SMITHSONIAN CONTRIBUTIONS TO THE

EARTH SCIENCES

NUMBER 4

William G. Melson,

Eugene Jarosewich,

and Charles A. Lundquist

Volcanic Eruption at

Metis Shoal, Tonga,

1967-1968:

Description and

Petrology

SMITHSONIAN INSTITUTION PRESS

CITY OF WASHINGTON

1970
ABSTRACT

Melson, William G.; Jarosewich, Eugene; and Lundquist, Charles A. Eruption of

Metis Shoal, Tonga, 1967-1968: Description and Petrology. Smithsonian Contributions to the Earth Sciences, 4: 1-18, 1970.—The 1967-1968 eruption of Metis

Shoal, Tonga, was evidentially similar to the frequent shallow submarine eruptions

of the inner island arc of Tonga. The eruption began about 10 December 1967, and

an island eventually emerged; by 19 February 1968, the island had been eroded to

beneath wave base. The eruptions were characterized by explosions of steam and

ash which hurled bombs a few to several hundred feet into the air. The rocks ejected

are pumiceous dacites which, for their silica content, have unusually low alkali contents

and rare earth-element contents. The chemical characteristics of the dacite are hard

to account for by partial melting of an ocean-ridge basalt parent. The peculiar properties of the dacite appear to characterize other Tongan lavas and support the idea

that Tonga is part of a distinct petrographic province.

Official publication date is handstamped in a limited number of initial copies and is recorded in

the Institution's annual report, Smithsonian Year.

UNITED STATES GOVERNMENT PRINTING OFFICE

WASHINGTON : 1970

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402-Price 30 cents
William G. Melson,

Eugene Jarosewich,

and Charles A. Lundquist

Volcanic Eruption at

Metis Shoal, Tonga,

1967-1968:

Description and

Petrology

Introduction

In late December 1967, we were informed by the

Smithsonian Center for Short-lived Phenomena that

a volcanic eruption was under way in the Tonga

Islands. The early reports indicated that a submarine

eruption had resulted in creation of a new island. In

February 1968, Charles A. Lundquist, on a previously

planned trip to Australia, managed to detour and visit

the site. At that time, the volcanic island had been

eroded away, and only shoals remained; however,

samples were obtained by diving. The petrology of

these samples and the description of the eruption,

obtained from photographs and from other documentation of the eruption obtained by correspondence and

by interviews with eye witnesses, are the subject of

this paper. Background information is summarized

on the probable relationships between volcanism and

seismicity in this island-arc environment.

Geologic Setting of Tonga

The Tonga Islands have long been known as sites

of frequent earthquakes and volcanic eruptions. The

William G. Melson and Eugene Jarosewich, Department of

Mineral Sciences, National Museum of Natural History,

Smithsonian Institution, Washington, D.C. 20560. Charles A.

Lundquist, Smithsonian Astrophysical Observatory, Cambridge, Massachusetts 02138.

volcanic activity is restricted to the western islands;

the eastern islands, including Tongatapu, long inactive volcanically, are capped by coral reefs and seldom have exposures of volcanic rocks.

Figure 1 shows that the western Tonga Islands lie

on a line that, with but slight bending, can be extended to include the volcanic Kermadec Islands, and

the volcanic and hydrothermal regions of North

Island, New Zealand.

There are 10 active volcanoes in western Tonga

(Richards, 1962). Of these, only Tofua, Late, and

Fonualei have remained above sea level throughout

historic times. The remaining seven volcanoes are

submarine. Of these seven, only Falcon Island, Metis

Shoal, and Home Reef have reached above sea level

during eruptions, and, at present, even these three

have been eroded to below sea level. The most active

shoal volcano appears to be the one which periodically

builds Falcon Island. In all, nine separate eruptions

have been reported. Perhaps because of their transitory existence, very little is known about the petrology

of these "jack-in-the-box" islands.

One of the few generalizations about the petrology

of the Tonga Islands was made many years ago by

Alfred Harker (1891), after examination of a suite

of Tongan rocks:

With the exception of the Falcon Island rocks, all

those [rocks] examined from the Tonga Islands ap1
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

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FIGURE 1.—Location of Tonga and Metis Shoal.
NUMBER 4

pear to be of submarine formation. The absence or

presence in different strata of any sensible proportion

of calcareous matter and organic remains is perhaps

related to the more or less rapid rate of accumulation

at different epochs of eruption. The volcanic material

ejected seems to have been almost exclusively of fragmental character, and in some cases there are indications of violent explosive action. This is quite in accord with the andesitic nature of the materials thrown

out, which are the types common in the Pacific region.

Harker evidently did not examine samples of lavas,

which do make up parts of Tofua and other active

subaerial Tongan volcanoes.

Bryan (1969) drew attention to certain chemical

peculiarities of Tongan volcanic rocks—unusually low

alkalies and high iron with high silica contents. He

notes that this tendency applies not only to the dacites, but to the basalts from Late and Tofua as well.

This peculiarity is true of our Metis Shoal's samples,

and bears out the distinctive chemistry of Tongan

lavas.

RELATION BETWEEN VOLCANISM AND SEISMOLOGY.—

Tonga is along the junction of two of the six major

tectonic blocks of the earth's crust (Figure 2). The

contact between these blocks is normally an oceanic

trench, oceanic ridge, or a fracture zone (transform

fault zone, Figure 3). At oceanic trenches, the thick

lithosphere, estimated to be around 100 kilometers

thick, is believed to move downward. Along its upper

surface, numerous earthquakes occur, defining a narrow belt. Rocks of the lithosphere, as they descend,

are envisioned as becoming hot enough to undergo

partial melting. Where tensional fractures occur along

the lithosphere, melts so generated move upward, giving rise to the volcanism that characterizes most island

arcs (Figure 4).

The Tonga-Kermadec region is one of the world's

most active seismic belts (Isacks et al., 1969). Figure

^12 0 1 8 0 120 6 0

FIGURE 2.—Location of Tonga in regard to the large tectonic blocks of the new global tectonics.

Heavy lines are island arcs or arc-like features. Tensional (divergent arrows) and compressional

(convergent arrows) indicate relative movements at margins of blocks; length of arrows is roughly

proportional to rate of relative movements. Some historically active volcanoes are indicated by X.

Open circles represent earthquakes that generated tsunamis (seismic sea waves) detected at

distances of 1,000 or more kilometers from their source. The six major tectonic blocks are shown

(America, Africa, Eurasia, Antarctica, Pacific, and Australian). Modified from Isacks, Oliver, and

Sykes (1968).
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

FIGURE 3.—Idealized block section showing probable nature of contacts between the tectonic

blocks (Figure 2). Contacts are trenches, ridges, and fracture zones (transform faults). Modified

from Isacks, Oliver, and Sykes (1968). Arrow indicates relative movements. The lithosphere

moves under at oceanic trenches and is generated by igneous activity at mid-ocean ridges in the

now widely accepted models of the new global tectonics. The contact between the lithosphere

and asthenosphere is not the contact between the crust and mantle, but is at the much greater

depth of about one-hundred kilometers.

300 100 0

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FIGURE 4.—Hypothetical section through an island arc. Down-going slab of lithosphere, seismic

zone (S) near surface of slab and in adjacent crust, tensional features beneath ocean deep where

slab bends abruptly and surface slab is still open are shown. Island-arc magmas move upward

along tension fractures developed near the base of the astheosphere. Modified from Isacks, Oliver,

and Sykes (1968).

5 shows epicenter locations through a portion of Tonga

during a single year. The large number of events and

their clear-cut restriction to a relatively narrow zone

(the BeniofF Zone) is characteristic of other island

arcs. It is along this zone that the upper surface of the

lithosphere is believed to be descending (Figure 4).

It is also thought that the lavas of island arcs originate

along this zone. A correlation between lava chemical

composition and the depth below the eruption site to

the Benioff Zone has been found. Specifically, the

potassium content has been found to increase with increasing depth to this zone (Hatherton and Dickinson, 1969). At present, there is insufficient data on

Tongan lavas to adequately test for such a relation.

The presence of such a relation does indicate that the

depth of magma generation is either at the Benioff

Zone or directly related to the depth of this zone.

Course of the Eruption

On 11 December 1967 (Greenwich Time), volcanic

activity in the Metis Shoal region of the Tonga Islands

was noted by the crews of several ships and by passengers on a Polynesian Airlines flight from Samoa to
NUMBER 4

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FIGURE 5.—Location of earthquake epicenters beneath Tonga during 1965. Vertical section

through Niumate, Tonga. Triangles represent events projected from within 0-150 km south of the

section; circles represent events projected from within 0-150 km north of the section. A vertical

exaggeration of about 31:1 is used for the topographic section. Modified from Isacks, Oliver, and

Sykes (1968).

Tonga. Volcanic activity was last reported from this

region in 1876 (Phillips, 1899), and accounts exist for

still earlier events (Richards, 1962).

The United States Navy Hydrographic Office map

 x

of the Tonga Islands indicates three shallow areas in

the region, of which the most southerly is designated

Metis Shoal. Each area is shown with a diameter of

xMap number 2016, Tonga Islands. 3d edition, 1947.

Hydrographic Office, Washington, D.C.

about 0.5'. The approximate coordinates of these

areas are:

Metis Shoal 19°10.8'S 174°51.8'W

northwestern shoal 19°9.8'S 174°52.6'W

northeastern shoal 19°9.5'S 174°51.1'W

The United States destroyer Fox, the first ship to

sight the 1967 eruption, reported the site to be 0.7

nautical miles north of Metis Shoal, which puts the

eruption approximately at the center of the three areas
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

in the Hydrographic Office chart. After its visit to the

site on 13 February 1968, the Soviet research vessel

Vityaz reported coordinates (Bezrukov et al., 1968) :

19°09'06"S 174°51.1'W.2

These are nearest to the coordinates of the northeastern shoal. The Alaimoana, en route from

Nuku'alofa, on 19 February 1968, passed near one

shoal south of the eruptive site where specimens were

collected. Hence, it appears that recent volcanic

activity was north of a more southerly shoal. No other

reports reaching us mention positions of the recent

eruption relative to other shoals, nor do we know how

many shoals might have been present at this time. On

the basis of sea discoloration, there may be some possibility that the southerly shoal, observed at a distance

from the Alaimoana, also had volcanic activity.

One observation on 11 December was from the

Tofua. Captain Peter Bennett reported:

At the closest approach of 9 miles the volcanic eruption had the appearance of an incandescent island

about

 l/% mile long and 150 feet high, glowing cherry

red at constant intensity. Above this island a dense

pillar of steam and smoke ascended to 3000 feet and

at approximately 1 minute intervals molten lava or

boulders in parabolic arcs were observed being ejected

to a height of 1000 feet.3

His Majesty, King Taufa'ahau Tupou IV of Tonga

and his party obtained early photographic records

of the events during a charter flight to the volcano

on 14 December. At this time, rocks projected an estimated 50 feet above the sea as shown in Figure 6, and

the island was some 100 to 200 yards across.4

On 14 December, two Orion Aircraft of No. 5

Squadron, Royal New Zealand Air Force, flew around

the volcano for approximately 30 minutes. Flight

Lieutenant P. K. Simpson reported:

2

 P. L. Bezrukov, J. O. Murdmaa, N. G. Prokoptsev, M. A.

Repechka, Preliminary results of geological investigations at

Metis Shoal Area, Tonga Islands. 1968. Enclosure with letter

of 6 May 1968 from P. L. Bezrukov to C. A. Lundquist.

3

 Letter from Captain P. Bennett, Tofua, Union Steam Ship

Company of New Zealand Limited, to C. A. Lundquist,

9 April 1968.

* Volcanic eruptions produces new "Jack-in-the-Box" island.

Article in The Chronicle, Nuku'alofa, Tonga Islands (J.

Reichelmann, editor), 15 December 1967, page 1. Also,

private communication and 8-mm motion-picture film from

J. J. Furniss, aviation adviser, Office of H. B. M. Commissioner and Consul, Tonga, to R. Citron in 1968.

At the site of the eruption an island had formed and

it was variously estimated to be 20 to 40 feet high

and as much as 150 to 300 yards long. At the eastern

(upwind) extremity of the island was a crater guessed

to be 10 to 20 yards across. Its shape was circular with

dark grey lips. Inside, the grey soon changed to a dark

cherry red becoming brighter and more orange with

increasing depth. These details were observed only

momentarily when for a few seconds the smoke cleared

about the crater. . . .

Lumps of dark colored matter were being flung

several hundred feet into the air. They seemed to be

ejected at intervals of 10 to 20 seconds and they

splashed into the sea in all directions out to a distance

of 250 yards. This matter was clearly visible from

l

/i to 1 mile away. Occasionally a much larger piece

(perhaps 2 or 3 feet across) would be seen to rise

to a height of 100 to 200 feet.

The sea water around the volcano was colored an

emerald green streaked with brown. This discoloration extended several hundred yards upwind and 7

nautical miles on a bearing of 240° T from the island.5

Five days later, on 19 December, Captain C. H. HillWillis of the motor vessel Aonui observed the island

from a distance of 4 miles. He noted: "the island was

one third of a mile long and approximately 80 feet

high. It was erupting from a single cone at intervals

of roughly 3 minutes, with steam vapour rising to some

8,000 feet."

 G

Particularly informative observations were provided

by Captain Thomas M. Holmes of Polynesian Airlines,

who flew over the site repeatedly. Figures 7 and 8

were taken by him on 31 December. He relates that:

The island, generally kidney-shaped at its best, lay

with its longitudinal axis approximately E—W. Steam

clouds on that day, 31 December, were blowing in a

direction of about 200 degrees true, the surface wind

was from the N.N.E.7

Concerning volcanic activity, Captain Holmes

5

 Royal New Zealand Air Force report on flight to Metis

Shoal, 14 December 1967. Enclosure with letter from D.

Rishworth, New Zealand Department of Scientific and Industrial Research, Geological Survey Office, to R. Citron,

8 May 1968.

0

 Letter from Captain C. H. Hill-Willis of the motor vessel

Aonui Nuku'alofa, Tonga Islands, to C. A. Lundquist, 10

April 1968.

7

 Letter from Captain T. M. Holmes, Polynesian Airlines

Limited, to R. Citron, 10 April 1968.
NUMBER 4

FIGURE 6.—Metis Shoal eruption. Steam and ash clouds with projecting blocks at base of cloud.

Rocks projected an estimated fifty feet above sea surface at this time. (Photograph courtesy of

Mr. J. Reichelmann.)
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

FIGURE 7.—Metis Shoal, 31 December 1967. Island has its long axis oriented about east-west.

Steam clouds are blowing to 200° true; surface winds are blowing from approximately N.N.E.

(Photograph courtesy of Mr. T. M. Holmes.)

(1968) reported: "On Sunday December 24th, violent

eruptions were taking place. By Sunday, December 31st, these eruptions appeared to have reduced

considerably, although still actively steaming [Figures

7 and 8]. By Sunday, January 7th, 1968, volcanic activity appeared to have ceased although steam was still

evident. By Sunday, 31 January 68, all activity had

ceased, volcanic and steaming."

 6

Captain Holmes estimates that the island reached

a maximum size by about 7 January, and that the

island disappeared in the first or second week of

February.

On 13 February, the Soviet research vessel Vityaz

reached the site

 2 (Bezrukov et al. 1968). They found

no features of continuing volcanic activity. There were

two or three low rocks above the sea level and a shoal

beneath them. They approached to within 200-300

meters of the rocks, and used a grab to obtain bottom

samples at depths from 37 to 190 meters.

The site was visited on 19 February by Charles A.

Lundquist in a small motorboat, the Alaimoana,

chartered from Nuku'alofa. Only an occasional point

of rock could be glimpsed in the towering breakers

that were present even though the sea was quite calm

(Figure 9). A linear shoal extended roughly east-west

from the breakers. The shoal increased sharply in

depth in the eastward direction and became invisible

within a few hundred meters of the breakers; westward,

the shoal increased in depth more gradually.

A dugout outrigger canoe, manned by two residents

of Matuku Island, was launched from the Alaimoana,

and samples were obtained by diving in water about

1.5 meters deep.

An immediate examination on shipboard of the

specimens revealed delicate fibers of glass exposed on

the surface, indicating only slight erosion; also, the

rocks were easily broken manually. This evidence would

seem to indicate that the rocks in their collected form

could not have been exposed very long to the environment of the shoal and must have been fractured from

larger objects shortly before their collection.

A final report by Captain Holmes describes the site

on 21 April 1968:
NUMBER 4

FIGURE 8.—Metis Shoal, 31 December 1967. Details are the same as for Figure 7, but viewed

from much closer. {Photograph courtesy of Mr. T. M. Holmes.)

The sea was calm and the island completely covered

vyithout any appearance of breaking waves. The color

looked more like yellow sand as opposed to black sand,

mentioned earlier. The water around about had a

definite yellow tinge, suggesting a sulphur content. The

coloring effect gave an impression of a discharge in

the sea. The size of the island (beneath the sea) was

roughly 100 yards across and circular in form.

We obtained no information on the volume of ejecta;

however, Murdmaa and others (1969) carried out a

dredging survey around the volcano and found very

little new debris on the sea floor. In many places, even

near the summit, there was no volcanic debris. Instead,

the area consisted of old ejecta encrusted with various

marine organisms. From this, Murdmaa and his colleagues (1969) concluded that the total volume of

ejecta was small.

TABLE 1.—Chronology of events, Metis Shoal, Tonga

Islands

1. Submarine eruption begins—unknown

2. Island emerges, subaerial eruptions—probably not earlier

than 10 December 1967

3. First observation and report of supermarine eruption—11

December 1967

4. Activity reached peak sometime in 31 December 1967 and

apparently ended by 7-8 January 1968

5. Maximum length, 700 meters; width, 100 meters; height,

15 meters (±50%)—reached on or about 7 January 1968

6. Erosion of island down to "rocky outcrops"—25 January

1968

7. Island only "jagged rocks and water washing across"—

1 February 1968

8. Island submerged, very high breakers on subsurface rock:

19 February 1968

9. Shoal completely beneath water with no breakers—1 April

1968

•Petrology of the Ejecta

DESCRIPTION.—The blocks, collected by diving, are

pumiceous dacites with about 43 percent glass, 37 percent pore space, 9 percent plagioclase phenocrysts, 11

percent pyroxene phenocrysts (augite>hypersthene),

and 0.3 percent magnetite. In order of abundance, the

phenocrysts are bytownite, augite, hypersthene, and
10 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

FIGURE 9.—Only shoals remained at the eruption site on 19 February 1968. (Photographed by

Charles A. Lundquist.)

titanmagnetite. The phenocrysts commonly occur in

clusters (Figure 10a) and bytownite phenocrysts are up

to 5 mm long. Critical features of the glass and minerals

follow:

Glass: The glass is colorless in thin section, has a refractive index of 1.502, and has the following approximate composition measured on the electron microprobe: SiO2=72.0±4.0 (ranges slightly from spot to

spot); Al2O3=13.7±1.0; TiO2 = 0.3±0.1; Fe=3.4±

0.2; MgO=l.l±0.1 ; CaO=3.4±0.2; Na 2O=2.7±

0.2; and K2O = 2.1 ±0.2. The glass has a silica content

comparable to that of rhyolites, although the Na2O and

K2O contents of the glass are lower than the contents

of these elements in typical rhyolites. A separate of

glass was prepared and analyzed by traditional wet

chemical analysis (Table 2). This analysis confirmed

the compositional pecularities indicated by the microprobe analysis. The glass contains abundant flattened

vesicles (Figure 11) and but few microlites.

Plagioclase: The plagioclase phenocrysts are remarkably calcic (An83-An87). One might expect oligoclase, and perhaps even sanidine, in a volcanic rock so

rich in silica. The pronounced oscillatory zoning in the

plagioclase reflects a remarkably small variation in composition. On the basis of microprobe traverses, the

maximum range on a single phenocryst is probably no

greater than 4-mole percent anorthite molecule. The

larger plagoclase phenocrysts commonly consist of complex multiple intergrown crystals. Some of the smaller

phenocrysts are untwinned, or have only a single twin

plane. These might be mistaken for phenocrysts of

alkali feldspar; their high index of refraction, and composition given by microprobe analyses show decisively

that they are not alkali feldspar.

Augite: the augite phenocrysts are light green and

weakly pleochroic and have a composition around

En42Fsi7Wo41. Zoning is readily visible in thin section,

but compositional variations across phenocrysts are
NUMBER 4 11

FIGURE 10.—Phenocrysts of augite, hypersthene, and calcic bytownite in individual crystals and

groupings (conglomerporphyritic clots).

small, amounting to less than 0.2% Fe variation. The

augite contains around 1.3 ±0.2% A12O3, and only

traces of Na2O and TiO2.

Hypersthene: Hypersthene occurs as pleochroic

light-green to light-pink phenocrysts, and has a composition around En63Fs3!tWo3.o. The hypersthene is only

chromian spinel occur in the olivine phenocryst (Figand only traces of other elements.

Titan-magnetite: There are octahedral opaque

phenocrysts of a magnetic spinel-group mineral containing about 4.4% Ti and 2.6% Mg (only these two

elements were determined).

Olivine: One partly resorbed olivine crystal rimmed

by hypersthene granules was noted in one of four thin

sections (Figures 12 and 13). One would expect it to

be iron-rich in such a high silica rock. Surprisingly,

it is Fo93, and thus even more forsteritic than is typical

of most basaltic olivines.

Chromite(?) : Octahedra of a brown translucent

chromian spinel occur in the olivine phenocryst (Figure 13).

CLASSIFICATION.—Tongan volcanic rocks seem to fit

roughly into the basalt-andesite-rhyolite association of

Turner and Verhoogen (1960), an association common in areas of crustal deformation and most typically

developed in connection with orogenic movements

along the margins of the Pacific. Many data are available on this association because the basalt-rhyolite

series has been erupted in numerous places along the

chains of active and recently active volcanoes that form

the island arcs of the Pacific border.

This association shows the average plagioclase composition to be increasingly sodic along the general series

of basalt—andesite—quartz latite—rhyolite, but the

composition of plagioclase phenocrysts shows little relation to the bulk chemical composition of the rocks in

which they occur; the range for rhyolitic quartz

latites being about An12 to An55.

The pumiceous Metis Shoal's rocks are difficult to

classify precisely because of their chemical and mineralogic peculiarities. They are different from most dacites

in their low total alkali content, and in their mineralogy. Typical dacites contain around five percent total

alkalies, and K2O typically is more abundant than
12 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

f f m ml*k

FIGURE 11.—Details of the glassy matrix. The glass was analyzed by electron probe (see text) and

by classical methods (Table II).

Na2O. For example, the type dacite (Troger, 1935, p.

75) has Na2O = 3.02 percent, and K2O = 2.51 percent,

although its silica content (66.76 percent) is quite like

that of the Metis Shoal's rocks (64.01 percent). Mineralogic peculiarities, which in part reflect the compositional peculiarities, include the presence of forsteritic

olivine and associated chromite, and the abundance of

bytownite phenocrysts. Dacites are typically porphyritic, but more sodic plagioclase phenocrysts, as well as

phenocrysts of sanidine and quartz, are common

phenocryst minerals.

RELATION TO TEKTITES.—The Metis Shoal's glass

is compositionally similar to certain tektites. Tektite

chemistry typically differs from that of most volcanic

glasses in a high silica content and low total alkali

content, a feature of the Metis Shoal's glass. Some

caution should thus be used in judging that "microtektites," small glass spheres found in certain deep

sea cores, are extraterrestrial based on chemistry alone.

The Metis Shoal's glass differs, however, from tektites compositionally in that Na2O is higher than K2O,

and petrographically in its pumiceous texture (Figure

11), compositional homogeneity (only slight compositional differences—mainly in SiO2 were noted), and in

presence of scattered microlites of hypersthene, plagioclase, and possibly other minerals. Also, the porphyritic

texture of the bulk sample (glass + phenocrysts)

clearly is not a feature of tektites.

PETROGENESIS.—The Metis Shoal rocks present

some difficult problems of origin. Crustal crystal-liquid

fractionation of a basaltic liquid can generate higher

silica liquids, but, a.long with silica, the alkalies are

highly enriched in residual liquids. For example, residual glasses in a tholeiitic basalt from the prehistoric

Makaopuhi lava lake, Hawaii, have SiO2=75%-78%

and total alkali contents typically about 8.6%, with

K2O = 0.41 in the uppermost analyzed sample, and

an SiO2 content of about 50 percent in the parent

basalt (Evans and Moore, 1968). It is thus clear that

a single-stage low pressure differentiation of basalt

could not have produced the Metis Shoal's rocks.

The presence of forsteritic olivine and associated

chromite octahedra is anomalous in a rock with such

high silica. Clearly the olivine was not in equilibrium
NUMBER 4 13

TABLE 2.—Chemical composition of whole rock {column 1)

and separated glass {column 2) from Metis Shoal's 1968

eruption. These samples have been entered into the petrology

collections of the United States National Museum

{USNM 111108).

TABLE 3.—Rocks from Tonga chemically similar to Metis

Shoal's bulk docite and glass {Table 7).

SiO2

A12O3

Fe2O3

FeO

MnO

MgO

CaO

Na 2O

K2O

H2O+

H 2 O -

TiO2

P2O,

CIPW Norm:

Q

OR

AB

AN

DI

HY

M T

IL

AP

1

64.01

12.95

1.51

5.43

0. 13

5.47

7.51

1.83

0.95

0.00

0.00

0.32

0.06

100. 17

24.52

5.61

15.49

24. 32

10.23

17.07

2. 19

0.61

0. 14

100. 18

2

73.38

12.66

1. 11

2.84

0.06

0.84

3.79

3.09

1.52

<0. 13

0.03

0.45

0.00

99.77

39. 11

8.98

26. 15

16. 19

2.21

4.64

1.61

0.85

0.03

99.77

Analyst: Eugene Jarosewich, Smithsonian Institution

with the liquid during the final phase of phenocryst

crystallization. This is shown by the jacketing of the

olivine by hypersthene granules (Figure 12). The

incongruent melting (and hence crystallization) of

hypersthene might conceivably lead to first crystallization of olivine in a high silica magma, but probably not in a magma as high in silica as that of Metis

Shoal, nor, even if olivine did crystallize, would one

expect it to be so forsteritic and associated with chromite. It is much more likely that the olivine and

chromite are xenocrysts, or reflect mixing and hybridization of already partially crystallized basaltic magma

with some other more siliceous magma.

At the present time, we can think of no plausible

scheme of partial melting or other processes which

SiO2

AI2O3

Fe2O3

FeO

M nO

MgO

CaO

Na2O

K2O

H2O +

H 2 O -

TiO2

P2O5

CIPW Norms

Q

Or

Ab

An

Di

Hy

Mt

IL

AP

H2O

1

73.50

11.90

0.76

4.84

0.63

3.80

2.81

0.75

0.46

0.58

0.09

100. 12

42.25

4.43

23. 78

17.64

0.58

8.57

1. 10

1. 10

0.20

0.46

100. 11

2

67.20

12. 13

0.52

8.47

0. 13

1.30

5.60

2.62

0.63

0.34

0.62

0. 16

99.72

31.07

3. 72

22. 17

19.48

6.38

14.28

0.75

1. 18

0.35

0. 34

99. 72

3

64.82

11.95

1.53

8. 16

0. 15

1. 19

6.54

2.64

0.85

1. 16

0.20

0.78

Tr.

99.97

27.70

5.02

22.34

18. 25

12. 28

9.32

2.22

1.48

1. 36

99.97

4

65.84

12.02

1.05

8.25

0. 13

1.29

6.22

2.66

0.77

0.84

0. 16

0.76

Tr.

99.99

28. 78

4.55

22.51

18.58

10.61

11.00

1.52

1.44

1.00

99.99

5

65.98

11.43

2.38

5.86

1.80

1. 10

5.20

2.77

1.09

0.56

0. 10

0.91

0.23

99.41

30.61

6.44

23.44

15.54

7.56

9.48

3.45

1.73

0.53

0.66

99.44

6

65.23

12.59

1.24

6.80

1.68

1.42

5.80

2.88

1.00

0.24

0. 14

0.88

0.21

100. 11

26.64

5.91

24.37

18.47

7.70

12. 70

1.80

1.67

0.49

0.34

100. 09

1. Herald Cays, light pumice, L. J. Sutherland, analyst

(Bryan, 1968).

2. Herald Cays, dark pumice, L. J. Sutherland, analyst (Bryan

1968).

3 and 4. Falcon Island, Tonga, Pumice from 1928 eruption

(Lacroix, 1939).

5 and 6. Fonualei, Tonga, dacite, (Richard, 1962).

could account for all the compositional and mineralogic

peculiarities of the Metis Shoal lava.

Dr. Norman Hubbard of the Lamont-Doharty Geological Observatory is carrying out an investigation of

the contents of the lanthanide elements in various

Tongan lavas. He performed an analysis for these

elements in the pumiceous dacite described in this

paper (Table 2, whole rock analysis). He kindly

allowed us to give the results of that analysis in this

paper. He obtained, via neutron activation analysis,

the following values (given in parts per million) :

lanthanum, 2.70; cerium, 7.31; niobium, 5.17; samar-
14 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

FIGURE 12.—Large, rounded olivine crystals (xenocryst) rimmed by hypersthene. Olivine

composition around F093.

ium, 1.56; gadolinium, 1.98; europium, 0.452; dysprosium, 2.51; erbium, 1.68; and ytterbium, 2.19.

Dr. Hubbard also reports a barium content of 334

parts per million.

Normally, higher silica lavas, like dacites and andesites, are higher in the lanthanide elements than are

the lower silica lavas, like basalts. Remarkably, however, the Metis Shoal dacite is lower in the lanthanide

elements than most basalts, particularly those from

the ocean-ridge system. Although lower in lanthanides

than the ocean-ridge basalts, the Metis Shoal sample

has the same "primitive" lanthanide abundance ratios

compared to chondritic meteorites as have the oceanridge basalts. Thus, any scheme of partial fusion or

fractional crystallization of a basaltic parent must provide for depletion of the lanthanide elements but at

the same time give high silica.

Comparisons of the Metis Shoal lava with midocean ridge basalts are appropriate. Most models of

sea-floor spreading and island arc development envision the oceanic crust as moving beneath island arcs

(Figure 4). Island arcs represent the place where

oceanic crust is destroyed either by addition to continental margins or by movement downward into the

mantle. In those models where ocean-ridge basalts

are envisioned as moving downward into the mantle,

partial melting might occur as the basaltic rocks descend deeper into the mantle, where they eventually

transform into eclogites. The partial melting of

oceanic-ridge basalts is thus a plausible and often considered mechanism for generating island-arc lavas,

such as those of Tonga. If partial melting of oceanridge basalts is giving rise to the Tongan lavas, however, the process must involve separation of crystals

which are higher in the lanthanide elements and lower

in silica than are those of the ocean-ridge basalts. If

this were not the case, the Metis Shoal lava would not

be depleted in rare earth elements and would be

higher in silica than ocean-ridge basalts. Dr. Hubbard

(personal communication) has stated that separation

of a sizable amount of hornblende from a basaltic parent might account for the peculiarities of the Metis

Shoal sample. This process would require that the

ocean-ridge basalts undergo partial fusion at high
NUMBER 4 15

FIGURE 13.—Detail of margin of olivine xenocryst showing chromite inclusions

and hypersthene rims.

water pressures. Liquids generated under such high

water pressures, which may be on the order of a few

kilobars, would be expected to be high in dissolved

water. The absence of hydrous phases in the Metis

Shoal dacite indicates that lava was not high in water.

This may mean that the above outlined explanation

for the silica and lanthanide contents is not correct,

or that the lava lost water by much degassing prior to

the crystallization of its phenocrysts minerals, or that

ocean-ridge basalts are not the parent material of the

Metis Shoal lava.

Comparison With Eruptions at Falcon Island

The nearest historic volcanism occurred on Falcon

Island (now only shoals remain), which is about 60

miles south of Metis Shoal. An account of the eruption

of 1927 is given by Hoffmeister, Ladd, and Ailing

(1929). Beginning on 4 October 1927, and ending

sometime in the spring of 1928, the eruption built an

island about 2 miles in diameter, and more than

1.2 X1010 ft3 of fragmental volcanic debris were

ejected. No lava was erupted; the island was composed

entirely of pyroclastic material ejected as agglomerate,

a mixture of brownish-gray ash, scoria, pumice, and

blocks of solid lava. Most of the material, except on

the beaches, was poorly sorted.

No bulk nor mineral chemical analyses are available

of the erupted materials; however, Ailing (Hoffmeister, et al. 1929) gives a brief petrographic description of the rocks:

A study of several thin sections of samples collected

on Falcon shows that the rock is a basic one of basaltic affinities, its textural relations suggesting a diabasic pyroclastic. It is composed in large part of a

nearly opaque brown glass, the small amount of crystalline matter present being very fine-grained and profoundly stained with oxides of iron.

Rods or lath-shaped crystals of plagioclase feldspar

are strikingly embedded in the glass. Many present a

zonal structure and when not actually concentrically

grown show an undulatory extinction. Extinction

angles point to a labradorite of bytownite composition
16 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

but it seems highly probable that the composition is not

uniform but varies. Absolutely untwinned crystals of

plagioclase are surprisingly common. These rods are

arranged in curiously grouped bunches in some portions of a slide, controlled by blow holes, and in others

exhibit a crude parallelism showing distinctly a flow

structure. These portions may be interpreted as inclusions of volcanic matter which represent a previously formed rock, possibly a flow broken up by the

recent activity which is responsible for the reappearance of the island.

Stout grains of pale green pyroxene, not far from

diopside, can be identified by exploring with the high

power. Olivine is probably present but its identification

is uncertain. Magnetite, apatite, ilmenite, are more

definitely determined. Much of the magnetite is titanium bearing.

Ailing believed the rocks to be basaltic, but on the

basis of his petrographic description, there is little

justification for this view. The index of refraction of

the glass for Falcon Island was not given. This would

have been a good guide to the SiO2 content of the glass,

which in turn would allow determination of the actual

nature of the erupted materials.

The Falcon Island volcanic rocks of the 1927 eruption differ from those from Metis Shoal. The large and

abundant phenocrysts in the latter obviously were not

present in equal abundance in the described Falcon

Island ejecta, nor is it likely that they would have been

missed during the sampling of the island, as they are

readily visible.

The Falcon Island eruptives of 1885 consisted in

part of pumiceous blocks, containing bytownite and

augite phenocrysts (Harker, 1891), but lacking hypersthene. Harker's description of the phenocrysts shows

them to be remarkably like those from Metis Shoal:

Some of the larger (augite) crystals are so associated

with the porphyritic feldspars as to prove that they

belong to an early phase of consolidation, but the bulk

of the minerals occur in ill-shaped idiomorphic crystals

scattered through the ground.

Harker's (1891) descriptions of glassy ash from

other of the Tonga Islands commonly mention crystals

of plagioclase, clinopyroxene (presumably augite), and

rhombic pyroxene (presumably hypersthene). It thus

appears that ash and pumiceous lava blocks, such as

those erupted at Metis Shoal recently, and at Falcon

Island in 1885, have been erupted at numerous times

during the geologic development of the Tonga Islands.

Relation Between the Pumiceous Blocks and the

Nature of the Metis Shoal Eruption

The generation of pumice from lava implies release

of pressure on lava containing dissolved volcanic gases,

perhaps during ascent of the lava from depth, or due

to the sudden explosive release of pressure on a nearsurface magma chamber due to failure of the chamber's

roof. The vesiculation process, combined with injection

of the vesiculating lava into cold, water-saturated older

volcanic debris, would lead to explosive steam generation and further shattering of the injected materials.

These materials would contribute to the formation of

a pyroclastic cone, containing very little massive, unvesiculated rock, and no extrusive flows. Such an eruptive scheme appears to fit the events recorded during

the eruptions at Metis Shoal and Falcon Island.

Figure 6 shows very large, angular blocks in the sea

at the base of the eruption cloud. It seems likely that

these large, undoubtedly pumiceous blocks actually

solidified within the volcanic cone, and were then forcefully pushed upward as solid masses during continued

injection of new pumice-forming lava at shallow depth.

Wave erosion and further shattering of these blocks

during intense steam eruptions would rapidly reduce

these blocks to fine materials, which eventually form

the surficial materials of the cone. These pyroclastic

cones are readily reduced to below sea level once the

eruption ceases. Wave erosion is the dominant process

of reduction (Hoffmeister et al., 1929) ; postulation of

some sort of volcanism-related subsidence to account

for the rapid disappearance of these islands appears

to be unnecessary.

The high silica content of the Metis Shoal volcanic

glass suggests a highly viscous lava, compared, for

example, to basaltic lava. Such high viscosity characterizes volcanic rocks that typically are violently

erupted in the Pacific region, and much less frequently

form flows.

Should an unusually voluminous eruption eventually

occur, a more permanent island may form at Metis

Shoal. Petrologically it may be much like Fonualei, an

active Tongan volcanic island containing dacitic

eruptives.
NUMBER 4 17

Suggestions for Additional Studies

Further data on the Metis Shoal pumiceous blocks

are needed. High-temperature experimental studies

would help to determine the origin of the phenocrysts

in the blocks. The preliminary petrographic and microprobe data appear to point to derivation of the bytovvnite phenocrysts by direct crystallization from a

magma with the composition of the pumiceous blocks;

mixing of genetically unrelated materials seems more

likely to account for the presence of olivine. If the

phenocrysts did, in fact, crystallize from a magma of

the bulk composition of the blocks, this could be

demonstrated under experimental conditions; the possible role of water and high total pressures may complicate the experimental approach. Nonetheless, oneatmosphere anhydrous experiments would be initially

useful. These experiments would also provide some

idea of the rocks' eruption temperature. Further useful data include trace elements and radiogenic isotopic

ratios. All these data are pertinent to the further characterization of the eruptives of the western arc of

Tonga, and hence to the possible nature of partial melting and subsequent evolution of magmas in the upper

mantle and crust beneath an active inner-island arc.

Careful monitoring of the volcanic eruptions of

Tonga should begin. Such monitoring should yield the

rate at which materials are being emitted in the region,

the dynamics involved in shallow submarine eruptions

and in the construction and destruction of volcanic

islands. Bathymetric surveys are needed in particular

before, during, and after eruptions to at least roughly

gauge volumes of eruptions. Early detection of eruptions and possible locations of as yet undiscovered

active Tongan submarine volcanoes may be possible

by the acoustic means which recently located active

submarine volcanoes just north of New Zealand

(Kibblewhite, 1966). This would require new listening stations near Tonga.

Acknowledgments

We wish to thank all those who provided the information on the eruption, and who are noted in the

referenced sources. In addition, we acknowledge the

helpful assistance provided by Mr. J. Riechelmann

and other members of the Tonga government, the

British Council in Nuku'alofa, Captain William Stanley of the ship Alaimoana, and 'Aplipate and Penisimani Tupouniua of Nuku'alofa. Mr. Robert Citron,

Smithsonian Center for Short-lived Phenomena, provided helpful assistance in communications with Tonga

throughout the course of the eruption.

Literature Cited

Bryan, W.

1969. Low-potash Drift Pumice from the Coral Sea.

Geological Magazine, 105(5): 431-539.

Evans, B. W., and Moore, J. G.

1968. Mineralogy as a Function of Depth in the Prehistoric Makaopuhi Tholeiitic Lava Lake, Hawaii.

Contributions to Mineralogy and Petrology,

17: 85-115.

Harker, A.

1891. Notes on a Collection of Rocks from Tonga. Geological Magazine, 8: 257-258.

Hatherton, T., and Dickinson, W. R.

1969. The Relationship Between Andesitic Volcanism

and Seismicity in Indonesia, the Lesser Antilles,

and other Island Arcs. Journal of Geophysical Research, 75: 5301-10.

Hoffmeister, J. E.; Ladd, H. S.; and Ailing, H. L.

1929. Falcon Island. American Journal of Science, 18:

461-71.

Isacks, B.; Sykes, L. R.; and Oliver, J.

1969. Focal Mechanisms of Deep and Shallow Earthquakes in the Tonga-Kermadec Region and the

Tectonics of Island Arcs. Bulletin of the Geological

Society of America, 80: 1443-70.

Isacks, B.; Oliver, J.; and Sykes, L. R.

1968. Seismology and the new Global Tectonics. Journal

of Geophysical Research, 73 : 5855-99.

Kibblewhite, A. C.

1966. The Acoustic Detection and Location of an Underwater Volcano. New Zealand Journal of Science,

9: 178-99.

Lacroix, A.

1939. Composition mineralogique et chimique des lavee

des volcans des iles de l'ocean Pacifique situees

entre l'Equataur et le tropique du Capricarne, le

175° de longitude ouest et le 165° de longitude est.

Memories de I'Academie des Sciences, Paris, 63 :1—

97.

Murdmaa, I. O.; Bezruko, P. L.; Zenkevich, N. L.; Prokoptsev, N. G.; and Repechka, M. A.

1969. Eruption of the Submarine Volcano Metis in the

Tongan Archipelago and its Effect on Sedimentation. Akademiia Nauk, SSSR, Doklady, 185(5):

1149-52. (In Russian.)

Phillips, C.

1899. On the Volcanoes of the New Zealand-Tonga
18 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

Volcanic Zones. Transactions and Proceedings of Troger, W. E.

the New Zealand Institute, 31: 510-51. 1935. Spezielle Petrographie der Eruptivgesteine. DeutsRichards, J. J. chen Mineralogischen Gesellschaft, Berlin. 360

1962. Catalogue of the Active Volcanoes of the World. pages.

Part 8: Kermadec, Tonga, and Samoa. Inter- Turner, F. J., and Verhoogen, J.

national Association of Volcanology, Rome. 38 1960. Igneous and Metamorphic Petrology. McGraw-Hill

pages. Book Company. 694 pages.

U.S. GOVERNMENT PRINTING OFFICE:1970 O—381-735