Bull Volcano1 (1994) 56:243-260

Voli/igology �9 Springer-Verlag 1994

The volcanic history of Volcfin Alcedo, Galdpagos Archipelago: A case study of rhyolitic oceanic volcanism Dennis Geist 1, Keith A. Howard 2, A. Mark Jellinek 1, Scott Rayder 3

1 Department of Geology University of Idaho, Moscow, ID 83843 U.S.A. 2 U.S. Geological Survey, 345 Middlefield Rd. Menlo Park, CA 94025 3 5202 Fairlawn Dr. Fayetteville, NY 13066

Received: August 6, 1993/Accepted: January 25, 1994

Abstract. Volcfin Alcedo is one of the seven western Gal~pagos shields and is the only active Galfipagos vol- cano known to have erupted rhyolite as well as basalt. The volcano stands 4 km above the sea floor and has a subaerial volume of 200 km 3, nearly all of which is bas- alt. As Volc~in Alcedo grew, it built an elongate domal shield, which was partly truncated during repeated cal- dera-collapse and partial-filling episodes. An outward- dipping sequence of basalt flows at least 250 m thick forms the steepest (to 33 ~ ) flanks of the volcano and is not tilted; thus a constructional origin for the steep up- per flanks is favored. About 1 km 3 of rhyolite erupted late in the volcano's history from at least three vents and in 2-5 episodes. The most explosive of these pro- duced a tephra blanket that covers the eastern half of the volcano. Homogeneous rhyolitic pumice is overlain by dacite-rhyolite commingled pumice, with no strati- graphic break. The tephra is notable for its low density and coarse grain size. The calculated height of the eruption plume is 23-30 km, and the intensity is esti- mated to have been 1.2x108 kg/s. Rhyolitic lavas vented from the floor of the caldera and from fissures along the rim overlie the tephra of the plinian phase. The age of the rhyolitic eruptions is _< 120 ka, on the basis of K-Ar ages. Between ten and 20 basaltic lava flows are younger than the rhyolites. Recent faulting resulted in a moat around part of the caldera floor. A1- cedo most resently erupted sometime between 1946 and 1960 from its southern flank. Alcedo maintains an active, transient hydrothermal system. Acoustic and seismic activity in 1991 is attributed to the disruption of the hydrothermal system by a regional-scale earth- quake.

Key words: caldera - Galapagos - ocean island - plin- ian - rhyolite - shield volcano - tephra fall

Correspondence to: D. Geist

Introduction

Volc~in Alcedo is one of the seven coalesced shield vol- canoes that make up Isabela and Fernandina Islands in the western Gal@agos Archipelago (Fig. 1). The west- ern Gal@agos Islands are the type locality of the Gal- @agos shield volcanoes, characterized by gradual out- er flanks, steep upper flanks, flat summits, and large calderas (Fig. 2), resembling an inverted soup bowl (Williams and McBirney 1979). Volc~in Alcedo differs from the other western Gal@agos volcanoes because it has erupted rhyolite; so far as is known, the other west- ern volcanoes have erupted only tholeiitic basalt.

' ' | ' ' ' I ' ol 'HI I / I o 90~ . cocosl 80W i l /

- 2 N , '~ / P l a t e I Pacific U / GSC .rJ." r--a Ooeanjl /

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? . . . . . . . . . . . . . ] /

Wolf Vol. ~archena V. Ecuador ~ a ~ ' ~ _ ~ chena ~ ~

A

G e n o v e s a

- 0 ~ k , , ' " * ~ ~ Darwin Vol. ~,~ - -

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I ~ r .... ~ C r i s t o b a l _lO.q C e r r o ~ W S i e r r a . . . . _

. v Azul ~ Negra

2 92 ~ I I 90~

Fig. 1. Location and tectonic setting of Volcfin Alcedo. Inset shows the location of the Galgpagos Islands in relation to the Gal@agos Spreading Center and South America. Alcedo lies at the center of Isabela Island. Contour is the 2000 m depth that outlines the Gal@agos Platform

244

subprovince within the Galfil?agos Archipelago. The volcanoes' steep upper slopes, flat tops, circumferential fssures, and prominent calderas contrast strikingly to the subdued shields that lack calderas in the central part of the archipelago. Furthermore, with the excep- tion of Volcfin Alcedo, the western volcanoes have erupted relatively uniform ocean-island basalts, unlike the diverse lavas of the other islands, which range from alkali-olivine basalts and trachytes to tholeiites indis- tinguishable from mid-ocean ridge basalts.

Fig. 2. Digital shaded-relief model of Volcfin Alcedo's caldera prepared from topographic data described in Fig. 4. Oblique lighting in the model accentuates apparent relief. A few digital artifacts are present. Image produced at the US Geological Sur- vey, Menlo Park, CA, by Geoffrey Phelps

Here, we present results of a detailed, field-based study and a geologic map of Volcfin Alcedo, the first study of its kind of a western Gal~pagos shield volca- no. Alcedo has been briefly described in regional geo- logic descriptions (e.g. Banfield et al. 1956; McBirney and Williams 1969; Simkin 1972; Nordlie 1973; Simkin 1984; Cullen et al. 1987; Munro and Mouginis-Mark 1990; Chadwick and Howard 1991) and a short note (McBirney et al. 1985). A historically active geyser has also been described (Nordlie and Colony 1973). The petrogenesis of Alcedo's lavas (e.g. Geist et al. 1992) is beyound the scope of this paper and will be addressed in a contribution that is in preparation.

The Galfipagos Islands (Fig. 1) are an archipelago of volcanoes that are the product of hotspot activity adjacent to the Galgpagos spreading center (Hey 1977; Hey et ah 1977). Although recent volcanism has oc- curred throughout the archipelago, the volcanoes of Is- abela and Fernandina Islands are by far the most ac- tive. These islands make up one of the most active vol- canic regions on the planet; nearly 50 major eruptions have been observed, and unwitnessed eruptions have undoubtedly also occurred in historic time (Simkin et al. 1981; Smithsonian/SEAN 1989). In addition to be- ing the focus of historical activity, the two western is- lands form a distinct volcanological and petrological

Morphology of Volc~in Alcedo

Despite the unique occurrence of rhyolite, Volcfin A1- cedo has all of the features characteristic of Galfipagos- type shields, including the planet's largest population of the famed Galfipagos tortoise. A narrow annulus of steep (to 33 ~ outward-facing slopes surmounts a broad gently sloping shield about i km outboard from the caldera (Figs. 2, 3). Alcedo's caldera is 7-8 km wide and has a maximum depth of 270 m (Figs. 2, 4). Fewer circumferential eruptive fissures occur on Atcedo than on the other western volcanoes (Chadwick and Ho- ward 1991), and, with the exception of Sierra Negra, Alcedo has the shortest steep upper flanks. Nordlie (1973) believed that Volcfin Alcedo is a geomorphic end-member reflecting declining activity, owing to its relatively shallow caldera and the shortness of its steep summit slopes.

Most of Volcfin Alcedo is underwater, and coales- cence of Alcedo with its neighboring volcanoes forms the west-central part of the Galfipagos platform (Fig. 1). Volcfin Alcedo's subaerial volume is about 200 km 3. The volume of the Gal~pagos platform be- neath Alcedo, to 3200 m depth, totals 2300 km 3, but the volcano's submarine volume must be double or tri- ple this value, depending on subsidence beneath the volcano and the extent that Alcedo's lavas extend lat- erally beyond the shoreline.

In addition to being of special petrologic and volca- nologic interest, the rhyolitic tephra blanket provides a useful stratigraphic marker, as it permits distinction be- tween younger and older basalts on Alcedo (Figs. 3, 4). Consequently, Alcedo can be divided into three princi- pal stratigraphic units, corresponding to an older basal- tic eruptive phase, an intermediate age rhyolite phase (including a small volume of stratigraphically interfin- gered basalt), and a younger basaltic phase (Table 1). A dearth of widespread eruptive beds, erosional un- conformities, systematic petrological differences, and magnetic reversals in the western Gal~ipagos hamper stratigraphic subdivision on the other volcanoes. The only other siliceous lava known in the Galfipagos Is- lands is a 1 Ma dome of rhyodacite on Pinzon volcano (Baitis and Lindstrom 1980). It has been suggested that Alcedo and Pinzon constitute a distinct petrologic sub- province containing siliceous rocks (McBirney 1990).

b

245

EXPLANATION

~ ] Younger basalt

~ ] Rhyolite flows

• ] RhyoIite pumice tephra

• ] Tephra and soil, undivided

~ ] Older basalt

Favlt--Dotted where concealed. Bar and ball on downthrown side

Fig,, 3. a Geologic map of Volc~n Alce- do. Some exposures of the younger bas- alt may be included as part of the older basalt in areas where the pumice unit is thin or the flows are heavily vegetated. Area underlain by undivided tephra and soil on the west flank of the volcano is deeply gullied and may consist largely of weathered basaltic scoria. Contours in ~eet. Box indicates the area of Fig. 4. b Eruptive vents and topographic con- tours (in feet; interval 500 feet - 152 m) modified from Chadwick and Howard (1991)

246

/. /" /

/ i

/,/ j1" " /

I/ / <,Q

I

.......... , p

DG27 A

\ , . /

......... x\ xN~x

> /

yb

DG43a' ~x~ " " ( 'J ! ( " ~ . , ~ , , , ~

.,.~-~-~ <Li \

- : ............ < . ...... :) , - .

/ .:., yb )

), <g~ x ~ - .......

�9 ~'" ~7 t ........ 2~ ...... : ...... ! ' l ......

.,hyoli,o i ,,r . ,

yb

i . :. 7 1

/ / . . / , i i " <'.~ i ob j

,~' .J i

~ - - ] Sulfur veneer

[ -~- - - ] Alluvium

t - - - ] Talus

F ~ - ] Younger basalt

�9 , . . . . . . . . , , , , . .......... ~[[.,, . . . . . . . . . . . . .;= .............. :::::::::::::::::::::::::::

. . . , , , , \ ,-:/: ........ ....................

Rhyolite Obsidian flows

ff

rb

I S

rr

rt

P

EXPLANATION

Frothy facies in ealdera

Blocky facies in ealdera

South-flank flow

Rim flow

Terrace flow

Pumice tephra

. . . . . . " 2 y .

- ~ Tephra and soft, undivided

- ~ Older basalt

' ~ Landslide

Fumarole

,. ~ . . . / " ...... / \W

rs \IZ..-" . /'21%" , - /

- ~ Basalt vent (including vents mantled by pumice)

Rhyolite vent (including vents mantled by pumice)

~ Crater [Crater is a rhyolite vent)

----~ Fault, bar and ball on downthrown side

~ K-Ar dating sample locality

Fig. 4. Geologic and topographic map of Alcedo's caldera (com- pare with Fig. 2). Topographic contours are at 100-ft (30.5-m) in- tervals; dashed auxiliary contours are at 50-ft intervals. These were drawn photogrammetrically using a Kern PG-2 stereoplot- ter. Elevations were calibrated photogrammetrically from map- ped shorelines to stations on the caldera rim whose elevations

were determined by altimeter. The contours are approximately correct with respect to elevation, longitude, and latitude, and they faithfully record local relief and general form. Mapped fu- maroles were active as of 1970, and most were active in 1991. Small parts of the southern caldera rim wall mapped as unit p have a thin veneer of unit yb that flowed down the caldera wall

Eruptive Stratigraphy of Alcedo

Pre-Rhyol i t ic Basaltic Phase

The older basalts (map unit ob) are exposed in caldera walls, on the volcano flanks where erosion has cut through or removed the rhyolitic tephra, and upwind

from the rhyolite vent (Figs. 3, 4). The sections ex- posed in the walls represent about 90 + 40 km 3 of the older basalt, constituting most of the upper half of the volcano's mass above sea level. We expect that similar rock also forms the underlying, unexposed bulk of the volcano.

247

Table 1. Stratigraphic sequence at Volc~n Alcedo

Unit Unit Rock type Maximum Volume Volume of K-Ar age, ka symbol thickness estimate dense-rock

m k m 3 equivalent km 3

Younger basalt yb Basalt flows > 20 1 0.8

Rhyolite sequence South-flank obsidian. Stratigraphic

position uncertain rs 25 0.4 0.3

East rim obsidian rr < 5 0.01 0.003

Caldera-floor obsidian < 50 0.07 0.05 Blocky flow unit rb Frothy flow unit rr 108 + 12

Terrace obsidian flow. Stratigraphic position uncertain rt 10 0.01 0.008 74 + 24

Main-tephra from plinian eruption p 15 :_> 3.4 _>0.6 B layer Commingled rhyolite and dacite pumice fall Proximal facies Rhyolite fall, surge, and lava deposits A layer Rhyolite pumice fall Icelandite layer Icelandite tephra 0.1 0.001

Basalt interbeds - - Basalt flows and scoria 6 0.001 0.001 Older pumice op Pumice tephra (2 exposures) L5 0.001 0.0002

Older basalt ob Basalt flows, rare icelandite 250 90 70 145 + 46, 0 + 50

Unexposed parts of shield Above sea level 760 110 80 Below sea level 3200 > 2300 > 1900

Rhyolite flow

Commingled rhyolite and dacite flow

Rhyolite flows

Frothy rhyolite, obsidian, and pumice

The exposed section of older basalts is as much as 250 m thick on the southeast wall of the caldera. A t this locality, basaltic lava flows dip outward between 10 and 40 ~ Two exposed flows dipping outward about 15 ~ preserve vertical lava stalactites where lava dr ipped into cavities, indicating the dips are depositional. On the nothern and western walls of the caldera, the ex- posed flows dip only 0-15 ~ .

The stratigraphic sections in the caldera wall expose most ly basaltic lava flows 1-4 m thick. Most flows have plagioclase phenocrysts +ol iv ine and augite, al though aphyric flows also occur. Icelandite, identifiable only by chemical analysis (54-57% SiO2), crops out on a low terrace on the N W wall of the caldera. Individual basalt flows can be traced for over a ki lometer along the caldera wall, but most are discontinuously exposed, owing to landslides and dense vegetation. Pahoehoe flow tops are nearly as abundant as a'a. Many of the flows have oxidized scoriaceous tops, but no well-de- veloped paleosols are exposed. All observed contacts are conformable , and no faults with notable displace- men t have been observed. Pyroclastic material is all but absent in exposures of the caldera walls, constitut- ing < 5% of the total exposed thickness, and most of this is basaltic scoria directly related to the surrounding flows. Several dikes that dip steeply toward the caldera cut some of the flows.

Several fissure vents of the older basalts are ex- posed on the flanks of Volc~n Alcedo (Fig. 3). Most

have the circunfferential and radial orientat ions typical of the western Galfipagos Islands (McBirney and Wil- liams 1969). Older basalt fissures within several kilo- meters of the caldera tend to be parallel to the rim. Distal and intermediate fissures are radial or subradial; most of these are subparallel to the NW4rend ing axis of Isabela Island. Notable exceptions are the east- trending lines of vents low on the western flank of the volcano, one of them connecting with Cowley Island (Fig. 3). The composi t ion of the basalts erupted f rom the Cowley Island fissure is indistinguishable f rom oth- er Volcfin Alcedo basalts. Because each of the western Gal~pag0s volcanoes has a distinctive composi t ional fingerprint (Geist 1992), these basalts must be related to Volc~n Alcedo 's m a g m a supply.

Interpretation of the older basaltic phase

The dipping lava flows on the southeast wall of the cal- dera were deposited on the steep upper flanks of Alce- do and were exposed when truncated by caldera widening (Figs. 5, 6). The nearly horizontal flows in the northern half of the caldera wall may have been depos- ited on the subhorizontal surface be tween the caldera rim and the outer flanks, similar to those that currently exist on Alcedo 's N W sector, Sierra Negra volcano 's north r im (Reynolds and Geist 1991), or Cerro Azul 's south rim. Most of these flows are only f rom 1 to 4 m

W E

r

A

0 I i,

- I k m

o 2

/ yb A'

~=~ ;aili i!

4 6 8 I0 12 k m

Plinian

248

-I 0 2 4

Fig. 5. Interpretive cross sections through Alcedo caldera. Lines A A ' and B B ' are located on Fig. 4. Rhyolite is stippled, and the pumice is too thin to show. Older basalt, ob; younger basalt, yb.

6 8 i 0 km

The depth and shape of the magma chambers are unknown, and subsurface features are interpreted stylistically

thick, suggesting that they were emplaced on the upper flanks of the volcano or capped pre-existing benches, rather than ponded on the floor of a caldera. Several flows now exposed on the floor of the caldera are much thicker (up to 20 m) due to ponding. Ponding of lavas in the caldera may have raised its floor level be- tween successive colIapses. There is no direct evidence for how long a caldera has existed on Volc~n Alcedo, but it is clear that it had attained its present dimensions before the rhyolite eruptions.

Volcfin Alcedo's caldera is asymmetric: a high, steep, single scarp borders the south side, and a step- wise three-tiered escarpment borders the northwest side (Figs. 4, 5). We attribute the southern escarpment to scalloping of the upper flanks by caldera-growth faults. By contrast, the northwest terraces are increas- ingly weathered and vegetated outward of the caldera, suggesting that they formed sequentially, with subsi- dence migrating to the southeast. The northwest benches are segmented by small faults and fissures caused by slumping toward the caldera center, but we interpret the major tiered benches as parts of older cal- dera floors stranded by progressive collapse events. The escarpments are draped by inward-dipping lava flows indicating flow over previously stranded benches. The sequence of collapse scarps on the northwest side, the steep southern walls, and the northeast elongation of the caldera suggest to us that the caldera has been migrating to the south. Hydrothermal activity, young rhyolite eruptions, and recent floor-fracturing on the south side of the caldera further suggest that the focus of magmatic activity has migrated south.

Rhyolitic Phase

Exposures of Volcfin Alcedo's rhyolites occur within the upper part of its stratigraphic section and include both pyroclastic material and lava flows. The volcano has erupted rhyolite at least twice; tenuous correlations suggest the possibility of as many as five separate erup- tions. All rhyolites have phenocrysts of oligoclase, iron-rich augite, and titanomagnetite totalling less than 5%. Sparse blobs (smaller than several centimeters across) of quenched, vesicular basalt occur in most of the rhyolitic lavas. The youngest rhyolitic tephra de- posit is referred to as the 'main tephra'; it is most wide- spread, and its correlation is straightforward.

Older pumice

Two pumice layers exposed in the caldera walls under- lie the main tephra and are separated from it by basal- tic lava flows (unit op in Table 1). Exposures of both are too small to be mappable at the scale of Fig. 4. One is exposed in a gully 100 m inside the southwestern rim of the caldera. At this locality, at least 1.5 m of reverse- ly graded pumice fall is exposed and overlain by an 0.8- m-thick basalt flow topped by scoria (see Fig. 7, medial facies). A thin, poorly developed paleosol on the top of the pumice has been baked by the basalt, suggesting at least a several-year hiatus. A 10-cm-thick layer of re- worked pumice, presumably derived from the underly- ing unit, overlies the basalt and is capped by another thin weathered layer. This weathered layer is then overlain by the main tephra.

249

N S

Fig. 6A-E. Schematic sections of our interpretation of the evolu- tion of Volcfin Alcedo from a Fernandina-like domal shape (A) to a broader caldera (B), in which the center of subsidence re- cently migrated southward (C) prior to rhyolite eruption (D), and finally moat faulting by resurgence and sagging of the caldera floor (E)

The second pre-main rhyolite tephra is a layer 0.3 m thick exposed on the northern wall of the caldera be- tween horizontal basalt flows. The pumice in this de- posit is better-rounded than the main-tephra; hence it is probably slightly reworked. Two basalt flows (total 6 m thick) lie between this pumice and the overlying main tephra.

The Main Tephra

The main tephra is best exposed on the east flank of Volcfin Alcedo, especially along the coast. Most of the main tephra erupted from the site of a large crater on the southern caldera floor (Figs. 2, 4), but evidence presented below suggests a second, hidden vent along the eastern caldera rim for the upper part of the depos- it. These deposits were studied in detail throughout their extent, in order to elucidate the character of the

eruption of these unusual oceanic rhyolites (Jellinek and Geist 1990; Rayder and Geist 1990).

The main tephra is divisible into several facies which are defined by changes in stratigraphic features with distance from the vent (Figs. 7, 8). The proximal facies occurs within 2 km of the vent, the medial fades is best exposed along the rim of the caldera (2-7 km from the vent), and the distal facies occurs on the east flank of Volcfin Alcedo (7-20 km). The stratigraphy becomes better-defined with distance from the vent; the units described below are defined as they appear in the distal facies, especially along the east coast of Isa- bela Island.

A thin, black, scoriaceous icelandite (54% SiOa) fall unit is discontinuously exposed at the base of the main tephra along the southeast coast. Its maximum thick- ness is 10 cm. There is no evidence of an hiatus, such as oxidation or paleosol development, between this unit and the overlying rhyolitic main tephra.

Two other topography-draping fall units and vastly subordinate, near-vent flowage deposits dominate the main tephra (Figs. 7, 8). The lower, or 'A', fall unit consists almost entirely of light-colored rhyolitic (71% SiOz) pumice. It is overlain by the 'B' unit, which com- prises light-colored rhyolitic pumice, black dacitic (659/0 SiO2) pumice, and mixed pumice ('mixed' pu- mice referring to tephra clasts and lavas that are heter- ogeneous mechanical mixtures of two magmas). Unit A is further divided into two subunits. The lower one (A1) is characterized by ash- to lapilli-sized pumice that coarsens only slightly toward the vent and its not- ably finer grain size. A1 consists of multiple thin beds with poorly developed normal grading. Individual beds are up to several centimeters thick and have basal ho- rizons with slightly larger clasts (up to 5 cm). A1 ranges from 2.5 to 45 cm thick along the coast. It is not recognizable in the proximal facies and is poorly de- fined in the medial facies (Fig. 7).

A2 is by far the most voluminous part of the main tephra. A2 is a multiple-bedded deposit (Fig. 8) con- sisting of gray and cream-colored pumice; it is charac- terized by the presence of exceedingly coarse, highly vesicular pumice that increases in abundance upward, making the deposit reversely graded. The bedding is defined by lenses that contain higher concentrations of the large ultravesicular clasts interlayered with beds in which smaller, denser clasts predominate. Individual layers range from 5 to 75 cm thick. The bedding is dis- continuous, and single layers can be traced for only tens of meters or less, imparting a lensoidal shape to the individual beds. The number of beds in the individ- ual sections ranges from three to 15.

Many of the coarse clasts in A2 are plate-like, and concentrations of them define the bedding. These clasts are unusual, because they have densities as low as 0.15 g/cm3; this density indicates 94% vesicularity, an observation that is at odds with Sparks's (1976) pre- diction that pumice will fragment at vesicularities of about 75%, when equal-sized spherical vesicles be- come close-packed. Petrographic and SEM examina- tion of the highly vesicular pumice indicates that it is

250

Proximal

Reversely-graded pumice fall. - . . . .

Poorly sorted, ungraded <r ~ ) \ " " ) tephra fall with largest clasts 2 cm in diameter. Fine-grained obsidian lapilli.

Poorly sorted pumice fall up to 2 cm with a layer o! fine obsidian lapi, l i . . ,(~--~(,.~/~

Sequence of 5 era-thick fine-grained " ~ . ~ . "1~ cross beds and dune forms interspersed with 10 cm-thick layers l~ ,c>~ '~ 1~ . ef normally graded tephra fall.

non-bedded tephra fall.

Base notexposed.

Media[ Disto[

I 1 meter

?

b ~ o o . . .

A2. Multiple, poorly defined beds of pumice fall. Crude reverse ~" "b" "~__ grading. Clasts to 20 cm across. ,b ,

.~, b~,.b AI. Angular pumice t o ~ 3 cm . - . . across. ~ -" " "

Reworked pumice ~ '~11 ~' . \- "bE'. . topped by paleosol. ~ " ~ ~-"

Scoria tephra 1o 4 cm acro Scoriaceous basalt flow.-

Reworked pumice topped

paleoso,. I "": ' l ,oo Multiply bedded pumice fall of O. b___., pre-mainreverse grading.phase" Poorly developed J.p ~ 0 ~ . J

r : I

�9 %~"~*1 B. Coarse black dacitic, gray . rhyolitic, and commingled

I ~ ~ I pumice fall. ",- O" b'l

u (bb "(31

J

A2. Multiply bedded, reversely graded pumice fall. Clasts to 30 cm across.

A1. Multiply bedded, fine- grained tephra fall. Icolanditie tephra fall. Basaltic lava flow topped by thin paleosol.

Fig. 7. RePresentative stratigraphic sections through the proximal, medial, and distal facies of the main-phase rhyolite pumice tephra

formed by close packing of extremely elongate vesi- cles, typically one millimeter wide and up to several centimeters long. The elongate vesicles become close- packed at much higher vesicularities than spherical vesicles (Klug and Cashman 1991). Most of the vesicles are circular in cross-section, and they have much greater volumes than the individual subspherical vesi- cles of normal pumice, indicating that they are not simply stretched equivalents of the latter. Some of the septa between the vesicles in the highly vesicular pu- mice are thinner than one micron. Most of the highly vesicular pumice clasts are flattened parallel to the length of the vesicles, but the clasts are not notably el- ongate. Smaller, denser pumice clasts (0.4-0.6 g/cm3), however, are more abundant than the highly vesicular pumice. The smaller pumice clasts are nearly equant, have vesicularities of about 80%, and the vesicles are subspherical and relatively uniform in size.

The base of the B layer is defined as the first ap- pearance of black or mixed pumice. In some places, two subunits can be discerned: a thicker, massive layer, with gray and black pumices randomly dispersed, is overlain by thinly (5-10 cm) interbedded layers richer in a single pumice type. Almost no dacitic pumice ex-

ists near the crater on the south floor of the caldera or elsewhere within the caldera, but mixed rhyolite-dacite lava occurs on the densely vegetated southeast rim of the caldera (unit rr, rim rhyolite; Fig. 4) and may mark a separate source for the B pumice layer.

Juvenile pumice overwhelmingly dominates the en- tire tephra sequence. Clasts of obsidian and basalt li- thic fragments make up 1-12% of the deposit at the rim of the caldera and up to 7% along the east coast. Within the A2 deposit, lithic fragments are most abun- dant where the deposit is thickest and has poorly de- veloped reverse grading. The bulk density of the teph- ra at the coast (ca. 12 km from the vent) is 0.41 g/cm 3, as determined by weighing a large box of the pumice.

The main tephra is not as readily divisible within several kilometers of the caldera floor crater (Fig. 7). No subdivisions of the A layer can be made in the proximal facies, and the dacitic pumice that defines the B layer is nearly absent. In the proximal area, poorly sorted fall deposits are sandwiched between obsidian flows, and several surge deposits have been identified. The latter are characterized by multiple thin (several centimeters) beds of normally graded lithic-rich mate- rial interbedded with thinner beds that are cross-stra-

251

Fig. 8. Photographs of the main-phase tephra along the east coast (about 15 km from the vent). Top photograph shows the reverse grading and bedding defined by the plate-like ultravesicular clasts. Shovel is 0.6 m long. Bottom photograph shows the contact between the A and B layers

tiffed. The cross-stratified beds have small (cm-scale) dune forms composed of black ash. These deposits have an insignificant volume relative to that of the fall tephra.

Distribution of the Tephra

Isopach and maximum-clast isopleth maps show a strongly asymmetric dispersal of the tephra; the diSper- sal axis trends northeast of the caldera and the plinian vent (Fig. 9). Both maps are constructed for the A layer only, because erosion has removed part of the B layer over much of the island. Two major uncertainties limit confident appraisal of the distribution of the teph- ra. First, much of the tephra fell in the sea. Second, the

thickness of the pumice on the southwestern half of the volcano's outer flanks is obscured by strong weather- ing and dense vegetation on the windward side of the island. An extensive gully system on the upper south- western flank is apparent on aerial photographs, which suggests thick accumulations of pumice could be pres- ent. These are mapped as tephra and soil, undivided (Fig. 4). We were able to inspect a single gully on the southwestern flank about half-way down the steep summit slope. The gully exposes 0.2-0.3 m of rhyolitic pumice underlain by nearly 2 m of dark brown soil de- rived from basaltic scoria.

The volume of the subaerial exposures of the main tephra is 1.1 km 3 (0.2 km 3 dense-rock equivalent, or DRE), assuming that the B layer is 25% the thickness of the A layer, a proportion that is typical of the least eroded outcrops. We used the technique of Pyle (1989), who assumes that the natural log of thickness is proportional to the square root of isopach area, to ex- trapolate our data and estimate the amount of tephra that was dispersed to sea (Fig. i0). We assumed that the isopachs have a length/width ratio of 4:1, which is typical of the on-land isopachs. Our conservative esti- mate of the total volume of tephra is then 3.4 km 3 (0.6 km 3 DRE). The 'thickness half-distance', the aver- age distance at which the thickness is half that at the vent, is 6.3 km. The y-intercept, which provides an esti- mate of the thickness of the fall deposit at the vent, is 6.6 m, in excellent agreement with the observed 7.6 m, which includes several surge deposits and some ballis- tic material.

Clast-size data can also be presented by Pyle's tech- nique, which results in an estimated maximum clast di- ameter of 38 cm (compared to 39 cm observed) and a clast half-distance (the distance at which the largest clasts are half the diameter of the clasts nearest the vent) of 8.4 km. The ratio of the clast-size/thickness half-distance is 1:3, which is typical of plinian fall de- posits (Pyle 1989).

The extent and shape of isopleths of fall deposits mostly depend on the height of an eruption column and the wind velocity at the time of eruption (Sparks 1986; Carey and Sparks 1986). Using Carey and Sparks's (1986) calibration of isopleth shape, we obtain estimates of the plume height of between 23 and 30 km (the range in estimates depends on which isopleth is used; a 4:1 aspect ratio was assumed for the isopleths). Further, we estimate that the stratospheric wind was from the southwest with a velocity of about 35 m/s; el- ongation of the proximal isopachs indicate that the tro- pospheric winds were also from the southwest. The cal- culated magma discharge rate, also called the intensity of the eruption (Carey and Sigurdsson 1989), is 1.2 • 108 kg/s, a value typical of plinian eruptions.

Granulornetric characteristics

The grain size and sorting of the Alcedo A2 subunit (Fig. 11) is characteristic of plinian fall deposits (Walk- er 1971), although tephra at two sites are more poorly

252

A Thickness

B Max, ctast q

C Median Pumice

- . \ 72 ',,, ,0k k �9

Fig. 9A-C. Isopach, isopleth, and median-grain-diameter maps of the A2 deposit. The isopleths are constructed from the average of the five largest pumice clasts taken from a 1-m-wide swath up through the deposit. The median grain size was determined by hand-sieving and weighing several kilograms of the A2 pumice in the field. The caldera floor vent is indicated by an "x' on each map

sorted than is typical for fall deposits. The main feature of the Alcedo deposit is its overall coarse grain size (median grain size ranges f rom 6 to 26 m m ) and deple- tion in the fine fraction (Fig. 12), particularly in the dis- tal facies. Tephra within the proximal facies tends to be

50-

~- 10- .c2_ ~ D

c o

o

2~

1000

lOO-

e -

k -

10-

A

B

. , . . , , , . , . , . . . . . . . . . . . . . . . .

20 40 60 A't/-A-~ (k m)

Fig. 10A, B. Plots of the maximum clast diameter and thickness of the main-phase deposit plotted against the square root of iso- pleth and isopach areas, after Pyle (1989). The 'on-land' line rep- resents the area of the isopachs assuming that no tephra were dispersed to sea. The 'best estimate' lines assume that the con- tours have an aspect ration of 4:1. Data from the Askja C (phrea- tomagmatic) and D (plinian) deposits are shown for comparison (Sparks et al. 1981; in Pyle 1989)

more poorly sorted, particularly in the fine grain sizes. In addition, the median grain size is finer within the proximal facies. Al though the max imum clast size tends to be greatest near the vent, unusually large clasts also occur near the coast along the dispersal axis (Fig. 9).

The dispersal value (Walker 1973) of the Volc~n Alcedo deposit (the area enclosed by the isopach that is 1% of the m a x i m u m thickness) is est imated to be 1700 km 2, and the f ragmenta t ion value (percentage of the deposit < 1 m m at the 10% thickness isopach) is 2 -6%. These estimates are based on the extrapolated 4:1 isopachs and isopleths. Even with these conserva- tive extrapolations, the main-phase pyroclastic erup- tion is well within the plinian field by Walker ' s classifi- cation.

The median grain-size values (Fig. 9) do not change systematically away f rom the vent. Specifically, an area along the nor theast coast (about 17 km f rom the vent) exhibits a median grain size of more than 20 m m and is

253

128 32 8 2 0.5 0.125 O.03mm 1 1 | 1 1 I I

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99.9 Distal, Medial

99 Facies

�9 ~ Proximal o ~ o o~ 50 >~ -fl

8 10

1

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Fig. 12. Probability plots of sieve data from the Alcedo A2 depos- it. The hatchured field contains 13 curves from the distal and me- dial facies. The most notable feature of these data is the finer grain size of the proximal deposits, which we suspect is due to the unusually low fallwelocities of large, low-density pyroclasts

beyond a zone where the median grain size is less than 10-15 mm. The deposits in this area also show the best- developed reverse grading, the best sorting, and the smallest proportion of lithic clasts. These attributes coincide with an unusually great concentration of the large, plate-like highly vesicular clasts. Although the origin of this anomaly is unknown, one possibility is that the high-altitude wind shifted slightly to the north during the peak of the eruption.

Even though lithic clasts constitute a small fraction of the deposit, they allow estimation of the muzzle vel- ocity during the plinian eruption (Wilson 1972). An

obsidian bomb 0.5 m in diameter located within the A2 layer one kilometer from the vent yields an estimated muzzle velocity of 255 m/s.

Rhyolite Lavas

Rhyolitic lava flows erupted from several vents follow- ing the plinian phase of the eruption. The older flow unit on the caldera floor (unit rf, frothy rhyolite; Fig. 4) erupted from the site of the 500-m-wide crater on the southern floor of the caldera that also served as the vent site for most of the plinian phase. The lava is com- posed mostly of frothy, vesicular rhyolite; some of its interior is massive obsidian, and obsidian squeeze-ups and clasts are found on the flow surface. A sparsely vesicular, blocky obsidian flow (unit rb, blocky rhyol- ite) followed the frothy rhyolite flow, flowing slightly north of the older flow. Lavas from these two flow units are geochemically indistinguishable, but sparse cumulate gabbro xenoliths have only been found in the blocky obsidian flow. Sparse, quenched magmatic in- clusions of vesicular basalt occur in both flow units. Dikes approximately 1 m wide that fed the obsidian flows crop out within the crater. The thicknesses of the flows is appreciably uncertain, but we estimate their volumes to be 0.05 to 0.1 km 3.

Spatter and flows of mixed rhyolite-and-dacite lava (unit rr; Fig. 4) overlie the pumice deposits high on the southeast rim of the caldera, and large obsidian blocks perch on the rim ridge (Fig. 4). Sporadic outcrops of this mixed lava and spatter are present along nearly 2 km of the caldera rim; this material apparently erupted from an unidentified ring fissure along the rim. Much of the lava dribbled down into the caldera, where it plasters the wall; elsewhere, landsliding of the wall exposes the flow interior.

A 10-m-thick rhyolite flow (unit rt, terrace rhyolite) exposed by recent landsliding of the caldera wall dips 4 ~ into the wall below the large bench on the eastern side of the caldera, about 180 m above the caldera floor. Most of the flow is strongly pumiceous, but slightly vesicular obsidian constitutes the lower part of its interior. It is underlain by 2 m of normally graded pumice fall, with clasts up to 60 cm in diameter. This flow is traceable for nearly 2 km along the caldera wall (unit rt in Fig. 4). Four flows assigned to the younger basalt overlie the rhyolite flow, but the main-phase pu- mice is not exposed in this cliff section. A 20-cm-thick layer of reworked pumice overlies the basalt flow that overlies the rhyolite flow, but we believe this pumice came from the rhyolite. The stratigraphic position of the flow and its vent site are uncertain.

A large flow of blocky rhyolite on the heavily vege- tated southern flank of Volcdn Alcedo (unit rs, south- flank rhyolite) was erupted from a circumferential line of vents 3 km long (Fig. 3). This flow is 5 km long and 25 m thick at its toe; it is roughly 0.4 km 3 in volume and features prominently on satellite images and aerial photographs (cover of Eos, 67:35). The stratigraphic position of this largest rhyolite flow is uncertain rela-

254

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80 100 120

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3 .5

Fig. 13. Composition of rhyol- ite units at Volcfin Alcedo ar- ranged in a tentatively as- signed eruption sequence. Lava flows on the caldera floor (units rf and rb) and rim (unit rr) stratigraphically overlie the main-phase pu- mice (uniLp), which in turn overlies basal and older (pre- main-phase) pumice (unit op; not shown on Fig. 4). Other stratigraphic relations be- tween the rhyolites are un- constrained. Note that neither basalt erupted between units op and p, icelandite erupted at the onset of unit p, no da- cite commingled with unit rr and the uppermost part of unit p are plotted. Analyses are by x-ray fluorescence (Hamilton College and USGS Menlo Park laboratories)

tive to the main-phase rhyolite. No pumice overlies the flow, but it is outside the area of the main-phase pu- mice fall. The flow contains more abundant magmatic inclusions of basaltic scoria and is slightly lower in SiO2 and K20 than the caldera-floor rhyolitic lavas (units rf and rb; Fig. 13). The flow is similar in the ex- tent of vegetation cover and lack of weathering to the other rhyolites, and its vents are at nearly the same el- evation as the caldera-floor rhyolite vent.

Compositional Constraints

The rhyolite units have subtle but systematic composi- tional differences (Fig. 13). The rhyolites fall into two principal groups. The first comprises the older pumice (unit op), the caldera-bench rhyolite flow (rt), and the main tephra rhyolitic pumice (p). These units are char- acterized by low concentrations of iron, calcium, stron- tium, and zirconium, and high concentrations of silica and potassium. The caldera floor rhyolitic flows (rf and rb), the caldera-rim rhyolite lava (rr), and the south flank lava (rs) have higher concentrations of iron, cal- cium, strontium, and zirconium, and lower concentra- tions of silica and potassium.

Field observations show the relative ages of the rhyolites must be unit op > unit p > units rf, rb, and rr, whereas units rt and rs are poorly constrained. These stratigraphic relationships suggest a systematic compo- sitional change with time: low-iron, high-silica, high- potassium magmas erupted earlier and the high-iron, low-silica, low-potassium magmas later. In light of these trends, the composition of unit rt suggests that it may be the oldest rhyolite flow. The voluminous south-

flank lava (unit rs) likely represents the youngest rhyolitic eruption.

Interpretation of the eruptive sequence

The main rhyolitic eruption of Volcfin Alcedo initiated with a strong explosion and a relatively low plume, ac- counting for the fine-grained and strongly fragmented deposits of layer A1. Once a stable eruptive column developed, it continued to grow in height throughout the eruption, probably as a result of widening and clearing of the conduit and vent (e.g. Wilson et al. 1980), which caused reverse grading in subunit A2. The reverse grading and bedding did not develop from simple aeolian sorting. Fall velocity is a function of density and shape, as well as size, and each of these characteristics varies widely in the clasts of the Alcedo A2 deposit. The highly vesicular clasts tend to have maximum diameters roughly five times those of normal pumice clasts. With the size and density differences both taken into account, the terminal-fall velocities would be about eight times greater than for the normal pumice, except that the plate-like shape of the ultrave- sicular clasts makes their drag coefficient much greater than that of spheres of equal volume (e.g. Komar and Reimers 1978; Kerr and Lister 1991). Calculations based on measured aspect ratios indicate that the plate-like clasts would have fallen with a similar termi- nal velocity as the smaller sphere-like clasts with which they are interbedded. In other words, the two types of clasts are probably dynamically equivalent.

Bedding in pumice-fall deposits is commonly attri- buted to pulsing of the eruptive plume, but that is

255

probably not the principal control in the Volcfin Alce- do deposit. Stratification in the Alcedo deposit is most- ly defined by layers richer in the highly vesicular clasts (Fig. 8), not just by clast size, as would occur by aeolian sorting within an eruption plume. It is therefore likely that the Alcedo plinian eruption alternately erupted normal pumice (density of about 0.5 g/cm 3) and the highly vesicular pumice (density of about 0.15 g/cm3).

After about 80% of the total volume of the material had erupted, black dacitic and mixed pumice erupted simultaneously, along with gray pumice that is indistin- guishable from the earlier material, creating the B tephra subunit. Two observations are particularly im- portant to the question of the relationship between the A and B subunits. First, the lack of dacitic pumice within the caldera suggests that the caldera was upwind (southwest) of the vent for the dacite. Second, the composition of the dacitic pumice resembles that of the dark dacitic component of the commingled post-pu- mice lava on the east rim of the c'~ldera, apparently erupted from a ring fissure. We speculate that with- drawal of magma from a broad, shallow magma cham- ber near the end of the plinian phase disturbed the stratification of a strongly zoned magma chamber un- der the east rim and led to eruption from both the floor crater (rhyolitic pumice and flows) and a ring fis- sure (rhyolitic, dacitic, and commingled pumice and flow), Alternatively, explosive eruption of the rhyolite may have caused intrusion of dacitic and basaltic mag- ma into the rhyolitic reservoir, leading to simultaneous eruption of all the lava types from two or more vents.

The eruption sequence of the main tephra and sub- sequent flows resembles that observed from other rhyolitic volcanoes (e.g. Heiken and Wohletz 1987). Progressive devolatilization of the magma during erup- tion results in a transition in eruptive products and eruptive styles. A manifestation of this process is ob- served in the products erupted from the caldera-floor vent at Volcfin Alcedo: (1) a fine pumice layer related to the initial explosive opening of the vent and deposi- tion from a rising plume; (2) coarse reversely graded fall pumice deposited from the fully established plume of the main plinian phase; and (3) frothy rhyolitic flows, and sparsely vesicular blocky obsidian. Follow- ing these events, a 500-m-wide crater pitted the obsid- ian flows on the caldera floor. The sequence of erup- tive products is probably not a result of pre-eruptive volatile gradients in the magma chamber, because identical modes and phase chemistry of Alcedo's pu- mice and obsidian preclude significantly different pre- emptive water contents. Instead, the sequence may re- sult from increasing the permeability of the vent during a violent eruption (Eichelberger et al. 1986), which would increase the degassing efficiency as the eruption proceeded. Alternatively, the degassing efficiency may be controlled by the decreasing velocity of the ascent of the magma during an eruption, caused by progres- sively lowered driving pressure from below. In the plinian phase, the magma rises quickly through the vent (200-300 m/s; Wilson et al. 1980), so phase separa- tion is unlikely to be efficient, even with highly perme-

able magma. In the later slow ascent for the magma, volatiles are able to separate from the magma, and lava erupts.

Post-Rhyolitic Basaltic Phase

Basalt flows that are younger than any of the rhyolites (younger basalt; unit yb) cover most of the caldera floor. These flows erupted from roughly circumferen- tial fissures near the caldera rim, cascading down the caldera wall to the floor. A long line of fissures crosses the eastern caldera wall at a low angle (Figs. 3, 4). Younger basalt flows also cover part of the volcano flanks (Fig. 3). These erupted from circumferential fis- sures high on the volcano, frequently at the base of the steep summit slopes and from oblique or radial fissures lower on the flanks. The older basalt and the younger basalt are compositionally similar although the young- er basalt averages slightly richer in silica than the older basalt. Aa is much more abundant than pahoehoe on the flanks of the volcano, and aa and pahoehoe are present in nearly equal amounts on the caldera floor.

One of the most prominent and recent features of Volcfin Alcedo is an arcuate 4-km-long fault consisting of at least a dozen en-echelon segments that forms a moat around the southern part of the caldera (Figs. 2, 4). Maximum displacement on the vertical scarp is 20- 25 m. A large, unvegetated landslide scar at the inter- section of this fault with the caldera wall may indicate recent movement. The fault cuts rhyolite and a ponded flow of younger basalt that erupted from a vent at the northern part of the caldera floor. Our photogrammet- ric mapping indicates a 15 m depression of this flow in the caldera center.

Historic Activity

Volcfin Alcedo has had one known historical eruption: a lava flow low on the southeast flank that appears in 1960 aerial photographs does not exist in a 1946 set. Reports of a 1954 eruption from the north flank (Rich- ards 1962; McBirney and Williams 1969) are incorrect. In 1954, Urvina Bay, at the boundary between Alcedo and Volcfin Darwin, 'was uplifted several meters. The uplift was not related to any eruption, but a magnitude 6.5 earthquake struck the Galgtpagos region on 4 June of that year (Roth6 1969, p 298). A report of warm- water columns in Urvina Bay in 1976 supports the idea that the uplift was related to intrusion of a shallow sill (McBirney and Williams 1969). Alcedo has a very ac- tive hydrothermal system. Most of the fumaroles are aligned along an 1.2 km arcuate zone along the south- east caldera wall, near the rhyolite vents. Dead vegeta- tion in some of the fumarolic areas and observations over two decades indicate that the hydrothermal activ- ity is transient.

Closely following a large earthquake centered in the western Galfipagos on 3 March 1991, Volcfin Alcedo began what was feared at the time could be premonito-

256

ry behavior, characterized by muffled subterranean ex- plosions followed by tremors. In 1970, rumbles lasting 2-3 s, followed by muffled booms, had been heard near the then-active geyser. These sounds probably resulted from shallow steam explosions, which occasionally trig- ger small earthquakes. The regional earthquake of 3 March may have altered the hydrothermal system, al- lowing water new access to hot rock, and resulting in both explosions and shallow earthquakes.

Ages and Eruption Rates

Four whole rock samples from Alcedo have been dated by the K-Ar technique (potassium-rich minerals are absent) (Table 2). All four age determinations are near the younger limit of the K-Ar technique for po- tassium-poor materials. We therefore consider these ages, and the geologic interpretation based on them, to be tentative. The rhyolite samples are much richer in K20 than the basalt samples; thus their determined ages are probably more reliable.

K-Ar ages of two specimens of the older basalt are 150 + 50 ka and 0 + 50 ka (Table 2). The first specimen is from the lowermost flow exposed on the north wall of the caldera, and the second is a sample from the lowermost flow on the northeast wall of the caldera (Fig. 4). Their stratigraphic depths are 215 m (below the rim) and 55 m (below the bench) respectively. The first age determination is consistent with the normal magnetic polarity of the flow and its stratigraphic posi- tion. A glassy rhyolitic lava from the caldera floor (unit rf, Fig. 4) yields a K-Ar age of 120+10 ka. This flow was dated as 140+16 ka previously (McBirney et al. 1985), but that date is now thought to be less accurate owing to a less precise blank correction (RA Duncan personal communication, 1992). A glassy sample from the obsidian flow of uncertain stratigraphic position on the eastern bench (map unit rt) yielded a K-Ar age of 74 + 24 ka (Table 2).

Excepting the older basalt that contains no measur- able radiogenic argon, the numerical ages agree with the stratigraphic positions. The rhyolite dates do not constrain the eruptive sequence well; they are equally consistent with eruption of the entire rhyolite package in rapid succession at about 100 ka or eruptions sepa- rated by up to 66000 years. Accepting the numerical ages at face value yields an eruption rate of 1.8 x 106

m3/year for -90 km 3 of older basalt from about 150 to 100 ka. The rate of younger basalt erupted since then (1 km 3) would be 0.1 x 106 m3/year. The higher calcu- lated eruption rate of the older basalts is 1-2 orders of magnitude lower than that for very active hotspot vol- canoes such as nearby Fernandina or Kilauea in Ha- waii, but it is at least an order of magnitude higher than for the long-lived volcanic centers of San Cristo- bal, Floreana, and Marchena (e.g. Geist et al. 1986; Bow and Geist 1992; Vicenzi et al. 1990). A rhyolite age possibly much younger than the K-Ar ages is sug- gested by the lack of erosional unconformities, well- developed paleosols, and erosion of the pumice blan- ket. Furthermore, fumarolic activity focuses on the main rhyolite vent in the crater floor. Further work to test the ages of rocks on Alcedo would be valuable.

Structural Evolution

Volcfin Alcedo's location and shape provide insight to its early history. We infer that Alcedo formed by vol- canic activity similar to that now focused in a 100 km swath to its west, south, and north. This volcanism con- structs the western edge of the shallow Galfipagos plat- form from the 3-km-deep ocean floor as the Nazca plate travels eastward over the Galfipagos hotspot.

Volcanoes and elongate seamounts in the northwes- tern Galfipagos tend to be aligned along elongate northwest ridges that are not merged into the Galfipa- gos platform (Simkin 1984; Chadwick and Howard 1991). Northwest-striking eruptive fissures mapped on San Cristobal, Santiago, Pinta, and Fernandina islands (McBirney and Williams 1969; Swanson et al. 1974; Geist et al. 1986; Cullen and McBirney 1986; Chadwick and Howard 1991) suggest that this trend reflects a re- gional stress field. The unusually oblique Cowley Is- land fissure that cuts the east shore (Fig. 3) suggests that some of the vents at Volcfin Alcedo are also in- fluenced by the regional field, as opposed to local grav- itational and magmatic-induced stresses. The southeas- tern and central part of the archipelago exhibits east- striking grabens and fissures, which parallel the Galgt- pagos spreading center and the southern margin of the platform (McBirney and Williams 1969; Simkin 1984). The northwest trends in the northwestern Galgtpagos are not readily explained, but recall the oblique pat- terns generated where the tip of a spreading center

Table 2. K-Ar ages of selected samples from Volc~n Alcedo Sample Map Determined Radiogenic % Radiogenic % K

unit age (ka) Argon ( x 10 -8 cc/gm) Argon

SR1 rf 120 + 10 1.1300 6.97 2.44 DG48 rt 74 + 24 0.8437 3.90 2.93 DG27 ob 150 + 50 0.2549 0.52 0.45 DG43a ob 0 + 50 - 0.0021 0 0.43

Ages are calculated using the following constants: At + he, = 0.581 x 10 l~ year - 1 h e =4.962 x 10-1~ year-1 Determined by RA Duncan, Oregon State University. Locations are shown on Fig. 4.

257

propagates into rigid lithosphere (Kleinrock and Hey 1989).

Two observations indicate that Alcedo has had the morphology of a Gal@agos-type shield for much of its subaerial history. First are the steep primary dips of the outward-dipping flows. Second, the vents of the older basalts follow the pattern of being circumferen- tial near the summit and radial on the flanks.

An earlier start for Alcedo compared to neighbor- ing western Gal@agos shields accords with its decreas- ing eruption rate and Nordlie's (1973) notion that its subaerial profile represents an advanced stage com- pared to the higher, more domical shields of Fernandi- na, Wolf, Darwin, and Cerro Azul volcanoes. The steep, narrow upper flanks of Alcedo represent rem- nants of a larger domal-shaped summit region that grew by accumulation of lava flows before being large- ly engulfed in a widening caldera (Fig. 4b of Nordlie 1973). The evolution of Galfipagos shields is now viewed as a more repetitive, steady-state process than Nordlie envisaged, in which calderas, and perhaps even steep summit areas, are features that exist and recur during much of the life of a Gal@agos volcano (Sire- kin 1984; Chadwick and Howard 1991). The stratigra- phic record at Alcedo permits appraisal of Nordlie's (1973) proposition of an old age for Alcedo relative to its neighbors.

Outward-dipping flows of older basalt underlie and parallel the steep upper flanks to a stratigraphic depth of at least 250 m (Fig. 6a). Debate has centered on whether the characteristic inverted-soup-bowl profile of Gal@agos shields forms by inflation (McBirney and Williams 1969; Nordlie 1973; Cullen et al. 1987) or by construction by lava flows (Simkin 1972; Chadwick and Howard 1991). Vertical lava drips and the lack of un- conformities in exposed dipping flows favor a construc- tional origin for Alcedo's steep dips, making it improb- able that Alcedo's steep summit is the result of defor- mation from intruded cone sheets (McBirney and Wil- liams 1969; Nordlie 1973) or shallow sills (Cullen et al. 1987), unless the whole summit area (bounded by the sharp break to lower slopes) is uplifted as a piston-like body. Simkin (1972) proposed that eruptions from cir- cumferential vents near the summit rim build the steep upper flanks of Gal@agos volcanoes, and eruptions from more distal radial vents build the gentle outer subaerial slope.

The remnant of Volc~n Alcedo's steep summit re- gion is elongate northwest-southeast as is the caldera, probably reflecting the distribution of summit magma reservoirs. The elongation aligns with a prominent zone of subradial eruptive fissures that connect Alcedo with adjacent Volcfin Darwin (Fig. 3). The orientation of these fissures may reflect either interacting pres- sures from the magma chambers of the adjoining vol- canoes, or gravitational stresses in the ridge formed by overlapping lavas of the coalesced volcanoes (Chad- wick and Howard 1991). Oblique fissures on the south flank of Volcfin Alcedo likewise link it with the adjoin- ing Sierra Negra volcano; the post-1946 eruption was from one of these.

Eruptions from dominantly circumferential fissures near the caldera and radial fissures on distal flanks have characterized Volcfin Alcedo's history and that of its neighbors (Chadwick and Howard 1991). The fis- sures are considered to reflect the stresses generated in each volcano, superimposed on the regional field (Chadwick and Howard 1991; Chadwick and Dieterich in press). The line of circumferential fissure vents that crosses the eastern caldera wall, rim, and outer slope (Fig. 4) is one of the youngest. It curves around a focus south of the caldera center, which suggests to us that the magma chamber from which the fissures issued lay under the southern part of the caldera.

The benches on the northwestern and eastern cal- dera walls are interpreted as stranded caldera floors. They indicate that Alcedo's caldera partially filled and collapsed cyclically, and that the center of collapse may be migrating southward (Fig. 6b, c). Partial filling and collapse of the caldera probably repeats many times in the life of a basaltic volcano (e.g. Holcomb 1987). Nearby Fernandina ca]dera has experienced partial fill- ing, collapse, and refilling just in the past 25 years (Simkin 1984; Chadwick et al. 1991; Rowland and Munro 1992). The highest bench of older basalts in the caldera is level with the northwest rim (Fig. 4) and may record a stage when the caldera was full to overflow- ing, a state now shown by Marchena volcano (Simkin 1984; Vicenzi et al. 1990).

The rhyolite eruptions late in Volcfin Alcedo's his- tory were from vents as much as 8 km apart and on opposite sides of the caldera. Because circumferential dikes at Alcedo and neighboring volcanoes dip steeply (Chadwick and Howard 1991), this distribution sug- gests that the rhyolitic magma chamber could have been as wide as the caldera. Modern solfataric activity at Alcedo, concentrated at the caldera-floor rhyolite vent and in a 1.2-kin-wide arc around it, suggests the possibility of a still-cooling magma body at depth. A rhyolitic intrusion i km 3 in volume (comparable to to the total erupted rhyolite) would be expected to cool in about 3000 years (Smith and Shaw 1979). This is more than an order of magnitude less than the K-Ar ages of the rhyolite.

Possible explanations for this discrepancy are: (1) the rhyolites may be younger than their K-Ar ages; (2) the fumaroles are due to cooling of a younger batch of magma, possibly related to intrusion of younger basalt, and the hydrothermal fluids are somehow channelled preferentially through the rhyolitic vents; (3) the rhyol- itic intrusion is much larger than the erupted products; an intrusive volume of roughly 150 km 3 would be re- quired to attain a cooling time of 10 s years (Smith and Shaw 1979); (4) the rhyolitic magma chamber main- tains heat through underplating by basaltic magma. One test for a long-lived rhyolitic chamber would be whether it traps and deflects less-buoyant rising basalt magma, resulting in a basaltic shadow zone (Smith and Shaw 1975; Bacon 1984). Vents for the younger basalt at Alcedo are rare or absent within the large polygon defined by the rhyolite vents, but some occur adjacent to this area (Fig. 3b). This geometry would be consis-

258

tent with long-lived rhyolitic magma beneath a basaltic shadow zone.

Recent moat faulting and sagging of the caldera floor demonstrate a different deformation style than that of the 1968 trapdoor collapse of Fernandina (Sim- kin and Howard 1970). The arcuate moat recalls the annular graben that circumscribes the Suswa caldera in Kenya (Johnson 1969). The Volcfin Alcedo ring fault may be related to subsidence of a ring graben, perhaps due to an eruption of the younger basalt. Alternative- ly, asymmetric trapdoor resurgence as at some sili- ceous calderas (Lipman 1984) could explain the fault's high uplifted lip. The moat's position on the floor pe- riphery nearest to the rhyolite vents suggests that for- mation of the moat may have been related to move- ment of rhyolitic magma. The moat fault aligns with an older margin of the caldera floor to the NW (Fig. 2), so we infer that moat formation resulted from reactiva- tion of an old caldera fault, with sense of motion re- versed (Fig. 6b-e). Deflatfon, due to eruption, lateral intrusion, or cooling of magmas that caused resur- gence, may explain sagging of the main caldera floor. A similar, but much more strongly developed, horst- like ridge surrounded by a partial moat occurs in sierra Negra's caldera; the origin of that feature is almost cer- tainly due to cyclical inflation and tumescence (Rey- nolds and Geist 1991).

Comparisons to Other Ocean-Island Volcanoes

Volcfin Alcedo is similar in many ways to the Icelandic volcano Askja, especially in tectonic setting and petro- logic character (Sparks et al. 1981); both are oceanic volcanoes that have erupted large volumes of rhyolite, and syn-eruptive magma mixing was important in both (Table 3). The principal differences are that the early phase of the major 1875 Askja rhyolitic eruption was phreatomagmatic and resulted in the formation of a new caldera.

The Miocene P1 ignimbrite of Gran Canaria is an- other well-studied example of explosive rhyolitic vol-

canism in the ocean-island environment (Freundt and Schmincke 1992). Three phases of magma mixing have been documented for the P1 ignimbrite. The first was between trachytic and rhyolitic magma within the mag- ma chamber before eruption, which resulted in homo- genized hybrids. At Volcfin Alcedo, the icelanditic tephra underlying the A deposit and some of the dacite formed by thorough mixing between basalt and rhyol- ite within the subcaldera magma chamber (Geist et al. 1992). The second episode of magma mixing in the P1 ignimbrite occurred during eruption, by tapping a lat- erally and vertically zoned magma chamber. This is analogous to the syn-eruptive mixing that occurred at Volcfin Alcedo between rhyolitic and dacitic magma during eruption of the B tephra and rr lava flow units, and between basalt and rhyolite immediately before eruption of the rb lava flow. The third episode of mix- ing within Gran Canaria's P1 occurred during flow, which is, of course, not observed in Volcfin Alcedo's tephra. The principal differences between Gran Canar- ia and Volcfin Alcedo are: Gran Canaria is a much old- er and longer-lived volcano than Volcfin Alcedo, P1 is much more voluminous than the Alcedo rhyolites (45 vs 1 km3), and Gran Canaria's magmas are substantial- ly more alkaline. Nevertheless, the evolved pyroclastic rocks at Volcfin Alcedo, Askja, and Gran Canaria have all been shown to have experience mixing between basalt, intermediate magmas, and rhyolites, both be- fore eruption in magma chambers and within the vent during eruption. Clearly, magma mixing is a common process during both the petrogenesis and eruption of oceanic rhyolites.

Conclusions

Although Volcfin Alcedo is most notable for the con- spicuous occurrence of rhyolite, it has the typical form and structural style of a Galfipagos shield: The rhyolite has provided a key stratigraphic horizon and permitted a unique opportunity to deduce the eruptive history and structural evolution of the volcano.

Table 3. Comparison of plinian rhyolitic eruptions of Alcedo and Askja, Iceland volcanoes. Askja data from Sparks et al. (1981)

Alcedo (A2) Askja (Layer D)

Typical composition 71% SiO2 73% Si02 Crystallinity 0.4-3% 1-1.5% Phases An23, Cpx, Ti-mt An48, Cpx, Ti-mt Compositional variation Extensive magma-mixing during eruption Volume (DRE) 0.6 km 3 0.2 km 3 Intensity 1.2 x 108 kg/s 7.9 • 107 kg/s Thickness half-distance 6.3 km 8.7 km Clast half-distance 8.4 km 8.7 km Estimated column height 23-30 km 26 km Grading Reversely Reversely Dispersal 1700 km 2 2000 km 2 Fragmentation 2-6% about 5% Proximal Md~b -3.1 to -4.4 -6 to -7.5 Sorting coefficient 1.3-2.8 q~ 0.6-1.8 ~b Lithic clasts Mostly 2-7% 0.4-4.1% Caldera No subsidence detected 2.5 km 3 subsidence

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A steep summit region, which is the m o s t character - istic a t t r ibute of Galf ipagos shields, has existed t h r o u g h o u t m u c h if no t all o f the emergen t life of the volcano. It mos t l ikely has f o r m e d by the const ruct ion- al bui ld-up of lava flows e rup ted f r o m near the summit and some that over f lowed the caldera (Simkin 1972). The ca ldera is a quasi -s teady-s ta te feature. I t repea ted- ly becomes part ial ly filled, bu t subsidence general ly keeps pace with filling by lava flows and wast ing of the walls. Mos t recent ly the focus of episodic caldera col- lapse has b e e n migra t ing to the southeast .

The 3.4 k m 3 of rhyoli t ic pumice that e rup ted late in the vo lcano ' s life did no t mod i fy the f o r m of the shield much. The charac te r of the rhyol i te is typical of small to m e d i u m erupt ions e lsewhere in oceanic settings and m u c h m o r e c o m m o n in cont inenta l settings (e.g. F ink and Man ley 1987). T he e rup t ion began with an explo- sive plinian phase and ended with the less violent erup- t ion o f rhyoli t ic lavas. T he erupt ive activity d is turbed the magmat i c p lumbing system, causing dacitic m a g m a to in t rude into the e rupt ing rhyoli t ic b o d y and new vents to open.

Critical da ta tha t will help resolve some of the fun- damen ta l ques t ions regard ing the evolu t ion o f Volcfin A l c e d o include be t te r age constraints , fur ther analysis o f possible de fo rmat iona i features associated with the summit region, and de te rmina t ion of the relat ionships be tween volcanic activity and the m a g m a t i c p lumbing systems.

Acknowledgements. This work was supported by NSF awards EAR8804903 and EAR9117640 to DG, and NASA and USGS funding to KAH. Thanks to Camilo Calapucha, Jorge Fierra, Jean Howard, and the Potters for help in the field and Milton Friere and Tui du Roy for reports of their observations of the 1991 activity. This work could not have been completed without the irreplaceable logistic support of the Charles Darwin Research Station and the permission of the Gal~ipagos National Park Ser- vice of Ecuador. We thank Geof Phelps for constructing the shaded-relief model and Bob Duncan for his state-of-the-art K- Ar work: Thorough reviews by Ed Wolfe and Andrei Sarna-Woj- cicki, of the US Geological Survey, are greatly appreciated, as are informal reviews by Alexander McBirney and Duncan Munro. Journal reviews by Tom Simkin, Scott Rowland, and Steve Carey clarified our work substantially.

References

Bacon CR (1984) Implications of silicic vent patterns for the presence of large crustal magma chambers. J Geophys Res 90:11243-11252

Baitis HW, Lindstrom MM (1980) Geology, petrography, and pe- trology of Pinzon Island, Gal~pagos Archipelago. Contrib Mi- neral Petrol 72: 367-386

Banfield AF, Behre CH, St Clair D (1956) Geology of Isabela (Albemarle) Island, Archipelago de Colon (Galapagos). Geol Soc Am Bull 67:215-234

Bow CS, Geist DJ (1992) The geology and petrology of Floreana Island, Galfipagos Archipelago, Ecuador. J Volcanol Geo- therm Res 52:81-105

Carey S, Sigurdsson H (1989) The intensity of plinian eruptions. Bull Volcanol 51:28-40

Carey S, Sparks RSJ (1986) Quantitative models of the fallout and dispersal of tephra from volcanic eruption columns. Bull Volcanol 48:109-125

Chadwick WW, Dieterich JH (in press) Mechanical modeling of circumferential and radial dike intrusion on Galfipagos volca- noes. J Volcanol Geotherm Res

Chadwick WW, Howard KA (1991) The pattern of circumferen- tial and radial eruptive fissures on the volcanoes of Fernandi- na and Isabela islands, Gal~pagos. Bull Volcanol 53:259-275

Chadwick WW Jr, De Roy T, Carrasco A (1991) The September 1988 intracaldera avalanche and eruption at Fernandina vol- cano, Gal@agos Islands. Bull Volcanol 53:276-286

Cullen AB, McBirney AR (1986) Volcanic geology and petrology of Isla Pinta, Gal@agos Archipelago. Geol Soc Am Bull 98: 294-301

Cullen AB, McBirney AR, Rogers RD (1987) Structural controls on the morphology of Galfipagos shields. J Volcanol Geo- therm Res 34:143-151

Delaney JR, Colony WE, Gerlach TE, Nordlie BE (1973) Geolo- gy of the Volcfin Chico Area on Sierra Negra volcano, Galfi- pagos Islands. Geol Soc Am Bull 84:2455-2470

Eichelberger JC, Carrigan CR, Westrich HR, Price RH (1986) Non-explosive silicic volcanism. Nature 323:598-602

Fink JH, Manley CR (1987) Origin of pumiceous and glassy tex- tures in rhyolite flows and domes. Geol Soc Am Spec Pap 212:77-88

Freundt A, Schmincke H-U (1992) Mixing of rhyolite, trachyte and basalt magma erupted from a vertically and laterally zoned reservoir, composite flow P1, Gran Cauaria. Contrib Mineral Petrol 112:1-19

Geist DJ (1992) An assessment of melting processes and the Gal- ~ipagos hotspot: major and trace element evidence. J Volcanol Geotherm Res 52: 65-82

Geist DJ, McBirney AR, Duncan RA (1986) Geology and petro- logy of lavas from San Cristobal Island, Galfipagos Archipela- go. Geol Soc Am Bull 97:555-566

Geist D, Howard KA, Larson P (1992) Crystal fractionation ori- gin for a bimodal tholeiite-rhyolite volcano: Volcan Alcedo, Galapagos. Eos 73: 622

Heiken G, Wohletz K (1987) Tephra deposits associated with silicic domes and flows. Geol Soc Am Spec Pap 212:55-76

Hey R (1977) Tectonic evolution of the Cocos-Nazca spreading center. Geol Soc Am Bull 88, 1401-1406

Hey R, Johnson GL, Lowrie A (1977) Recent plate motions in the Galfipagos area. Geol Soc Am Bull 88, 1385-1403

Holcomb RT (1987) Eruptive history and long-term behavior of Kilauea volcano. US Geol Surv Prof Pap 1350:509-535

Jellinek AM, Geist DJ (1990) The prehistoric plinian eruption of Alcedo Volcano, Gal~ipagos Islands. Geol Soc Am Abs w Prog 22: 26

Johnson RW (1969) Volcanic geology of Mount Suswa, Kenya. Philos Trans R Soc London Ser A: 265:383-412

Kerr RC, Lister JR (1991) The effects of shape on crystal settling and on the rheology of magma. J Geol 99:457-467

Kleinrock MC, Hey RN (1989) Detailed tectonics near the tip of the Gal~pagos 95.5~ propagator: How the lithosphere tears and a spreading axis develops. J Geophys Res 94:13801- 13838

Klug C, Cashman KV (1991) Effects of vesicle size and shape on the vesicularity of silicic magma. Eos 72:312

Komar PD, Reimers CE (1978) Grain shape effects on settling rates. J Geol 86:193-209

Lipman PW (1984) The roots of ash flow calderas in western North America: Windows into the tops of granitic batholiths. J Geophys Res 89:8801-8841

MacDonald GA, Katsura T (1964) Chemical composition of Ha- waiian lavas. J Petrol 5:82-133

McBirney AR (1990) Differentiated rocks of the Galfipagos hot- spot. Eos 71 : 1695-1696

McBirney AR, Williams H (1969) Geology and petrology of the Galgpagos Islands. Geol Soc Am Mem 118, pp 1-197

McBiruey AR, Cullen AB, Geist D, Vicenzi EP, Duncan RA, Hall ML, Estrella M (1985) The Galfipagos volcano Alcedo: a unique ocean caldera. J Volcanol Geotherm Res 26:173-177

260

Munro DC, Mouginis-Mark PJ (1990) Eruptive patterns and structure of Isla Fernandina, Gal@agos Islands, from SPOT-1 and large format camera images. Int J Remote Sensing 11 : 1501-1509

Nordlie BE (1973) Geology and structure of the western Gal~pa- gos volcanoes and a model for their origin. Geol Soc Am Bull 84: 2931-2956

Nordlie BE, Colony WE (1973) Fumarole with periodic water fountaining, Volcfin Alcedo, Galfipagos Islands. Geol Soc Am Bull 84:1709-1720

Pyle DM (1989) The thickness, volume and grainsize of tephra fall deposits. Bull Volcanol 51:1-15

Rayder S, Geist DJ (1990) Geology of the vent complex at Alce- do Volcano, Galfipagos Islands. Geol Soc Am Abs w Prog 22:31

Reynolds R, Geist DJ (1991) Structure and eruptive style of Sier- ra Negra volcano, Isabela Island, Gal~ipagos Archipelago. Eos 72:313

Richards A (1962) Active volcanoes of the Archipelago de Colon (Galapagos), Catalogue of the active volcanoes of the world, Part XIV. International Association of Volcanology and Chemistry of the Earth's Interior, Naples

Roth6 JP (1969) The seismicity of the Earth 1953-1965. UNESCO, Paris, 336 pp

Rowland SK, Munro DC (1992) The caldera of Volcan Fernandi- ha: a remote sensing study of its structure and recent activity. Bull Volcanol 55: 9%109

Simkin T (1972) Origin of some flat-topped volcanoes and guyots. Geol Soc Am Mere 132:183-193

Simkin T (1984) Geology of Gal~ipagos Islands. In: Perry R (ed) Key environments: Galgpagos. Pergammon, Oxford: 16-41

Simkin T, Howard KA (1970) Caldera collapse in the Gal@agos Islands. Science 169:429M37

Simkin T, Siebert L, McClelland L, Bridge D, Newhall C, Latter J (1981) Volcanoes of the world. Hutchinson Ross, Strouds- burg, PA

Smith RL, Shaw HR (1975) Igneous-related geothermal systems. In: White DE, Williams DL (eds) Assessment of geothermal

resources of the United States - 1975. US Geol Surv Circ 726: 58-83

Smith RL, Shaw HR (1979) Igneous-related geothermal systems. In: Muffler LJP (ed) Assessment of geothermal resources in the United States - 1978. US Geol Snrv Circ 790:12-17

Smithsonian Institution/SEAN (1989) Global volcanism 1975- 1985. Prentice Hall, Englewood Cliffs, NJ and American Geo- physical Union, Washington, DC pp 1-657

Sparks RSJ (1976) The dynamics of bubble formation and growth in magmas: a review and analysis. J Volcanol Geotherm Res 3:1-37

Sparks RSJ (1986) The dimensions and dynamics of volcanic eruption columns. Bull Volcanol 48:3-15

Sparks RSJ, Wilson L, Sigurdsson H (1981) The pyroclastic de- posits of the 1875 eruption of Askja, Iceland. Philos Trans R Soc London 299A: 241-273

Swanson FJ, Baitis HW, Lexa J, Dymond J (1974) Geology of Santiago, Rabida, and Pinzon Islands, Gal~ipagos. Geol Soc Am Bull 85:1803-1810

Vicenzi EP, McBirney AR, White WM, Hamilton M (1990) The geology and geochemistry of Isla Marchena, Galfipagos Ar- chipelago: an ocean island adjacent to a mid-ocean ridge. J Volcanol Geotherm Res 40:291-315

Walker GPL (1971) Grainsize characteristics of pyroclastic de- posits. J Geol 79:696-714

Walker GPL (1973) Explosive volcanic eruptions - a new classif- ication scheme. Geol Rundsch 62: 431446

Williams H, McBirney AR (1979) Volcanology. Freeman, San Francisco, pp 1-391

Wilson L (1972) Explosive volcanic eruptions - II. The atmos- pheric trajectories of pyroclasts. Geophys J R Astrom Soc 45: 543-556

Wilson L, Sparks RSJ, Walker GPL (1980) Explosive volcanic eruptions IV. The control of magma properties and conduit geometry on eruption column behaviour. Geophys J R As- trom Soc 63:117-148

Editorial responsibility: S. Carey