Observations of the Eruptions of 

July 22 and August 7,1980, at 

Mount St. Helens, Washington

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1335
COVER Mount St. Helens as viewed from the north on July 22, 1980, showing the formation of a 

pyroclastic density flow. Photograph by Harry X. Glicken.
OBSERVATIONS OF THE ERUPTIONS

OF JULY 22 AND AUGUST 7, 1980,

AT MOUNT ST. HELENS, WASHINGTON
FRONTISPIECE Mount St. Helens as viewed from the north at about 1906 PDT, during the second of the three eruptive pulses of July 22,1980. 

The vertical-eruption column is derived directly from the vent, whereas the ash cloud rising through the atmospheric clouds in the 

foreground is derived from a pyroclastic density flow. Tephra can be seen falling from the left side of the vertical column. Mount Hood, 

Oreg., is visible in the background. Photograph by James W. Vallance.


Observations of the Eruptions of 

July 22 and August 7, 1980, at 

Mount St. Helens, Washington

By RICHARD P. HOBLITT

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1335

A description and discussion of pyroclastic density flows 

and other eruptive phenomena

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1986
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Hoblitt, Richard P.

Observations of the eruptions of July 22 and August 7, 1980, at Mount St. Helens, Washington

(Geological Survey Professional Paper; 1335)

Bibliography: p.

Supt. of Docs. No.: 119.16:1335

1. Saint Helens, Mount (Wash.) Eruption, 1980.

I. Title. II. Series.

QE523.S23H63 1985 551.2T0979784 84-600141

For sale by the Superintendent of Documents,

U.S. Government Printing Office

Washington, D.C. 20402
CONTENTS

Page Page

Abstract ........................................... 1 Discussion Continued

Introduction ........................................ 1 Speculations concerning a relief-valve mechanism ........ 32

Acknowledgments ................................... 2 Features associated with the front of the pyroclastic denTerminology ........................................ 2 sity flow formed by the third eruptive pulse of July 22,

Crater geometry ..................................... 3 1980 ........................................ 34

Eruptions of July 22, 1980 ............................. 4 Temperatures of the pyroclastic-flow deposits of July

Eruption of August 7,1980 ............................. 15 22, 1980 ..................................... 35

Emplacement data for pyroclastic flows of July 22 and A possible genetic relationship between pyroclastic flows

August 7, 1980 ................................... 20 and pyroclastic surges during the eruption of August

Discussion .......................................... 20 7, 1980 ...................................... 35

The eruptions of July 22 and August 7, 1980, in compari- References cited ..................................... 38

son with other eruptions ........................ 20 Appendix. Determination of the velocity of the pyroclastic

The eruptions of July 22 and August 7, 1980, in terms of density flow ..................................... 42

column-collapse models ......................... 21

ILLUSTRATIONS

Page

FRONTISPIECE. Mount St. Helens as viewed from the north at about 1906 PDT, during the second eruptive pulse of July 22,1980.

FIGURE 1. Index map of the Mount St. Helens region .............................................................. 2

2. Topographic map of the area around the crater, dome, and north flank of Mount St. Helens, from photographs

taken on July 1,1980 ........................................................................... 3

3-10. Photographs showing:

3. Amphitheater as viewed from the north at about 1800 PDT on July 22, 1980, between first and second

eruptive pulses ......................................................................... 5

4. Mount St. Helens as viewed from the north, sequence showing about the first 30 s of the second eruptive

pulse of July 22,1980 .................................................................... 6

5. Mount St. Helens as viewed from the northwest at about 1826:17 PDT on July 22, 1980, about 1 min after

the beginning of the second eruptive pulse ................................................... 12

6. Mount St. Helens as viewed from the northwest at about 1826:30 PDT on July 22,1980 ................... 13

7. Mount St. Helens as viewed from the west-southwest, second eruptive pulse between 1830 and 1831 PDT on

July 22, 1980 .......................................................................... 14

8. Amphitheater as viewed from the north at about 1859 PDT on July 22,1980, moderate level of activity about

1 min before start of third eruptive pulse ..................................................... 16

9. Amphitheater as viewed from the north on July 22, 1980, sequence of the early part of the third eruptive

pulse ................................................................................. 17

10. Amphitheater as viewed from the north at 1916:48 PDT on July 22, 1980, vertical column with helical

form ................................................................................. 21

11. Topographic maps of area around the vent of Mount St. Helens, from photographs taken on July 1 and July 31,1980 ... 22

12. Photographs showing Mount St. Helens as viewed from Coldwater Peak on August 7, 1980, sequence of the early part

of the first eruptive pulse ........................................................................ 23

13. Graph showing velocity of the flow front as a function of distance from the vent for the flow observed on August 7,

1980 ........................................................................................ 29

14. Photograph showing Mount St. Helens as viewed from the north on August 15, 1980, and path of pyroclastic density

flow of August 7,1980 .......................................................................... 30

15. Topographic map of area around the path of the flow of August 7, 1980, from photographs taken on September 6,

1980 ........................................................................................ 31

16-19. Diagrams showing:

16. Hypothetical relief-valve mechanism ........................................................... 33

17. Two alternative solutions for the flow field in the host medium of a density flow ......................... 35

18. Mechanism that may have operated during the flow observed on August 7,1980 ......................... 37

19. Procedure used to extract flow-front positions from photographs ..................................... 42

20. Diagrammatic topographic map illustrating construction of average vertical profile through flow path ............... 43

21. Diagrammatic vertical profile through average flow path ................................................... 44

VII
VIII CONTENTS

TABLE

TABLE 1. Emplacement data for pyroclastic flows of July 22 and August 7, 1980, compared to previously reported emplacement

data for small and large pyroclastic flows ............................................................ 20
OBSERVATIONS OF THE ERUPTIONS OF JULY 22 AND AUGUST 7, 1980, 

AT MOUNT ST. HELENS, WASHINGTON

By RICHARD P. HOBLITT

ABSTRACT

The explosive eruptions of July 22 and August 7, 1980, at Mount 

St. Helens, Wash., both included multiple eruptive pulses. The beginnings of three of the pulses two on July 22 and one on August 

7 were witnessed and photographed. Each of these three began with 

a fountain of gases and pyroclasts that collapsed around the vent and 

generated a pyroclastic density flow. Significant vertical-eruption 

columns developed only after the density flows were generated. This 

behavior is attributable to either an increase in the gas content of the 

eruption jet or a decrease in vent radius with time. An increase in the 

gas content may have occurred as the vent was cleared (by expulsion 

of a plug of pyroclasts) or as the eruption began to tap deeper, gasrich magma after first expelling the upper, gas-depleted part of the 

magma body. An effective decrease of the vent radius with time may 

have occurred as the eruption originated from progressively deeper 

levels in the vent. All of these processes vent clearing; tapping of 

deeper, gas-rich magma; and effective decrease in vent 

radius probably operated to some extent. A "relief-valve" 

mechanism is proposed here to account for the occurrence of multiple 

eruptive pulses. This mechanism requires that the conduit above the 

magma body be filled with a bed of pyroclasts, and that the vesiculation rate in the magma body be inadequate to sustain continuous 

eruption. During a repose interval, vesiculation of the magma body 

would cause gas to flow upward through the bed of pyroclasts. If the 

rate at which the magma produced gas exceeded the rate at which gas 

escaped to the atmosphere, the vertical pressure difference across the 

bed of pyroclastic debris would increase, as would the gas-flow rate. 

Eventually a gas-flow rate would be achieved that would suddenly 

diminish the ability of the bed to maintain a pressure difference between the magma body and the atmosphere. The bed of pyroclasts 

would then be expelled (that is, the relief valve would open) and an 

eruption would commence. During the eruption, gas would be lost 

faster than it could be replaced by vesiculation, so the gas-flow rate in 

the conduit would decrease. Eventually the gas-flow rate would 

decrease to a value that would be inadequate to expel pyroclasts, so 

the conduit would again become choked with pyroclasts (that is, the 

relief valve would close). Another period of repose would commence. 

The eruption/repose sequence would be repeated until gas-production 

rates were inadequate to reopen the valve, either because the depth of 

the pyroclast bed had become too great, the volatile content of the 

magma had become too low, or the magma had been expended.

A timed sequence of photographs of a pyroclastic density flow on 

August 7 indicates that, in general, the velocity of the flow front was 

determined by the underlying topography. Observations and details 

of the velocity/topography relationship suggest that both pyroclastic 

flows and pyroclastic surges formed. The following mechanism is consistent with the data During initial fountain collapse and when the

flow passed over steep, irregular terrain, a highly inflated suspension 

of gases and pyroclasts formed. In this suspension, the pyroclasts 

underwent rapid differential settling according to size and density; a 

relatively low-concentration, fine-grained upper phase formed over a 

relatively high-concentration coarse-grained phase. The low-particleconcentration phase (the pyroclastic surge) was subject to lower internal friction than the basal high-concentration phase (the pyroclastic 

flow), and so accelerated away from it. The surge advanced until it 

had deposited so much of its solid fraction that its net density became 

less than that of the ambient air. At this point it rose convectively off 

the ground, quickly decelerated, and was overtaken by the 

pyroclastic flow.

The behavior of the flow of August 7 suggests that a pyroclastic 

density flow probably expands through the ingestion of air wherever 

it passes over surfaces whose relief is a significant fraction of the flow 

thickness. Thus, a pyroclastic flow may spawn one or more pyroclast 

surges at locations remote from the source volcano. The ingestion of 

air by a pyroclastic surge would increase the time that particles would 

be held in suspension and, thus, extend the lifetime and length of the 

pyroclastic surge.

INTRODUCTION

The reawakening of Mount St. Helens (fig. 1) offered 

an unusual opportunity to document the rarely observed, short-lived phenomena that occur during explosive eruptions. An early explosive phase of activity 

was defined by the magmatic eruptions of May 18, May 

25, June 12, July 22, August 7, and October 16-18, 

1980. Of these, the eruptions of July 22 and August 7 

were, at least with regard to flowage phenomena, the 

most closely observed and photographically best documented.1 This report presents some observations and 

photographs of eruption phenomena that occurred on 

July 22 and August 7, and suggests mechanisms to explain them. The emphasis is on the pulsating nature of 

the eruptions, the timing of pyroclastic density flow formation relative to vertical-eruption column development, and the generation and motion of pyroclastic

1These two eruptions were also monitored by radar. For details of column heights and ash 

content deduced by this method see Harris and others (1980, 1981).

1
OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

50 100 150 KILOMETERS 

_J_______I

FIGURE 1. Index map of the Mount St. Helens region.

flows and surges. Also considered are the emplacement 

temperatures of pyroclastic-flow deposits, and phenomena probably related to the displacement of air by an advancing pyroclastic density flow.

ACKNOWLEDGMENTS

The U.S. Forest Service staff at Gifford Pinchot 

National Forest helped provide communications and 

logistic support for this study. Their efforts and those of 

the University of Washington/U.S. Geological Survey 

seismic-monitoring team are sincerely appreciated. 

Michael Montgomery skillfully piloted our helicopter, 

sometimes in demanding circumstances, during the 

eruptions of both July 22 and August 7. I am indebted 

to Janis Markman for efficiently carrying out the duties 

of helicopter dispatcher at all hours of the day and 

night. Many of the photographs presented in this 

report, or referred to in its preparation, were taken by

others. I wish to thank J. W. Vallance, C. D. Miller, 

H. X. Glicken, M. P. Doukas, L. A. McBroome, T. J. 

Bornhorst, Terry Leighley, P. W. Lipman, and R. D. 

Denslinger for making their photographs available to 

me. One of the topographic maps used in this report was 

kindly prepared by D. E. Fair. The artwork for many of 

the illustrations was provided by Barbara M. Myers. 

Useful discussions and criticisms of the manuscript 

were provided by W. Z. Savage, D. R. Crandell, W. E. 

Scott, D. R. Mullineaux, R. V. Fisher, J. W. Vallance, 

and R. S. J. Sparks.

TERMINOLOGY

A bewildering variety of names has been applied to 

the types of flowage phenomena that are described in 

this report. The same is true of the resulting deposits. 

Here, the term pyroclastic flow is used to mean a hot, 

dry density flow of gases and volcanic rock debris whose
CRATER GEOMETRY

density is similar to the density of the resulting deposit. 

The deposit is termed a pyroclastic-flow deposit. The 

term pyroclastic surge is used here to mean a hot, dry 

density flow of gases and volcanic rock debris whose 

density is much less than that of the resulting deposit, 

which is called a pyroclastic-surge deposit. When used 

without a modifier, "flow" is used as a synonym for 

pyroclastic density flow; that is, as a nonspecific term 

for pyroclastic flow or pyroclastic surge.

CRATER GEOMETRY

The paroxysmal events of May 18, 1980 failure of 

the north side of the mountain, the resultant directed 

blast, and subsequent plinian eruption (Christiansen 

and Peterson, 1981) produced a deep crater breached 

to the north; this crater, U-shaped in plan, will be referred to as the amphitheater. A vent was situated at 

the center of the crater floor (fig. 2). Prior to July 22, a

FIGURE 2. Topographic map of the area around the crater, dome (shaded), and north flank of 

Mount St. Helens, prepared from photographs (USGS) taken on July 1, 1980. Contours and 

elevations in feet; hachures indicate areas of topographic lows; 1 ft = 0.3048 m.
OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

circularly symmetric dome, whose diameter was about 

325 m and whose maximum exposed thickness was 

about 45 m, lay centered over the vent (fig. 2). This mass 

was extruded after the explosive eruption of June 12 

(Moore and others, 1981). A low, semicircular rampart 

of ejecta spanned the crater breach north of the dome 

(fig. 2). The rampart was roughly concentric to, and 

about 250 m from, the dome center.

It is interesting to note that Mount Lamington, 

Papua New Guinea, a volcano whose 1951-52 eruptive 

sequence rather closely parallels the 1980-81 eruptions 

at Mount St. Helens, had a crater geometry similar to 

that described above, including a breach-spanning 

rampart, at a correlative stage in the eruptive activity 

(Taylor, 1958, p. 59).

ERUPTIONS OF JULY 22, 1980

On July 22, the beginning of the first of three eruptive 

pulses was reported at 1714 PDT by U.S. Forest Service 

observer Richard D. Denslinger from a fixed-wing aircraft (unpub. records of the Gifford Pinchot National 

Forest). Although U.S. Geological Survey field parties 

were working near Mount St. Helens at the time of the 

eruption, the north side of the mountain was largely 

obscured from ground observation by a cloud layer. I 

first observed and photographed the eruption through 

broken clouds from a helicopter to the west of the mountain at 1719 PDT. An unobstructed view just above the 

cloud tops at 1722 PDT gave no indication that a pyroclastic flow or surge had occurred. However, a set 

of seven untimed eruption photographs taken by 

Denslinger from the northeast indicates otherwise. 

Denslinger's sequence probably begins within 2 min of 

the onset of the eruption. This is several minutes before 

I had a clear view. In addition to a voluminous, verticaleruption plume, his photographs show ash clouds rising 

convectively from the amphitheater floor. This indicates that at least one density flow was produced by 

the first eruptive pulse. Because of obscuring atmospheric clouds, the northward limit of the ash clouds 

cannot be located accurately, but it certainly did not exceed 2 km from the vent. Thus, compared to the density 

flows produced by the succeeding eruptive pulses, this 

flow was small. Denslinger's photographs do not include the beginning of the eruption, so the timing of 

flow formation relative to the onset of eruption is uncertain. However, it is clear that the flow formed early and 

might have formed at the beginning of the eruption.

The vigor of the eruption column waned quickly 

(noticeably by 1719 PDT), and by 1732 the voluminous, 

light-gray, ash-laden clouds of the early eruption had 

been replaced by much less voluminous white steam

clouds. As the level of activity declined, visibility within 

the amphitheater improved, so at 1757 PDT we flew 

past the mouth of the amphitheater to observe the effects of the eruption. The floor of the amphitheater appeared to be covered with new pyroclastic-flow deposits. 

The northern part of the dome was still intact but, as 

evidenced by the source of the steam clouds, it had been 

disrupted somewhere between its center and southern 

margin (fig. 3). At 1808 PDT Denslinger reported that 

the center portion of the dome was missing2 (unpub. 

records of the Gifford Pinchot National Forest).

At 1814 PDT, it was necessary to leave the mountain 

to refuel the helicopter; consequently, I was not present 

for the start of the second eruptive pulse.

When the second eruptive pulse commenced at 1825 

PDT, visibility in the vicinity of the vent was good. The 

eruption was photographed until 1831 by James W. 

Vallance from a U.S. Forest Service fixed-wing aircraft. 

Vallance's photographs show that the pulse began with 

the appearance of a dark (ash-rich), colloform eruption 

cloud (fig. 44). Within 30 s the expanding cloud filled 

the entire amphitheater (figs. 4A-J). However, it 

showed little tendency to develop vertically; rather, colloform ash clouds rose convectively from a fountaining 

suspension of gases and pyroclasts (figs. 4C-J). Projections of this "pyroclastic fountain" followed trajectories 

with maximum heights of as much as (approximately) 

800 m above the vent. (A projection is indicated with an 

arrow on fig. 4H.) A growing pyroclastic density flow 

was apparently initiated and fed by the projections as 

they collided with the amphitheater floor. The flow 

front is discernible on figures 4C and 4D, but is best 

displayed on succeeding photographs of the sequence, 

after the front emerges from the shadow obscuring the 

amphitheater floor. (The flow front is indicated with a 

dashed line on fig. 4H.) The flow front was initially 

unembayed (or embayed on a fine scale) and roughly 

semicircular in plan; the leading portion was centered on 

the amphitheater floor (figs. 4D-F). When the flow front 

reached a point about 1 km north of the ejecta rampart, 

it became increasingly digitate with three well-defined 

lobes at the center flanked by two or three others that 

were less well formed (figs. 4G-J). About 30 s after the 

start of the eruption, the flow front passed from view 

beneath the atmospheric clouds. The mean velocity to 

this point about 1.2 km north of the vent was about 

40 m/s. Although the flow was now hidden from view, 

its progress was marked by convecting ash clouds that 

rose from it and penetrated the atmospheric cloud layer. 

About 1 min after the start of the pulse, a verticalAfter the eruptions of July 22, breadcrust bombs, some weighing more than 50 kg, were 

found about 1.5 km northwest of the vent. These were probably fragments of the central part 

of the dome that was explosively disrupted at the outset of the first eruptive pulse.
ERUPTIONS OF JULY 22, 1980

FIGURE 3. Amphitheater as viewed from the north at about 1800 PDT on July 22, 1980, between the first and second eruptive 

pulses. Northern part of the dome (arrow) is visible above rampart (dashed line). Mild steam clouds emanate from the center of 

the dome. Distance from dome center to base of crater wall is about 0.5 km. (Photograph by Michael P. Doukas.)

eruption column began to form rapidly. Figure 5 shows 

the eruption clouds as viewed from the northwest about 

1 min after the pulse began. The mean rise velocity of 

the cloud top during the first minute was about 60 m/s. 

By the time an additional 13 s had elapsed, the appearance of the eruption clouds had changed dramatically 

(fig. 6). The mean rise velocity of that part of the cloud 

directly above the vent during these 13 s was about 

130 m/s. On figure 6, the ash cloud rising from the pyroclastic flow and the vertical column can not be differentiated on the basis of color. However, within about 5 

min (fig. 7) the vertical column became distinctly 

darker, apparently because it carried a greater proportion of ash. This suggests that fragmental material was 

then being fed preferentially into the vertical column 

rather than being fed to the density flow.

When I returned to the north side of the mountain at 

1853 PDT, the ash cloud from the pyroclastic density 

flow had dissipated and the amphitheater was again 

clear. The eruption had waned but the level of activity 

was, both in terms of the ash content and emission rate 

of the plume, greater than that observed during the lull

between the first and second eruptive pulses. This 

moderate level of activity persisted until a third (final) 

eruptive pulse began.

A few minutes before 1900 PDT, we began an east-towest helicopter traverse about 4.5 km north of the amphitheater, which afforded a good view of the dome and 

vent (fig. 8). The surface of the dome, which appeared 

dark colored and rough after the first pulse, appeared 

light colored and smooth after the second pulse. Its 

general profile remained unchanged; evidently, the chief 

effect of the second pulse on the dome was to mantle its 

surface with a thin layer of ejecta.

At about 1900 PDT, the moderate ash emission gave 

way to a vertical to subvertical fountain of "spearhead" 

ash projections (Taylor, 1958, p. 32). These projections shaped like long, thin, inverted V's rose 

perhaps as much as 100 m above the crater floor before 

the ejecta that composed them fell back into the vent. 

The overall appearance of this activity was suggestive 

of a great (albeit overactivated) fluidized bed. The 

height of the fountain then began to increase noticeably. 

Within 10 s of the time that this increase first became
OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

FIGURE 4. Mount St. Helens as viewed from the north, sequence (44-J) showing about the first 30 s of the second eruptive pulse of 

July 22, 1980. Photographs 4A-I taken at about 3-s intervals beginning at 1825:20±5 PDT; photograph 4J taken about 6 s
ERUPTIONS OF JULY 22, 1980

after 47. Arrow (477) indicates projection from pyroclastic fountain. Dashed line (477) indicates front of pyroclastic density flow. 

(Photographs by James W. Vallance.)
OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

FIGURE 4. Continued
ERUPTIONS OF JULY 22, 1980

FIGURE 4. Continued
10 OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

FIGURE 4. Continued

apparent, the fountain rose to about 400 m above the 

crater floor and ejecta began to fall outside the vent. A 

small pyroclastic density flow immediately appeared as 

it began to spill over the rampart (fig. 9A). I recorded 

the first 9 s of flowage with a clock-equipped camera; 

subsequent photographs (taken by H. X. Glicken) were 

not timed. The photographs clearly show (figs. 9B-D) 

that the flow was derived from the fountain: it originated from areas where the trajectories of projections 

intersected the floor and walls of the amphitheater. 

Four seconds after the flow began (fig. 9£?), its leading 

point was about 150 m north of the ejecta rampart. In 

general, the flow was parabolic in plan, with a width of 

about 300 m at the ejecta rampart. In detail, the flow 

margins exhibited lobe-and-cleft structures (Simpson, 

1969; Alien, 1971a; Fisher, 1977); lobes ranged in width 

from about 10 m to a few tens of meters. As it advanced 

northward, the flow maintained a rough bilateral symmetry about the axis of the amphitheater. Nine seconds 

after the flow began, the front was about 250 m north of 

the rampart. The flow was no longer parabolic in plan as 

the leading part had become rather blunt (fig. 9C); it advanced along an east-west-trending front about 500 m 

wide. Within another five seconds3 the front had advanced to about 400 m north of the rampart, and a

distinct large-scale cleft had appeared at the center of 

the flow front (fig. 9D). This cleft divided the front more 

or less symmetrically into two lobes, each about 300 m 

across. Each of these, in turn, was divided into two 

poorly defined secondary lobes. These secondary lobes 

were somewhat better defined by the time the front was 

about 750 m north of the rampart (fig. 9E). Until this 

point, the flow had maintained a rough bilateral symmetry about the axis of the amphitheater. About 1 km 

north of the rampart, however, the flow became asymmetrical as the front advanced with shifting, irregular 

embayments (fig. 9F). At this point the flow, which had 

been expanding laterally as it advanced, had completely 

covered the amphitheater floor and had begun to move 

up the amphitheater walls. The flow reached a height of 

about 200 m above the amphitheater floor on the west 

wall and a somewhat lower height on the east wall. 

Perhaps the complex flow-front geometry was due to 

the influence of the walls on the flow.

Because of our flight path, our view of the flow front 

was obscured beyond this point by the atmospheric 

cloud layer that shielded the north flank of the

This is an estimate based on the assumption that flow velocities at the times of the untimed 

photographs were similar to those determined from the timed photographs.
ERUPTIONS OF JULY 22, 1980 11

FIGURE 4. Continued

mountain.

Within a few seconds of our last observation, Vallance 

began to take photographs from the U.S. Forest Service 

aircraft that was now about 11 km west-northwest of 

the crater at an altitude of 5 km. At this time the flow 

front was emerging from the mouth of the amphitheater, roughly 1.5 km north of the ejecta rampart. The 

front now consisted of four or five lobes similar to those 

observed during the second eruptive pulse. The multiple 

lobes coalesced into two lobes about 1.6 km from the 

rampart. Evidently this was caused by the underlying 

topography, because at this point the gently sloping 

floor of the amphitheater merges northward with two

north-trending channels. After advancing an additional 

few hundred meters, the front became obscured as it 

passed beneath the clouds.

As it moved through the amphitheater, the flow front 

had a distinct overhang (best displayed on figs. 9C-E). 

For the first 10 to 15 s, this overhang was less pronounced at the flow nose, where it was 10 m thick or 

less, than on the flow flanks, where it was as much as 

several tens of meters thick. This difference seemed to 

diminish with time. For about the first kilometer of 

travel, the front was in the shadow of the west wall of 

the amphitheater. The overhang, manifest on the photographs as a dark band at the flow front, seemed to dis-
12 OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

FIGURE 5. Mount St. Helens as viewed from the northwest at about 1826:17 PDT on July 22, 1980, about 1 min after the beginning of the second eruptive pulse. Highest point on the eruption cloud is about 3,000 m above highest visible point on the rim of 

the amphitheater. (Photograph by James W. Vallance.)

appear as the flow moved from the shadow into direct 

sunlight, apparently due to the change in lighting 

conditions.

About 10 s after the flow first appeared, diffuse lightcolored areas appeared on the amphitheater floor 

(fig. 9Q. One of these adjoined the west side of the flow 

front, the others were between the west margin of the 

flow and the adjacent wall of the amphitheater. Within 

the next 5 s, the light-colored areas became more extensive. Now, the flow front was immediately preceded by, 

and in contact with, a diffuse, light-colored zone that in 

places exhibited striae or rays normal to the front 

(fig. 9D). The diffuse zone was, in turn, preceded by and 

merged into a series of discontinuous bands that paralleled the flow front. These bands are discernible in fig. 

9D, but are best displayed in fig. 9E. The width of the 

region in which this phenomenon was most obvious was 

about 50 m, although isolated, faint bands could be seen 

within a few hundred meters of the flow front. In those 

areas where they were well developed, the width of the 

bands and the spacing between them was about 5-10 m.

The band thickness was probably no more than a few 

meters; the diffuse, light-colored zone along the flow 

front may have been somewhat thicker, particularly immediately adjacent to the front. Unfortunately, it was 

not possible to tell if the bands were moving or stationary. Once the flow front reached the mouth of the 

amphitheater and emerged from shadow into direct 

sunlight, the phenomenon was much less apparent.

A vertical-eruption column began to develop about 

1/2 to 1 min after the third pulse began, at a time when 

the density flow was already well under way (compare 

figs. 9E-G). By 1915 PDT, the region near the vent, 

which had been obscured since the pyroclastic fountain 

appeared, cleared again and the eruption column could 

be seen rotating in a clockwise direction as it rose 

(fig. 10). Such rotation apparently is not unique, as a 

photograph of a 1951 eruption of Mount Lamington 

shows a similar feature (Taylor, 1958, fig. 32).

The effect of the third pulse on the dome was not 

observed until the day after the eruption. Remnants of 

the dome could clearly be seen in the walls of the vent.
ERUPTIONS OF JULY 22, 1980 13

FIGURE 6. Mount St. Helens as viewed from the northwest, showing the second eruptive 

pulse at about 1826:30 PDT on July 22, 1980, about 13 s after the view shown in figure 

5. Note that (compare fig. 5) a vertical column is forming rapidly. Highest point on the 

eruption cloud is about 4,100 m above the highest visible point on the rim of the amphitheater. (Photograph by James W. Vallance.)

However, sufficient ejecta had accumulated around the 

vent to otherwise conceal the remnants. The topography of the vent area after the eruption of July 22 is 

shown in figure 11.

Two units are recognizable in the pyroclastic-flow 

deposits produced by the eruptions of July 22. (See 

Rowley and others, 1981; Kuntz and others, 1981, for

discussions of these deposits.) A lower, cream-colored 

unit is the more extensive of the two and apparently 

was emplaced by the pyroclastic flow of the second 

eruptive pulse, and an upper, salmon-colored unit is a 

product of the third pulse. Each unit consists of a 1- to 

2-m-thick sheet of rounded dacite pumice blocks in an 

ash matrix; a small fraction of the blocks are bread-
14 OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

FIGURE 7. Mount St. Helens as viewed from the west-southwest, showing the second eruptive pulse between 1830 and 1831 PDT 

on July 22, 1980. Note that the vertical column is distinctly darker than the ash cloud rising from the pyroclastic density flow 

that is moving to the north beneath the atmospheric cloud layer. (Photograph by James W. Vallance.)

crusted. Margins of these deposits are lobate and have 

distinct levees and noses that have less ash than their 

central portions.

Emplacement-temperature data for these units are 

summarized by Banks and Hoblitt (1981). The lower 

deposit generally yielded lower emplacement temperatures than did the upper deposit. The maximum temperatures measured in the lower deposit were generally 

less than 600 °C, whereas the maximum temperatures of 

the upper deposit exceeded 600 °C. A temperature in excess of 600 °C was measured in the lower deposit on only 

one occasion; this was about 1 hr after emplacement, at 

a site on the flow margin. The surface here consisted 

almost entirely of pumice cobbles, but at depths of a few 

to a few tens of centimeters the interstices were filled 

with ash. I recorded the maximum temperature about 

660 °C in the void space between the surface cobbles. 

Then, as the thermocouple probe was pushed down into 

the underlying ash matrix, the temperature began to 

fall (the magnitude of the decrease was not recorded). 

Apparently the ash was emplaced at a lower temperature than the pumice cobbles. Further evidence of an 

inhomogeneous emplacement-temperature distribution 

was obtained during the next few days when temperature probes were inserted into the deposits. Occasionally, a probe would be stopped by a large pumice block 

at some depth. The temperature of these blocks was 

greater than that of the finer grained debris that enclosed them.

Shortly after emplacement, the central portions of 

many pyroclastic-flow lobes exhibited a markedly fluid 

behavior. As a result, these areas were treacherous to 

walk on. It was common to sink 0.5 m or more into ash 

and pumice whose temperature exceeded 500 °C. The 

problem could generally be avoided by "patting" an 

area with one's boot before placing full weight on it. 

This caused gases to escape from the fluid mass at 

numerous points around the disturbed area. Within a 

few seconds the area would "set up" and support a person's full weight. (See Wilson and Head, 1981, for a 

discussion of flow rheology.) The fluid behavior of the 

deposits ceased, probably due to capillary phenomena,
ERUPTION OF AUGUST 7, 1980 15

after they had been wetted by rain. The levees and toes 

of the flow deposits were not fluid, even shortly after 

emplacement, presumably because of their high proportion of pumice blocks to ash matrix.

ERUPTION OF AUGUST 7, 1980

The onset of harmonic tremor at 1217 PDT on August 

7 suggested that an eruption was imminent (M alone and 

others, 1981). Observation from the ground was possible because of clear skies; consequently, we began an 

eruption watch from Coldwater Peak, a good vantage 

point about 11 km to the north of Mount St. Helens. 

However, visibility was hampered by windblown ash 

deflated from the amphitheater walls by low-altitude 

winds. Low-altitude winds blew to the northwest, 

whereas high-altitude winds blew to the northeast.

When we arrived at Coldwater Peak (at 1423 PDT), 

vent activity consisted of mild steam emissions. At 

1438 PDT a dark (presumably ash-rich) plume rose to a 

height of about 300 m above the vent. By 1447 PDT 

this plume had apparently dissipated. By 1454 a second 

dark plume was visible; this appeared to reach about the 

same height as the first plume. For the next ll/2 hours 

the plume height continued to fluctuate; the maximum 

plume height during this time was about 600 m above 

the vent (about level with the rim of the amphitheater).

The eruption began with the appearance of a white 

steam plume over the western part of the vent area at 

about 1623 PDT. The plume quickly grew into a vertical 

column; by 1626:12 the top of the column was about 

3,500 m above the vent (fig. 12A). As the column grew, 

its lower portion became progressively darker and 

presumably increasingly rich in ash. The profile of the 

east side of the column, clearly visible from the vantage 

point of Coldwater Peak (fig. 12A), was essentially vertical. Shortly before 1627 PDT, the lower part of the 

vertical profile was rapidly replaced by crescentic projections (fig. 12B) resembling those of the pyroclastic 

fountains of July 22. This change was accompanied by a 

pronounced, low-frequency rumbling noise. The projections traversed the distance between the vent and east 

amphitheater wall (about 600 m) in about 8 s. The projection velocity, about 75 m/s, may approximate that of 

the ejection velocity. The growth of the initial verticaleruption column stopped when the ash projections appeared. The ash cloud resulting from the initial vertical 

eruption became diffuse and began to drift northnorthwest.

Within 10 s of the first appearance of the projections, 

the front of a pyroclastic density flow could be vaguely 

discerned through the haze as a faint, light-colored band

that stretched along the amphitheater floor from wall to 

wall. At this time, the flow front was about 1 km north 

of the vent. I photographed the flow at intervals of a few 

seconds to a few tens of seconds with a clock-equipped 

camera until forward movement terminated; examples 

are shown in figures 12C-L. These photographs allow 

mean velocities to be determined4 at intervals along 

that part of the flow path over which the flow front is 

clearly visible (fig. 13). The front could first be clearly 

seen through the haze about 45 s after the ash projections appeared; at this time the front was about 1.3 km 

north of the center of the vent. The front advanced 

rapidly at first and then, about 1.7 km north of the vent 

center, seemed to stagnate (fig. 12C). About 1 min after 

the flow started, a vertical-eruption column that 

became much larger than the initial column began to 

develop over the vent.

The pyroclastic density flows of July 22 and August 7 

followed two paths down the north flank of the volcano. 

The leading part of the flow of August 7 advanced out of 

the west side of the amphitheater and entered the head 

of the western path. The ash cloud rising from this 

western flow then cast a shadow on portions of the 

eastern path. Because of this, the eastern flow was only 

barely visible except when it moved into direct sunlight; 

the eastern flow will not be considered further.

The path followed by the leading part of the western 

flow is shown in profile in figure 13, in an oblique aerial 

view in figure 14, and in plan view in figure 15. At 

altitudes between about 1,500 and 1,180 m, the path 

coincided with a channel eroded by previous pyroclastic 

flows; this channel consisted of a series of alternating 

benches and cliffs that resembled stairsteps. The upper 

part of the channel, between altitudes of about 1,500 

and 1,270 m, constituted the steepest segment of the 

path. At about 1,350 m, the channel changed course 

rather abruptly from due north to about 30° east of 

north. At 1,270 m, the mean slope of the steps decreased, and near 1,240 m the channel curved about 20° 

back toward the north. At 1,180 m, the mouth of the 

channel emptied onto the apex of the "pumice plain," a 

broad aggradational fan of pyroclastic-flow deposits.

Overall, the channel controlled the course of the flow; 

the nose followed it closely, whereas the body followed it 

generally. Once the nose passed a given point in the 

channel, the body of the flow expanded laterally beyond 

the confining channel walls. For example, 10 s after it 

began to descend the steep upper part of the channel, 

the flow extended as much as 100 m to either side (compare figs. I2E, F). At curves, the expanding body of the 

flow overrode the channel walls in a manner analogous

TTie procedure used to determine flow velocities is given in the appendix.
16 OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

FIGURE 8. Amphitheater as viewed from the north at about 1859 PDT on July 22, 1980, showing moderate level of activity about 

1 min before the start of the third eruptive pulse.

to a bobsled leaving a course at a sharp bend. The portion of the flow that overrode a curve then lost momentum and an ash cloud rose convectively from it. This 

behavior served to exaggerate the curves followed by 

the flow nose. As the flow front descended the steep 

parts of the path, its thickness appeared to decrease; at 

times it was about 10 m or perhaps less (fig. 12F).

Convectively rising ash clouds marked the progress of 

the flow; in general, the height and volume of the cloud 

rising above a given point on the path were directly 

related to the time elapsed since the front passed that 

point. The rate at which clouds of ash were produced 

varied along the path, thus the resulting aggregate 

cloud developed protuberances. Major protuberances 

emanated from along each of the two braced path segments shown in figures 14 and 15. The flow-front positions of figures 12Z) and 12E approximately delimit the 

path segment indicated by the lower brace. The 

delimiting photographs for the upper brace are shown in 

figures 12E and 127. Each protuberance rose above a 

segment of the flow path over which the flow front accelerated and then decelerated. Bends in the flow path 

also seem to have affected the rate of ash-cloud production because significant contributions to the second

major protuberance originated from the bends at altitudes of 1,350 and 1,240 m.

The character of the flow front changed shortly after 

it moved onto the gently sloping pumice plain; the convex, colloform ash clouds (figs. 12H-I) rose off of the 

ground (fig. 12J) and were succeeded by a thin, lightcolored wedge (figs. 12K-L). This change was synchronous with the velocity low that occurred on the 

pumice plain at about 1,150 m altitude (fig. 13, time interval 13). The leading part of the wedge was no more 

than a few meters thick. Following the change, the flow 

became digitate as distinct lobes advanced radially 

away from the apex of the aggradational fan. The advance then stopped about 6.5 min after the pyroclastic 

fountain initiated the flow. During this terminal phase, 

the rate of ash-cloud generation decreased markedly.

FIGURE 9 (facing page). Amphitheater as viewed from the north 

on July 22, 1980, sequence (9A-G) showing the formation of a 

pyroclastic density flow during the early part of the third eruptive 

pulse. A, 1900:59; B, 1901:03 PDT, timed photographs by the author; C, 1901:08 PDT, photography by Harry X. Glicken, time from 

comparison with a timed photograph that is not shown; D, E, F, 

G, untimed sequence taken within 30 s after C, by Harry X. Glicken.
ERUPTION OF AUGUST 7, 1980 17
18 OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

D

FIGURE 9. Continued
ERUPTION OF AUGUST 7, 1980 19

E

F

FIGURE 9. Continued
20 OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

G

FIGURE 9. Continued

About 10 min after the start of the eruption, a tephra 

curtain obscured the northwest to northeast sector of 

the mountain. Thus, although additional eruptive 

pulses occurred on August 7 (Harris and others, 1981; 

Malone and others, 1981), only the first was observed to 

emplace a pyroclastic flow. The distribution of the 

deposits of August 7 coincides with the observed 

geometry of the flow front at the time flowage stopped. 

Therefore, if any of the subsequent pulses produced 

flows and field evidence suggests that at least one of 

them did their deposits were less extensive than the 

deposit produced by the first pulse.

EMPLACEMENT DATA FOR

PYROCLASTIC FLOWS OF

JULY 22 AND AUGUST 7, 1980

Emplacement data for the pyroclastic flows of July 22 

and August 7 are compared with those of other pyroclastic flows (Sheridan, 1979) in table 1. This comparison indicates that the mobility of the Mount St. 

Helens flows approached that of much larger pyroclastic flows.

TABLE 1. Emplacement data for pyroclastic flows of July 22 and 

August 7, 1980, compared to previously reported emplacement 

data for small and large pyroclastic flows

[Leaders ( ), no data available]

Pyroclastic 

flow

Deposit Runout Vertical Fountain

slope (L) drop (H) height HIL

(degrees) (km) (km) (m)

July 22 

Second pulse 

Third pulse -

August 7 

Small flows 

Most mobile - 

Least mobile

Large flows1 Most mobile - 

Least mobile

1.5 

1.5

2.4 

25.

.7 

1.5

6.5 

5.9

15.0 

1.9

76. 

22.

1.7 

1.2

4.5 

1.4

800

400

3,000(?) 

500

'From Sheridan, 1979.

0.26 

.20

300-500 0.19-0.23

.30 

.74(7)

.06 

.20

DISCUSSION

THE ERUPTIONS OF JULY 22 AND AUGUST 7, 1980, 

IN COMPARISON WITH OTHER ERUPTIONS

The literature contains accounts of eruptions apparently similar in behavior to those described earlier in 

this report. However, most accounts are insufficiently
DISCUSSION 21

FIGURE 10. Amphitheater as viewed from the north at 1916:48 PDT on July 22, 1980, showing helical nature of vertical-eruption 

column that was visible once the ash cloud from the pyroclastic density flow shown on figure 9 dissipated.

detailed to make close comparisons. Perhaps the closest 

analog is given by Taylor (1958) in his description of 

"shallow pocket" explosions and resultant nuees 

ardentes that he witnessed during the 1951 eruptions of 

Mount Lamington, Papua New Guinea. Similar behavior was probably also exhibited by Mayon Volcano, 

Republic of the Philippines, in 1968 (Moore and Melson, 

1969).

Nairn and Self (1978) described in detail the periodic, 

cannon-like explosions that generated pyroclastic 

avalanches during the eruptions of Ngauruhoe Volcano, 

New Zealand, in 1975. These were discrete events that 

commenced with visible shock waves. Within a few 

tenths of a second after the appearance of the shock 

wave, a slug of compressed gases and fragmental material was ejected, and began to expand in all directions. 

These shallow cannon-like explosions were probably 

similar to the first eruptive pulse of July 22, which explosively disrupted a plug dome. However, the cannonlike behavior contrasts to the pyroclastic fountaining 

observed at the outset of the second and third pulses of 

July 22 and the first pulse of August 7. The fountains

were more sustained events that increased in intensity 

over a period of many seconds. Each can be likened to a 

vertically directed garden hose whose water pressure is 

gradually increased. An additional difference is that the 

Mount St. Helens fountains gave way to verticaleruption columns, whereas the Ngauruhoe explosions 

were separated by periods of quiescence.

THE ERUPTIONS OF JULY 22 AND AUGUST 7, 1980, 

IN TERMS OF COLUMN-COLLAPSE MODELS

In two observed cases on July 22, a pyroclastic fountain generated a pyroclastic density flow before a 

vertical-eruption column developed. The major vertical 

column observed on August 7 also developed after a 

pyroclastic fountain appeared, although the fountain 

was preceded by a minor vertical column.

It is useful to consider the fountains, flows, and 

subsequent vertical columns in terms of the columncollapse models of Sparks and Wilson (1976), Sparks 

and others (1978), and Wilson and others (1980). These
22 OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

FIGURE 11. Topographic maps of the area around the vent of Mount St. Helens. Topography in 

gray prepared from photographs (USGS) taken on July 1, 1980, before the July 22, 1980 eruptions; topography in red prepared from photographs (USGS) taken on July 31, 1980, after the 

July 22, 1980 eruptions. On the July 31 photographs, part of the crater floor is obscured by 

volcanic fumes; hence, contours in the obscured area (shown as red dashed lines) could only be 

located approximately. Map prepared by Diana E. Fair. Contour intervals 80 ft (gray) and 40 ft 

(red); 1 ft=0.3048 m.

models predict the combinations of vent radius and gas 

content that will cause a jet of erupted gases and 

pyroclasts to either rise convectively or undergo gravitational collapse. Column collapse occurs when the condition for convective rise is not met. This condition is 

that the jet must have, upon deceleration to a minimum 

upward velocity by drag forces, a density less than that 

of the ambient atmosphere. Column collapse results in 

the formation of pyroclastic density flows. The collapse

models assume steady-state conditions, a great depth of 

magma fragmentation, and an exit pressure of 1 bar. 

Initially, the eruptions of July 22 and August 7 did not 

meet the steady-state requirement, were probably derived from shallow depths, and probably had an exit 

pressure exceeding that of the atmosphere. Thus the 

models are initially inappropriate for those eruptions. 

However, the same principles of vent radius and gas 

content should determine whether collapse or convec-
DISCUSSION 23

FIGURE 12. Mount St. Helens as viewed from Coldwater Peak (about 11 km north of vent) on August 7, 1980, sequence (12/4-L) 

showing the early part of the first eruptive pulse: A, 1626:12; B, 1626:47; C, 1627:55; D, 1628:44; E, 1629:17; F, 1629:27; G, 

1630:02; H, 1630:18;/, 1630:33; J, 1630:53; K, 1631:22; L, 1632:15 PDT.
24 OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

FIGURE 12. Continued
DISCUSSION 25

FIGURE 12. Continued
26 OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

FIGURE 12. Continued
DISCUSSION 27

FIGURE 12. Continued
28 OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

FIGURE 12. Continued
DISCUSSION 29

2,500. 

2,400

2,300 -

-I 35

1,300 -

1,200 

1,100 -

1 2 78 9 10 

TIME INTERVAL

1213 14 15 16 1718

234 

DISTANCE FROM VENT (IN KILOMETERS)

FIGURE 13. Velocity of the flow front as a function of distance from the vent center for the flow observed on August 7, 1980. Each bar 

shows a mean velocity for a time interval determined from two sequential, timed photographs. The horizontal distance traversed during 

a time interval is shown by the bar width, and the terrain traversed during an interval is that portion of the topographic profile 

delimited by the bar or its extension. Shaded areas at the tops of bars are estimated velocity uncertainties. The two straight diagonal 

lines are "energy lines" (Hsu, 1975). An energy line is constructed by drawing a line between the terminus of a flow and the top of the 

pyroclastic fountain that produced the flow. The two energy lines shown correspond to two estimates of the fountain height that probably bracket the range of reasonable fountain-height estimates. According to the energy-line concept, the flow will accelerate if the slope 

of the topography is greater than that of the energy line, and the flow will come to rest where the energy line intersects the topographic 

surface. Topographic profile has fivefold vertical exaggeration.
30 OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

-X

FIGURE 14. Mount St. Helens as viewed from the north on August 15, 1980. Dashed line shows path followed by the front of the 

western pyroclastic density flow of August 7. Braces show segments of flow path from which major ash-cloud protuberances 

emanated.

tion occurs in shallow, high-pressure explosions if the 

vent-radius variable is redefined as an "effective vent 

radius" (R. S. J. Sparks, written commun., 1982). This 

effective vent radius is the radius achieved by the 

ejected gas/solid mixture upon expansion to a pressure 

of 1 bar.

The pyroclastic fountains could be considered as eruption jets whose effective vent radii and gas contents 

were such that convective rise was not possible. However, after the initial collapse, conditions must have 

changed because convective rise began to occur. According to Wilson and others (1980), a shift from column 

collapse to convective rise implies that either the vent 

radius (in this case, effective vent radius) decreased or 

that the gas content increased. Either or perhaps both 

changes may have occurred during the eruptions of July 

22 and August 7.

The observed change from column collapse to convective rise may have been due to a decrease in the vent 

radius. This may seem odd because the physical vent 

radius is likely to widen as an eruption proceeds. However, it is also likely that the explosion depth would increase with time, and so the large effective vent radius 

of the early, shallow explosions would be succeeded by

the small physical vent radius of the later, deeper explosions. In effect, the radius would decrease with time.

It is also possible that the observed behavior was due 

to an increase in gas content. At least two explanations 

for such an increase are possible.

1. The upper part of the magma column could have 

become depleted in gas between eruptions; thus, less 

gas would be available to drive the early part of the 

eruption than the later part. This possibility is consistent with the observation of low-level gas-streaming activity during the repose intervals between eruptions.

2. An initial, unusually high proportion of solids 

could result from "vent clearing" as fragmental material filling the upper part of the vent was suddenly 

expelled at the outset of an eruptive pulse. This alternative is supported by the appearance of spearhead projections over the vent immediately prior to the formation of pyroclastic fountains during the third eruptive 

pulse of July 22. The spearhead projections are clearly 

due to pressurized gases escaping through pyroclasts. 

This suggests that the conduit above the magma body 

may have been choked with pyroclasts at the beginning 

of the eruption. 

The shift from column collapse to convective rise was
DISCUSSION 31

122°12'30" 122°10'00"

46°] 2' 30"

FIGURE 15. Topographic map of the area around the crater and north flank of Mount St. 

Helens, prepared from photographs (USGS) taken on September 6, 1980. Dashed line 

shows path followed by the front of the western pyroclastic density flow of August 7. 

Braces show segments of flow path from which major ash-cloud protuberances emanated. 

Contours in feet; 1 ft = 0.3048 m.
32 OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

probably not due to any single cause. Rather, the three 

possibilities mentioned above decrease in vent radius, 

and increase in gas content from upper magma gas 

depletion and from vent clearing probably were all 

operative to some extent.

SPECULATIONS CONCERNING A 

RELIEF-VALVE MECHANISM

If the conduit above the magma body is choked with 

pyroclastic debris at the beginning of an eruption, that 

debris may act as a relief valve. Fluidization experiments show that as gas flows upward through a bed 

of poorly sorted particles, the system will change 

behavior as the flow rate is increased (C. J. N. Wilson, 

1980). At low flow rates the particles remain stationary. 

However, as the flow is increased, a rate will eventually 

be reached at which the drag force of the gas will be sufficient to support the buoyant weight of the smallest 

particles, and the bed will become partially fluidized. 

Additional increases will cause the bed to expand until 

the condition of maximum bed expansion is reached. 

Until this point, an increase in the flow rate is accompanied by an increase in the vertical pressure difference 

across the bed. However, as the flow rate is increased 

beyond the point of maximum bed expansion, the gas 

begins to segregate into bubbles and channels, and the 

pressure drop across the bed decreases sharply.

It is possible that an analogous process operates at 

the outset of eruptions if the conduit above the magma 

body is filled with pyroclastic debris. Vesiculation of the 

magma would cause gas to flow through this bed of 

debris and escape to the atmosphere (fig. 16A). If the 

rate at which the magma produced gas exceeded the 

rate at which gas escaped, the pressure difference across 

the bed would increase, as would the flow rate. Eventually the flow rate would increase to a value that would 

cause formation of gas bubbles and channels through 

the bed of pyroclasts (fig. 16B). This would cause a rapid 

decrease in the pressure difference across the bed, and 

the magma would experience a sudden decompression, 

as the ability of the debris bed to contain the accumulated gases became drastically reduced. The upper

part of the decompressed magma body would begin to 

fragment and suddenly liberate more gas, which would 

further increase the flow rate. When the rate of gas flow 

became great enough, it would begin to expel the ventfilling bed of pyroclasts (fig. 16Q. If this ejected mass of 

gases and pyroclasts had a relatively high particle 

concentration, and did not meet the requirements for 

convective rise, it would form a pyroclastic fountain. 

The fountaining gas/solid suspension would generate a 

pyroclastic density flow. After the vent was cleared, the 

relatively low particle concentration of the continuing 

eruption jet could meet the requirements for convective 

rise, and a vertical-eruption column would develop (fig. 

16D).

The eruption would be modulated into a series of 

pulses if the gas-loss rate during eruptions exceeded the 

rate at which gas is produced by Vesiculation. As the 

eruption tapped progressively deeper levels of the 

magma body, progressively less vesicular magma would 

be available to drive the eruption. The gas content of the 

eruption jet would therefore begin to decrease. Eventually the gas content would decline to a value that 

would be inadequate to eject the largest fragments from 

the vent. This process would continue, with progressively smaller fragments remaining in the vent, until the 

vent again became choked with pyroclasts (fig. 16A). In 

effect, the pyroclastic debris has acted as a relief valve. 

The eruption/repose sequence would continue until gasproduction rates had become inadequate to open the 

relief valve, either because the depth of the bed of pyroclasts had become too great, the volatile content of the 

magma had become too low, or the magma had been 

expended.

The eruptions of Mount St. Helens provide several 

lines of evidence that support the relief-valve 

hypothesis.

1. The relief-valve mechanism would modulate an 

eruption into a series of pulses separated by periods of 

relative quiescence. Multiple eruptive pulses occurred at 

Mount St. Helens on July 22, August 7, and October 

16-18, 1980.

2. Mild gas-streaming activity would be expected 

during periods of quiescence between eruptive pulses. 

This was observed on July 22.

FIGURE 16. Hypothetical relief-valve mechanism: A, Vent above magma body is choked with pyroclastic debris. Vesiculation in the upper part 

of the magma body releases gases, which flow up through debris. The vertical pressure difference across the debris, and the gas flow rate, 

increase as vesiculation proceeds. Debris remains stationary and volcano is in repose. B, Continuing vesiculation has increased flow rate to 

a value that causes formation of gas bubbles and channels through debris. Pressure difference across the debris bed decreases sharply. C, 

Decompression of the magma triggers fragmentation of the vesiculated magma Gases liberated by the fragmentation further increase the 

flow rate. This causes vent-filling debris to be expelled as a gas/solid mixture (pyroclastic fountain) whose density is greater than air. A 

pyroclastic density flow (not shown) is formed as an eruption begins. D, As the vent is cleared of the debris, the density of the eruption jet 

decreases until the condition for convective rise is met. As the eruption taps progressively less vesicular magma, the gas content decreases 

until debris is not expelled as quickly as it is produced. This leads to the reestablishment of the condition shown in step A.
DISCUSSION

Pyroclastic debris

Vesiculated magma

Gas bubble

Gas channel

B

Pyroclastic 

fountain

Fragmentation of 

top of magma body

Convectively rising

vertical-column

eruption
34 OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

3. Partial fluidization of the vent-filling debris 

would immediately precede an eruptive pulse. This was 

observed just prior to the third eruptive pulse of 

July 22.

4. Pyroclastic density flows would be generated 

before a convectively driven vertical-eruption column 

appeared. This occurred at least twice on July 22, and at 

least once on August 7.

5. If a pyroclastic density flow is formed by the 

relief-valve mechanism, the volume of the resultant flow 

deposit should approximate the volume of the ventfilling debris from which it was derived. This relationship, along with the vent diameter, makes it possible to 

calculate the approximate depth of the hypothetical 

debris bed that forms the relief valve. Small depth 

values would favor the relief-valve mechanism, whereas 

large depth values would favor some other mechanism. 

The volumes of pyroclastic flows generated by the second and third eruptive pulses of July 22 were each 

about 0.003 km3 (Rowley and others, 1981, p. 497). A 

reasonable vent-radius estimate is about 50 m. Thus, a 

bed depth of only about 400 m would be necessary to 

produce the observed flow-deposit volume.

The relief-valve mechanism could not have operated 

at the outset of the first eruptive pulse of July 22 

because the vent was blocked by a dome. Observations 

indicate that a density flow formed early during this 

pulse. However, this flow was quite small as compared 

to those produced by the second and third eruptive 

pulses of July 22 and the first pulse of August 7, when 

the vent was not blocked by a dome. One interpretation 

is that the difference in the size of the flows is due to the 

eruptive mechanism. That is, the small density flow was 

formed from fragmental debris produced by the explosive disruption of the dome, whereas the succeeding 

larger flows were products of the relief-valve mechanism. The relief valve would presumably eject a larger 

volume of fragmental material than would be produced 

by a shallow dome explosion.

The relief-valve mechanism is apt to operate only if 

the maximum vertical pressure difference that can 

develop across the debris column is a significant fraction of the pressure that can develop at the upper surface of the magma body. Apparently, no method currently exists to calculate the maximum pressure drop 

that can be maintained across a bed of poorly sorted 

pyroclasts. However, fluidization experiments on ignimbrite fines (C. J. N. Wilson, 1980) suggest that a value of 

about 0.1 bar per meter of bed depth is reasonable. 

Thus, rather thin beds of vent debris are capable of containing substantial gas pressures. For example, thicknesses of a few hundreds of meters would contain pressure drops of a few tens of bars. Such values are of the 

same order as those predicted (Wilson and others, 1980)

for the upper surface of a rhyolitic magma with a low 

volatile content (about 1 percent).

The closest antecedent to the proposed relief-valve 

mechanism is that suggested by Eichelberger and 

Westrich (1981). They suggested pulsating eruptive 

behavior to explain an upward increase in the proportion of dense fragments in pyroclastic deposits of 

rhyolitic magma of low volatile content. Further, they 

proposed that the inferred pulsating behavior was a 

result of eruption rates that exceeded vesiculation rates.

FEATURES ASSOCIATED WITH THE FRONT OF 

THE PYROCLASTIC DENSITY FLOW FORMED BY 

THE THIRD ERUPTIVE PULSE OF JULY 22, 1980

Some of the most intriguing phenomena associated 

with the pyroclastic density flows were the light-colored 

features that appeared ahead of the flow front during 

the third eruptive pulse of July 22. These features included a diffuse zone immediately ahead of the flow 

front and a series of discontinuous bands that preceded 

and merged with the diffuse zone (best displayed on 

fig. 9E). The mechanism responsible for the bands is not 

known, but they could have been caused by displacement of air by the advancing flow. Presumably, the 

wind generated by the displacement eroded and transported loose ash from previously emplaced deposits.

The diffuse zone immediately ahead of the flow front 

locally exhibited striae or rays normal to the front 

(fig. 9D). The diffuse zone may be a manifestation of 

"separated flow." Separated flow has been postulated 

by Alien (197la) as one of two possible solutions for the 

flow field in the host medium ahead of a density flow 

with an overhanging head. This type of separated flow 

(fig. 11 A) requires that air below the flow-front 

overhang be pushed ahead of the front. Such separated 

flows have been aptly termed "rollers" (Alien, 1971b). I 

suggest that the roller was made visible by the ash 

caught up in it. The striae or rays that seem to radiate 

from the front of the pyroclastic flow are envisaged as 

streams of ash mobilized by the basal part of the roller, 

whose flow direction parallels that of the front of the 

pyroclastic density flow. Alien (197la) concluded that 

observations of density flows favored an alternative 

("mass flow") solution, which involves a lobe-and-cleft 

head structure (cleft: fig. 11B; lobe: fig. 17C). In this 

solution, air below the flow-front overhang is channeled 

into the clefts and overridden by the flow. In the 

example that I witnessed, the diffuse zone ahead of the 

flow front, which may have been caused by a roller, appeared even though the flow front exhibited lobe-andcleft structures. Both solutions apparently operated 

concurrently.
DISCUSSION 35

FIGURE 17. Two alternative solutions for the flow field in the host medium of a density flow. Head of density flow is 

shaded. The velocity of the host medium is assigned a value equal but opposite to that of the head, so the head is 

stationary. Streamlines are drawn relative to the stationary head. A, Solution involving separated flow. Bt C, 

Solution for flow on axes of clefts and lobes, respectively. (Modified from Alien, 1971a)

Whatever processes were actually responsible for the 

phenomena that preceded the front of the density flow, 

probably too little debris was mobilized to have produced a recognizable deposit. However, these processes 

might produce significant deposits if they were associated with large-scale flows.

TEMPERATURES OF THE PYROCLASTIC-FLOW 

DEPOSITS OF JULY 22, 1980

The temperature data obtained from the pyroclasticflow deposits of July 22 provide some evidence of the 

heat-transfer processes that operated on the pyroclasts. The most reasonable interpretation is that the 

debris mixed with and transferred heat to air. The 

emplacement-temperature inhomogeneities reflect the 

fact that (1) small particles, because of their greater 

surface-area/volume ratio, transfer heat more efficiently 

than do large fragments, and (2) insufficient time had 

elapsed prior to the measurements for temperature 

equilibration to occur. The greatest lateral 

emplacement-temperature gradient within a given 

deposit was observed near the vent (Banks and Hoblitt, 

1981). This suggests that most cooling occurred near 

the vent, probably as fountaining ejecta mixed with air 

to form a turbulent suspension of gases and pyroclasts. 

This may explain why the pyroclastic-flow deposit produced by the third eruptive pulse was, in general, 

emplaced at a higher temperature than that of the

second pulse. The second pulse produced a somewhat 

higher fountain than the third (table 1), and thus had a 

greater opportunity to mix with air. The pyroclasticflow deposit of the second pulse also appeared to be 

finer grained than that of the third. Thus, the debris of 

the second pulse had a greater aggregate heat-transfer 

efficiency than that of the third. The importance of air 

cooling in pyroclastic flows was first suggested by 

Smith (1960) as an explanation for the discrepancies 

between the welding patterns observed in ash flows 

and those expected from theory. The emplacementtemperature predictions of Sparks and others (1978) are, 

at least qualitatively, in accord with the temperature 

data and interpretations presented here.

A POSSIBLE GENETIC RELATIONSHIP 

BETWEEN PYROCLASTIC FLOWS AND 

PYROCLASTIC SURGES DURING THE 

ERUPTION OF AUGUST 7, 1980

The sequence of timed photographs obtained on 

August 7 yielded an almost complete velocity history 

for one flow.5 This history apparently is more detailed 

than any velocity record of a similar event. In general,

In this discussion it is important to remember that the term "flow" without a modifier is 

used as a synonym for "density flow"; that is, as a nonspecific term for pyroclastic flow or 

pyroclastic surge.
36 OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

the flow-front velocity is directly related to topographic 

slope (fig. 13). Two cycles of acceleration and deceleration occurred over the two steep parts of the flow path; 

each acceleration and deceleration corresponds to an increase and decrease, respectively, of the slope. These 

velocity data can be compared to the predictions of the 

energy-line concept of Hsu (1975), which has been summarized by Sheridan (1980, p. 400) as follows:

"A density flow initiated at some elevation will move as potential 

energy is converted to kinetic energy minus friction. The energy 

line is the slope along which the frictional loss is balanced by conversion of potential to kinetic energy. * * * * The slope of the 

energy line is calculated as the arc tan of the loss in height (H) 

divided by run-out distance (L)."

A flow will accelerate (decelerate) as it travels over terrain whose slope is greater than (less than) that of the 

energy line; it will come to rest where the topographic 

surface intersects the energy line.

For the flow of August 7, the selection of a value for H 

is somewhat arbitrary because of an uncertainty in the 

height of the pyroclastic fountain above the vent. I have 

made two fountain-height estimates that probably 

bracket the range of reasonable values (table 1). The 

energy lines that correspond to the two estimates have 

been plotted in figure 13. Overall, the acceleration 

predictions derived from comparison of the energy line 

and topographic slopes are fairly accurate, particularly 

from the energy line based on the lower estimate of 

fountain height. However, the low value of time interval 

13 on the velocity profile is anomalous. Interval 13 

marks the end of the second acceleration/deceleration 

cycle and a change in flow behavior. The deceleration is 

not surprising, but the subsequent acceleration is unexpected because it is not related to an increase in slope. A 

possible explanation for the velocity change lies in the 

concomitant change in the appearance of the flow front. 

The voluminous, billowing, colloform clouds that 

marked the flow front ceased to advance and began to 

rise. Concurrently, the front became thin, wedge 

shaped, and digitate, and was followed by only small 

ash clouds. I suggest that these changes in velocity and 

appearance occurred when a pyroclastic surge derived 

from and preceding the pyroclastic flow was overtaken 

by the pyroclastic flow.

As a pyroclastic flow travels down a steep slope, its 

gas content can increase. This could partly be due to an

increase in the trituration rate of pumice, and a consequent increase in the release rate of juvenile gases. 

Another important source is air incorporated by the 

moving mass. During the flow of August 7, air was 

overridden and incorporated as the flow plunged off 

cliffs and splashed over obstacles. Analyses of gases 

from rootless fumaroles in the pyroclastic-flow deposit 

of June 12, 1980, showed air to be the dominant component (Casadevall and Greenland, 1981, p. 222). Air probably was also an important component of the flow of 

August 7 because that flow followed the same path as 

the flow of June 12. Whatever the mechanism, an increase in the gas/solid ratio almost certainly occurred as 

the August 7 flow descended the steeper portions of the 

flow path. It is inferred that the resultant expansion 

would produce an inflated, turbulent suspension of hot 

gases and pyroclasts, and that the pyroclasts would 

settle differentially according to their size and density. 

A high-particle-concentration, coarse-grained phase 

moving along the ground surface would grade vertically 

into a fine-grained suspension of lower concentration. 

The uppermost part of this hot suspension, whose net 

density would be less than that of the ambient air, 

would rise convectively. The remainder of the lowparticle-concentration part of the flow would be less 

subject to energy losses through internal friction than 

the basal flow, and so would accelerate away from the 

basal flow. Thus, the hot, low-concentration suspension a pyroclastic surge would separate from and 

"surge" ahead of the high-concentration phase a 

pyroclastic flow. The hot surge cloud would lose mass 

through deposition until it became less dense than the 

ambient air. At this point it would rise, decelerate, and 

be overtaken by its parent pyroclastic flow.

This hypothetical sequence (fig. 18) inflation, segregation, separation, and differential acceleration may 

have been repeated three times in the flow of August 7. 

The first sequence was initiated by the pyroclastic fountain as it generated a turbulent, inflated mixture of air 

and ejecta around the vent. This segregated into both a 

pyroclastic flow and a pyroclastic surge. The surge initially accelerated and moved in front of the pyroclastic 

flow. Then, on the gently sloping floor of the amphitheater, the surge decelerated and was overtaken by the 

pyroclastic flow. Unfortunately, the detailed velocity 

data shown on figure 13 begin (time interval 1) at the 

end of this sequence, as the pyroclastic flow overtook

FIGURE 18. Mechanism that may have operated during the flow observed on August 7, 1980. A, Highly inflated suspension generated by collapse of pyroclastic fountain or by passage of pyroclastic flow over steep, irregular terrain. B, Segregation of the suspension into high- and 

low-concentration phases. C, Low-concentration phase (pyroclastic surge) that has separated from high-concentration phase (pyroclastic 

flow) deposits surge beds until it becomes less dense than air, rises, and decelerates. D, Rising remnant of pyroclastic-surge cloud is overtaken by pyroclastic flow.
DISCUSSION 37

\

B
OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

the stagnating surge. The details of the early velocity 

history could not be determined because of poor viewing 

conditions. However, the mean velocity from the beginning of fountain formation to the time that the flow 

front first became clearly visible is 30 m/s. The velocity 

was low when the flow front became clearly visible, so 

the velocity shortly after fountain collapse must have 

been greater than 30 m/s, which is about the highest 

velocity determined from the clearly visible flow front 

(fig. 13). Thus the highest velocity was probably 

achieved soon after fountain collapse; this was followed 

by a deceleration and a subsequent acceleration of the 

flow front.

A second surge was probably initiated as the pyroclastic flow crossed an abrupt break in slope at the 

1,650-m level. A third and final pyroclastic surge was initiated as the pyroclastic flow descended the stair 

steps the steepest, most irregular part of the flow 

path. In contrast to the first surge, which was derived 

directly from the pyroclastic fountain, the second and 

third surges were derived from the pyroclastic flow. The 

acceleration/deceleration cycles of the second and third 

surges are shown on figure 13.

The hypothetical inflation, segregation, separation, 

and differential acceleration sequence is consistent with 

the behavior of the ash-cloud protuberances that were 

described previously in this report. The two major protuberances originated from the parts of the path that 

were traversed by the postulated surge clouds. The 

parts of the path in which only small ash clouds were 

produced were traversed solely by the pyroclastic flow. 

The ash cloud produced by the collapse of the pyroclastic fountain could also be considered as a protuberance; 

this merged upward into the vertical-eruption column 

that developed after the pyroclastic fountain.

Little stratigraphic evidence is available to test the 

proposed pyroclastic-flow/pyroclastic-surge relationship. The flow of August 7 produced both pyroclasticsurge and pyroclastic-flow deposits; the distribution of 

the pyroclastic-flow deposits is known in detail, but little is known of the distribution of the pyroclastic-surge 

deposits. I identified surge deposits from the eruption of 

August 7 at the apex of the pumice plain. The presence 

of pyroclastic-surge deposits at this location is consistent with my interpretation of the velocity data.

The possible genetic pyroclastic-flow/pyroclas ticsurge relationship during the eruption of August 7 is 

consistent with the following models and observational

data.

1. Similar segregation and separation models have

previously been described by Fisher, 1976, 1979, 

p. 311-313; Sparks and others, 1978, p. 1735; Wohletz 

and Sheridan, 1979; Fisher and others, 1980, p. 475; 

and Fisher and Heiken, 1982, p. 365-367.

2. Accounts of explosive stratovolcanic eruptions 

indicate that flow separation is a common phenomenon 

(for example, Anderson and Flett, 1903, p. 511; MacDonald and Alcarez, 1956, p. 174; Moore and Melson, 

1969, p. 616-617).

3. On the basis of stratigraphic relationships, 

Sparks and others (1973) suggested that a pyroclastic 

flow is often accompanied and preceded by a pyroclastic 

surge.

4. A momentary pause in the rate of flow advance, 

which follows an initial rapid advance, has been observed at the outset of other eruptions (Ferret, 1937, 

p. 91-92). Such a pause was probably caused by the 

deceleration of a pyroclastic surge, which was subsequently overtaken by a cogenetic pyroclastic flow. Both 

the pyroclastic flow and pyroclastic surge are probably 

generated simultaneously by a pyroclastic fountain.

The ingestion of air and resultant dilution of a flow 

probably occurs wherever a flow passes over surfaces 

whose relief is a significant fraction of the flow 

thickness. This may occur on the flanks of the source 

volcano or on the surrounding terrain. A pyroclastic 

flow may thus spawn one or more pyroclastic surges at 

locations remote from the volcano. This dilution probably occurs in pyroclastic surges as well as pyroclastic 

flows. The ingestion of air by a pyroclastic surge would 

increase the time that particles would be held in suspension and, thus, extend the lifetime and length of the 

pyroclastic surge.

REFERENCES CITED

Alien, J. R. L., 197la, Mixing at turidity current heads, and its 

geological implications: Journal of Sedimentary Petrology, v. 41, 

no. 1, p. 97-113.

___1971b, Transverse erosional marks of mud and rock Their 

physical basis and geological significance: Sedimentary Geology, 

v. 5, p. 167-385.

Anderson, Tempest, and Flett, J. S., 1903, Report on the eruption of 

the Soufriere in St. Vincent in 1902 and on a visit to Montagne 

Pelee in Martinique: Philosophical Transactions of the Royal 

Society of London, ser. A, v. 200, p. 353-553.

Banks, N. G., and Hoblitt, R. P., 1981, Summary of temperature 

studies of 1980 deposits, 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. 295-313.

CasadevaU, T. J., and Greenland, L. P., 1981, The chemistry of gases 

emanating from Mount St. Helens, May-September 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. 221-226.

Christiansen, R. L., and Peterson, D. W., 1981, Chronology of the 

1980 eruptive activity, 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. 17-30.

Eichelberger, J. C., and Westrich, H. R., 1981, Magmatic volatiles in 

explosive rhyolitic eruptions: Geophysical Research Letter, v. 8, 

no. 7, p. 757-760.
REFERENCES CITED 39

Fisher, R. V., 1976, The mobility of hot pyroclastic flows [abs.]:

Geological Society of America Abstracts with Programs, v. 8, no.

5, p. 586. 

___1977, Erosion by volcanic base-surge density currents Ushaped channels: Geological Society of America Bulletin, v. 88,

p. 1287-1297. 

.1979, Models for pyroclastic surges and pyroclastic flows:

Journal of Volcanology and Geothermal Research, v. 6, 

p. 305-318.

Fisher, R. V., and Heiken, Grant, 1982, Mt. Pelee, Martinique May 

18 and 20, 1902, pyroclastic flows and surges: Journal of Volcanology and Geothermal Research, v. 13, p. 339-371.

Fisher, R. V., Smith, A. L., and Roobol, M. J., 1980, Destruction of St. 

Pierre, Martinique, by ash-cloud surges, May 8 and 20, 1902: 

Geology, v. 8, p. 472-476.

Harris, D. M., Roe, Robert, and Rose, W. I., Jr., 1980, Radar observations of the July 22 and Aug. 7, 1980 eruptions of Mount St. 

Helens: EOS, American Geophysical Union Transactions, v. 61, 

no. 46, p. 1137.

Harris, D. M., Rose, W. I., Jr., Roe, Robert, and Thompson, M. R., 

1981, Radar observations of ash eruptions, 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. 323-333.

Hsu, K. J., 1975, Catastrophic debris streams (sturzstroms) 

generated by rockfalls: Geological Society America Bulletin, v. 

86, p. 129-140.

Kuntz, M. A., Rowley, P. D., MacLeod, N. S., Reynolds, R. L., 

Mcbroome, L. A., Kaplan, A. M., and Lidke, D. J., 1981, Petrography and particle-size distribution of pyroclastic flow, ash-cloud, 

and surge deposits, 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. 525-539.

MacDonald, G. A., and Alcarez, Arturo, 1956, Nuees ardentes of the 

1948-1953 eruption of Hibok-Hibok: Bulletin Volcanologique, v. 

18, p. 169-178.

Malone, S. D., Endo, E. T., Weaver, C. S., and Ramey, J. W., 1981, 

Seismic monitoring for eruption prediction, 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. 803-813.

Moore, J. G., Lipman, P. W., Swanson, D. A., and Alpha, T. R., 1981, 

Growth of lava domes in the crater, June 1980-January 1981, 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. 541-556.

Moore, J. G., and Melson, W. G., 1969, Nuees ardentes of the 1968 

eruption of Mayon volcano, Philippines: Bulletin Volcanologique, 

v. 33, p. 600-620.

Nairn, I. A., and Self, Stephen, 1978, Explosive eruptions and 

pyroclastic avalanches from Ngauruhoe in February 1975: Journal of Volcanology and Geothermal Research, v. 3, p. 39-60.

Ferret, F. A., 1937, The eruption of Mt. Pelee 1929-1932: Carnegie Institution of Washington Pub. no. 458, 126 p.

Rowley, P. D., Kuntz, M. A., and MacLeod, N. S., 1981, Pyroclastic 

flow deposits, 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. 489-512.

Sheridan, M. F., 1979, Emplacement of pyroclastic flows A review: 

Geological Society of America Special Paper 180, p. 125-136.

___1980, Pyroclastic block flow from the September, 1976, eruption of La Soufriere volcano, Guadeloupe: Bulletin Volcanologique, v. 43-2, p. 397-402.

Simpson, J. E., 1969, A comparison between laboratory and atmospheric density currents: Quarterly Journal of the Royal 

Meteorological Society, v. 95, p. 758-765.

Smith, R. L., 1960, Ash-flows: Geological Society of America Bulletin, 

v. 71, p. 795-842.

Sparks, R. S. J., Self, Stephen, and Walker, G. P. L., 1973, Products of 

ignimbrite eruptions: Geology, v. 1, no. 3, p. 115-118.

Sparks, R. S. J., and Wilson, Lionel, 1976, A model for the formation 

of ignimbrite by gravitational column collapse: Geological Society of London Journal, v. 132, p. 441-451.

Sparks, R. S. J., Wilson, Lionel, and Hulme, G., 1978, Theoretical 

modeling of the generation, movement, and emplacement of 

pyroclastic flows by column collapse: Journal of Geophysical 

Research, v. 83, no. B4, p. 1727-1739.

Taylor, G. A., 1958, The 1951 eruption of Mount Lamington, Papua: 

Australia Bureau of Mineral Resources, Geology, and Geophysics 

Bulletin 38, 117 p.

Volk, William, 1958, Applied statistics for engineers: New York, 

McGraw-HUl, 354 p.

Wilson, C. J. N., 1980, The role of fluidization in the emplacement of 

pyroclastic flows An experimental approach: Journal of 

Volcanology and Geothermal Research, v. 8, p. 231-249.

Wilson, Lionel, and Head, J. W., 1981, Morphology and rheology of 

pyroclastic flows and their deposits, and guidelines for future 

observations, 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. 513-524.

Wilson, Lionel, Sparks, R. S. J., and Walker, G. P. L., 1980, Explosive 

volcanic eruptions - IV. The control of magma properties and 

conduit geometry on eruption column behavior: Geophysical 

Journal of the Royal Astronomical Society, v. 63, p. 117-148.

Wohletz, K. H., and Sheridan, M. F., 1979, A model for pyroclastic 

surge: Geological Society of America Special Paper 180, 

p. 177-194.

APPENDIX
42 OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

APPENDIX. DETERMINATION OF THE

VELOCITY OF THE PYROCLASTIC

DENSITY FLOW

Because the time intervals between photographs are 

known to within ±0.5 s, flow-velocity determination 

was reduced to the problem of establishing the distance 

covered by the flow front between successive exposures. 

This is not a trivial problem. Except for a few photographs, insufficient landmarks are available to accurately locate the position of the front on a topographic 

map.

The photographs are 35-mm slides that were taken 

with a hand-held, clock-equipped camera The camera 

was approximately horizontal and the aiming point was 

approximately the same for all photographs. The first 

slide was projected, and the flow front and salient 

reference features (such as the silhouette of the mountain) were traced onto a sheet of paper. On projections of 

succeeding photographs, the paper was moved so that

the traced reference features were superimposed, then 

the new position of the flow front was traced (as in 

fig. 19).

The leading point of the flow front on each tracing 

was identified; together these points constituted the 

flow path. By inspection, this path was located on the 

available topographic map that best represented the 

topography at the time of eruption (fig. 15; shown 

diagrammatically on fig. 20).

It was then necessary to prepare a topographic profile 

that, ideally, would include the flow path and the point 

from which the photographs were taken (Coldwater 

Peak). Because these points do not all lie on a single 

vertical plane, a vertical plane was chosen that included 

the Coldwater Peak camera station and that passed 

through an "average" flow path chosen by inspection 

(fig. 20). The topographic contours intersected by the 

flow path were orthographically projected onto this 

plane, then the profile (fig. 21) was constructed.

Lines were then drawn from each contour point on the 

profile to the camera site. These lines represent the

FIGURE 19. Procedure used to extract flow-front positions from photographs: a, tracings of flow margins as they appeared on successive 

photographs; b, flow path determined by drawing a line through successive flow-front positions; c, topographic contours as veiwed 

from the camera station; and d, vertical-projection plans as viewed from the camera station.
APPENDIX 43

paths of light rays to the camera. The next step was to 

draw a vertical line to obtain its intersection points with 

the rays. Any vertical line will do. Parallel lines with the 

same spacings as the intersection points were placed on 

a transparent overlay. These lines show the spacing between specific topographic contours traversed by the 

flow front as seen from the camera station. The overlay 

was then optically projected onto the composite tracing 

of the flow fronts (fig. 19), and the scale was optically 

adjusted until two or more flow-front positions of 

known altitude matched the altitudes given by the 

overlay points. The altitudes of all other flow-front tracings were then read from the overlay. These altitudes 

could now be found on the flow path (fig. 20), and the 

distances between successive flow-front positions could 

be determined.

The accuracy of the results was not significantly affected by the selection of flow path or projection plane 

because the altitudes of interest were similar to the 

altitude of the Coldwater Peak camera site. Thus, as 

seen from the camera station, topographic contour lines 

traversed by the flow were essentially straight lines. 

Varying the flow path or the position of the verticalprojection plane would not significantly change the altitude estimates. The uncertainty in the distance between 

successive flow-front positions is estimated to be 

±50 m (based on the uncertainty in locating flow-front 

positions on the few photographs with adequate landmarks). The resulting uncertainty in the velocity was 

calculated according to the procedure of Volk (1958, 

p. 141-145).

Camera station

FIGURE 20. Diagrammatic topographic map illustrating construction of average vertical profile through flow path: a, flow path; 

b, vertical-projection plane; c, topographic contour lines.
44 OBSERVATIONS OF ERUPTIONS, JULY 22 AND AUGUST 7, 1980, MOUNT ST. HELENS, WASHINGTON

Camera station

FIGURE 21. Diagrammatic vertical profile through average flow path: a, topographic contour points used to construct the profile; b, vertical 

line drawn through rays connecting camera station and topographic contour points; c, intersection points of vertical line and rays (used 

to construct d); d, overlay showing the spacing between topographic contours traversed by the flow front as seen from the camera 

station.

*U.S. GOVERNMENT PRINTING OFFICE: 1986-6?6-OiJ-? :260l6