000 or perhaps 

50,000 years old, in contrast to rocks at other major Cascade 

Range volcanoes, which are more than a hundred thousand 

years old. The cone of Mount St. Helens is also young compared to other large Cascade Range volcanoes. Before 1980, 

Mount St. Helens was a relatively symmetrical, little-eroded 

structure emplaced over older ridges and valleys (fig. 1). Its 

symmetry and smooth slopes were recognized by early 

explorers and scientists alike as evidence that the edifice was 

young. Just how young was surprising; most rock formations 

visible on the cone before 1980 were less than 2,500 years 

old, and those on the upper part were less than 1,000 years 

old.

Mount St. Helens has also been much more active, 

especially explosively active, during the last 40,000 years 

than its sister Cascade Range volcanoes. During the past 

4,000 years, for example, it produced more than 50

SUBDIVISIONS OF ERUPTIVE HISTORY

The eruptive history of Mount St. Helens can be subdivided into (1) old and modern segments according to major 

compositional changes and (2) several stages and periods 

according to the episodic nature of its eruptions (table 7).

OLD MOUNT ST. HELENS

Compositionally, an early silicic volcano that existed 

until about 2,500 years ago can be distinguished from the 

modern, more mafic cone. The early volcano, termed "old 

Mount St. Helens" by Verhoogen (1937), consisted of dacite 

and silicic andesite. It produced abundant tephra, pyroclastic-flow deposits, domes, and short lava flows, some of 

which in turn generated other pyroclastic flows and lahars. 

Close to the volcano, pumiceous tephra deposits of old 

Mount St. Helens commonly are as thick as several meters 

(fig. 3). Pyroclastic-flow deposits from the old volcano are 

major constituents of thick, flat-topped stacks of strata that 

partly fill valleys leading away from the cone (Crandell, 

1987). Although remnants of old domes and lava flows are 

sparse, lithic pyroclastic-flow deposits provide evidence of 

their existence (Crandell, 1987). The dominantly fragmental 

deposits of the old volcano include the rocks of the "old 

Mount St. Helens series" of Verhoogen (1937, p. 268) and 

the "ancestral volcanic center" of Hopson (1971).

MODERN MOUNT ST. HELENS

The modern volcano began with the eruption of mafic 

magma shortly after 2,500 years ago. Most eruptions of the 

modern volcano occurred during one of two main episodes—between 2,500 and 1,500 years ago and between 

about 500 years ago and the present. A brief eruptive episode 

occurred about 1,200 years ago.

The mafic eruptions that initiated the modern volcano 

produced andesitic and basaltic tephra, pyroclastic flows, 

and hot lahars, as well as lava flows; however, not all magma 

erupted by the modern volcano was mafic. Even after the 

mafic magma appeared, dacite continued to be erupted intermittently, producing tephra, pyroclastic flows, and domes. 

The resulting modern cone is a complex pile of mafic lava 

flows, dacitic domes, and dacitic and andesitic deposits of 

pyroclastic flows, lahars, and tephra, all of which have been 

intruded by dikes and plugs of basalt, andesite, and dacite.
84 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Table 7. Classification of pre-1980 eruptive history, Mount St. Helens.

Age 1 Volcano designation Eruptive stage Eruptive period

Goat Rocks 

200 Modern

Mount Spirit Kalama 

500

St. Helens Lake 

1,200 Sugar Bowl

1,500 (mafic and silicic) 

Castle Creek 

2,500

Pine Creek 

3,000

Smith Creek 

4,000

Tephra set

T

X

w

D

B

P

Y

10,000 "Old" or Dormant interval

11,000 

"Ancestral" Swift Creek 

13,000

J 

S

18,000

21,000

35,000

40,000

Mount

St. Helens

(silicic)

Mostly dormant interval

Cougar

K 

M

Mostly dormant interval

Ape Canyon

In calendar years to 500 years B.P.; older ages in radiocarbon years before A.D. 1950.

ERUPTIVE STAGES AND PERIODS

Both the old and modern volcanoes were strongly episodic, as well as variable in composition, and thus the eruptive history can be divided into many parts. The pre-1980 

history of the volcano as now recognized includes four eruptive stages, the Ape Canyon, Cougar, Swift Creek, and Spirit 

Lake; the Spirit Lake is further divided into six eruptive periods (Crandell, 1987) (table 7). Each stage and each period 

represents an episode of multiple eruptions characterized by 

close association in time, similarity in composition, or both.

Initially, Mount St. Helens' eruptive record was subdivided into nine eruptive periods (Crandell and others, 1981;

Mullineaux and Crandell, 1981). In that classification, eruptive periods before 4,000 years ago had durations of thousands of years, in contrast to younger ones that had durations 

of only centuries. To remedy that disparity, Crandell (1987, 

p. 12-13) reclassified the first three periods as stages and 

combined the last six periods into a single stage. In the 

revised classification, each stage includes repeated episodes 

of activity and intervening dormancy spread over more than 

a thousand years.

Deposits of the first two stages, Ape Canyon and Cougar, are relatively poorly preserved, and we have only 

sketchy knowledge of their eruptive history. All deposits of 

these stages were subjected to severe erosion and other
ERUPTIVE HISTORY 85

disturbances during the last major glaciation and have been 

weathered for a long time as compared to other Mount St. 

Helens products. Deposits of the following Swift Creek 

stage are much better preserved; however, they have been 

subjected to rigorous processes caused by a cold, late-glacial 

climate and more than 10,000 years of postglacial weathering. In contrast, products of the youngest (Spirit Lake) stage 

are remarkably well preserved, and the eruptive history of 

that stage is much better known.

Each stage was separated from the next by a long dormant, or at least relatively quiet, interval that can be inferred 

from buried weathering profiles or from the absence of eruptive products. Some evidence suggests that at least minor 

eruptions did occur during intervals between the first three 

stages, whereas no evidence has been seen that suggests any 

such activity between the latest two stages, the Swift Creek 

and Spirit Lake.

APE CANYON STAGE

The Ape Canyon stage began with the small-volume 

eruptions that apparently record the birth of the volcano. The 

first evidence known of a Mount St. Helens is in the multiple, thin beds of layer Cb, which record small, mild to moderately explosive eruptions. These eruptions probably also 

created domes and perhaps pyroclastic flows, but tephra of 

layer Cb is their only identified product. The outbursts that 

produced layer Cb may have occurred in rapid succession 

because no evidence of a pause long enough to form even an 

incipient soil was found within the layer.

The volcano then was dormant long enough for an oxidation profile to form in the upper part of layer Cb. The 

length of time represented by the profile is not known, but 

comparison with profiles in younger, better dated deposits 

suggests more than a thousand years. Evidence from studies 

of Mount St. Helens tephras far downwind suggests the possibility that a much longer time, perhaps as long as 10,000 

years, passed between deposition of layer Cb and younger 

layers of set C (Busacca and others, 1992).

Small-volume eruptions of pumice also began the next, 

main series of Ape Canyon events. These were followed, 

without evidence of a pause, by large-volume eruptions that 

produced layers Cw and Cm. Those were followed in turn by 

repeated outbursts that produced thin beds of probable tephra 

and thicker deposits of probable ash-cloud origin. That series 

of eruptions was interrupted at least three times by pauses 

long enough for weakly oxidized soils to form. The Ape 

Canyon tephra eruptions culminated in highly explosive outbursts that produced the voluminous layers Cy and Cs. Layer 

Cs, which may be correlative with layer Cy, is the largest 

volume tephra known of Pleistocene age.

During that main series of Ape Canyon events, the volcano also produced pyroclastic flows and surges and their 

associated ash clouds, as well as mudflows (Hyde, 1975;

Crandell, 1987). Prismatically jointed lithic blocks in one 

mudflow deposit suggest the presence of one or more domes 

(Crandell, 1987, p. 19).

The main series of Ape Canyon eruptions probably 

spanned at least 2,000 years. Two radiocarbon ages from just 

underneath tephras erupted during that time (W-2661 and 

W-2976; table 2) and one from a volcanic mudflow (Hyde, 

1975) at Mount St. Helens are between 38,000 and 36,000 

years B.P. A radiocarbon age of about 33,650 years B.P. 

from 25 cm above tephra of Ape Canyon age in Nevada 

(Davis, 1978, p. 45) is consistent with the ages obtained from 

deposits near the volcano.

Thick flowage deposits of the Ape Canyon stage 

extended down the North Fork Toutle valley and probably 

aggraded it to at least as far downstream as the Cowlitz River 

valley (Crandell, 1987; Scott, 1988). Those deposits must 

have also dammed the North Fork Toutle valley north of 

Mount St. Helens to produce the first of many versions of 

Spirit Lake.

APE CANYON-COUGAR INTERVAL

About 15,000 years passed between Ape Canyon and 

Cougar stage eruptions. No unequivocal primary eruptive 

products have been identified at Mount St. Helens that represent that period, although some evidence suggests that the 

volcano was not completely dormant. Fine, ash-rich detritus 

accumulated on large areas of uplands during at least three 

separate episodes. In addition, thin, discontinuous lenses of 

small pumice lapilli within those deposits suggest that some 

tephra was erupted; however, these pumice deposits are not 

sufficiently voluminous or well preserved to be identified 

satisfactorily as to origin, and thus unambiguous evidence of 

eruptions is lacking. That lack of evidence could be due, 

however, only to lack of preservation because voluminous 

tephras may have been erupted and carried only northeasterly. In the Cascade Range to the northeast, any such tephra 

would be severely eroded and not easily found. Farther to the 

northeast in Canada, two tephras that are between 35,000 

and 20,000 years old and are characterized by cummingtonite have been identified (Westgate and Fulton, 1975).

COUGAR STAGE

The Cougar stage, which apparently lasted only 

2,000-3,000 years, is characterized by tephra eruptions that 

were less voluminous than those of Ape Canyon time but 

show more compositional variation. Those eruptions also 

produced large pyroclastic flows and lahars (Crandell, 

1987), one or more lava flows of dacite or siliceous andesite 

(C.A. Hopson, written commun., 1974), and probably one or 

more dacite domes of similar composition. Two episodes of 

tephra production are identified within the Cougar stage and
86 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

are separated by enough time to form an oxidized soil profile.

The Cougar stage apparently began with mildly explosive or nonexplosive events that produced lahars, a debris 

avalanche, and a siliceous andesite lava flow (Crandell, 

1987, p. 24). Explosive eruptions then created large pumiceous pyroclastic flows that travelled down the southeast, 

south, and west sides of the volcano (Crandell, 1987); deposits of those flows are characterized by hypersthene and hornblende. These eruptions were followed by outbursts that 

produced the multiple pumiceous layers of tephra set M and 

some coeval ash deposits that probably were derived from 

pyroclastic flows. The first set M tephras are characterized 

by cummingtonite, which is progressively replaced upward 

in the set by hypersthene.

Eruptions of now recognizable tephra layers stopped 

temporarily after deposition of set M. Ashy fine material that 

accumulated on top of the set suggests, however, that at least 

minor eruptions, perhaps of tephra or pyroclastic flows, continued.

The next eruptive episode of this stage produced a 

sequence of tephras very different from earlier ones. Eruptions that produced tephra set K as recognized were small in 

volume, intermittent, and repetitive in terms of scale and 

composition. Some larger volume eruptions may have 

occurred at about that time, as suggested by strata in an outcrop northeast of the volcano, but the age relation of these 

strata to set K tephras is not known. Set K was followed by 

eruption of voluminous pyroclastic flows that moved down 

the south and southeast flanks of the volcano. Those flows 

are the last known products of the Cougar stage.

During Cougar time, large pyroclastic flows and lahars 

filled the Lewis River valley south of Mount St. Helens to a 

depth of more than a hundred meters and aggraded the valley 

far downvalley (Hyde, 1975; Crandell, 1987). Detritus from 

the volcano so overwhelmed the Lewis River that pyroclastic-flow deposits of Cougar age have surface gradients of as 

much as 25 m/km from north to south across the Lewis River 

valley directly south of Mount St. Helens.

The ferromagnesian mineral suites in products of Cougar time record different magmatic conditions at various 

times during this stage. Experiments of several investigators (see Geschwind and Rutherford, 1992, and references 

therein) indicate that magmas characterized by 

cummingtonite last equilibrated at lower temperature and 

perhaps higher water content than those characterized by 

hypersthene. Several differences or changes between relatively cool and hot magmatic conditions are recorded in the 

Cougar-stage products. It is not known, however, whether 

these changes represent different source magmas, different 

parts of a nonhomogeneous magma, or changes in a magma 

with time.

The hypersthene-rich ferromagnesian suites of early 

products suggest that at the start of Cougar time the source 

magma had equilibrated at a higher temperature than that of

the last magma erupted at the end of the previous, Ape Canyon, stage. But, by the time the first layers of set M were 

erupted, the dominance of cummingtonite suggests that the 

source magma or magma batch for the set M tephras had 

equilibrated at a lower temperature. No intervening products 

characterized by both cummingtonite and hypersthene are 

known.

During set M eruptions, temperatures of the source 

magma apparently became progressively higher, as indicated by hypersthene substitution for cummingtonite upward 

in the sequence. Such a change could have resulted from progressive heating of the source magma by injection of new 

magma or perhaps by tapping of successively deeper parts of 

a magma body (Hopson and Melson, 1990). Presence of olivine in layer Mo suggests that a new magma, if present, was 

mafic.

The sequence of set K tephras and the pyroclastic-flow 

deposits that followed set M indicates again a relatively low 

temperature source magma and a return to a higher temperature magma.

COUGAR-SWIFT CREEK INTERVAL

Little is known about the relatively quiet interval of 

about 5,000 years that followed the Cougar stage. Accumulation of ash-rich fine sediments on uplands suggests some 

volcanic activity, but no deposits containing pumice lapilli 

were seen in those sediments. Because this interval occurred 

during the latter part of the last major glaciation, eruptive 

products of the time could have been so severely eroded or 

altered that they were not identified in this study.

SWIFT CREEK STAGE

During the Swift Creek stage, between about 13,000 

and 10,500 years ago, the volcano produced large volumes 

of pyroclastic flows and moderate to large volumes of 

tephras that extend hundreds of kilometers downwind. The 

Swift Creek stage includes two distinct episodes of tephra 

production, one about 13,000 years ago and the second 

between about 12,000 and 10,500 years ago. Between these 

episodes, multiple pyroclastic flows and lahars built extensive valley fills (Crandell, 1987).

The Swift Creek stage began with the production of 

tephras and ash-cloud deposits, and ash-cloud deposits continued to be produced throughout the time of deposition of 

set S tephras. The final two eruptions of set S produced the 

two largest volume tephras since Ape Canyon time. 

Although only the last two tephras of set S were of such large 

volume, as many as three beds of set S have been recognized 

at multiple sites in eastern Washington as far as 300 km east 

of the volcano (Foley, 1976, 1982; Hammatt, 1976; Moody, 

1977; Mullineaux and others, 1978; Busacca and others, 

1992). The presence of ash-cloud deposits interbedded with
ERUPTIVE HISTORY 87

tephra of all parts of set S indicates that pyroclastic flows 

were produced repeatedly during that part of the Swift Creek 

stage.

Set S tephra was followed by many lahars and lithic 

pyroclastic flows; some lithic pyroclastic flows probably 

were derived from domes (Crandell, 1987). Flo wage deposits were voluminous enough to produce extensive valley 

fills, especially southeast of the volcano (Crandell, 1987, p. 

.36). As the pyroclastic flows and lahars filled valleys, ashrich deposits accumulated on upland surfaces.

The next Swift Creek events produced thin ash deposits 

at the base of set J. The volcano then erupted at least three 

large-volume tephras but apparently no associated pyroclastic flows. Ash from these highly explosive outbursts 

extended far to the east of the volcano. The resulting ash 

beds have been identified not only in eastern Washington but 

also as far as Montana (Carrara and others, 1986).

The final eruptions of Swift Creek time produced the 

thick, coarse tephra layer Jg, which extends toward the west. 

Limited distribution relative to thickness and grain size suggests that either the eruptive column was low or the winds 

were of low velocity during eruption.

As in the Cougar stage, the ferromagnesian minerals in 

Swift Creek deposits record differences in source-magma 

conditions from one eruptive episode to another. The cummingtonite in set S suggests a magma that had equilibrated 

at a lower temperature than the source magma of the lastknown eruptions of Cougar time. The disappearance of cummingtonite and its replacement by hypersthene in tephras of 

set J indicate again a relatively high temperature in the 

source magma. The last Swift Creek tephra erupted is also 

the least silicic and may record mixture of invading mafic 

magma and preexisting dacitic magma.

SWIFT CREEK-SPIRIT LAKE INTERVAL

During this interval, from about 10,500 to 4,000 years 

ago, Mount St. Helens apparently was completely dormant. 

No evidence has been found that indicates the volcano 

erupted at all during the interval; a search of many years has 

turned up no evidence of eruptive products attributable to the 

interval. Nor, in contrast to the earlier intervals between 

stages, is there any oxidized, ash-rich bed that would suggest 

unrecognized events during this interval. Obviously, the possibility of minor eruptions cannot be ruled out because their 

products could have filtered down into the disturbed soil 

zone at the top of the deposits of Swift Creek age. This interval, however, is the longest in the history of the volcano for 

which we have no evidence whatever of eruptive activity.

SPIRIT LAKE STAGE

Eruptions during the Spirit Lake stage were responsible 

for building the volcano generally recognized as Mount St.

Helens. The six eruptive periods of the Spirit Lake stage produced the rocks that make up the visible cone and record the 

compositional change from the older Mount St. Helens to the 

more mafic and variable modern volcano. Highly explosive 

eruptions of voluminous, purniceous tephra are notable features of the earliest period. During later periods, domes, 

pyroclastic flows, and lava flows became more important.

SMITH CREEK PERIOD

The first period of Spirit Lake time consists of three 

eruptive episodes that differ distinctly in character. The 

Smith Creek period began with several explosive pulses of 

small to moderate volume that produced the thin but widespread pumiceous layer Yb. The distribution of these beds 

shows that wind directions changed between or during the 

eruptions. Deposition of layer Yb was followed, with no 

apparent pause, by numerous but probably less gas rich and 

less vigorous ejections of lithic to somewhat vesicular 

tephras of layer Yd.

Mount St. Helens then was dormant for a few hundred 

years. During that time, surficial processes formed a thin, 

incipient soil that is commonly topped by sparse carbonaceous material. This soil is weak but persistent and can be 

identified from northeast to southeast of the volcano.

Highly explosive and voluminous eruptions characterize the second phase of Smith Creek time. The second phase 

started with the discharge of layer Yn, the largest volume 

tephra of Holocene time. Layer Yn is present along a narrow 

lobe to the north-northeast that extends for at least several 

hundred kilometers across the northwestern United States 

and into Canada (Crandell and others, 1962; Westgate and 

others, 1970). The tephra eruption was closely followed by 

one or more pyroclastic flows that produced thick ash-cloud 

deposits; these deposits directly overlie layer Yn close to the 

volcano. Another highly explosive and voluminous pumice 

eruption then produced the Ye tephra, which was carried 

eastward across Washington State and at least as far as Idaho 

(Smith and others, 1977). Eruption of the Ye tephra was also 

followed by pyroclastic flows, which produced ash-cloud 

deposits that cover the tephra layer near the east base of the 

volcano.

Tephra eruptions that followed layer Ye were much 

smaller and are characterized by less vesicular pumice and 

higher proportions of lithic fragments. Interbedding of ashcloud deposits and abundance of lithic clasts suggest that the 

eruption of tephra was interspersed with formation of pyroclastic flows and domes. This activity apparently continued 

with no pauses long enough to form recognizable soils until 

the end of the Smith Creek period. One conspicuous, hot 

lithic pyroclastic flow of late Smith Creek time entered the 

Smith Creek valley, picking up and charring logs (Crandell, 

1987); its associated ash cloud spread over adjacent slopes 

and formed the charcoal-bearing ash bed ya.
PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

Pyroclastic-flow and lahar deposits of the Smith Creek 

period raised the level of the fan on the north flank of the volcano, and lahars extended down the North Fork Toutle River 

valley at least 50 km below Spirit Lake. The thick deposits 

of Smith Creek age that crop out downvalley indicate that an 

early Spirit Lake was formed or expanded at that time.

Radiocarbon dates show that the Smith Creek period 

began shortly after 4,000 years ago and ended at some time 

after 3,350 years ago. Ages at the base of layer Yn of about 

3,500 years demonstrate that the dormant time between 

eruption of layers Yb and Yd lasted no more than a few hundred years.

SMITH CREEK-PINE CREEK INTERVAL

This interval lasted at most about 300 years and may 

have been much shorter. Some eruptions occurred during the 

interval, but apparently none was strong enough to produce 

lapilli-size clasts on the lower flanks of the volcano or 

nearby. Only the ash bed pbp, with its faint bedding, records 

activity during the Smith Creek-Pine Creek interval. The 

thin lamina within ash bed pbp exhibit successive changes in 

ferromagnesian mineral composition that record a series of 

some kind of small-volume eruptive events. They also 

apparently record a progressive change from a relatively low 

temperature magma, characterized by cummingtonite, to a 

higher temperature magma characterized by hypersthene. 

The magma under Mount St. Helens at the time may have 

been heated by injection of mafic magma that did not reach 

the surface until several hundred years later, during the eruptions of Castle Creek time.

PINE CREEK PERIOD

Pine Creek time, relative to Smith Creek time, was 

marked by much lower explosivity. It was characterized by 

discharge of small-volume tephras and large-volume pumiceous and lithic pyroclastic flows (Crandell, 1987). Lithic 

pyroclastic-flow deposits, probably derived from relatively 

nonexplosive eruptions of dacite domes, extend as far as 18 

km from the volcano (Crandell, 1987).

The earliest eruptions of the Pine Creek period probably 

formed a dacite dome or cryptodome. The first tephra eruption, of layer Pm, was strongly explosive but not voluminous. The tephra contains abundant fresh, lithic fragments of 

hypersthene-hornblende dacite. Evidently, degassed magma 

or rock of Pine Creek composition was emplaced on or near 

the surface before being fragmented during the eruption of 

layer Pm. Several successive small-volume tephra eruptions 

produced the thin, multiple beds that make up layer Ps; these 

beds contain abundant lithic clasts, as well as clasts of pumice. The eruptions of layers Pm and Ps presumably were

interspersed with pyroclastic flows and domes that provided 

a source of fresh, lithic clasts.

Although no recognized tephra deposits are present 

between layers Ps and Pu, ash of probable ash-cloud origin 

suggests that pyroclastic flows formed during that time.

When tephra eruptions resumed, they produced the 

thin, multiple beds of layer Pu, which are remarkably similar 

to those of layer Ps. As during the early part of Pine Creek 

time, abundance of fresh lithic clasts indicates that domes or 

cryptodomes formed between tephra eruptions.

The final tephra of Pine Creek time records at least 

three explosive events, during which gas-rich magma was 

erupted along with lithic fragments. Yellow-gray ash of 

probable ash-cloud origin that is associated with beds of 

layer Py indicates that at least one pyroclastic flow occurred 

during the eruptions of tephra.

Pine Creek time spanned most of an interval from about 

3,000 to 2,500 years ago. During that time, lahars and fluvial 

deposits consisting mostly of newly erupted material 

strongly aggraded the valley floors of the North Fork Toutle 

River and other rivers that headed on the volcano. They 

raised the surface of the fan that impounds Spirit Lake and, 

40 km down the North Fork Toutle valley, blocked a small 

tributary valley to create the basin that holds Silver Lake 

(Mullineaux and Crandell, 1962). They also formed a continuous fill across the floor of the Cowlitz River valley near 

Castle Rock and another fill in the Lewis River valley, 

which, near Woodland, was about 7.5 m higher than the 

present floodplain (Crandell and Mullineaux, 1973).

PINE CREEK-CASTLE CREEK INTERVAL

The Pine Creek-Castle Creek interval was short, lasting 

at most about 300 years and very likely much less. Evidence 

of volcanic activity is limited to that provided by ash bed v, 

which suggests that some volcanic events of small volume 

occurred. The lack of charred vegetation in or at the top of 

ash bed v suggests that no dense stand of vegetation was 

established during this interval.

CASTLE CREEK PERIOD

The defining feature of this period is the first appearance of mafic magma at the surface, which initiated the modern Mount St. Helens and its variety of rock composition. 

Andesite, dacite, and basalt all were erupted during Castle 

Creek time, producing pyroclastic flows, surges, and tephra 

as well as lava flows. Volumes of explosively erupted deposits were moderate or small.

The first known eruptive products of the period are 

andesitic and are characterized by ferromagnesian suites of 

hypersthene and augite. The early Castle Creek eruptions 

resulted in at least one lava flow and created lahars
ERUPTIVE HISTORY 89

(Crandell, 1987). Next, explosive eruptions produced the 

tephra layer Bh. Similar explosions that followed resulted in 

layer Bo, in which olivine dominated ferromagnesian suites 

for the first time in Mount St. Helens history. Repeated 

explosions created several different shower beds within 

layer Bo, as well as pyroclastic flows and hot lahars. Intercalation of tephras with pyroclastic-flow and lahar deposits 

suggests frequent changes in eruptive behavior, which probably included initial fountaining of tephra during eruptions 

that later produced pyroclastic flows, lahars, and lava flows.

After layer Bo, milder eruptions of undetermined character produced mafic ash, followed by ash that was more 

silicic. These deposits, which formed ash bed ba, probably 

include detritus from both small tephra eruptions and from 

ash clouds derived from pyroclastic flows. As ash bed ba 

accumulated, the small-volume, slightly vesicular dacitic 

tephra layer Bd was erupted. Dacitic pyroclastic flows of the 

same composition as layer Bd provide radiocarbon dates of 

2,200-2,000 years for those deposits (Crandell, 1987). After 

a pause of 200-400 years, another explosive eruption produced the dacitic tephra layer Bi, which is highly vesicular 

and has a ferromagnesian mineral suite slightly different 

from that of layer Bd. Radiocarbon ages of approximately 

1,800 years, obtained from both below and above layer Bi, 

indicate that eruption of layer Bi was distinctly later than that 

of layer Bd.

After formation of layer Bi, only olivine basalt is 

recorded by the tephra sequence; the basaltic scoria of layer 

Bu records at least two mildly explosive events. At one site, 

however, an andesite lava flow overlies dacitic pyroclasticflow deposits and underlies a basalt lava flow (Crandell, 

1987, p. 57-59). Thus, at least one andesitic eruption 

occurred between the eruptions of dacite and basalt.

Basaltic lava flows as well as tephra were produced at 

the end of Castle Creek time. Multiple thin flow units of 

basalt overlie at least part of layer Bu on the north flank of 

the volcano; the lava flows were emitted after at least the first 

eruption of layer Bu. Fountaining that produced tephra may 

have been followed by quieter emission of the lava flows.

The largest basalt flows of late Castle Creek time, however, are pahoehoe flows on the south flank of Mount St. 

Helens. There is no physical overlap of the basalt flows on 

the north with the pahoehoe flows on the south, and radiocarbon dates do not firmly determine which is younger (Crandell, 1987). Consequently, the age relation of the pahoehoe 

flows and other basalts of Castle Creek time is uncertain.

Castle Creek eruptions followed those of Pine Creek 

time after only a short period and continued for almost 1,000 

years.

SUGAR BOWL PERIOD

The Sugar Bowl episode was short and markedly different from other periods in Mount St. Helens history. It

produced the only unequivocal laterally directed blast 

known from Mount St. Helens before the 1980 eruptions 

(Crandell and Mullineaux, 1978).

During Sugar Bowl time, the volcano first erupted quietly to produce a dome, then erupted violently at least twice 

producing a small volume of tephra, directed-blast deposits, 

pyroclastic flows, and lahars (Crandell and Hoblitt, 1986; 

Crandell, 1987). The Sugar Bowl blasts, described initially 

as only one (Crandell and Mullineaux, 1978), were much 

smaller than the one in 1980, extending only about one-third 

as far as that of 1980. In composition, fragmental deposits of 

the Sugar Bowl period are similar to rock of Sugar Bowl 

dome.

Radiocarbon analyses and stratigraphic relations indicate that the Sugar Bowl eruptions occurred about 1,200 

years ago, about midway between the Castle Creek and 

Kalama periods. The morphologically similar East dome, in 

contrast, has no fragmental deposits associated with it, and 

its age is not closely known. Absence of set B tephras on East 

dome indicates that it was emplaced after the Castle Creek 

period, and presence of set W tephras on it show that it 

existed before Kalama time. It could have risen at any time 

between those two periods, and whether it is coeval with the 

Sugar Bowl dome is not known.

Except for the dome itself, Sugar Bowl eruptions produced only deposits of small volume and probably occurred 

within a short time, perhaps a few years. Sugar Bowl eruptions caused no major changes in the shape or size of the volcano but did form a prominent dome on the north flank of 

Mount St. Helens.

KALAMA PERIOD

In A.D. 1480, or possibly late in A.D. 1479, a highly 

explosive pumice eruption opened the Kalama period with 

discharge of tephra layer Wn. The Kalama period continued 

for perhaps 300 years and comprised a wide variety of 

magma compositions and kinds of eruptions. It included 

highly explosive ejection of voluminous dacitic and andesitic tephras and pyroclastic flows, dacitic pyroclastic flows, 

nonexplosive eruption of andesite lava flows, and rise of the 

dacitic summit dome.

Eruption of layer Wn produced the most extensive and 

most voluminous tephra of Kalama time; layer Wn has been 

identified more than 400 km northeast of Mount St. Helens 

(Smith and others, 1977). Reworking of that pumice layer, 

where thick, produced fluvial and laharic deposits locally at 

the northeast and south bases of the volcano (Crandell, 

1987). A dome probably formed next, before eruption of the 

relatively small tephra layer Wa. The dome provided layer 

Wa with abundant fresh lithic fragments of the same mineral 

composition as the new magma. The presence of more than 

one bed or zone in both layers Wa and Wb, along with abundant fresh lithic clasts, indicates that multiple small
90 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

explosive pulses and dome formation occurred between 

A.D. 1480 and 1482, when the next voluminous and highly 

explosive eruption produced layer We. One or more pyroclastic flows probably were created during that event 

because ash probably derived from pyroclastic flows is intermingled with and overlies the upper part of the layer. Layer 

We also extended far downwind and has been identified 

eastward more than 400 km from Mount St. Helens (Smith 

and others, 1977).

Perhaps about the time of eruption of layer We, pyroclastic flows of pumiceous and lithic dacite began to move 

down the southwest flank of the volcano (Hoblitt and others, 

1980). The magma erupted gradually became more mafic. 

Explosive, dacitic eruptions were followed by weaker eruptions of andesitic tephra and the quiet emission of andesitic 

lava flows, which mantled much of the south flank of the 

volcano. Some beds of tephra set X probably were ejected 

during early phases of eruptions that produced lava flows.

The andesitic eruptions were followed by rise of a large 

dome, perhaps starting during the first half of the 17th century (Hoblitt, 1989); that dome made up the summit of 

Mount St. Helens until early 1980. Avalanches of hot debris 

from the dome swept down all sides of the volcano, formed 

pyroclastic flows and lahars, and covered the upper parts of 

many Kalama lava flows (Hoblitt and others, 1980; Crandell, 

1987). The rounded summit dome and smooth profiles of the 

avalanches and lava flows were primarily responsible for the 

smooth, youthful appearing cone of the pre-1980 Mount St. 

Helens.

The principal growth of the volcano during the Kalama 

period probably occurred within about 200 years, but debris 

avalanches from the dome continued for about another 100 

years (Hoblitt, 1989; Pallister and others, 1992).

Changes in composition during Kalama time were 

gradual but distinctive, from dacite to andesite and back to 

dacite. Patterns of trace elements along with other compositional changes indicate that the changes resulted from injection of mafic magma into a preexisting dacite (Hoblitt, 1989; 

Pallister and others, 1992)

GOAT ROCKS PERIOD

The Goat Rocks eruptive episode was short and produced relatively small volumes of magma. Its first eruptive 

event was a single, highly explosive, moderate-volume eruption in A.D. 1800 that produced the pumiceous, dacitic layer 

T. That layer spread along a narrow path northeastward 

across Washington, Idaho, and into Montana (Okazaki and 

others, 1972, p. 81). The tephra eruption apparently was not 

accompanied by other events but was followed later in the 

same year (Yamaguchi and others, 1990) by the andesitic 

lava flow called the "floating island" flow by Lawrence 

(1941).

No eruptions have been documented for the next three 

decades, but minor ones were reported from A.D. 1831 to 

1857 (Holmes, 1955). In A.D. 1842, the largest volume of 

those eruptions dumped a reported half-inch of ash on The 

Dalles, Oregon. Goat Rocks dome was extruded at about that 

same time (Crandell, 1987, p. 88). The dome probably was 

constructed incrementally through several years during the 

1840's, during which time many eruptions were observed 

(Holmes, 1955). Growth of the dome caused dome-rock avalanches, pyroclastic flows, and lahars; the flowage deposits 

created a broad fan below the dome on the north flank of the 

volcano (Hoblitt and others, 1980, p. 558; Crandell, 1987). 

The last eruption usually attributed to the Goat Rocks period 

occurred in 1857, although a study of old records suggests 

that minor eruptions of Mount St. Helens might have 

occurred in 1898, 1903, and 1921 (Majors, 1980, p. 36-41).

The relatively small volume eruptions of Goat Rocks 

time produced no major changes in the size or shape of the 

volcano.

HAZARDS FROM 

TEPHRA ERUPTIONS

Future eruptions of Mount St. Helens are certain, and 

some of those eruptions will endanger lives and property. 

Tephra generally is less of a threat to lives than are flowage 

events such as pyroclastic flows and lahars. Nevertheless, 

tephra can cover such large areas that it can cause even 

greater overall loss to people and property than flowage 

events.

Although future eruptions certainly will occur, their 

specific character and timing cannot be predicted. Only generalized forecasts can be made, based on patterns of past 

activity, and even these are necessarily uncertain. Probabilities of various kinds and magnitudes of events can be estimated, however, from the volcano's eruptive record, and 

hazard maps based on these probabilities can be drawn to 

guide response planning. And, although the timing of future 

eruptions cannot be predicted, seismic monitoring would be 

expected to detect movement of large amounts of magma at 

Mount St. Helens, which in turn would provide warning 

before another large eruption occurs (Pallister and others, 

1992).

Tephra causes physical, medical, psychological, and 

social problems, especially close to the volcano. The severity 

of most effects decreases rapidly with increasing distance 

because of decreasing thickness of tephra deposited, yet the 

overall damage beyond the immediate vicinity of the volcano can be great because of the vast areas affected.

Tephra endangers people and property chiefly by 

impact and heat of large fragments, by burial and load 

from thick accumulations, by ash inhalation, and by many 

other effects resulting from dispersion of ash in air and
HAZARDS FROM TEPHRA ERUPTIONS 91

water (see Wilcox, 1959b, Blong, 1984, and Scott, 1989, 

for more detailed descriptions). Close to a volcano, the 

chief dangers are large fragments and thick ash. Most 

deaths from tephra eruptions are caused by fragment 

impact or collapse of roofs caused by the weight of tephra. 

Large fragments also can kill or injure people and animals by heat, penetrate and otherwise damage structures, 

and start fires. In addition to crushing roofs, thick ash 

deposits commonly disable power and communication 

lines, block transportation routes, and bury crops. People 

have been killed or injured by inhaling large amounts of 

ash, especially if it is hot, and injury to eyes and respiratory systems can be serious for both people and animals. 

Ash also commonly overloads water supply and sewer 

systems, and development of a cemented crust on ash 

deposits can lead to excessive runoff, which in turn 

causes secondary mudflows.

At great distances, where tephra is fine grained and 

dispersed, it can still cause serious injuries to eyes and 

respiratory systems. The thin deposits can also severely 

hamper transportation and communication facilities. Even 

at hundreds of kilometers from a volcano, ash suspended 

in the air can limit transportation by causing darkness during daylight hours and otherwise reducing visibility and 

by damaging engines and other equipment. Growing 

crops can be set back or destroyed by thin ash, but future 

crops typically benefit from the addition to the soil.

Recent events make it clear that ash is extremely dangerous to jet aircraft, even at great distances from a 

source volcano. On several occasions, highly dispersed ash 

has damaged jet engines sufficiently to cause temporary 

failure and accompanying loss of altitude and risk of a 

crash. In most of these encounters, ash particles have also 

caused extensive damage to other parts of the aircraft 

(Casadevall, 1991).

Medical, psychological, and social effects (see 

Blong, 1984) are commonly long term as well as immediate. Depression, anxiety, and stress were high in areas 

affected by tephra in the first two years after the 1980 

Mount St. Helens eruptions (Shore and others, 1987), and 

ensuing political disruption, anger, and litigation continued 

for many more years.

Fortunately, losses from tephra effects can be greatly 

minimized, except for those from large-volume eruptions 

and near the volcano. In some cases the fallout can simply be avoided, and in many others protective measures 

can limit adverse effects. Avoidance is feasible except 

close to the volcano primarily because of the time windcarried tephra takes to reach any given location after its 

eruption has begun. That time can allow people to move 

out of the path of the tephra; obviously such a response is 

increasingly feasible with increasing distance from the 

volcano. Avoidance by evacuation before an eruption 

begins, however, generally is not appropriate. Neither the 

character nor timing of such an eruption could be specifically predicted, a large number of people might have to 

be moved, and the area predicted to be at risk would 

change with each anticipated change in wind direction.

In contrast, protective measures are relatively 

straightforward, and many are easy to implement. Taking 

shelter in sturdy structures and using masks or covers to 

protect people and equipment from ash can significantly 

reduce damage. For best results, mitigative measures 

should be both planned well in advance and easy to implement on short notice. Obviously, early notification that 

tephra has been erupted and information about its direction and speed are critical in order for protective measures to be taken before the tephra arrives.

In practice, effective reponse to eruptions, even 

when the hazards have been assessed and described, has 

been difficult (see Peterson, 1988, and Peterson and Tilling, 1993, and references therein). An important factor is 

that predictions (and thus warnings) of future eruptions 

must be probabilistic; they are not specific as to either 

time or character of eruptions. Sorenson and Mileti 

(1987) pointed out that warnings need to be not only specific but also certain, consistent, accurate, and clear. In 

the absence of predictions that are specific and certain, 

response plans must be drawn to cover many possible 

times and situations, perhaps too many to be readily 

understood and accepted. Those same uncertainties make 

it difficult to maintain a high state of readiness.

To date, volcanologists' mitigation efforts have consisted chiefly of hazards assessments and monitoring to 

predict eruptions. Recently, the importance of effective 

communication of that information has been emphasized 

(Peterson, 1988). Yet another important factor is that of 

detection of hazardous events when they actually occur so 

that civil officials and the general public can be warned. 

Such warnings are more desirable than conditional warnings of possible or probable events, which usually are all 

that can be provided in advance. Warnings of real events 

in progress meet the criteria of being specific and certain 

(Sorenson and Mileti, 1987) and enable civil officials and 

the public to react with greater confidence.

The risks posed by tephra are particularly appropriate for mitigation by warnings based on detection of 

events already in progress. In many cases, erupted tephra 

can be detected and tracked visually from the ground or 

air, and satellites can provide primary or supplemental 

information. Another method developed recently (R.P. 

Hoblitt, written commun., 1992) tracks the downwind 

progress of erupted tephra by detecting the lightning 

strikes that it causes.

Thus, tephra can be dangerous, but because of its 

characteristics mitigation can be relatively successful. Success, however, requires adequate advance planning, detection of hazardous events when they actually occur, and 

the capability of prompt response to those events.
92 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

REFERENCES CITED

Barnosky, C.W., 1981, A record of late Quaternary vegetation from 

Davis Lake, southern Puget Lowland, Washington: Quaternary 

Research, v. 16, p. 221-238.

Blong, R.J., 1984, Volcanic hazard—A sourcebook on the effects of 

eruptions: Orlando, Florida, Academic Press, 424 p.

Bloss, F.D., 1981, The spindle stage—Principles and practice: New 

York Cambridge University Press, 340 p.

Borchardt, G.A., Norgren, J.A., and Harward, M.E., 1973, Correlation of ash layers in peat bogs of eastern Oregon: Geological 

Society of America Bulletin, v. 84, p. 3101-3108.

Busacca, A.J., Nelstead, K.T., McDonald, E.V., and Purser, M.D., 

1992, Correlation of distal tephra layers in loess in the channeled scabland and palouse of Washington State: Quaternary 

Research, v. 37, p. 281-303.

Carey, S.N., and Sigurdsson, H., 1982, Influence of particle aggregation on deposition of distal tephra from the May 18, 1980 

eruption of Mount St. Helens volcano: Journal of Geophysical 

Research, v. 87, no. B8, p. 7061-7072.

Carey, S., Gardner, J., and Sigurdsson, H., 1989, Intensity and magnitude of post-glacial plinian eruptions at Mount St. Helens 

[abs.]: New Mexico Bureau of Mines and Mineral Resources 

Bulletin 131, p. 43.

Carithers, Ward, 1946, Pumice and pumicite occurrences of Washington: Washington Division of Mines and Geology Report of 

Investigations 15, 78 p.

Carrara, P.E., and Trimble, D.A., 1992, A Glacier Peak and Mount 

St. Helens J volcanic ash couplet and the timing of deglaciation 

in the Colville Valley area, Washington: Canadian Journal of 

Earth Sciences, v. 29, p. 2397-2405.

Carrara, P.E., Wilcox, R.E., and Richmond, G.M., 1984, Deglaciation and revegetation in the Glacier National Park region, Montana: Geological Society of America Abstracts with Programs, 

v. 16, p. 217.

Carrara, P.E., Wilcox, R.E., and Short, S.K., 1986, Deglaciation of 

the mountainous region of northwestern Montana, U.S.A., as 

indicated by late Pleistocene ashes: Arctic and Alpine 

Research, v. 18, p. 317-325.

Casadevall, T.J., ed., 1991, The first international symposium on 

volcanic ash and aviation safety, programs and abstracts: U.S. 

Geological Survey Circular 1065, 58 p.

Crandell, D.R., 1969, Surficial geology of Mount Rainier National 

Park, Washington: U.S. Geological Survey Bulletin 1288,41 p.

————1971, Postglacial lahars from Mount Rainier volcano, 

Washington: U.S. Geological Professional Paper 677, 75 p.

————1980, Recent eruptive history of Mount Hood, Oregon, and 

potential hazards from future eruptions: U. S. Geological Survey Bulletin 1492,81 p. 

-1987, Deposits of pre-1980 pyroclastic flows and lahars

from Mount St. Helens volcano, Washington: U. S. Geological

Survey Professional Paper 1444, 91 p. 

Crandell, D.R., and Hoblitt, R.P., 1986, Lateral blasts at Mount St.

Helens and hazards zonation: Bulletin of Volcanology, v. 48,

p. 26-36. 

Crandell, D.R., and Miller, R.D., 1974, Quaternary stratigraphy and

extent of glaciation in the Mount Rainier region, Washington:

U.S. Geological Survey Professional Paper 847, 59 p.

Crandell, D.R., and Mullineaux, D.R., 1973, Pine Creek volcanic 

assemblage at Mount St. Helens, Washington: U.S. Geological 

Survey Bulletin 1383-A, 23 p.

————1978, Potential hazards from future eruptions of Mount St. 

Helens volcano, Washington: U.S. Geological Survey Bulletin 

1383-C, 26 p.

Crandell, D.R., Mullineaux, D.R., Miller, R.D., and Rubin, M., 

1962, Pyroclastic deposits of Recent age at Mount Rainier, 

Washington, in Short papers in geology, hydrology, and topography: U.S. Geological Survey Professional Paper 450-D, p. 

D64-68. i

Crandell, D.R., Mullineaux, D.R., Rubin, M., Spiker, E., and 

Kelley, M.L., 1981, Radiocarbon dates from volcanic deposits 

at Mount St. Helens, Washington: U.S. Geological Survey 

Open-File Report 81-844, 15 p.

Crandell, D.R., and Waldron, H.H., 1956, A Recent volcanic mudflow of exceptional dimensions from Mount Rainier, Washington: American Journal of Science, v. 254, no. 6, p. 349-362.

Davis, J.O., 1978, Quaternary tephrochronology of the Lake Lahontan area, Nevada and California: University of Nevada, Reno, 

Nevada Archeologic Survey Research Paper 7, 96 p.

————1985, Correlation of late Quaternary tephra layers in a long 

pluvial sequence near Summer Lake, Oregon: Quaternary 

Research, v. 23, p. 38-53.

Davis, P.T., Barnosky, C.W., and Stuiver, M., 1982, A 20,000 yr 

record of volcanic ashfalls, Davis Lake, southwestern Washington: American Quaternary Association Biennial Conference, 7th, University of Washington, June 28-30, Programs 

and Abstracts, p. 87.

Doukas, M.P., 1990, Road guide to volcanic deposits of Mount St. 

Helens and vicinity, Washington: U.S. Geological Survey Bulletin 1859, 53 p.

Fiacco, R.J., Jr., Palais, J.M., Germani, M.S., Zielinski, G.A., and 

Mayewski, P.A., 1993, Characteristics and possible source of a 

1479 A.D. volcanic ash layer in a Greenland ice core: Quaternary Research, v. 39, p. 267-273.

Fiske, R.S., Hopson, C.A., and Waters, A.C., 1963, Geology of 

Mount Rainier National Park, Washington: U.S. Geological 

Survey Professional Paper 444, 93 p.

Foley, L.L., 1976, Slack water sediments in the Alpowa Creek 

drainage, Washington: Pullman, Washington State University, 

M.A. thesis, 55 p.

————1982, Quaternary chronology of the Palouse loess near 

Washtucna, eastern Washington: Bellingham, Western Washington University, M.A. thesis, 139 p.

Fremont, J.C., 1845, Report of the exploring expedition to the 

Rocky Mountains in the year 1842, and to Oregon and North 

California in the years 1843-44: Washington, D.C.

Fulton, R.J., and Armstrong, J.E., 1965, Day 2, w'Schultz, C.B., and 

Smith, H.T.U., eds., Guidebook field conference J, Pacific 

Northwest, 1965: International Association (Union) Quaternary Research Congress, 7th, p. 87-98.

Gairdner, Meridith, 1836, Letter: Edinburgh New Philosophical 

Journal, v. 20, p. 206.

Geschwind, Carl-Henry, and Rutherford, M.J., 1992, Cummingtonite and the evolution of the Mount St. Helens (Washington) 

magma system—An experimental study: Geology, v. 20, 

p. 1011-1014.
REFERENCES CITED 93

Goddard, E.N., Trask, P.O., DeFord, R.K., Rove, O.N., Singewald, 

J.T., Jr., and Overbeck, R.M., 1948, Rock-color chart: Washington, D.C., National Research Council.

Hammatt, H.H., 1976, Late Quaternary stratigraphy and archaeological chronology in the Lower Granite Reservoir, lower 

Snake River, Washington: Pullman, Washington State University, Ph.D. dissertation, 272 p.

Hoblitt, R.P., 1989, Day 3—The Kalama eruptive period, southwest 

and south flanks, in Field excursions to volcanic terranes in the 

western United States II—Cascades and intermountain west: 

New Mexico Bureau of Mines and Mineral Resources Memoir 

47, p. 65-69.

Hoblitt, R.P., Crandell, D.R., and Mullineaux, D.R., 1980, Mount 

St. Helens eruptive behavior during the past 1,500 years: Geology, v. 8, no. 11, p. 555-559.

Holmes, K.L., 1955, Mount St. Helens' recent eruptions: The Oregon Historical Quarterly, v. 56, p. 196-210.

Hopson, C.A., 1971, Eruptive sequence at Mount St. Helens, Washington: Geological Society of America Abstracts with Programs, v. 3, no. 2, p. 138.

Hopson, C.A., and Melson, W.A., 1990, Compositional trends and 

eruptive cycles at Mount St. Helens: Geoscience, Canada, v. 

17, no. 3, p. 131-141.

Hopson, C.A., Waters, A.C., Bender, V.R., and Rubin, Meyer, 

1962, The latest eruptions from Mount Rainier volcano: Journal of Geology, v. 70, no. 6, p. 635-647.

Hyde, J.H., 1975, Upper Pleistocene pyroclastic-flow deposits and 

lahars south of Mount St. Helens volcano, Washington: U.S. 

Geological Survey Bulletin 1383-B, 20 p.

Jackson, L.L., Brown, F.W., and Neil, ST., 1987, Major and minor 

elements requiring individual determination, classical whole 

rock analysis, in Baedecker, P.A., ed., Methods for geochemical analysis: U.S. Geological Survey Bulletin 1770-G, 23 p.

Lawrence, D.B., 1938, Trees on the march: Mazama, v. 20, no. 12, 

p. 49-54.

————1939, Continuing research on the flora of Mount St. Helens: 

Mazama, v. 21, no. 12, p. 49-54.

————1941, The "floating island" lava flow of Mount St. Helens: 

Mazama, v. 23, no 12, p. 56-60.

-1954, Diagrammatic history of the northeast slope of Mount

logical Society America Abstracts with Programs, v. 9, no. 7, 

p. 1098-1099.

-1978, Microstratigraphy, paleoecology, and tephrochronolSt. Helens, Washington: Mazama, v. 36, no. 13, p. 41—44.

Le Bas, M.J., Le Maitre, R.W., Streckeisen, A., and Zanettin, B., 

1986, A chemical classification of volcanic rocks based on the 

total alkali-silica diagram: Journal of Petrology, v. 27, pt. 3, 

p. 745-750.

Mack, C.B., 1980, Postglacial tephrochronology of the Indian 

Heaven area, south Cascade Range, Washington: Pullman, 

Washington State University, M.S. thesis, 75 p.

Majors, H.M., 1980, Three newly discovered accounts of activity 

on Mount St. Helens, 1898, 1903, and 1921: Northwest Discovery, v. 1, no. 1, p. 36-41.

Mathewes, R.W., and Westgate, J.A., 1980, Bridge River 

tephra—Revised distribution and significance for detecting old 

carbon errors in radiocarbon dates of limnic sediments in 

southern British Columbia: Canadian Journal of Earth Sciences, v. 17, p. 1454-1461.

Moody, U.L., 1977, Correlation of flood deposits containing Mt. St. 

Helens set S ashes and the stratigraphic position of Mt. St. 

Helens set J and Glacier Peak ashes, central Washington: Geoogy of the Lind Coulee site, central Washington: Pullman, 

Washington State University, Ph.D. thesis, 273 p. 

Mullineaux, D.R., 1964, Extensive Recent pumice lapilli and ash 

layers from Mount St. Helens volcano, southern Washington 

[abs]: Geological Society of America Special Paper 76, p. 285.

————1974, Pumice and other pyroclastic deposits in Mount Rainier National Park, Washington: U.S. Geological Survey Bulletin 1326, 83 p.

————1977, Volcanic hazards—Extent and severity of potential 

tephra hazard interpreted from layer Yn from Mt. St. Helens, 

Washington: Geological Society of America Abstracts with 

Programs, v. 9, no. 7, p. 1105.

————1986, Summary of pre-1980 tephra-fall deposits erupted

from Mount St. Helens, Washington State, U.S.A.: Bulletin of 

Volcanology, v. 48, no. 1, p. 17-26.

Mullineaux, D.R., and Crandell, D.R., 1962, Recent lahars from 

Mount St. Helens, Washington: Geological Society of America 

Bulletin, v. 73, no. 7, p. 855-869.

————1981, The eruptive history of Mount St. Helens, in Lipman, 

P.W., and Mullineaux, D.R., eds., The 1980 eruptions of 

Mount St. Helens, Washington: U.S. Geological Survey Professional Paper 1250, p.3-15.

Mullineaux, D.R., Hyde, J.H., and Rubin, Meyer, 1972, Preliminary 

assessment of upper Pleistocene and Holocene pumiceous 

tephra from Mount St. Helens, southern Washington: Geological Society America Abstracts with Programs, v. 4, no. 3, 

p. 204-205.

———1975, Widespread late glacial and postglacial tephra deposits from Mount St. Helens, Washington: U.S. Geological Survey Journal of Research, v. 3, no. 3, p. 329-335.

Mullineaux, D.R., Wilcox, R.E., Ebaugh, W.F., Fryxell, R., and 

Rubin, Meyer, 1978, Age of the last major scabland flood of the 

Columbia Plateau in eastern Washington: Quaternary 

Research, v. 10, p. 171-180.

Munsell Color Company, Inc., 1954, Soil color charts: Baltimore, 

Maryland.

Norgren, J.A., Borchardt, G.A., and Harward, M.E., 1970, Mt. St. 

Helens Y ash in northeastern Oregon and south-central Washington [abs.]: Northwest Science, v. 44, p. 66.

Okazaki, R., Smith, H.W., Gilkeson, R.A., and Franklin, J., 1972, 

Correlation of West Blacktail Ash with pyroclastic layer T 

from the 1800 A.D. eruption of Mount St. Helens: Northwest 

Science, v. 46, p. 77-89.

Pallister, J.S., and Hoblitt, R.P., 1985, Magma mixing at Mount St. 

Helens [abs.]: Eos, v. 66, p. 1111.

Pallister, J.S., Hoblitt, R.P., Crandell, D.R., and Mullineaux, D.R., 

1992, Mount St. Helens a decade after the 1980 eruptions—Magmatic models, chemical cycles, and a revised hazards assessment: Bulletin of Volcanology, v. 54, p. 126-146.

Peterson, D.W., 1988, Volcanic hazards and public response: Journal of Geophysical Research, v. 93, p. 4161^170.

Peterson, D.W., and Tilling, R.I., 1993, Interactions between scientists, civil authorities and the public at hazardous volcanoes, in 

Kilburn, C.R.J., and Luongo, G., eds, Active lavas: London, 

UCL Press, p. 339-365.

Porter, S.C., Pierce, K.L., and Hamilton, T.D., 1983, Late Wisconsin mountain glaciation in the western United States, in Porter,
94 PRE-1980 TEPHRA-FALL DEPOSITS, MOUNT ST. HELENS

S.C., ed., Late Quaternary environments of the United States, 

v. 1: Minneapolis, University of Minnesota Press, p. 71-111.

Riehle, J.R., Mann, D.H., Peteet, D.M., Engstrom, D.R., Brew, 

D.A., and Meyer, C.E., 1992, The Mount Edgecumbe tephra 

deposits, a marker horizon in southeastern Alaska near the 

Pleistocene-Holocene boundary: Quaternary Research, v. 37, 

p. 183-202.

Sarna-Wojcicki, A.M., Champion, D.E., and Davis J.O., 1983, 

Holocene volcanism in the conterminous United States and the 

role of silicic volcanic ash layers in correlation of latest-Pleistocene and Holocene deposits, in Wright, H.E., Jr., ed., LateQuaternary environments of the United States, v. 2, The 

Holocene: Minneapolis, University of Minnesota Press, 

p. 52-77.

Sarna-Wojcicki, A.M., Shipley, S., Waitt, R.B., Dzurisin, D., and 

Wood, S.H., 1981, Areal distribution, thickness, mass, volume, 

and grain size of air-fall ash from the six major eruptions of 

1980, in Lipman, P.W., and Mullineaux, D.R., eds., The 1980 

eruptions of Mount St. Helens, Washington: U.S. Geological 

Survey Professional Paper 1250, p. 577-600.

Scott, K.M., 1988, Origins, behavior, and sedimentology of lahars 

and lahar-runout flows in the Toutle-Cowlitz river system: U.S. 

Geological Survey Professional Paper 1447-A, 74 p.

Scott, W.E., 1989, Volcanic and related hazards, in Volcanic hazards—Short course in geology, v. 1: Washington, D.C., American Geophysical Union, p. 9-50.

Shapiro, Leonard, 1975, Rapid analysis of silicate, carbonate, and 

phosphate rocks—Revised edition: U.S. Geological Survey 

Bulletin 1401,76 p.

Shapiro, Leonard, and Bannock, W.W., 1962, Rapid analysis of silicate, carbonate and phosphate rocks: U.S. Geological Survey 

Bulletin 1144-A, 56 p.

Shore, J.H., Tatum, E.L., and Vollmer, W.M., 1987, Mental health 

consequences of the Mount St. Helens disaster, in Keller, 

S.A.C., ed., Mount St. Helens—Five years later: Cheney, Eastern Washington University Press, p. 435-438.

Smith, D.R., and Leeman, W.P., 1982, Mineralogy and phase chemistry of Mount St. Helens tephra sets W and Y as keys to their 

identification: Quaternary Research 17, 211-227.

Smith, H.W., Okazaki, Rose, and Aarstad, John, 1968, Recent volcanic ash in soils of northeastern Washington and northern Idaho: Northwest Science, v. 42, p. 150-160.

Smith, H.W., Okazaki, R., and Knowles, C.E., 1977, Electron 

microprobe analysis of glass shards from tephra assigned to set 

W, Mount St. Helens, Washington: Quaternary Research, v. 7, 

no. 2, p. 207-217.

Sorem, R.K., 1982, Volcanic ash clusters—Tephra rafts and scavengers: Journal of Volcanology and Geothermal Research, v. 

13, p. 63-71.

Sorenson, J.H., and Mileti, D.S., 1987, Public warning needs: U.S. 

Geological Survey Open-File Report 87-269, p. 9-75.

Stradling, D.F., and Kiver, E.P., 1986, The significance of volcanic 

ash as a stratigraphic marker for the late Pleistocene in northeastern Washington, in Keller, S.A.C., ed., Mount St. 

Helens—Five years later: Cheney, Eastern Washington University Press, p. 120-126.

Suess, H.E., 1970, Bristlecone-pine calibration of the radiocarbon 

time scale 5200 B.C. to the present [with discussion], in Olsson, I.W., ed., Radiocarbon variations and absolute chronology: Nobel Symposium, 12th, Uppsala, 1969, Proceedings; New

York, John Wiley, p. 303-311. 

Taggart, J.E., Jr., Lindsay, J.R., Scott, B.A., Vivit, D.V., Bartel,

A.J., and Stewart, K.C., 1987, Analysis of geologic materials

by wavelength-dispersive X-ray fluorescence spectrometry, in

Baedecker, P.A., ed., Methods for geochemical analysis: U.S.

Geological Survey Bulletin 1770-E, 19 p. 

Thorarinsson, S., 1944, Tephrochronological studies in Iceland:

Geografiska Annaler, v. 26, p. 1-217. 

————1954, The tephra-fall from Hekla on March 29,1947, in The

eruption of Hekla 1947-1948: Icelandic Scientific Society, v.

2, no. 3, 68 p. 

-1974, The terms tephra and tephrochronology, in Westgate,

J.A., and Gold, C.M., eds., World bibliography and index of 

Quaternary tephrochronology: Calgary, University of Alberta, 

528 p.

Verhoogen, Jean, 1937, Mount St. Helens, a recent Cascade volcano: California University, Department of Geological Sciences 

Bulletin, v. 24, no. 9, p. 263-302.

Vogel, J.S., Cornell, W., Nelson, D.E., and Southon, J.R., 1990, 

Vesuvius/Avellino, one possible source of seventeenth century 

BC climatic disturbances: Nature, v. 344, p. 534—537.

Waitt, R.B., 1980, About forty last-glacial Lake Missoula 

jokulhlaups through southern Washington: Journal of Geology, 

v. 88, p. 653-679.

Waitt, R.B., and Dzurisin, Daniel, 1981, Proximal air-fall deposits 

from the May 18 eruption—Stratigraphy and field sedimentology, in Lipman, P.W., and Mullineaux, D.R., eds., The 1980 

eruptions of Mount St. Helens, Washington: U.S. Geological 

Survey Professional Paper 1250, p. 601-616.

Westgate, J.A., 1975, Holocene tephra layers in southwestern Canada—New units and revision of age estimates: Geological 

Society of America Abstracts with Programs, v. 7, no. 6, p. 879.

————1977, Identification and significance of late-Holocene 

tephra from Otter Creek, southern British Columbia, and localities in west-central Alberta: Canadian Journal of Earth Sciences, v. 14, p. 2593-2600.

Westgate, J.A., and Fulton, R.J., 1975, Tephrostratigraphy of 

Olympia interglacial sediments in south-central British Columbia, Canada: Canadian Journal of Earth Sciences, v. 12, no. 3, 

p. 489-502.

Westgate, J.A., Smith, D.G.W., and Nichols, H., 1970, Late Quaternary pyroclastic layers in the Edmonton area, Alberta: Symposium on Pedology and Quaternary Research, Edmonton, 1969, 

Proceedings; Calgary, Alberta University Press, p. 179-187.

Wilcox, R.E., 1959a, Use of the spindle stage for determination of 

principal indices of refraction of crystal fragments: American 

Mineralogist, v. 44, no. 11-12, p. 1272-1293.

————1959b, Some effects of recent volcanic ash falls, with especial reference to Alaska: U.S. Geological Survey Bulletin 

1028-N, p. 409-476.

————1962, Cherkasov's "focal screening" for determination of 

refractive index by the immersion method: International 

Microscopy Symposium, 1960, Proceedings; Chicago, 

McCrone Associates, p. 160-165.

————1965, Volcanic-ash chronology, in Wright, H.E., Jr., and 

Frey, D.G., eds., The Quaternary of the United States: Princeton, N.J., Princeton University Press, p. 807-816.
REFERENCES CITED 95

————1983, Refractive index determination using the central focal 

masking technique with dispersion colors: American Mineralogist, v. 68, p. 1226-1236.

Yamaguchi, D.K., 1982, New tree ring dates for recent eruptions of 

Mount St. Helens: American Quaternary Association Biennial 

Conference, 7th, Program and Abstracts, p. 183.

————1983, New tree-ring dates for recent eruptions of Mount St. 

Helens, short paper: Quaternary Research, v. 20, p. 246-250.

————1985, Tree-ring evidence for a two-year interval between 

recent prehistoric explosive eruptions of Mount St. Helens: 

Geology, v. 13, p. 554-557.

————1993, Forest history, Mount St. Helens: National Geographic Research and Exploration, v. 9, p. 294-325.

Yamaguchi, O.K., and Hoblitt, R.P., 1986, Tree-ring dating of 

Mount St. Helens flowage deposits emplaced in the valley of 

the South Fork Toutle River, Washington, during the past 350 

years [abs.], in Keller, S.A.C., ed., Mount St. Helens—Five 

years later: Cheney, Eastern Washington University Press, 

p. 441.

Yamaguchi, O.K., Hoblitt, R.P., and Lawrence, D.B., 1990, A new 

tree-ring date for the "floating island" lava flow, Mount St. 

Helens, Washington: Bulletin of Volcanology, v. 52, 

p. 545-550.

Published in the Central Region, Denver, Colorado 

Manuscript approved for publication November 30, 1994 

Edited by Judith Stoeser 

Graphics by Patricia Wilber 

Photocomposition by Shelly A. Fields

APPENDIX
APPENDIX 

CHEMICAL ANALYSES OF PUMICE FROM DESCRIBED TEPHRA DEPOS

In weight percent. Leaders (--) indicate not determined. Analysts and methods are as follows. 1966 samples: rapid rock analysis as described in Shapiro and Bannock (1

absorption analysis; analysts Paul Elmore, S. Potts, J. Glenn, H. Smith, D. Taylor, G. Chloe, and L. Artis. 1975 and 1977 samples: rapid rock analysis as described in sect

(1975); analysts Lowell Artis (1975) and Z.A. Hamlin (1977). 1981 samples: X-ray spectroscopy as described in Taggart and others (1987), iron determined as FeO as de

analysts J.S. Wahlberg, J. Taggart, and J. Baker. H2O for selected samples determined as described in Jackson and others (1987). LOI is loss on ignition.

Tephra 

unit

T

T (upper)

T (lower)

Xm

Xs

Xb

Xb

Wd

We

We

Wb

Wa

Wn (upper)

Wn (lower)

Wn

D

D

Bu

Bu

Bi

Bi

Bd

Bo

Bo

Bh

Bh

Py

Pu

Ps

Pm

Pm

Date of 

Field No. analysis

8-28-61-1

8-14-76-8

8-14-76-6

8-18-71-7

8-18-71-4

8-5-74-8

8-11-71-10

8-11-71-9

8-24-76-2

8-24-76-1

8-11-71-7

8-11-71-6

8-14-76-5

8-14-76-1

8-15-64-2

8-24-74-1

8-22-74-1

9-1-77-1

9-25-73-9

8-10-70-3

8-10-70-3

8-28-74-5

9-25-73-3

9-25-73-30

9-25-73-33

9-25-73-33

8-16-70-5

8-30-77-5

8-22-70-1 1

8-19-71-11

8-27-71-5

1966

1981

1981

1981

1981

1981

1975

1981

1977

1981

1981

1981

1981

1981

1966

1981

1975

1981

1977

1981

1977

1981

1981

1977

1977

1981

1981

1981

1981

1981

1977

Si02

63.6

62.9

62.8

54.3

59.3

58.4

57.6

64.7

65.0

65.5

65.3

65.4

65.2

65.4

67.5

67.1

68.5

44.9

47.3

61.3

64.2

61.9

53.2

54.8

57.6

56.7

60.7

62.5

62.7

62.2

64.6

A12 03

17.0

17.5

17.7

17.2

16.3

17.4

17.3

16.9

17.2

16.6

16.9

16.8

16.9

16.8

16.2

16.3

15.8

18.4

17.5

17.7

15.4

17.2

17.8

16.7

16.9

17.3

18.3

17.9

17.8

17.6

16.8

Fe2O3/FeO

1.8 3.2

4.80

4.78

7.85

6.98

6.44

2.3 4.2

4.09

1.0 2.4

3.94

3.90

3.92

3.81

3.86

1.3 2.1

3.49

1.3 2.0

10.5

3.7 5.6

4.77

1.4 2.8

4.85

9.83

3.9 5.0

3.6 4.8

8.64

4.79

4.35

4.66

4.65

1.4 2.6

MgO

2.2

2.03

1.85

5.42

3.77

3.56

4.1

1. 42

1.6

1.31

1.27

1.29

1.27

1.29

0.99

1.01

0.87

6.21

4.5

1.71

1.4

1.77

3.36

2.7

2.0

2.18

1.99

1.74

2.02

1.94

1.8

CaO

4.8

5.12

4.95

7.37

5.57

6.36

6.6

4.15

3.7

3.98

3.92

3.93

3.92

3.83

3.6

3.30

3.1

7.78

8.0

4.42

3.0

4.45

6.58

5.5

4.9

5.53

4.92

4.90

4.94

4.92

4.0

Na2O

4.5

4.48

4.52

4.00

4.09

4.33

4.5

4.67

4.4

4.55

4.67

4.70

4.65

4.64

4.8

4.76

5.0

3.12

3.5

4.23

4.2

4.21

3.68

3.9

4.4

4.25

4.19

4.44

4.42

4.27

4.3

K2O

1.3

1.30

1.28

1.13

1.36

1.32

1.1

1.49

1.4

1.54

1.53

1.52

1.52

1.55

1.6

1.81

1.8

1.07

1.3

.41

.7

.40

0.90

.1

.4

.24

.19

.29

.20

1.20

1.5

H?O

(+) (-) TiO2

0.58 0.06 0.68

0.61

0.61

1.29

1.07

1.10

0.40 0.12 1.2

0.53

1.1 0.71 0.47

0.50

0.48

0.49

0.47

0.48

1.0 0.14 0.44

0.39

0.83 0.17 0.42

2.15

2.0 2.3 2.3

0.65

2.5 0.92 0.62

0.67

1.56

1.6 1.3 1.6

0.94 1.1 1.5

1.52

0.64

0.60

0.61

0.61

1.4 0.30 0.51

P2 05

0.16

0.14

0.17

0.33

0.25

0.29

0.40

0.15

0.19

0.14

0.14

0.14

0.15

0.15

0.12

0.14

0.19

0.61

0.59

0.16

0.19

0.14

0.25

0.34

0.35

0.31

0.22

0.21

0.18

0.18

0.19

M
Ye

Yn

Yn

Yb

Jg

Jy

Js

So

Sg

Mm

Cy

Cb

8-24-76-5

9-3-77-10

8-27-61-14

9-3-77-13

8-14-70-2

7-20-72-4

7-10-72-9

8-24-77-1

8-24-76-7

8-12-71-8

8-12-71-15

9-7-79-4

1981

1981

1966

1981

1981

1981

1981

1981

1981

1981

1981

1981

63.5

63.2

63.4

63.6

54.4

60.5

59.7

58.8

60.5

57.9

57.9

63.5

17.4

17.5

17.2

17.4

14.2

18.3

19.1

19.0

18.5

18.9

20.0

17.5

4.11

4.01

1.6 2.3

4.14

6.33

4.52

4.70

4.47

4.22

5.17

4.25

4.15

1.46

1.46

1.3

1.44

3.15

1.82

1.91

1.58

1.49

2.14

1.57

1.56

4.43

4.37

4.1

4.53

5.66

4.47

4.58

4.21

4.15

4.84

4.30

4.36

4.43

4.38

4.3

4.41

3.93

4.21

4.32

4.06

4.29

4.03

3.98

4.26

1.26

1.25

1.2 2.3

1.24

0.96

1.14

1.02

1.08

1.14

1.03

0.98

1.21

0.51

0.50

0.76 0.51

0.52

0.90

0.64

0.65

0.59

0.56

0.71

0.57

0.56

0.17

0.17

0.17

0.15

0.26

0.15

0.12

0.20

0.15

0.21

0.25

0.12