U.S. Department of the Interior

U.S. Geological Survey

Magma Migration and Resupply

During the 1974 Summit Eruptions

of Kïlauea Volcano, Hawai‘i

Professional Paper 1613
Telephoto view to the south across Kïlauea Caldera from the USGS Hawaiian Volcano Observatory, July 19, 1974, 13:25 hrs. Lava fountains up to 25 m high on the skyline south of

Keanakäko‘i Crater feed lava cascades into the crater. Three en echelon fissure fountains are active north of Keanakäko‘i. Low fountains at the Crater’s north rim feed flows that have

just cut the Crater Rim Road, middle fountains feed fluid pähoehoe lavas into a gully on the right. High fountains on the floor of Kïlauea Caldera (foreground) mark the easternmost

extent of the caldera-floor fountains that extend more than a kilometer to the west of this photo. USGS photo by John C. Forbes.
UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1999

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1613

Magma Migration and Resupply

During the 1974 Summit Eruptions

of Kïlauea Volcano, Hawai‘i

By John P. Lockwood, Robert I. Tilling, Robin T. Holcomb, Fred Klein,

Arnold T. Okamura, and Donald W. Peterson

There has never been a year like 1974 in historical time when three

discrete yet magmatically related eruptions occurred in the summit

area of Kïlauea Volcano
U.S. Department of the Interior

Bruce Babbitt, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and does not imply endorsement

by the U.S. Government.

United States Government Printing Office: 1999

For additional copies please contact:

USGS Information Services

Box 25286

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1–888–ASK–USGS

World Wide Web: http://www.usgs.gov/

Layout and design by Stephen L. Scott

Manuscript approved for publication, April 8, 1999

 Library of Congress Cataloging-in-Publication Data

Magma migration and resupply during the 1974 summit eruptions of Kïlauea Volcano,

Hawai‘i / by John P. Lockwood... [et al.].

p. cm. -- (U.S. Geological Survey professional paper ; 1613)

Includes bibliographical references.

1. Magmas--Hawai‘i--Kïlauea Volcano. 2. Kïlauea Volcano (Hawai‘i)--Eruption, 1974.

I. Lockwood, John P. II. Series

QE461 .M2347 1999

551.21’09969’--dc21

 99-052676
CONTENTS

Abstract--------------------------------------------------------------------------------------------- 1

Introduction---------------------------------------------------------------------------------------- 2

Acknowledgments--------------------------------------------------------------------------------- 2

Preceding Activity: Setting the Stage----------------------------------------------------------- 3

Long-lived Rift Activity at Mauna Ulu--------------------------------------------------- 4

Other Precursory Signs of Return to Summit Activity---------------------------------- 4

Chronological Narratives------------------------------------------------------------------------- 4

July 19-22, 1974, Eruption----------------------------------------------------------------- 4

September 19, 1974, Eruption------------------------------------------------------------- 8

December 31, 1974, Eruption-------------------------------------------------------------- 10

Tectonic Significance of the December 31, 1974, Eruption---------------------------- 13

January 1-10, 1975, Intrusion--------------------------------------------------------------- 15

Geophysical Observations------------------------------------------------------------------------ 15

Geodetic Monitoring------------------------------------------------------------------------- 15

Summit tilt – the Uwëkahuna Record----------------------------------------------- 15

Summit spirit-level tilt surveys------------------------------------------------------- 18

Horizontal distance surveys---------------------------------------------------------- 19

Levelling surveys---------------------------------------------------------------------- 20

Seismic Observations------------------------------------------------------------------------ 21

Eruption of July 19-22, 1974, and the following 2 months---------------------- 23

The caldera eruption of September 19, 1974-------------------------------------- 24

Eruption of December 31, 1974----------------------------------------------------- 25

Geoelectrical Surveys----------------------------------------------------------------------------- 27

Geochemistry and Petrology--------------------------------------------------------------------- 27

Conclusions and Speculations------------------------------------------------------------------- 32

References Cited----------------------------------------------------------------------------------- 35

FIGURES

1. Map of Kïlauea Volcano--------------------------------------------------------------------------------------------------------- 2

2. Distribution of 1974 lavas and eruptive vents in the summit area of Kïlauea Volcano--------------------------------- 5

3. Kïlauea Volcano summit area--------------------------------------------------------------------------------------------------- 6

4. Aerial view of eruptive vents north of Keanakäko‘i Crater----------------------------------------------------------------- 6

5. Thin pähoehoe flow advancing across the east side of Kïlauea caldera on July 19, 1974------------------------------ 6

6. Aerial view of the July 1974 lava flows--------------------------------------------------------------------------------------- 7

7. E-W tilt component from the Ideal-Aerosmith tiltmeter at Uwëkahuna Vault------------------------------------------- 8

8. Deformation and seismicity of the Kïlauea summit------------------------------------------------------------------------- 11

9. Aerial view of the easternmost flows of the December 31 eruption------------------------------------------------------ 11

10. Surface texture of December 31, 1974 pahoehoe northeast of Pu‘u Koa ́e---------------------------------------------- 12

11. Selvage on margin of early-formed dike-------------------------------------------------------------------------------------- 13

12. Location of the May 1973 and December 1974 eruptive vents------------------------------------------------------------ 14

13. Plot of tilt components for the Uwëkahuna water-tube tiltmeter---------------------------------------------------------- 16

14. Cumulative plots of Uwëkahuna water-tube tilt during the 1974 eruptions---------------------------------------------- 17

15. Spirit-level tilt vectors at Kïlauea summit stations--------------------------------------------------------------------------- 18

16. Horizontal displacement vectors from EDM surveys----------------------------------------------------------------------- 19

17. Leveling data for the Kïlauea summit area------------------------------------------------------------------------------------ 20

18. Inflation centers determined by levelling------------------------------------------------------------------------------------- 21

19. Daily number of “volcanic” earthquakes during 1974 and early 1975---------------------------------------------------- 21

III
CONTENTS

FIGURES—Continued

20. Earthquakes related to the eruption of July 1974--------------------------------------------------------------------------- 22

21. Earthquakes related to the eruption of September 1974------------------------------------------------------------------- 23

22. Earthquakes related to the eruption of December 1974------------------------------------------------------------------- 24

23. Earthquakes following the eruption of December 1974------------------------------------------------------------------- 25

24. Distribution of electrical self-potential fields in the southern Kïlauea Caldera----------------------------------------- 27

25. Chemical analyis plots---------------------------------------------------------------------------------------------------------- 28

26. MgO variation diagrams-------------------------------------------------------------------------------------------------------- 30

27. Time/oxide series plots--------------------------------------------------------------------------------------------------------- 33

IV

TABLES

1. Eruptive activity of Kïlauea Volcano since the 1967-1968 Halema‘uma‘u eruption----------------------------------- 3

2. Modal compositions of the 1974 lavas---------------------------------------------------------------------------------------- 9

3. Geodetic surveys in the Kïlauea summit area before, during, and after the 1974 summit eruptions----------------- 16

4. Chemical analyses of lavas erupted in July 1974---------------------------------------------------------------------------- 29

5. Chemical analyses of lavas erupted in September 1974-------------------------------------------------------------------- 32

6. Chemical analyses of lavas erupted in December 1974--------------------------------------------------------------------- 34
1

Magma Migration and Resupply during the 1974

Summit Eruptions of Kïlauea Volcano, Hawai‘i

By John P. Lockwood, Robert I. Tilling, Robin T. Holcomb, Fred Klein,

Arnold T. Okamura, and Donald W. Peterson

ABSTRACT

Three brief eruptions—the longest lasting 3 days, the

shortest only 6 hours—took place in the summit region of

Kïlauea Volcano during the last half of 1974: July 19-22,

September 19, and December 31. These eruptions followed

an inferred constriction of the upper East Rift Zone (ERZ)

supply conduits to the long-lived Mauna Ulu eruption in early

1974, causing magma and excess pressure to accumulate beneath Kïlauea’s summit. The 1974 eruptions each shifted

farther westward across Kïlauea’s summit, reflecting the apparent westward migration of magmatic storage, from the

upper ERZ (Mauna Ulu), across Kïlauea’s summit caldera,

and eventually down the volcano’s southwest flank. Subsurface migration of magma storage centers across Kïlauea is

documented not only by the loci of eruptive activity, but also

by seismic and geodetic observations, which clearly trace the

westward propagation of magma-generated seismic activity

and deformation centers. These eruptions occurred during a

period of increasing unrest at neighboring Mauna Loa Volcano, a factor possibly related to tectonic processes that also

influenced the 1974 Kïlauea summit eruptions.

The July eruption was fed by separate, subparallel fissures that bridged the zone between the upper ERZ and the

southeastern margin of the modern Kïlauea Caldera. These

lavas, characterized by bimodal compositions, erupted from

separate vent systems and in part represented the eruption of

stored, “old” magma that was forced to the surface by an intrusion of more primitive magma from depth. The highly

differentiated lavas from the southern vent system are characterized by megascopic plagioclase phenocrysts, a rarity at

Kïlauea’s summit. Lavas erupted in September are much more

homogeneous than those erupted in July, but only slightly

more mafic than the mean composition of the two groups of

July lavas combined. The December lavas, however, include

some of the most mafic compositions erupted at Kïlauea’s

summit since 1959 (12.2-15.6 percent MgO) and may include

compositions little modified from a batch of “new” magma

that is postulated to have entered the Kïlauea summit storage

area before the July eruption.

The September eruption began within Halema‘uma‘u

Crater, in the same area of the Kïlauea eruption in September, 1971. In contrast to 1971, however, 1974 caldera-floor

“curtain-of-fire” fountains abruptly halted their advance when

they reached the southwest wall of the caldera. In 1971, eruptive fissures extended beyond Kïlauea Caldera and down a

strand of the Southwest Rift Zone (SWRZ) during earliest

phases of that eruption. The SWRZ was blocked in September 1974, however, and magma was contained within Kïlauea

caldera. This blockage was possibly caused by compressive

stresses across the SWRZ related to the inflation of the

reawakening Mauna Loa Volcano, which had begun to inflate in early 1974. We propose that the inflation state of

Mauna Loa influences Kïlauea’s SWRZ, and that long-term

net inflation of Mauna Loa is also responsible for the paucity

of SWRZ eruptive activity relative to the activity of Kïlauea’s

ERZ.

The December eruption took place southwest of Kïlauea

Caldera in a historically noneruptive area between the Koa ́e

Fault System (KFS) and the SWRZ. This eruption apparently continued a geologically recent pattern of magmatic

injection into the KFS, a new mode of activity first suspected

in 1965 and which later included the first recorded KFS surface eruption (1973). This activity may be a new element in

the tectonic evolution of Kïlauea and suggests that the Koa ́e

Fault System may become an increasingly important locus of

magmatic injection and intrusion in the future.

The volcanic conditions that existed on Hawai‘i in the

mid-1970s bears some similarities to the present situation (late

1990s). The long-lived ERZ eruption of Mauna Ulu was apparently beginning to wane in mid-1973, following several brief

eruptive outbreaks and intrusions uprift, and Mauna Loa was

beginning to inflate. Twenty-five years later, another long ERZ

eruption has at times waned in intensity, eruptive and intrusive activity has taken place uprift of the main vent, and Mauna

Loa is again slowly inflating. These parallels in eruptive history may well be coincidental, but they would merit further

1
2 THE 1974 SUMMIT ERUPTIONS OF KÏLAUEA VOLCANO, HAWAI‘I

study should the Pu ́u ‘Ö‘ö eruption cease, eruptions return

to Kïlauea’s summit, and then activity resume at Mauna Loa.

INTRODUCTION

Nearly all known eruptions of Kïlauea, one of the most

active volcanoes on Earth, have taken place either within its

summit region or along its two rift zones—the east-rift zone

(ERZ) and the southwest-rift zone (SWRZ)–that radiate from

the summit (fig. 1; Holcomb, 1987). In 1974, Kïlauea exhibited an especially wide range of eruptive behavior. During

that year, the long-lived Mauna Ulu eruption in the upper ERZ

(Swanson and others, 1979; Tilling and others, 1987) declined

and ceased, two short-lived eruptions—one in July, another in

September—occurred within Kïlauea’s summit caldera and

the upper ERZ, and in December a brief, vigorous outbreak

took place in an area between the upper SWRZ and the Koa ́e

Fault System (KFS) (fig. 1). Each of these eruptions broke out

in different areas, and the lavas of each had distinctive chemical compositions; yet, seismic and ground-deformation activity associated with the eruptions suggested that each was related to a general pattern of westerly migration of a shallow

magma source. Although selected aspects of the 1974 eruptions have been treated briefly (Peterson, 1976; Tilling and others, 1976; Pollard and others, 1983; Lipman and others, 1985;

Klein and others, 1987; Dvorak and Okamura, 1987; Hazlett,

1993), they have never been documented in any detail.

The purpose of this paper is twofold: (1) to present a

complete account of these contrasting yet related eruptions,

thus filling a gap in the published narratives of recent activity

of Kïlauea; and (2) to examine their significance within a

broader context of regional magmatic and eruptive dynamics. In hindsight, one unanticipated benefit of the long delay

in preparing this paper is that we have gained a longer term

historical perspective and can view these three eruptions

within a multidecade context of the eruptive behavior of not

only Kïlauea, but also of the adjacent Mauna Loa.

ACKNOWLEDGMENTS

Summary reports of Hawaiian eruptive activity such as

this involve the contributions of virtually the entire staff of

the Hawaiian Volcano Observatory, past and present. As one

reviews the rain- and sweat-soaked field notes of others or

pours through the voluminous, meticulously annotated, archived HVO monitoring records largely written by the late

John Forbes, a deep appreciation for the careful work of these

mostly unsung heroes of the HVO “family” becomes almost

overwhelming. Robert Koyanagi in particular contributed

substantially to this paper; numerous ideas about the signifiFigure 1.—Map of Kïlauea Volcano, showing principal geographic and tectonic features. Abbreviations

on inset map: K, Kïlauea; ML, Mauna Loa; MK, Mauna Kea; H, Hualälai; KO, Kohala.

0 10 20 KILOMETERS

X

19°

10' Pacific Ocean

HILINA FAULT

SYSTEM

KOA ́E FAULT SYSTEM

Pu‘u ‘Ö‘ö Crater

Mauna Ulu

Crater

Kïlauea

Caldera

Area of

figure 2

Halema‘uma‘u, Crater Mauna Iki

Crater

 RIFT ZONE

KÏLAUEA

VOLCANO

EAST RIFT ZONE

Heiheiahulu 

Crater

SOUTHWEST

MAUNA LOA

VOLCANO

Pahäla

19°

30'

155°30' 155°

HAWAI‘I

Hilo

K O

H

M K

M L K

Area of

figure 1
3

cance of the three 1974 eruptions stem from his freely shared

perspectives on the long-term evolution of Kïlauea. Others,

including Elliot Endo, Jennifer Nakata, Reginald Okamura,

Don Swanson, John Unger, and Charles Zablocki shared important observations. Christina Neal provided unpublished

geologic mapping of the Kïlauea summit area as well as important insights on the eruptive record of the upper SWRZ.

Dave Siems provided exceptionally rapid turn-around of

chemical analyses for us in 1992, Susanne Dorn helped measure flow thicknesses, and John Luczaj provided modal analyses. Martha Lockwood provided invaluable support in the

field and office. Wendell Duffield, Richard Moore, and Robert Koyanagi provided constructive reviews of the manuscript;

their suggestions for improvements are greatly appreciated.

PRECEDING ACTIVITY: SETTING THE

STAGE

Halema‘uma‘u Crater, within Kïlauea’s caldera (fig.

1), was the site of nearly continuous lava-lake activity

throughout most of the period 1823 to 1923; in addition to

the persistently active summit lava lake, more than a dozen

other eruptions, at the summit as well as both rift zones, occurred during the same interval (Macdonald and Abbott,

1970). Following the subsidence of Halema‘uma‘u lava lake

and the ensuing series of phreatic eruptions in May 1924,

the century-long continuous summit activity ceased (Jaggar

and Finch, 1924). During the next 10 years, eruptive activity at Kïlauea was sporadic and feeble, entirely restricted to

Table 1.—Eruptive activity at Kïlauea since the 1967-68 Halema‘uma‘u eruption. [Abbreviations for location of outbreak: SUM, summit; ERZ, east-rift zone; SWRZ, southwest-rift zone; and KFS, Koa ́e Fault

System. Table modified from Dzurisin and others (1984, Table 1)].

1Smallest recorded Kïlauea eruption!

INTRODUCTION
4 THE 1974 SUMMIT ERUPTIONS OF KÏLAUEA VOLCANO, HAWAI‘I

Halema‘uma‘u. Kïlauea then was completely inactive in the

period 1935 to 1951, during which time neighboring Mauna

Loa Volcano erupted five times (Macdonald, 1954). After

Mauna Loa’s voluminous SWRZ eruption in 1950, it remained

dormant and Kïlauea once again dominated Hawaiian volcanism for the next 25 years, beginning with the June 1952 eruption at Halema‘uma‘u. Numerous short-lived eruptions,

within the summit caldera and in the upper as well as lower

ERZ, took place through the mid-1960s: in 1954, 1955, 1959-

60, 1961, 1962, 1963, and 1965.

The 8-month eruption within Halema‘uma‘u in 1967-

68 (Kinoshita and others, 1969) was at the time the longestduration activity at Kïlauea since 1924, and exhibited continuous lava-lake activity typical of that prevailing for more

than a century prior to 1924. Beginning in 1969, however,

eruptions in the ERZ have dominated the activity at Kïlauea

(table 1). The longest-lived and most voluminous of these

are the 1969-74 eruptions at Mauna Ulu (Swanson and others, 1979; Tilling and others, 1987) and the Pu‘u‘Ö‘öKupaianaha eruption (Wolfe, 1988; Heliker and Wright, 1991),

which began in January 1983 and continues unabated as of

December 1999. Characteristics of Kïlauea eruptive activity

since the 1967-68 Halema‘uma‘u eruption are summarized

in table 1.

LONG-LIVED RIFT ACTIVITY AT MAUNA ULU

The nearly steady-state activity that prevailed at the

Mauna Ulu and Alae shields from February 1972 to July 1974

was disrupted in 1973 by several lava-lake drainings and two

brief eruptive breakouts 3-4 km uprift—one in May in the vicinity of Hi ́iaka and Pauahi Craters, and the other in November

within and to the east of Pauahi Crater (table 1; Tilling and

others, 1987, fig. 16.3). Lava-lake activity resumed at Mauna

Ulu following each of these interruptions, but the activity became increasingly sluggish, less continuous, and marked by

periods of repose, one lasting nearly 2 months in July-September 1973. During this repose the lake surface crusted entirely and no molten lava could be seen. The July-September

1973 repose period in part may be related to a day-long intrusion on 9 June 1973 into the KFS (Dzurisin and others, 1984,

table 1; Klein and others, 1987, table 43.1); this intrusion was

accompanied by a drastic lowering of Mauna Ulu lava lake but

did not result in any new eruptive breakouts (Tilling, 1987).

During the first half of 1974, even though a regime of

nearly steady-state magma transfer had been reestablished, a

noticeable change developed in Mauna Ulu’s eruptive style.

Rather than exhibiting the essentially continuous lava-lake

behavior typical of the 1972-73 period, the activity was now

characterized by brief episodes of high lava fountaining alternating with longer periods of low-level vent activity. Following a particularly vigorous eruptive episode in May-June

1974, lava-lake activity at Mauna Ulu progressively declined

through mid-July, the lake level gradually subsided, circulation became increasingly sluggish, and lava finally disappeared in late July (Tilling and others, 1987). Molten lava

never again returned to the Mauna Ulu crater, as it had in

February 1972 after a 4-month quiescent period following

two short-lived summit and SWRZ eruptions in August and

September 1971 (table 1; Swanson and others, 1979; Duffield and others, 1982).

OTHER PRECURSORY SIGNS OF RETURN TO

SUMMIT ACTIVITY

The changing character and progressive decline of visible activity at Mauna Ulu were accompanied by increasing

shallow summit seismicity and a net summit inflation of about

25 microradians (μrad) during the period May 5 to November

10, 1973. These observations suggested that the magmatic

connection between Mauna Ulu and Kïlauea’s summit reservoir was becoming less efficient (Tilling and others, 1987).

Indeed, when the state of summit inflation in mid-October

1973 surpassed the previous historical high, reached in early

February 1972 just before the Mauna Ulu lava lake was reactivated, staffers at the Hawaiian Volcano Observatory (HVO)

wrote: “If it doesn’t break out at the summit, then there is a

chance that it might break out somewhere on the upper east

rift” (unpublished HVO Monthly Report, September 21 to

October 20, 1973). Their hunch proved correct: the November

10, 1973 Pauahi eruption (table 1) began 3 weeks later. Still

another possible indicator of the pending demise of Mauna

Ulu was a 3-hour intrusion into the Pauahi area that accompanied the March 23-24, 1974 eruptive episode at Mauna Ulu

(Dzurisin and others, 1984, table 1; Klein and others, 1987,

table 43.1).

Finally, the apparent deterioration in the nearly steadystate magma transfer from the summit region to the Mauna

Ulu shield—probably underway since mid-1973—was also

reflected in a halved average lava-eruption rate, from 8 x 106

m3/month during 1972-73 to 4 x106 m3 during 1974, prompting Tilling and others (1987, p. 411) to speculate: “This reduction * * * perhaps augured the eventual cessation of the

eruption.”

CHRONOLOGICAL NARRATIVES

The three spectacular, short-lived, small-volume

eruptions that occurred at Kïlauea’s summit following the

cessation of sustained Mauna Ulu activity in 1974 are the

principal focus of this paper. Figure 2 shows the vents and

flow fields for each of these individual eruptions. All times

given in the following narratives are in Hawaiian Standard

Time (UT minus 10 hr).

JULY 19-22, 1974 ERUPTION

The seismic alarm system sounded in an HVO residence

at 03:45 on July 19, triggered by a sharp and sustained flurry
Halema‘uma

Crater

Keanakäko‘i

 Crater

Kï

C

Crater

Rim

Drive

8/

9/24/71

12/31/74

9/19/74

9/24/71

9/24/71

Sand

Hill

HVO

Chain

of 

R

Craters

D B A C E

Kamakai ́a

Hills

Cone Crater

Koa ́e

Pu‘u

Mauna

Iki

EXPLANATION

1974 vent

fissures

December 31 lava September 19 lava July 19-22 lava September 24-29 lava August 14 lava 

B A

1974 1971

2700

2600

2600 2400

2200

2800

3200 3100

2900

2600

2800

2700 3000

3100

3000

2702

2600

2900

3200

3300

3200

3300

3400

3500

3600

3800

3700

3700 3900

155° 17'30"

1925'°

9/26-29/71

I H GF

N

0 1 MILE

0 1 KILOMETER

155° 20' 19°20' 30"

19°17' 30"

Figure 2.—Map showing the vents and flow fields of the summit and upper southwest rift eruptions of 1971 and 1974 (modified from Holcomb, 1987, Fig. 12

covers much of the August 1971 lava in the southeastern part of Kïlauea Caldera; the September 19, 1974 lava covers much of the September 1971 lava in an

For the December 31, 1974 eruption, the bold letters refer to the chronological order of the opening of vent fissures (A = first; I = last). Contour elevations 
6 THE 1974 SUMMIT ERUPTIONS OF KÏLAUEA VOLCANO, HAWAI‘I

Figure 4.—Aerial view to south-southeast of eruptive vents north of

Keanakäko‘i Crater; Kïlauea Caldera floor in the lower left. The

August 1971 fissure is closest to the caldera rim; the July 19, 1974

fissure that opened at 12:35 is just to the south; the 13:15 vents are

at the rim of Keanakäko‘i. USGS photo by J.P. Lockwood.

Figure 5.—Thin pähoehoe flow advancing across the east side of

Kïlauea caldera on July 19, 1974 at about 14:00. The active flow is

covering an earlier formed flow; note the fresh lava emerging from

cracks in the crust of the earlier flow. USGS photo by R.T. Holcomb.

I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I IIII I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I IIII I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

I I I I I I I I I I

I I I I I I I I I I I I I I I I I I

I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

HVO

UWE

UWEV KÏLAUEA

 CALDERA

Halema‘uma‘u

 Crater Crater

Rim

Dr.

Crater Rim

Drive

Keanakäko‘i

 Crater

Lua Manu

Crater

Kïlauea Iki

Crater

OTLV

0 1 KILOMETER

AHU

EXPLANATION

Eruptive vents

September 19 lavas

Crater rim

July 19-22 lavas

19° 25'

155°17'30"

Puhimau

 Crater

Chain of Craters Road

Figure 3.—Kïlauea Volcano summit area, showing location of eruptive vents and lava flows of the July and September 1974 eruptions,

along with geographic features described in text. HVO, USGS Hawaiian Volcano Observatory; OTLV, Outlet Vault; UWEV,

Uwëkahuna Vault; UWE, Uwëkahuna geodetic station; AHU, Ahua

geodetic station.

of earthquakes centered near the uppermost part of the ERZ

and the southern part of Kïlauea caldera. Some of these earthquakes were strong enough to be felt by local inhabitants.

The first observers to HVO at 04:00 noted the normal glow

above the summit of Mauna Ulu, reflecting the incandescent

parts of a dwindling but still-active lava lake. A field party at

Mauna Ulu later reported (~09:00) seeing molten lava during

crustal overturn of the lake surface more than 40 m below the

crater rim; this was the final sighting of molten lava at Mauna

Ulu. The same field party also noted many new holes, or reopening of patched-over areas in the pavement of Chain of

Craters Road south of Puhimau Crater (figs. 2 and 3). Volcanic tremor persisted at Mauna Ulu for three more days, before ceasing on July 22. This date (July 22) is arbitrarily

taken as the end of the 1969-74 Mauna Ulu eruption (Tilling,

1987).

The onset of seismic activity on the early morning of

July 19 coincided with the beginning of abrupt deflation of

the summit region. As seismicity increased, strong tremor

began about 10:30 and the rate of deflation also increased

markedly. Magma was apparently moving towards the surface in the Keanakäko‘i Crater area (fig. 3). About 12:30,

lava broke out from a small fissure at the base of the south

wall of Keanakäko‘i, accompanied by small rockfalls from

the crater wall. The lava fountains were initially about 30 m

high. Within a few minutes, a second eruptive fissure, with

15-20 m-high lava fountains, formed north of Keanakäko‘i

(fig. 4), in the area between the Crater Rim Road and the

fissures of the August 14, 1971 eruption (fig. 2; Duffield and
7

others, 1982). Lava from this fissure flowed into the August

1971 fissure as well as cascaded over the wall of the summit

caldera. Shortly thereafter, two en echelon fissures broke out

to the west, on the Kïlauea Caldera floor (Figs. 3, 18), and

lava from all three fissures north of Crater Rim Road began

to advance across the flow field of the August 1971 eruption.

The advance eastward across the caldera floor was rapid, and

involved complex sheet flow as new lava spread over thin

crusts of slightly older flow sheets (fig. 5).

During essentially the same time interval, the activity

to the south also increased, and several other new en echelon

fissures extended rapidly both to the west and to the east of the

initial outbreak in Keanakäko‘i Crater (fig. 3). A fast-moving,

fluid flow consisting of slabby pähoehoe and loose a’a traveled quickly to the south and southeast for more than 2 km

(fig. 6). These flows encased large ohia trees with chilled lava

near their source vents, forming a field of “lava trees” when

the fluid portions of the lava later flowed away from the encased trees (Lockwood and Williams, 1978). Moore and

Kachadoorian (1980) used subsequent rheological observations

of these lava trees to determine that flow velocities for this

branch of the July 1974 flow varied between 5.7 km/hr (near

eruptive vents) to 0.5 km/hr (near this flow’s distal end).

By 13:00, lava from fissures on the southern rim of

Keanakäko‘i began to cascade into the crater; these spectacular lava cascades, together with lava erupted by the withincrater fissure, completely covered the crater floor to a depth

of about 5 m, burying the blanket of 1959 tephra that had

previously mantled lava of 1877 age. Also by 13:00, the easternmost of these “southern” fissures had extended 300 m east

of Lua Manu Crater (fig. 3), cutting the Chain of Craters Road.

Lava cascades poured into Lua Manu and, together with lava

0 10 20 KILOMETERS

X

19° 10' Pacific Ocean

HILINA FAULT

SYSTEM

KOA ́E FAULT SYSTEM

Pu‘u ‘Ö‘ö Crater

Mauna Ulu

Crater

Kïlauea

Caldera

Area of

figure 2

Halema‘uma‘u,

Crater Mauna Iki

Crater

 RIFT ZONE

KÏLAUEA

VOLCANO

EAST RIFT ZONE

Heiheiahulu 

Crater

SOUTHWEST

MAUNA LOA

VOLCANO

Pahäla

19°

30'

155°30' 155°

HAWAI‘I

Hilo

K O

H

M K

M L K

Area of

figure 1

Halema‘uma‘u

Keanakäko‘i

Kïlauea Caldera

Puhimau

Figure 6.—Aerial view to the northwest of the July 1974 lava flows, which extend south to the bottom of the photo and across the east side

of Kïlauea Caldera. Mauna Loa Volcano forms the slopes beyond Kïlauea Caldera; Mauna Kea Volcano in the distant background. USGS

photo by J.P. Lockwood.

CHRONOLOGICAL NARRATIVES
8 THE 1974 SUMMIT ERUPTIONS OF KÏLAUEA VOLCANO, HAWAI‘I

derived from a fissure on this crater’s east wall, partly filled

the crater with about 15 m of lava. About two-thirds of this

lava later drained back into the fissure, leaving a prominent

“lava-subsidence terrace” (high lava mark) half way up the

crater’s walls.

All fissures north and south of Crater Rim Road were

vigorously active when, at 13:15, two new en echelon fissures broke out north of the northern edge of Keanakäko‘i

Crater, in the area between the previously formed fissures

(figs. 3, 4). Some lava from these latest fissures crossed Crater Rim Road about 10 minutes later and began to cascade

down the north wall of Keanakäko‘i Crater. However, most

of the lava from the fissures north of the Crater Rim Road

poured over the wall of the caldera to feed the flow spreading

over the southeastern part of the caldera floor. One of these

lava streams into the caldera raced down a curving gully, with

lava banking up against and spraying the outside wall of several bends, much as had occurred in the same gully in 1971

(Duffield and others, 1982). This particular stream was later

the topic of a detailed flow-dynamics analysis by Heslop and

others (1989), who concluded that it was characterized by

laminar flow, traveled at a mean velocity in excess of 8 m/s,

and had low or negligible yield strength and viscosities in the

range of 85-140 Pa-s.

By 13:45, lava from the westernmost of the southern

line of fissures (fig. 3) also began to cascade into Keanakäko‘i

Crater, while the vent activity at the fissure inside the crater

was decreasing. Meanwhile, within the summit caldera, the

fissures remained vigorous, feeding lava that had covered

about one-half of the August 1971 flow field by 14:35. Although there were momentary surges in lava-fountain heights

to 45-55 m, discharge rates at all fissures gradually began to

decline; by 16:15, all activity had ceased along the fissures

south of Keanakäko‘i. However, low fountaining from the

two fissures in the caldera floor persisted until the early morning of July 20. One vent on the caldera floor continued to

spatter and feed a small flow through July 21, but all eruptive

activity stopped by early morning of July 22.

All lava flows during this 3-day eruption were highly

fluid and fast flowing, typically less than a meter thick near

their source vents. Near eruptive fissures, lava flows appeared

to have been gas rich, as indicated by the formation of cavernous “shelly” pähoehoe (Swanson, 1973), with vesicle and gascavity space locally comprising as much as 80 percent of the

bulk volume. The flows consisted entirely of pähoehoe near

Keanakäko‘i Crater and on the caldera floor, but formed platy

pähoehoe and rubbly  ́a‘ä in the south-trending flow west of

Puhimau Crater (fig. 3), where the flow was only 1.5-m thick

at its distal end. Lavas from the July 19-22 eruption covered

an area of 2.91 km2

, and the estimated total eruptive volume

was about 6.5 x 106 m3 . These vesicular lavas have an estimated DRE (dense rock equivalent) volume of 4.9 x 106

 m3.

1

The July 1974 lavas are nearly aphanitic tholeiitic

basalts with widely scattered minor olivine and/or plagioclase

phenocrysts about 1 mm across; representative modal compositions are given in table 2. As later discussed, lavas erupted

from the fissures north of Keanakäko‘i have chemical compositions quite distinct from those of lavas erupted from fissures south of Keanakäko‘i.

SEPTEMBER 19, 1974 ERUPTION

Following the July eruption, leveling and tilt measurements showed a center of tumescence to be migrating westward from the Keanakäko‘i area to the area south of

Halema‘uma‘u (figs. 3, 18). Very small, shallow, short-period earthquakes centered south of the caldera near Outlet

Vault (fig. 3) also increased, especially after September 16.

The news media soon learned of these developments, and

HVO spokesmen were being quoted in the press as anticipating an imminent eruption on Kïlauea’s SWRZ, largely drawing upon a reasonable comparison with events 3 years earlier, when the August 14, 1971 summit eruption was followed

within less than 6 weeks by a summit-SWRZ eruption (Duffield and others, 1982). Thus, by early September 1974, HVO

staffers and the public alike were primed for a possible repeat

of the 1971 pattern.

There was little warning of the brief eruption soon to

occur. In contrast to the July summit eruption, there was much

less precursory seismicity (no felt earthquakes) prior to the

September eruption. Just after midnight on September 19,

however, at 00:44 HST, small instrumentally detected earthquakes (1-3/min) local to the North Pit seismometer (in

Kïlauea Caldera) began to occur. At 00:47 these earthquakes

increased to 5-10/min and weak tremor began. The IdealFigure 7.—E-W tilt component from the Ideal-Aerosmith tiltmeter

at Uwëkahuna Vault for a 14-hr period preceding, during, and after

the September 19 eruption. Based on 6-min reading intervals.

Methodologies for determination of areas and volumes: 1

Area covered was determined by digital analysis of mapped flows

and is accurate to + 5 percent. 2

Volume erupted was determined by field estimation of flow thicknesses in each of 26 subareas of known area. Such estimates are inherently imprecise (especially for thicker flows) and are accurate to only

perhaps + 25-30 percent.

22:00 00:00 02:00 04:00 06:00 08:00 10:00 12:00INFLATION (East up) ------->

TILT (microradians)

00:55

Missing data

01:37

09:30

E—W tilt (observed)

Extrapolated

184

186

188

190

192

194

196

198

200

Sept. 18 Sept.19

TIME (6-minute data intervals)
9

Aerosmith continuously recording tiltmeter at Uwëkahuna

Vault (fig. 3) also showed an abrupt but slight (0.04 μrad)

deflation in E-W tilt at this same time, and at 00:55 a precipitous inflation (west down) began. The tiltmeter recorder went

off scale at 01:10, causing the tilt record to be lost for the

next 44 min, until the recorder was reset by HVO staff. A

reconstruction of the lost record (fig. 7) shows that a little

over 13 μrad of inflation occurred at Uwëkahuna (caused by

the injection of ascending magma) and that a gradual deflation began about 01:37. Meanwhile, seismic tremor magnitude increased, as did the frequency and magnitude of earthquakes, as the ascending dike neared the surface; tremor

alarms sounded at 01:21. At 01:27, glow above Halema‘uma‘u

indicated an eruption was already in progress. By 01:32, tops

of lava fountains could be seen above Halema‘uma‘u’s north

rim in the northeast corner of the crater. These initial fountains must have been nearly 100 m high. The fountains then

rapidly migrated southwestward across Halema‘uma‘u’s floor,

and at 01:45 a “curtain of fire” climbed the crater’s west wall.

A line of fountains about 20 m high migrated across the 0.5

km from the west rim of Halema‘uma‘u to the west wall of

Kïlauea Caldera (fig. 3) within 1 min, where the rapid southwest advance of the fountains came to an abrupt stop. This

was a significant deviation from the September 1971 pattern,

when eruptive fissures quickly migrated out of the caldera,

across the Crater Rim Drive, and far down the SWRZ. The

fountains on Halema‘uma‘u’s floor then quickly subsided in

height, but lava was still being erupted at an estimated rate of

nearly 2 x 106

 m3/hr. Fountaining extended across the crater

floor in two continuous, N. 60o E fissures about 70 m apart

(fig. 3).

By 02:30, lava spreading from the two fissures had covered about three-quarters of Halema‘uma‘u’s floor, and lava

falls were cascading into a small central pit crater that had

formed near the end of the 1967-68 eruption (Kinoshita and

others, 1969). The N. 30o E-trending line of fountains outside Halema‘uma‘u on the caldera floor produced an active

pähoehoe flow that flowed south and cut the Crater Rim Road

at 02:55. By 03:00, Halema‘uma‘u’s floor was completely

covered. At 03:05, a flow moving north from the calderafloor fissures circled back and began cascading

into Halema‘uma‘u. The southern line of fountains on

Halema‘uma‘u’s floor began to be drowned by the rising lava

lake, and by 04:45 only three small (2- to 5-m-high) fountains remained visible along this fissure, although a linear

zone of upwelling indicated that discharge continued along

the length of this fissure. The northern row of fountains

rapidly diminished in height after 06:00 and at 08:00 began

loud degassing. All vigorous fountains on the floor of

Halema‘uma‘u were soon drowned by the rising lake, but

broad upwelling dome fountains 15-20 m high were still produced along the linear trends of former fountains. At about

this time, the lava lake level began to subside, either through

degassing processes or by drainage through unknown openings on the floor of Halema‘uma‘u. Low fountains nonetheless remained active at a few places along the eruptive fissures until mid-afternoon. Irregular overturning of the lake

crust continued until the morning of September 22.

Table 2.—Modal compositions (volume percent) of the lavas of the 1974 summit eruptions. [Modal

determinations by J.P. Lockwood, John Luczaj, and F.A. Trusdell].

CHRONOLOGICAL NARRATIVES
10 THE 1974 SUMMIT ERUPTIONS OF KÏLAUEA VOLCANO, HAWAI‘I

At its highest level, reached at about 09:30, the lake

covered the entire floor of Halema‘uma‘u except for the tops

of three high spatter cones formed during the 1967-68 eruption (Kinoshita and others, 1969)2

. The lake surface lowered

rapidly and by 13:00 had dropped about 7 m. During this

possible drainback episode, large areas of the lake crust slowly

rotated counterclockwise, much as would a large bathtub being drained in the northern hemisphere.

At 09:30, coincident with highest lake level, E-W deflation as recorded by the Uwëkahuna Ideal-Aerosmith tiltmeter “bottomed out” and the Kïlauea summit began to reinflate (fig. 7). The morning’s 8-hr deflation did not equal the

magnitude of the postmidnight inflation, however, and so the

09:30 reinflation began at a level 5.6 μrad higher than that

prevailing before September 19.

The total area covered during this eruption was only

1.06 km2

. Relatively accurate determination of eruptive volumes was possible, since most of the erupted lava was confined to Halema‘uma‘u. About 11.7 x 106

 m3 of lava was

erupted initially, all but 0.5 x 106 m3 of which accumulated in

Halema‘uma‘u. Drainback, combined with degassing

volume reduction, withdrew about 4.8 x 106 m3 from

Halema‘uma‘u, leaving a prominent lava-subsidence terrace

9 m above the final, solidified lake surface; the estimated average depth of the lake after drainback was about 10 m. The

residual volume for the entire eruption was about 6.9 x 106

m3. The extensive convective circulation of lava in

Halema‘uma‘u allowed efficient degassing, resulting in residual dense, sparsely vesicular lava for the bulk of the September lavas. Our estimate of the overall DRE volume of the

September lavas is 5.7 x 106 m3.

The September lavas consist of sparsely olivine-phyric

fountain-fed pähoehoe (table 2). In contrast to the lavas of

the July summit eruption, the September 19 lavas were relatively uniform in composition (table 2).

A visit to Mauna Ulu on September 23 showed that the

crater floor was actively subsiding at that time, as evidenced

by loud grinding noises coming from the fume-shrouded crater bottom. Glowing cracks circumferential to the crater rim

also indicated ongoing crater collapse, which had apparently

accelerated during the September eruption. The crater had

enlarged considerably in diameter since the cessation of

Mauna Ulu eruptive activity coincident with the July summit

eruption.

DECEMBER 31, 1974 ERUPTION

Beginning immediately after the September eruption

through December, the summit region of Kïlauea inflated more

than 30 μrad, as measured by the E-W component of

Uwekähuna tiltmeters (see fig. 13), attaining the highest inflation state since daily tilt measurements were initiated at

Uwëkahuna in 1956. A reoccupation of the summit “Electronic Distance Measurement” (EDM) monitor lines on December 23 showed that the two survey lines across the southwest margin of the caldera continued to show higher rates of

extension than the other summit lines, possibly indicating

localized inflation not detectable in the Uwëkahuna E-W tilt

record.

The December 31 eruption was preceded by a monthlong premonitory increase in summit and upper ERZ seismicity, particularly during the final week of December. On

Christmas Day at 07:48, a 32-km deep, magnitude 4.5 summit earthquake was widely felt on Hawai‘i. During the following 3 days, more than a hundred aftershocks occurred,

one of which at 12:20 on December 25 registered ~M 3.5.

Also on Christmas Day, three closely spaced offshore earthquakes occurred south of the Hilina Fault System (fig. 1) at

18:13 (M=4.0), 18:17 (M=3.7), and 18:24 (M=4.5). These

were felt by residents of Volcano, parts of Puna, and Hilo.

Over the next several days, two shallow summit earthquakes,

magnitudes 2.5 and 1.5 respectively, were felt by residents of

Volcano and Hawai‘i National Park. At 19:24, December 28

a Kïlauea southeast-flank earthquake of magnitude about 3.8

was felt in Puna and Hilo.

On the evening of December 30, shallow summit earthquakes local to the Outlet Vault seismic station, located in the

southwestern part of the summit (fig. 3), started to increase in

number, with shocks occurring at rates of 2-4 per minute by

22:54. At 23:03, a moderate earthquake (M~2.2) was felt in

the Hawai‘i National Park housing area (northernmost corner

of fig. 2). At 23:58, another south caldera earthquake (M~2.3)

was felt, followed by a steady succession of smaller earthquakes, mostly M<1. Shortly after midnight (00:10, December 31), the seismic alarms sounded at HVO residences and

the E-W component of the Ideal-Aerosmith tiltmeter (fig. 8)

began a precipitous summit deflation (approximately 3 μrad/

hr). By 02:05, continuous tremor activity had increased to such

an intensity (5 mm peak-to-peak amplitude—fig. 8) that it was

detected at the MLO station, located more than 15 km away,

on the southeast flank of adjacent Mauna Loa. At the same

time, the deflation rate had nearly tripled, increasing to more

than 11 μrad/hr (fig. 8). The combination of increasing seismicity, intense tremor, and abrupt deflation signaled an imminent eruption, and all eyes at HVO looked to the southwest,

where seismic activity was concentrated.

At 02:56, lava fountains were sighted toward the southsouthwest, in the direction of Sand Hill (fig. 2). By 03:10,

lava fountains 35-40 m high had formed a 100 m-long, N 80o

E.-trending “curtain of fire” approximately 1.5 km south of

Sand Hill. The fissure, consisting of several closely spaced

en echelon segments (fig. 9), was advancing at both ends,

and the line of fountains rapidly extended to a length of about

700 m. Several more widely spaced, left-stepping en echelon fissures, with individual trends of N. 70-85o

 E., then be2These spatter cones, and the central area of the Halema‘uma‘u crater

floor, had subsided 45 m from their 1968 level during the September 1971

eruption (Duffield and others, 1982). Our September triangulation showed

that the spatter cones, and presumably the entire crater floor, had subsided

an additional 2.5 m between late September 1971 and late September 1974.
11

Figure 8.—Deformation and seismicity of the Kïlauea summit area as measured by instruments at Uwëkahuna for the period December 30, 1974 to January 05, 1975. A, E-W tilt

component from the Ideal-Aerosmith tiltmeter, based on hourly readings (μrad). B, Contraction of an Electronic Distance Measurement (EDM) line across Kïlauea Caldera, between Uwëkahuna and Ahua geodetic stations (mm). C, Peak-to-peak amplitude of seismic tremor (mm) at UWE seismometer from hourly averages.

Figure 9.—Aerial view to the southeast of the easternmost, en echelon vents and flows of

the December 31 eruption. USGS photo by Elliot Endo. A, C, and E mark the vent fissures

shown in figure 2.

C

A

E

0

10

20

30

40

50

900

800

700

600

500

400

300

200

100

0

60

70

80

90

100

110

120

30 31 1 2 3 4 5

130

140

150

DECEMBER 1974 JANUARY 1975

DEFLATION (microradians)

CONTRACTION (mm)

TREMOR AMPLITUDE (mm)

5

4

3

2

1

A

B

C

CHRONOLOGICAL NARRATIVES
12 THE 1974 SUMMIT ERUPTIONS OF KÏLAUEA VOLCANO, HAWAI‘I

gan to extend to the east- northeast and to the west-southwest. The approximate chronologic progression of fissure

openings is indicated on fig. 2.

The sequence of fissure opening and the appearance of

lava were fascinating to observe in the eerie, flickering, predawn red light given off by the dancing lava fountains. As

ascending dikes rose close to the surface, a series of new

ground cracks would open, typically about 50 m ahead of

established fountains, as the entire dike system advanced laterally across the Ka’u Desert, at rates as great as 30 m/min.

In the area of opening cracks, numerous small earthquakes

caused a near-constant rocking sensation and continuous lowfrequency tremor could be felt by on-site observers, usually

accompanied by the sharp cracking sounds of breaking rocks.

Numerous conspicuous, subparallel cracks formed over

elongate areas about 10-m wide, but the cracks from which

lava would soon fountain opened wider and were usually

marked by small graben structures in the preexisting cover

of Keanakäko‘i Ash (Decker and Christiansen, 1984). Such

cracks began to fume profusely when they had dilated about

10 cm. Soon the white steam (appearing red in fountain

light) would change to pungent, SO2-rich fume and within

two minutes magma could be heard rumbling at depth, as

fissures rapidly dilated to 50-80 cm width. Glow then soon

could be seen in the cracks, and one quick (but perilous)

glance revealed viscous, gas-depleted lava rising rapidly

at depths of 5-10 m. Gas bursts would throw clots of spatter out of the crack first, and as magma reached the surface, continuous fountains quickly rose to 10 m and, within

a few minutes, would reach 30 m in height. The entire

sequence from the initial ground cracking to established

lava fountaining took 3-10 min at any one site. Later surveys revealed that minor ground cracking subparallel to

the eruptive fissures extended to more than 50 m away

from the actual vents (Pollard and others, 1983).

The deflation rate at the summit began to decrease even

as the effusion of lava continued unabated. The average deflation rate was 11.4 μrad/hr during the interval 02:00-04:00

Figure 10.—Surface texture of December 31, 1974 pähoehoe northeast of Pu‘u Koa ́e. Note the 10-mdiameter “lava coil” in the right middle of the photo. USGS photo by Elliot Endo.

lava coil
13

but decreased to 5.2 μrad/hr during the interval 04:00-06:00

(fig. 8). By about 04:40, the eastward migration of the active

fissure systems ceased, with the easternmost fissures (segment E, fig. 2) ending within half a kilometer of the September 1971 flow lobe near Outlet Vault (fig. 3). The westward

migration of the eruptive fissures was slightly more rapid.

By 05:00, fissure vents (segment H, pl. 1) had opened nearly

1 km north of Cone Crater, and a lava flow had cut the Mauna

Iki trail and began to cascade into the westernmost pit crater.

This area of the southwest rift zone is marked by numerous

deep fissures, many of which likely formed in 1868 (Brigham,

1909). Large amounts of lava cascaded into them and into a

small pit crater southwest of Cone Crater (fig. 2, 1 mi S. of

segment G). The surface lava flows were gas rich and fast

flowing, and the pähoehoe surfaces were constantly breaking

apart, reforming, and forming large, thin plates, which formed

remarkably large eddies and lava coils (Peck, 1966) where

partially congealed, thin lava skins were trapped between

plates flowing at different velocities (fig. 10).

At the peak of eruptive activity, fountain heights intermittently reached a maximum of 100 m but generally

averaged about 30-40 m. Shortly after 05:00, fountaining

waned noticeably and became more gas rich at the east end

of the fissure system, but strong activity persisted to the

west. Eruptive fissures continued to migrate southwesterly.

The westernmost fissure of this eruption (segment I, fig. 2)

opened about 06:30 in the area northeast of Pu‘u Koa ́e.

From that time on, all lava fountains gradually subsided,

and by 08:50 all vent activity stopped, although the walls of

eruptive fissures still glowed red and a thin fountain-fed

 ́a‘ä flow continued to move toward the Kamakai ́a Hills

(fig. 2). The gas-charged shelly pähoehoe of this fast-moving flow converted to  ́a‘ä within 2 km of eruptive vents,

and is a good example of the pähoehoe to  ́a‘ä transition

(Peterson and Tilling, 1980).

Pollard and others (1983) mapped the December 31

eruptive fissures and associated ground cracks in detail, and

readers may refer to their publication for detailed vent geometry, analysis of subsurface dike characteristics, and the detailed distribution of lava flows near the vent areas.

Lava from the eruption covered an area of about 7.25

km2

, the largest area covered southwest of Kïlauea Caldera

since the 1919-20 SWRZ eruption centered at Mauna Iki

(Macdonald and Abbott, 1970, table 3; Hazlett, 1993). The

lava flows were typically less than 1 m in thickness, however,

and the total volume of erupted material was estimated to be

only about 5.9 x 106 m3. The lavas were extremely fluid and

thin near eruptive vents, and hundreds of small kïpuka of the

underlying Keanakäko‘i Ash are exposed within the flows

(see Pollard and others, 1983, fig.15) for detailed mapping of

these features). The December 31 lavas everywhere consisted

of highly vesicular shelly pähoehoe and low-density fountain-fed  ́a‘ä, and their estimated DRE volume was only 3.0 x

106

 m3. This estimate should be considered a minimum, however, because some lava flowed into preeruption fissures and

cannot be accounted for.

As discussed later, the olivine-phyric lavas erupted during this 6-hr eruption were distinctly more mafic than those

of the July and September eruptions, and the lavas were characterized by abundant euhedral olivine phenocrysts (fig. 11).

Representative modes of the December 1974 lavas are given

in table 2.

TECTONIC SIGNIFICANCE OF THE DECEMBER 31

ERUPTION

The pathways by which magma reaches Kïlauea’s surface during the initial eruptive stages are controlled by the

orientation of preexisting stress fields in the Kïlauea edifice.

In the Kïlauea summit area (above about 1,000 m elevation),

most eruptions occur within the boundaries of Kïlauea

Caldera, or along either of Kïlauea’s two rift zones (fig. 12).

The December 31 eruption was not located in either the caldera

or the historically active SWRZ, however, but instead belongs

to a unique class of eruptions that take place south of the

Kïlauea Caldera, in a wedge-shaped area lying between the

SWRZ and the Koa ́e Fault System (KFS). Individual fissures in this area, including those formed on December 31,

trend about N. 75o E. and appear to have been controlled by

Figure 11.—Selvage on margin of early formed dike on the east

end of the December 31, 1974 eruptive vents, showing abundance

of euhedral olivine in this early formed lava (mode in table 2, sample

L-92-63).

CHRONOLOGICAL NARRATIVES
14 THE 1974 SUMMIT ERUPTIONS OF KÏLAUEA VOLCANO, HAWAI‘I

regional stress fields associated with the KFS instead of the

stress field governing eruptive pathways of the SWRZ, which

typically trend about N. 45o E.. The overall zone of individual en echelon fissures formed on December 31 trends N.

55o E., however, and Pollard and others (1983) speculated

that the individual ENE-striking dikes at the surface may

merge into a single NE-trending dike at depth, suggesting a

complex tectonic regime not strictly part of either the KFS or

the SWRZ.

The Koa ́e Fault System (Duffield, 1975) is a relatively

young feature on Kïlauea and may be generally described as

a reflection of the westward extension of stress fields associated with Kïlauea’s middle and lower east rift zone. The KFS

is a zone of normal faulting and had not been characterized

by eruptive activity prior to the 1970’s. Magma did, however,

migrate westward into the eastern end of the KFS during the

small eruption of May 5, 1973, and a small amount of lava

reached the surface a kilometer southwest of the Chain of

Craters Road (Tilling and others, 1987). Deformation and

seismic observation showed clearly that a dike had intruded

into the eastern Koa ́e at this time and extended west of the

surface vents (Koyanagi and others, 1983; Klein and others,

1987). Electrical Self-Potential (SP) measurements by

Zablocki (1978) demonstrated the subsurface intrusion of

magma extended nearly 3 km into the KFS.

The first recorded injection of magma into the KFS may

have taken place 8 years earlier, during and following the

eruption of December 1965 (Fiske and Koyanagi, 1968). SurFigure 12.—Location of the May 1973 and December 1974 eruptive vents, showing their relationship to the Koa ́e Fault System and to

other historical and prehistoric vents in the Kïlauea summit area.

Kilauea

Caldera

HVO

EAST RIFT ZONE

SOUTHWEST RIFT ZONE

KOAE FAULT SYSTEM

12-31-74 vents

5-05-73 vents

Mauna Ulu

Crater

0 1 2 KILOMETERS

N

EXPLANATION

May 1973 and December 1974 vents

Other historical and prehistoric vents

Halemaumau

Kïlauea

Halema‘uma‘u

KOA ́E
15

face eruptive vents for this brief eruption were all located

within the ERZ, east of the Chain of Craters Road, but major

dilation of KFS faults and ground cracking extended about

10 km westward into the KFS (Fiske and Koyanagi, 1968,

fig. 1). Although surface eruptive activity ceased by the early

morning of Christmas day, volcanic tremor continued for over

36 hours, major fractures continued to dilate, and fresh ground

cracks developed. It is probable that magma was injected

into the KFS at this time.

The en echelon eruptive vents of December 31, 1974

were individually parallel to, though not located within, the

KFS (fig. 12); they represented the largest eruption ever to

occur along the KFS trend in this area. Four other small, late

prehistoric vents have been mapped with Koa ́e orientation

near the December 31 vents, but these each had very limited

production (Walker, 1969).

The apparent entry of magma into fringes of the KFS is

a relatively new development in the evolution of Kïlauea. Is

the KFS now likely to become a significant new locus of future Kïlauea eruptive activity, as Kïlauea’s south flank continues to be displaced southward (Fiske and Swanson, 1992)?

Or, will the pace of extensional dilation in the KFS exceed

the supply of magma to this zone, so that most volcanic activity will be intrusive and not extrusive?

JANUARY 1-10, 1975 INTRUSION

As described above, very rapid summit deflation preceded and accompanied the December 31 eruption. However, even after the cessation of visible activity, sharp summit

deflation, as measured by the E-W component of the IdealAerosmith tiltmeter, persisted for the next 24 hr at rates between 2 and 3 μrad/hr (fig. 8). Electronic Distance Measurement (EDM) measurements across Kïlauea Caldera were

obtained multiple times during following the December 31

eruption. Major contractions of these EDM lines (fig. 8) also

showed that deformation of the summit area continued for

about 3 days following cessation of eruptive activity. Collectively, the persistence of sharp summit deflation as measured

by tilt and EDM surveys after cessation of visible activity,

the continuing volcanic tremor through January 2, 1975, the

continued intense seismicity downrift of the December 31

vent fissures through early January, and ground cracking in

vent as well as nonvent areas (Pollard and others, 1983)

strongly suggested substantial posteruption intrusive activity

downslope from the area where the KFS meets the SWRZ

(fig. 12). This inference is also well substantiated by earthquake distribution (see fig. 23).

An April 1975 level profile across the area of intense

January seismicity revealed a 4-km-wide “bulge” of 9 cm height

relative to the previous survey in 1921 (unpublished HVO

Monthly Report, 8/21/75 - 9/20/75). It is not known when this

uplift actually formed, but it is interesting to speculate that it

could reflect the injection of magma into this area in January

1975.

GEOPHYSICAL OBSERVATIONS

GEODETIC MONITORING

Kïlauea Volcano is well known to deform in a quasielastic manner in response to the shifting supply of stored

magma within its edifice (Mogi, 1958; Dvorak and others,

1983; Dzurisin and others, 1984). Geodetic measurements

by HVO staff document such deformation across Kïlauea’s

summit area during the last half of 1974 and show that the

centers of preeruption magmatic inflations and syneruptive

deflations were migrating systematically westward across the

volcano during this time.

Continuous measurements of Kïlauea deformation are

only possible at specific points on the volcano (measurements

of tilt at tiltmeter sites or of strain along local survey lines or

across cracks). Regional studies (EDM, GPS, or leveling

surveys) are required to understand behavior of larger areas

of the volcanic edifice, but such surveys are time consuming

and thus infrequent. Because of this, the HVO field surveys

were not made at the ideal times immediately before and immediately after eruptive activity, but instead spanned longer

periods, which include effects of both preeruptive, syneruptive,

and posteruptive deformation (table 3). Continuous or daily

tilt measurements at Uwëkahuna can be used to infer the nature of deformation between geodetic surveys.

SUMMIT TILT — THE UWËKAHUNA RECORD

Beginning in 1956, daily measurements of summit tilt

at Kïlauea have been made by means of a short-based

water-tube tiltmeter at Uwëkahuna Vault, northwest of

Halema‘uma‘u (fig. 3). These measurements were supplemented by the installation of a continuously recording mercury-capacitance tiltmeter (Ideal-Aerosmith) at the same site

in June 1965. The water-tube tiltmeter readings give N-S

and E-W tilt components, but the Ideal-Aerosmith tiltmeter

record yields only the E-W component. These Uwëkahuna

tilt measurements record tilt at only a single point, but nonetheless provide a reliable indicator of the general degree of

tumescence of Kïlauea’s summit reservoir, as it inflates or

deflates in response to influx or withdrawal of magma (Eaton

and Murata, 1960).

A plot of summit tilt for the period January 1973 to

August 1975 (fig. 13) provides context for the following discussion of the geodetic survey results. The first four months

of 1973 show an essentially flat tilt curve with minor oscillations—a pattern characteristic of the quasi-steady-state regime that had prevailed since February 1972. The May-November 1973 period included the two eruptive outbreaks in

the Pauahi area in May, each of which produced abrupt summit deflations of ~20 μrad on the E-W component (fig. 13).

Despite these two deflation events in 1973, the entire 1972-

74 Mauna Ulu eruption resulted in negligible net inflation of

Kïlauea’s summit (Tilling and others, 1987).

After a small summit deflation associated with the voluminous May-June 1974 eruptive episode at Mauna Ulu (TilCHRONOLOGICAL NARRATIVES
16 THE 1974 SUMMIT ERUPTIONS OF KÏLAUEA VOLCANO, HAWAI‘I

Table 3.—Geodetic surveys in Kïlauea Volcano summit region before, during, and after the

1974 summit eruptions.

Figure 13.—Plot of E-W and N-S tilt components for the Uwëkahuna water-tube tiltmeter, 1973 to 1975.

NORTH – SOUTH COMPONENT

Down = deflation

EAST – WEST COMPONENT

Down = deflation

300

200

100

0

TILT, IN MICRORADIANS

1973 1974 1975

Hi ́iaka-Pauahi

5/5/73

Pauahi

11/10/73

ERZ Intrusion

3/24/74

Summit

7/19–22/74

Summit

9/19/74

SWRZ 12/31/74

May–June eruptive 

episode at Mauna Ulu
17

ling and others, 1987), the summit region began to inflate rapidly, at a rate comparable to that for the period of net inflation

in the latter half of 1971, which encompassed the summit (August) and SWRZ (September) eruptions. Similarly, net summit inflation during the latter half of 1973 indicated a blocking of the supply conduits to Mauna Ulu (Tilling and others,

1987) and helped set the stage for a return to summit eruptive

activity. The July 1974 eruption, which also involved the uppermost ERZ, resulted in an 18-μrad summit deflation (E-W

component, fig. 13). On the other hand, the September eruption produced a sharp inflation of ~14 μrad (fig. 7), as is commonly seen for summit eruptions, such as in 1971 (Duffield

and others, 1982, fig. 2A). However, neither of the relatively

small July or September eruption-related deformation events

affected the overall steady summit inflation, that continued

until onset of the December eruption. The December eruption

was accompanied by a major, precipitous summit deflation

(see Figs. 8, 13), which continued at gradually declining rates

until the evening of January 5, when the Uwëkahuna tilt finally “bottomed out.” This prolonged summit deflation and

accompanying seismicity demonstrated that magmatic intrusion into Kïlauea’s southwest flank persisted for days after

the cessation of the December surface eruption. Summit deflation as measured by the Uwëkahuna short-base water-tube

tiltmeter indicated that the E-W tilt bottomed out on January

3, but that the N-S tilt did not bottom out until January 16.

Seismic activity was intense in the area southeast of the lower

SWRZ for 2 weeks, with dozens of sharp earthquakes felt each

day by residents of Pähala. We have arbitrarily considered the

January 1975 intrusion as lasting through about January 10.

The Ideal-Aerosmith E-W tilt data indicate that the summit deflation associated with the December eruption totaled

about 132 μrad; however, the short-base water-tube tiltmeter

measured somewhat less total deflation between maxima and

minima: E-W component, 118 μrad; and N-S component, 117

μrad (fig. 13). This deflation of Kïlauea’s summit region is

the greatest since that of the September 1961 ERZ eruption,

or possibly even since that of the 1959-60 Kïlauea Iki-Kapoho

activity in the lowermost, subaerial part of the ERZ3. It also

should be noted that before the large deflation associated with

the December 31 eruption, the inflation state of the volcano

attained its highest level since systematic summit tilt measurements began in 1956. Since that time, the Kïlauea summit has been showing overall deflation, and its reservoir is in

its least swollen state since 1913 (Tilling and Dvorak, 1993).

The Uwëkahuna water-tube tilt record is conventionally plotted as separate traces of the N-S and E-W tilt compoFigure 14.—Cumulative plots of Uwëkahuna water-tube tilt measurements during the July, September, and December 1974

eruptions. The resultant vectors shown are calculated daily readings of E-W and N-S tilt components.

3One unexpected secondary effect of this massive deflation was an apparent change in gas emission from the summit area. During the period of

summit inflation prior to December, Halema‘uma‘u was fuming profusely

and dense fume obscured all views into the deep crater of Mauna Ulu. Immediately after the December-January deflation, Halema‘uma‘u stopped all

fuming, and the bottom of Mauna Ulu was visible for the first time since

July 1974. These changes likely reflect the withdrawal of magma to deeper

levels beneath Halema‘uma‘u and Mauna Ulu.

JULY SEPTEMBER DECEMBER–JANUARY

7-19-74

7-20-74

7-21-74

9-19-74

9-18-74

12-30-74

12-31-74

1-01-75

1-02-75

1-03-75

1-04-75

1-21-75

1-10-75

N

S

W E

0 25 microradian (μrad)

Vector direction Vector scale

GEOPHYSICAL OBSERVATIONS GEODETIC MONITORING
18 THE 1974 SUMMIT ERUPTIONS OF KÏLAUEA VOLCANO, HAWAI‘I

nents (fig. 13), but a plot of resultant tilt vectors for the three

1974 eruptions is of substantial interest (fig. 14). These plots,

based on daily readings at roughly 24-hr intervals, show that

the centers of deflation moved southwestward relative to

Uwëkahuna during the July and December eruptions, but that

a net inflation of the Halema‘uma‘u area took place during

the September eruption and resulted in northwestward tilt at

Uwëkahuna. During the course of the July deflation, the deflation center migrated 18o

 clockwise (as seen from

Uwëkahuna), and during the December-January deflation it

migrated 30o

 in the same direction. Tilt vectors from a single

point do not, of course, indicate the distance to deformation

centers, but if one assumes that the deflation centers are

roughly in the Halema‘uma‘u area (as is indicated by regional

spirit-level tilting, EDM, and leveling traverse surveys - - see

next section), the shifting deflation vectors measured at

Uwëkahuna indicate a westward or southwestward migration

of several hundred meters during these deflation events. Another advantage of plotting resultant vectors from the

Uwëkahuna water-tube tiltmeter (as in fig. 14) is that the total tilt magnitude can be calculated (about 164 μrad in the

case of the December-January deflation).

SUMMIT SPIRIT-LEVEL TILT SURVEYS

Sixteen spirit-level tilt4 stations around the Kïlauea summit were routinely occupied at the same time as other regional deformation monitoring efforts and provide a general

Figure 15.—Spirit-level tilt vectors at Kïlauea summit stations for periods between surveys from March 1974 and October 1975. Survey

periods include the earliest and latest readings for each survey interval. A, survey period 3/12/74 to 7/26/74; B, survey period 7/22/74 to

10/10/74; C, survey period 9/30/74 to 1/13/75; D, survey period 1/06/75 to 10/01/75.

0 5 KILOMETERS

19°

20'

19°

23'

19°

26'

155°20' 155°25'

B

0 5 MICRORADIANS

0 5 KILOMETERS

19°

20'

19°

23'

19°

26'

155°20' 155°25'

D 0 10 MICRORADIANS

4The term “spirit-level tilting” (Kinoshita and others, 1974) will be used

in this paper to describe the common field method of monitoring tilt that is

also referred to as “dry-tilt” (Yamashita, 1981) and “single-setup leveling”

(SSL—Yamashita, 1992). Yamashita suggests that SSL be the preferred term

henceforth, but we prefer to use the original term, “spirit-level tilting,” because in Hawai‘i some stations involve more than one setup.

19°

20'

19°

23'

19°

26'

155°20' 155°25'

C 0 5 KILOMETERS

0 10 MICRORADIANS

0 5 KILOMETERS

HVO

Kïlauea

Caldera

19°

20'

19°

23'

19°

26'

155°20' 155°25'

A

0 5 MICRORADIANS
19

understanding of slope changes on the volcanic edifice. Individual calculated tilt vectors can be influenced by local and

diurnal perturbations, but when combined with other survey

methods (EDM and leveling) are important tools for obtaining a more comprehensive deformation picture for the time

periods between surveys.

The Kïlauea spirit-level tilt stations were occupied just

after the July eruption (fig. 15A), and record the summit inflation that had occurred since April. The apparent center of

this inflation was located east of Halema‘uma‘u, near

Keanakäko‘i Crater, as is also apparent from concurrent EDM

and leveling surveys (see figs. 16A, 17). The next survey was

made after the September eruption (fig. 15B) and shows that

the center of inflation after the July eruption had shifted westward, as was also shown by EDM and leveling surveys. The

next spirit-level tilt survey, made in January 1975, shows the

overwhelming influence of the major deflation which followed

the December 31 eruption and points to a deflation center

southeast of Halema‘uma‘u (fig. 15C). Another survey late

in 1975 (fig. 15D) shows that the major reinflation following

the December-January deflation was centered in roughly the

same area.

HORIZONTAL DISTANCE SURVEYS

EDM surveys just after the July eruption showed that

overall inflation in the preceding 3 months was located in the

south-central caldera, east of Halema‘uma‘u (fig. 16A). A

similar pattern is shown by tilt and leveling surveys (figs. 15,

17). The next EDM survey of the summit area was not made

until November, after the September eruption, but at this time

the net inflation since July shifted westward, to the

Halema‘uma‘u area (fig. 16B). The next regional summit

EDM survey, conducted in January 1975 after the December

31 eruption, showed the combined effects of the massive summit deflation, combined with the effects of magmatic injection into the area southwest of Kïlauea Caldera (fig. 16C).

Repeated EDM measurements during and immediately following the December 31 eruption (fig. 8) showed well the

rate of summit contraction. Following the December-JanuFigure 16.—Calculated best-fit horizontal displacement vectors from EDM surveys conducted across Kïlauea during the period March 1974

to October 1975 (floating horizontal datum). Survey periods include the earliest and latest readings for each survey interval. A, survey period

4/12/74 to 7/25/74; B, survey period 7/22/74 to 11/7/74; C, survey period 11/4/74 to 1/17/75; D, survey period 1/14/75 to 9/19/75.

+

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A

HVO

155°20' 155°10'

20°

26'

19°

26'

0 20 cm

Vector scale

0 5 km

+

155°20' 155°10'

20°

26'

19° 26'

+

+

+

+

+

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+

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+

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Vector scale

0 5 km

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B

155°20' 155°10'

20°

26'

19°

26'

0 20 cm

Vector scale

0 5 km

+

+

+

155°20' 155°10'

20°

26'

19° 26'

D

+

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0 20 cm

Vector scale

0 5 km

+

GEOPHYSICAL OBSERVATIONS GEODETIC MONITORING
20 THE 1974 SUMMIT ERUPTIONS OF KÏLAUEA VOLCANO, HAWAI‘I

ary 1975 deflation the summit reservoir inflated for most of

1975 , but the center of inflation was now centered well south

of Halema‘uma‘u (fig. 16D).

LEVELING SURVEYS

Leveling precisely documents vertical changes on a

volcano’s surface and may be used to help constrain volume

changes associated with inflation or deflation of the volcano’s

surface. During the post-1971 period when supply conduits

to the active Mauna Ulu eruption were unconstricted, little

net deformation occurred at the Kïlauea summit (fig. 17A),

but as the conduits to Mauna Ulu became blocked in May

(Tilling and others, 1987), the summit began to engorge with

magma. Leveling of Kïlauea’s summit area after the small

July and September eruptions documented the overall inflation that had taken place in the months prior to these surveys

(figs. 17B, 17C). Analysis of these data showed that the centers of preeruption inflation had shifted westward by nearly 2

km over the April-October interval (see fig. 18). The survey

after the December eruption (fig. 17D) reflected a combination of preeruption inflation, syneruptive deflation, and the

beginning of posteruption inflation and shows a center of net

deflation about 1 km southeast of Halema‘uma‘u (fig. 18).

All the above geodetic data (tilt, EDM, and leveling)

consistently show that geodetically defined centers of surface deformation migrated from east to west across Kïlauea

Caldera in 1974, as the supply conduits to the long-active

Mauna Ulu vent became constricted and magma began to

accumulate within the summit area. The westward migration

of the deformation or “pressure” centers indicates that the

subsurface magmatic storage reservoirs moved from an initial center near Keanakäko‘i to the Halema‘uma‘u area and

Figure 17.—Leveling data for the Kïlauea summit area for five surveys conducted from November 1973 to January 1975. Contour intervals

in millimeters, referenced to a non-stable datum on the southeast slope of Mauna Loa. Survey periods include the earliest and latest readings

for each survey interval. A, survey period 11/13/73 to 4/05/74; B, survey period 4/04/74 to 7/31/74; C, survey period 7/26/74 to 10/01/74; D,

survey period 9/30/74 to 1/13/75. Tilt vectors (in microradians) shown are determined from available summit spirit-level tilt measurements

for approximately the same survey periods.

155°16'

19°

25'

C HVO

0 40 80 MICRORADIANS

0 1 2 KILOMETERS

50

25

25

75

100

125

150

175

200

225

250

–10

0

10

10

10 −10

0

20

30

–20

20

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–20

–30

–40

–50

–60

–30

–20

KÏLAUEA IKI

JC

OT

PUHIMAU HOT

AREA

HVO

SW

SSW SMT

SSW

155°16'

19°

25'

A

HALEMA‘UMA‘U

0 20 40 MICRORADIANS

0 1 2 KILOMETERS

60

0

40

20

100

100

120

200

160

180

80

60

80

140

60

SW

SW

NOSE

NAIL

JC

ER 3

HVO

155°15'

19°

25'

B

0 40 80 MICRORADIANS

0 1 2 KILOMETERS

HVO −300

−400

−600

0

−100

−500

−400 155°16'

19°

25'

D

0 60 120 MICRORADIANS

0 1 2 KILOMETERS

−

−

300

−

400

500

−200
21

then to an area southeast of Halema‘uma‘u, possibly outside

the bounds of Kïlauea Caldera.

SEISMIC OBSERVATIONS

Seismicity provides critical evidence of magma movement within Kïlauea’s shallow magma conduit system and

was especially instructive during the last half of 1974, when

extensive magma migration occurred beneath the summit area.

Volcanic earthquakes during eruptive or intrusive swarms are

primarily related to the emplacement of dikes and may thus

extend far beyond the eruptive vent area as magma moves

elsewhere beneath the surface. The migration of earthquakes

with time shows the growth and propagation of new dikes

before, during, and after surface eruptive activity.

Seismicity between these intense swarms occurs at

much lower rates than during eruptive crises. This

interevent seismicity is generally concentrated in Kïlauea’s

summit region and usually accompanies inflation of the

shallow summit magma reservoir. Earthquakes during

these times are probably caused by a gradual pressure increase in the magma reservoir(s) and occur either adjacent

to magma bodies undergoing expansion, or on nearby fractures gradually stressed by the uplift, extension, and pressure increase in the entire summit region. General seismicity associated with various periods in 1974 are discussed

in the following sections; more detailed analyses are given

in Klein and others (1987).

Figure 18.—Inflation centers (stars) for the intervals between leveling surveys on April 4, July 31, and October 1, 1974. Data derived

from best-fit point-source solutions for leveling data by J. J. Dvorak

(1992, written communication). Lavas erupted from the “northern”

vent fissures (N) and the “southern” fissures (S) for the July 19-22,

1974 eruption are chemically distinct (see table 4).

Figure 19.—Daily number of “volcanic” earthquakes (as determined by their locations and depths)

timed and located by HVO during 1974 and early 1975. These events are at 0-5 km depth below

the surface and under Kïlauea’s rift zones or summit caldera. They include swarms accompanying eruptions and intrusions and the more continuous seismicity of the magma system as it

inflates between eruptions.

KÏLAUEA

CALDERA

Uwëkahuna HVO

4-04 to 7-31-74

7-31 to 10-01-74

10-01-74 to 1-23-75

Keanakäko‘i

vent

fissures

July (S)

December

September

Halema‘uma‘u

Crater Rim

Drive

July (N)

0 1 2 KILOMETERS

155°17'30"

19°

25'

INTRUSION

3/24/74

LAST MAJOR

MAUNA ULU

ACTIVITY

ERUPTION

7/19/74

ERUPTION

12/31/74

ERUPTION

9/19/74

1974 1975

JAN JAN

EARTHQUAKES PER DAY

GEOPHYSICAL OBSERVATIONS GEODETIC MONITORING
22 THE 1974 SUMMIT ERUPTIONS OF KÏLAUEA VOLCANO, HAWAI‘I

The daily numbers of shallow (0-5 km) volcanic earthquakes that were timed and located in 1974 and early 1975

are plotted in figure 19. These earthquakes mostly occurred

beneath Kïlauea’s summit region or rift zones and are for the

most part related to magmatic rather than tectonic processes.

All processed events are included in this plot; the magnitude

above which event sampling is complete may vary slightly

with time but is generally about M=1.5. Four magmatic events

(one minor intrusion and three eruptions) occurred during this

period. The level of interevent seismicity increased slightly

through 1974, reflecting the gradual inflation of the summit

magma reservoir, and culminated with the very large eruption and intrusion at year’s end.

After an intrusion near Mauna Ulu on March 24

(Klein and others, 1987), the remainder of March and April

was nearly aseismic (fig. 20A), but earthquakes resumed

in the caldera in May (fig. 20B). Periods like May, in which

Kïlauea Caldera earthquakes occur in low-intensity swarms

lasting a few weeks, are characteristic of times of summit

inflation. A similar pattern persisted through June and

early July (fig. 20C). Earthquakes between March and July

also occurred on the uppermost section of the ERZ between Keanakäko‘i and Ko ́oko ́olau, suggesting that the

ERZ was participating in the summit inflation and was

slowly receiving injections of magma.

Figure 20.—Earthquakes in the Kïlauea summit region related to and preceding the eruption of July 19, 1974. A, the period 3/25/74 - 4/30/

74; B, the period 5/1/74 - 5/31/74; C, the period 6/1/74 - 7/18/74; D, earthquakes during 7/19/74, when the caldera and upper ERZ eruption

occurred. Depth, 0-5 km.

MAGNITUDES

0.0+

1.0+

19°

20'

19°

25'

A

155°20' 155°15'

MAGNITUDES

0.0+

1.0+

B

10 KM 19°

20'

19°

25'

155°20' 155°15'

MAGNITUDES

0.0+

1.0+

C

19°

20'

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25'

155°20' 155°15'

MAGNITUDES

0.0+

1.0+

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19° 10 KM 20'

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25'

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23

MAGNITUDES

0.0+

1.0+

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19° 20'

19° 25'

155°20' 155°15'

MAGNITUDES

0.0+

1.0+

A

19° 20'

19° 25'

155°20' 155°15'

MAGNITUDES

0.0+

1.0+

B

19° 20'

19° 25'

155°20' 155°15'

 ERUPTION OF JULY 19, 1974 AND

THE FOLLOWING 2 MONTHS

The earthquake swarm immediately preceding the July

eruption was very intense and lasted only 9 hr (fig. 20D).

The swarm was accompanied by a modest deflation of about

3 μrad, after which tilt nearly leveled off. A larger and more

rapid summit deflation occurred at about 10:30, however,

when high-amplitude tremor began and the number of earthquakes that could be timed and located sharply diminished.

Magma reached the surface about 2 hr later.

The geometry of the seismic zone, presumably reflective of the dike feeding the eruption, is slightly elongate

and extends from about 3.5 km depth below Ko ́oko ́olau to

within a kilometer or less of the surface near Puhimau (Klein

and others, 1987, fig. 43.63.C). The inferred dike thus dips

steeply to the southeast, but its geometry cannot be adequately

resolved by the distribution of earthquake foci; it is likely a

thin blade structure. The locations and depths of earthquakes

within about 1 km of the surface are poorly determined. The

apparent lack of events in this depth range may result from

this poor record or from the fact that the low-rigidity and

low-stress surface layer may be too weak to generate earthquakes large enough to be detected.

Earthquakes migrated uprift at an apparent rate of about

160 m/hr (Klein and others, 1987, fig. 43.63D), probably along

a dipping structure. The fact that the vents were uprift of the

swarm location near Keanakäko‘i and in the caldera also suggests that uprift migration took place. If the earthquakes

propagated up a structure dipping about 60o, the updip migration would be about 300-400 m/hr; this was apparently a

forceful intrusion of a dike leading from the East rift conduit

Figure 21.—Earthquakes in the Kïlauea summit region related to and preceding the eruption of September 19, 1974. A, the period 7/20/

74 - 8/11/74; B, the period 8/12/74 - 9/15/74; C, during the period 9/16/74 - 9/19/74, which immediately preceded the caldera eruption

of 9/19/74. Depth, 0-5 km.

GEOPHYSICAL OBSERVATIONS GEODETIC MONITORING
24 THE 1974 SUMMIT ERUPTIONS OF KÏLAUEA VOLCANO, HAWAI‘I

below Ko‘oko‘olau up to the surface. The intense seismicity

and low tilt rates during this swarm show that new rock was

breaking under high magma pressure and relatively low volume flow. When the dike reached the surface, pressure and

seismicity decreased, and volume flow, tremor, and tilt rate

increased.

The earthquakes following the July eruption (fig. 21A)

were located away from the deeper parts of newly emplaced

dike and shifted to the southeast caldera and western part

of the KFS fault system. This change in pattern presaged a

shift of magmatic activity through the caldera and to the

southwest.

THE ERUPTION OF SEPTEMBER 19, 1974

The September caldera eruption was seismically unlike

the others during the year and did not generate an intense earthquake swarm. It was instead preceded by 3 days of slightly

elevated seismicity south of the caldera along the west end of

the KFS (fig. 21B). This seismicity may outline a magma conduit or family of dikes that extended south from Kïlauea

Caldera, then turned SW between the SWRZ and the western

KFS. Low-intensity swarms in the summit region lasting several days are typical of intereruption periods when the volcano is inflating (Klein and others, 1987), and the activity of

Figure 22.—Earthquakes in the Kïlauea summit region related to and preceding the eruption of December 31, 1974. A, the period 9/20/

74 - 10/19/74; B, the period 10/20/74 - 12/30/74; C, eruption-related earthquakes on the morning of 12/31/74 (00:00 - 09:00). Depth, 0-

5 km.

MAGNITUDES

0.0+

1.0+

2.0+

A

19° 20'

19° 25'

155°20' 155°15'

MAGNITUDES

0.0+

1.0+

2.0+

3.0+

B

19° 20'

19° 25'

155°20' 155°15'

MAGNITUDES

0.0+

1.0+

2.0+

C

19° 20'

19° 25'

155°20' 155°15'
25

31 eruption, as it had done before the September eruption.

Beginning on December 24, this heightened activity apparently migrated very slowly into both the upper ERZ and

western KFS at roughly a kilometer per day (Klein and others, 1987, fig. 43.66). This apparent slow leaking of magma

was another symptom of excessive supply in the summit

reservoir.

Seismic patterns divide the eruption of December 31

into two different phases: an intense but short swarm just south

of the caldera (related to the eruption-feeding dike), and a

subsequent, more energetic, 2-week-long seismic crisis related to a massive deflation of Kïlauea’s summit and an intrusion that propagated as far as 18 km to the southwest.

The small cluster of earthquakes about 3 km south of

Halema‘uma‘u lies directly below and slightly east of the eruptive vents (fig. 22C). Earthquakes began in the middle of this

zone around 16:00 on the December 30 and were about 2.5

km deep. The seismic zone slowly grew in size until 00:00-

01:00 on the December 31 when it was about 2 km in diameter

and reached maximum intensity. The events then ranged between 1 and 4 km depth. Earthquakes migrated upward at about

5 km/hr during the most intense period, but the dike grew

aseismically upward at about 300 m/hr during the last 3 hr of

its journey to the surface (Klein and others, 1987, fig. 43.67).

Earthquakes then diminished and tremor intensity increased

(fig. 7, 19) prior to lava breakout shortly before 03:00. This

September 16-19 (fig. 21C) is not distinguishable from background seismicity during the preceding 2 months, except for

the activity south-southwest of the caldera. The preeruption

earthquakes may have accompanied a relatively passive, lowvolume intrusion into the western KFS. Although the swarm

was a weak one, it suggests southwestward earthquake migration (Klein and others, 1987, fig. 43.64.D). When the brief

September 19 eruption began, earthquakes abruptly ceased.

This could be expected if magma pressure had induced the

earthquakes, but had suddenly dropped when magma flow was

redirected to an active vent.

The month following the eruption saw most seismicity

confined to the south caldera (fig. 22A). Beginning in late

October seismicity again moved into the western KFS (fig.

22B), suggesting that an intrusion into this area took place,

similar to the one prior to the September eruption. Seismicity in the north caldera and the highest Uwëkahuna tilt values

recorded since 1956 (see section “Geodetic Monitoring”) also

indicated that Kïlauea was highly inflated with magma, in

preparation for another eruption.

ERUPTION OF DECEMBER 31, 1974

Summit seismicity southwest and southeast of the

caldera intensified during the week prior to the December

GEOPHYSICAL OBSERVATIONS GEODETIC MONITORING

Figure 23.—Earthquakes that accompanied the eruption of 12/31/74 and the intrusion of early

January. A, distribution of earthquakes (plan view); B, time distribution of earthquakes southwest of the summit and south of the SWRZ, 12/30/74 to 1/7/75.
26 THE 1974 SUMMIT ERUPTIONS OF KÏLAUEA VOLCANO, HAWAI‘I JANUARY 1975 DECEMBER 1974

DISTANCE (KM) SW TO NE A A'

A A'

A

155°25' 155°20' 155°15'

19°

20'

19°

15'

19°

25'

B

1

A

A'

MAGNITUDES

0.0+

1.0+

2.0+

3.0+

MAGNITUDES

0.0+

1.0+

2.0+

3.0+

Figure 23.—Continued.
27

intense swarm accompanied the lateral and vertical growth of

the dike that fed the eruption, similar to the July eruption.

About the time this first swarm reached maximum intensity, additional earthquakes began migrating to the southwest. Earthquakes not only occurred parallel to and southeast of the primary SWRZ surface cracks and fissures, but

also in a cluster that extends 12 km south into Kïlauea’s south

flank from the SWRZ to the coast (fig. 23A). The seismically

active dike initially propagated southwestward at about 1.3

km/hr for about 14 km, then slowed to half that speed as it

extended an additional 4 km (fig. 23B). The locus of seismicity then migrated southward at the west end of the Hilina

Fault System. Earthquakes in this area ranged in depth from

0 to 9 km. They appear typical of tectonic earthquakes in

Kïlauea’s mobile south flank. These flank earthquakes are

likely triggered by compressional stresses from newly

emplaced dikes upslope and are analogous to flank earthquakes of the ERZ.

GEOELECTRICAL SURVEYS

Geoelectrical studies during the early 1970s delineated

the distribution of numerous subsurface thermal anomalies

in the summit area of Kïlauea (Zablocki, 1976, 1978). An

electrical self-potential (SP) survey in the summer of 1972

revealed a positive SP anomaly along the southern margin of

the caldera (an area of frequent historical eruptive activity)

that also extended more than 2 km to the south, outside the

eruptively active caldera, but above a zone of shallow (<5 km

depth) earthquakes. Zablocki (1978) suggested that this SP

anomaly represented a previously unrecognized active part

of the Kïlauea summit magma reservoir.

The eruption of December 31, 1974 was anomalous, as

it occurred in an area where no eruption had ever taken place

in historical times (Peterson, 1967). The eruptive fissures

opened directly over the earlier recognized SP high south and

southwest of the Kïlauea Caldera (fig. 24) and migrated southwestward into the SWRZ. Earlier 1974 summit eruptions

had also occurred in areas of positive SP anomalies (fig. 24),

although the geochemically anomalous “southern” vents of

the July eruption are located south of the caldera SP anomaly.

Zablocki conducted extensive geoelectrical surveys

across the December fissures, using both SP and very lowfrequency (VLF) induction techniques. His studies showed

that several surface segments of the en-echelon eruptive system were accompanied by SP anomalies on their uprift, or

eastward, extensions (never to the west). This suggested that

the subsurface dikes feeding the surface vents plunged down

to the east, toward the inferred extension of Kilauea’s magma

reservoir system. Further support for this eastward plunge of

feeding dikes is provided by an abrupt increase in bicarbonate content of water in a research drill hole located 1 km

northeast of the easternmost vents (Tilling and Jones, 1996).

This change, apparently coincident with the December 31

eruption, was interpreted by Tilling and Jones to reflect a

pulselike influx of CO2

 associated with magma migrating at

depth beneath the drill site. The SP and VLF data also indicated that the dikes were near-vertical or dipped steeply to

the north, observations supported by later modeling of detailed deformation measurements (Pollard and others, 1983).

GEOCHEMISTRY AND PETROLOGY

In the following discussion, a working knowledge of

magma-transfer processes at Kïlauea will be useful, in particular the dynamics between the summit magma reservoir

and intrusive/eruptive activity in the summit area or the rift

zones. For a general summary of “how Kïlauea works,” the

interested reader is referred to the classic work of Eaton and

Murata (1960) and to review papers by Decker (1987) and

Tilling and Dvorak (1993).

Major-element chemical analyses were obtained for 34

samples of lava erupted during the three summit eruptions of

1974 (tables 4-6); 27 of these were collected during or shortly

after the eruptions and analyzed in the late 1970s, and 7 were

collected and analyzed in 1992 to round out the sample suites.

In terms of gross averages (that is, not MgO-adjusted for olivine-control variations), the compositions of the July and

Figure 24.—Distribution of electrical self-potential fields in the area

of the southern Kïlauea Caldera and to the south, as measured in

July and August 1972. S-P values measured as potential between

electrodes spaced 100 m apart, and contoured as millivolts/100m

(from C. Zablocki, 1972, unpublished HVO monthly report, 9/21/

72 to 10/20/72).

vent

fissures

 

+

+

+

+

+

KÏLAUEA,

CALDERA

Uwëkahuna HVO

July (S)

December

September

Halema‘uma‘u

C rater Rim

Drive

July (N)

0 1 2 km

-600 mv

-200 mv

-400 mv

0 mv

-200 mv

-400 mv

0 mv

Keanakäko‘i

155°17'30"

19°

25'

GEOCHEMISTRY AND PETROLOGY
28 THE 1974 SUMMIT ERUPTIONS OF KÏLAUEA VOLCANO, HAWAI‘I

Figure 25.—Average abundance of major oxides for lava erupted at Mauna Ulu (see fig. 1) during the period 1970-1974 and in the

July, September, and December 1974 eruptions (fig. 2). A, SiO2; B, Al2O3 and “FeO” (total Fe expressed as FeO); C, MgO and CaO;

D, Na2O and TiO2; E, K2O and P2O5. In these plots, the compositional distinction between the lavas erupted from the “northern”

and “southern” vent fissures of the July 1974 eruption (see text) is not considered.

50.50

50.00

49.50

49.00

Mauna Ulu

1970–74

July 1974

September

1974

December

1974

July 1974

September

1974

December

1974

July 1974

September

1974

December

1974

July 1974

September

1974

September

1974

December

1974

July 1974

December

1974

A

14.00

13.50

12.50

11.50

13.00

12.00

11.00

Mauna Ulu

1970–74

Al203, IN WEIGHT PERCENT

B

2.50

2.40

2.20

2.00

2.30

2.10

1.90

Mauna Ulu

1970–74

Na2O, IN WEIGHT PERCENT

D

0.50

0.45

0.35

0.25

0.40

0.30

0.20

Mauna Ulu

1970–74

OXIDE, IN WEIGHT PERCENT

OXIDE, IN WEIGHT PERCENT SIO2, IN WEIGHT PERCENT

E

14.00

15.00

13.00

12.00

11.00

11.00

11.00

11.00

11.00

Mauna Ulu

1970–74

C

Al203

"FeO"

Na2O

TiO2

K2O

P2O5

MgO

CaO
29

Kïlauea olivine-control lines of Wright (1971) also shown

for reference. While some of the chemical variations obviously imply effects of olivine control, the lack of coherent

linear variation against MgO also requires the involvement

of other petrologic processes (for example, fractionation or

mixing of magma batches). In particular, some plagioclasebearing lavas of the July eruption (tables 2, 4) are differentiated or “hybrid” with values of MgO < 7.00 weight percent,

an observation noted previously by Wright and Tilling (1980).

Such an occurrence apparently marks a first for historical

Kïlauea summit lavas.

Whereas the lavas of the September and December eruptions define relatively homogeneous compositional groups,

the lavas of the July eruption do not. While the average composition of the July lavas, taken as a single group, is quite

similar to the average composition of the September lavas

September lavas are fairly similar and slightly less mafic than

the mean composition for lavas for the 1969-74 Mauna Ulu

eruptions (fig. 25A). In contrast, the December 1974 lavas

are more mafic than any produced during the July and September eruptions, as well as during the Mauna Ulu eruptions

(fig. 25A). These December lavas (table 6; Fig. 25C) are the

most magnesian historical summit lavas since those produced

in the 1959 Kïlauea Iki eruption (Murata and Richter, 1966;

Wright, 1973) or the August 1968 Hi ́iaka upper ERZ eruption (Jackson and others, 1975; Wright and others, 1975).

Moreover, the December 1974 lavas are also more magnesian than the prehistoric Kïlauea summit lavas exposed in the

caldera walls (Casadevall and Dzurisin, 1987).

Figure 26 shows the compositions of the July, September, and December 1974 lavas plotted on the MgO-variation

diagram commonly used for Hawaiian lavas, with the three

Table 4.—Chemical analyses of lavas erupted in July 1974, grouped according to vent location (see fig. 18 and text). [Values given in

weight percent; LOI = loss on ignition].

(N) (S)

GEOCHEMISTRY AND PETROLOGY
30 THE 1974 SUMMIT ERUPTIONS OF KÏLAUEA VOLCANO, HAWAI‘I

(fig. 25), the July lavas actually exhibit a bimodal composition reflected in their vent locations (table 4). Lavas erupted

from fissures south of Keanakäko‘i Crater (figs. 2, 3) (hereafter referred to as the “southern” fissures) are distinctly more

silicic and less magnesian than lavas erupted from fissures

north of Keanakäko‘i Crater (hereafter called the “northern”

fissures). The lavas from the “southern” fissures are distinctly

more differentiated in composition (table 4, fig. 26) and contain megascopically conspicuous plagioclase — a rarity at

Kïlauea’s summit. Significantly, the compositional trends of

lavas from the “southern” and “northern” fissures, as determined by their estimated time of eruption, define two curves

that appear to converge toward a single, uniform composition as the eruption progressed, presumably reflecting greater

efficiency of magma mixing as the stored magma was flushed

out (fig. 27).

Wright and Tilling (1980) considered the differentiated

July 1974 lavas to be the evolved “daughters” of magma residing in the Kïlauea shallow storage system since 1972, or possibly since mid-1970. Their calculations further suggested that

the eruptive products of September and December resulted

from mixing of various combinations of several olivine-conFigure 26.—MgO-variation diagrams for July, September, and December 1974 lavas, compared with “olivine-control” lines (KIL 1840,

PH KIL CAL, and KIL 1959) for Kïlauea lavas (see Wright, 1971, Table 3). A, SiO2; B, Al2O3; C, CaO; D, P2O5; E, K2O; F, Na2O

(because of near identity to the other “olivine-control” lines plotted, the line for PH KIL CAL is not shown); G, “FeO” (total Fe expressed

as FeO); H, TiO2.

A

++

++ +

+

+

++

+

+

+

+

+

+

+

+++ ++ +

++

52.00

51.00

50.00

49.00

48.00

47.00

SiO2, IN WEIGHT PERCENT

Al2O3, IN WEIGHT PERCENT

16.0014.0012.0010.00

8.006.00

MgO, IN WEIGHT PERCENT MgO, IN WEIGHT PERCENT

14.00

13.00

12.00

11.00

16.0014.0012.0010.00

8.006.00

B

8.00

9.00

10.00

11.00

12.00

CaO, IN WEIGHT PERCENT

16.0014.0012.0010.00

8.006.00

C

0.16

0.20

0.18

0.28

0.26

0.24

0.22

0.30

P2O5, IN WEIGHT PERCENT

16.0014.0012.0010.00

8.006.00

D

"Olivine-control" Lines

for Kïlauea lavas

(Wright, 1971, Table 3)

July 1974, "northern" vents (N)

July 1974, "southern" vents (S)

+

KIL 1840

PH KIL CAL

 KIL 1959

September 1974

December 1974
31

Figure 26.—Continued.

trolled and chemically distinct variants (magmatic “batches”)

presumably tapped by Mauna Ulu since 1972 and the July 1974

differentiated lava (Wright and Tilling, 1980, table 3).

The MgO-rich lavas erupted in December 1974 were

considered by Wright and Tilling (1980) to be representative

of a hypothetical magma batch (their chemical variant No. 10)

that was inferred to have been first supplied to, and later mixed

with, other variants in the Mauna Ulu system as early as January 1973. This hypothetical, unadulterated variant was considered by Wright and Tilling (1980) to never have reached the

surface, however, until the December 1974 eruption, and then

at a point on the southwestern flank of the volcano far from

Mauna Ulu (fg. 1). Perhaps the hypothethized chemical variant No. 10 in Mauna Ulu lavas needs to be reexamined.

Some of the July 1974 lavas (for example, analyses 7,

9, and 10, table 4) cannot be linked by olivine-control alone,

and likely were stored in the Kïlauea summit area for several

years before eruption. It seems reasonable that the differentiated 1974 summit lavas were pushed to the surface by upward transport of a new batch of more mafic magma from

depth that mixed with stored magma to modulate the hybrid

compositions of most lavas erupted in July and September.

GEOCHEMISTRY AND PETROLOGY

MgO, IN WEIGHT PERCENT MgO, IN WEIGHT PERCENT

10.25

10.50

11.00

11.50

11.25

10.75

11.75

“FeO”, IN WEIGHT PERCENT

16.0014.0012.0010.00

8.006.00

G

0.16

2.00

2.75

2.50

2.25

3.00

TiO2, IN WEIGHT PERCENT

16.0014.0012.0010.00

8.006.00

H

0.30

0.40

0.35

0.45

0.50

0.55

0.60

K2O, IN WEIGHT PERCENT

16.0014.0012.0010.00

8.006.00

E

1.70

1.90

1.80

2.30

2.20

2.10

2.00

2.40

Na2O, IN WEIGHT PERCENT

16.0014.0012.0010.00

8.006.00

F

+

+ +

+

++ +

++ + ++

+

+

+

+ +++

++

++

+

++

"Olivine-control" Lines

for Kïlauea lavas

(Wright, 1971, Table 3)

July 1974, "northern" vents (N)

July 1974, "southern" vents (S)

+

KIL 1840

PH KIL CAL

 KIL 1959

September 1974

December 1974
32 THE 1974 SUMMIT ERUPTIONS OF KÏLAUEA VOLCANO, HAWAI‘I

Table 5.—Chemical analyses of lavas erupted in September 1974. [Values given in weight percent; LOI = loss

on ignition].

The first of this postulated new magma batch, as represented

by MgO-rich samples such as SWR-375-14 and SWR-1274-

6 (analyses 2 and 14, table 6), did not appear at the surface

until December 31. Petrologic mixing calculations are pending to test and refine the earlier preliminary model of Wright

and Tilling (1980).

CONCLUSIONS AND SPECULATIONS

Sustained lava lake activity was common at Kïlauea’s

summit before 1924, but there has never been a year like 1974

in historical time, when three discrete eruptions occurred in

the summit area. These three eruptions were each distinct

events, each taking place from new vents in separate areas,

and the lavas of each had different chemical signatures.

Despite their distinct individual characteristics, each of

these eruptive episodes was related to underlying, common

magmatic processes that affected the entire Kïlauea summit

area in the latter half of 1974. Deformation of the Kïlauea

edifice and the distribution and character of volcanic earthquakes show that the magma storage areas migrated westward beneath the summit region in 1974: from the upper ERZ,

to the area between the uppermost east rift zone and Halemaumau (July), to Halema‘uma‘u and the west (September),

then into an area between the western KFS and the upper

SWRZ (December), with subsequent intrusion into an area

south and southeast of the SWRZ (early January).

The geochemistry of these summit-erupted lavas shifted

from compositions typical of mixed magmas, including “old”

slightly differentiated tholeiite basalts (July) to fairly primitive “new” olivine-rich tholeiites (December). We propose
33

that these geochemical changes reflect the injection of a new,

primitive magma batch into the shallow storage reservoir beneath Kïlauea’s summit in mid-1974. This new magma forced

differentiated, stored magma to the surface (July) and gradually mixed with stored magma to form the eruptive products

of September and December. Only in December did relatively uncontaminated samples of this new magma reach the

surface.

In a general way, the summit eruptions of 1974 repeated

much of the script for 1971 Kïlauea eruptive activity. In midJune 1971, the magma supply to Mauna Ulu was partially

blocked (Swanson and others, 1979) and magma accumuFigure 27.—Variation with time in abundances of major oxides for lavas collected during the July 19-22, 1974 eruption, showing the initially

contrasting compositions of lavas erupted from the “northern” vents (N, solid symbols) and “southern” vents (S, open symbols). Dashed

arrows show the compositions of the northern-vents lavas approaching those of the southern-vents lavas as the eruption progressed (see text).

A, Al2O3 and “FeO” (total Fe expressed as FeO); B, SiO2; C, MgO and CaO; D, TiO2 and Na2O; E, K2O and P2O5.

CONCLUSIONS AND SPECULATIONS

Al2O3, N Al2O3, S

Al2O3

CaO

MgO

Al2O3

“FeO”, N “FeO”, S

MgO, N CaO, S MgO, S CaO, N Na2O, N Na2O, S

Na2O

TiO2, N TiO2, S

TiO2

K2O, N K2O, S

K2O

P2O5, N P2O5, S

P2O5

 N S

MgO

TiO2

Na2O

15

14

13

12

11

10

7/19/74

12:00

7/19/74

18:00

7/20/74

0:00

7/20/74

6:00

7/20/74

12:00

7/20/74

18:00

7/21/74

0:00

7/19/74

12:00

7/19/74

18:00

7/20/74

0:00

7/20/74

6:00

7/20/74

12:00

7/20/74

18:00

7/21/74

0:00

7/19/74

12:00

7/19/74

18:00

7/20/74

0:00

7/20/74

6:00

7/20/74

12:00

7/20/74

18:00

7/21/74

0:00

7/19/74

12:00

7/19/74

18:00

7/20/74

0:00

7/20/74

6:00

7/20/74

12:00

7/20/74

18:00

7/21/74

0:00

7/19/74

12:00

7/19/74

18:00

7/20/74

0:00

7/20/74

6:00

7/20/74

12:00

7/20/74

18:00

7/21/74

0:00

A

Oxide, in weight percent

Time

12

11

9

10

8

7

6

C

Oxide, in weight percent

Time

0.5

0.45

0.35

0.4

0.3

0.25

0.2

E

Oxide, in weight percent

Time

2.75

2.50

2.25

2.00

1.75

D

Oxide, in weight percent

Time

51.0

50.2

50.4

50.6

50.8

49.8

50.0

49.6

B

SiO2, in weight percent

Time
Table 6.—Chemical analyses of lavas erupted in December 1974. [Values given in weight percent; LOI = loss on ignition].
35

lated in Kïlauea’s summit area, which began to inflate rapidly. Two small summit eruptions occurred, the first on August 14, in the same area as the “northern” vents of the July

1974 eruption (fig. 3). The August 1971 eruption was accompanied by a small deflation, followed immediately by

inflation south of Halema‘uma‘u, as was the case in 1974.

The second 1971 summit eruption began on September 24 on

the west rim of Halema‘uma‘u, was preceded by a very sharp

summit inflation, and was accompanied by a small deflation

(Duffield and others, 1982), as was the September 1974 eruption. Here the parallels between 1971 and 1974 begin to diverge, however. Following the September 1971 eruptive

breakout at Halema‘uma‘u, vents opened down the SWRZ,

reaching nearly 12 km southwest of Kïlauea Caldera (Dvorak,

1990). In September 1974 however, eruptive fissures which

initially migrated to the southwest stopped at the caldera rim

and never extended to the southwest.

What blocked the migration of eruptive vents into the

SWRZ in 1974? A possible answer to this question may lie

to the northwest of Kïlauea, within the edifice of her giant

neighbor volcano, Mauna Loa. Deformation measurements

on Mauna Loa, initiated in 1965, had shown no changes

through 1973, but in August 1974, at the time of an annual

survey, EDM survey lines across Moku‘äweoweo indicated

significant extensions since the previous summer (Lockwood

and others, 1976). This, as well as the sharp increase in seismic activity beneath the summit area that had begun in April

1974 (Koyanagi and others, 1975) indicated that Mauna Loa

had begun to inflate, as a prelude to subsequent eruptions in

1975 and 1984 (Lockwood and others, 1987). Later monitoring, referenced to stable points on Mauna Kea and Hualälai,

showed that the horizontal dilations measured across

Moku‘äweoweo were not symmetrically distributed about the

summit inflation center, but were instead accommodated by

southeastward translation of Mauna Loa’s southeast flank.

We suggest that this southeast-directed inflationary pressure

against neighboring Kïlauea effectively “closed” Kïlauea’s

SWRZ in 1974 and thus blocked magma from migrating down

this rift zone as it had done in 1971. Although the M=5.5

Ka‘öiki earthquake of November 30, 1974 and subsequent

aftershock activity on Mauna Loa’s southeast flank may have

relieved some of Mauna Loa’s influence on Kïlauea, inflation continued, leading to the Mauna Loa eruption of July 5,

1975 (Lockwood and others, 1987).

Duffield and others (1982) suggested that the 1971

migration of lava into the SWRZ took place because Mauna

Loa was not inflated at the time and wrote (p. 303) that “Eruption along the southwest rift zone could be inhibited by an

increase in compressive stress across the zone when a magma

reservoir in neighboring Mauna Loa is inflated, effectively

squeezing the rift zone closed.” Their hypothesis is compatible with the events of September 19, 1974, when a westward-advancing eruptive fissure abruptly stopped at the

caldera’s rim. It seems reasonable to speculate that Kïlauea’s

SWRZ eruptions are unlikely to occur during times of Mauna

Loa inflation.

The influence of Mauna Loa may also explain basic

differences between the SWRZ and ERZ. Duffield and others (1982) have described several differences between the two

rift zones, including the much higher frequency of eruptions

on the ERZ. This disparity is likely caused by the fact that

the north side of the ERZ is relatively immobile, and so any

seaward movement of the south flank results in reduced compressive stress and an enhanced environment for magmatic

intrusion and storage within the ERZ. The northwest side of

the SWRZ is not stable, however. Inflation of the Mauna Loa

edifice results in southeastward movement of the SWRZ’s

northwest flank, thereby increasing compressive stresses

across the rift and restricting the environment for magma transport and eruption.

Did the December injection of magma between the

SWRZ and the KFS and the subsequent earthquake swarm

south of the lower SWRZ, coupled with the succession of

earlier ERZ and 1974 summit intrusion/eruption events serve

to “unlock” Kïlauea’s south flank and facilitate the November 29 M=7.2 Kalapana earthquake? Or, were all these preceding events only symptoms of larger dynamic forces that

enabled both the magmatic and tectonic events of the 1970’s?

Equally intriguing is the changing role of the Koa ́e

Fault System in the evolution of Kïlauea Volcano. The KFS

is a zone of normal faults and has never before been considered a locus of volcanic activity. The eruptions of May 24,

1969, May 5, 1973, and December 31, 1974, however, as

well as the possible intrusion of December 25-27, 1965, all

occurred within or near the margins of the KFS. As also suggested by Fiske and Swanson (1992), is it possible that in the

future the Koa ́e will become a major site for the injection

and possible storage of magma, perhaps replacing the summit caldera as the “heart” of Kïlauea?

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