Subglacial volcanics — On the formation of acid hyaloclastites

Journal o f Volcanology and Geothermal Research, 8 (1980) 95--110 Elsevier Scientific Publishing Company, Amsterdam -- Printed in Belgium

SUBGLACIAL VOLCANICS HYALOCLASTITES

- ON THE FORMATION

95

OF ACID

H. F U R N E S ~, I.B. FRIDLEIFSSON: and F.B. ATKINS 3

1Geologisk lnstitutt, Avd. A, Alldgt. 41, 5014 Bergen (Norway) : National Energy Authority, Laugavegur 116, 105 Reykjavik (Iceland) 3 Department o f Geology and Mineralogy, University o f Oxford, Parks Road, Oxford OXI 3PR (United Kingdom) (Received March 1, 1979; revised and accepted September 7, 1979)

ABSTRACT Furnes, H., Fridleifsson, I.B. and Atkins, F.B., 1980. Subglacial volcanics -- on the formation of acid hyaloclastites. J. Volcanol. Geotherm. Res., 8: 95--110. Several rhyolitic volcanic accumulations of subglacial origin in Iceland consist of two principal components: (1) hyaloclastite;and (2) ellipsoidal to irregular lobes averaging about 7 m in diameter. Two different types of hyaloclastite, reflecting major changes in tile style of subglacial eruptions, occur either as separate layers or in intimate association. A pumice-bearing type is thought to have resulted from explosive events during an eruption. The second type, characterized by fragments of obsidian and lithic rhyolite, is genetically related to the lobes. The lobes generally consist (from the margin inwards) of an obsidian rind, a zone of flow-banded/flow-folded pumiceous material, and a central zone of columnar rhyolite. They are thought to represent remnants of disintegrated subglacial (subaquatic) rhyolitic flows or intrusions into the water-logged pumice-bearing hyaloclastite Due to the rapid chilling of subglacial flows, dome-shaped bodies representing the initial stage of a growing lobe, develop on flow units. When the internal pressure of the lobe due to inflow of magma exceeds the tensional strength of the obsidian rind the latter shatters, and new lava is exposed to the quenching medium. As long as this process of build-up/ sudden release of pressure can take place during a relatively quiet phase of an eruption, the second type of acid hyaloclastite will form.

INTRODUCTION H y a l o c l a s t i t e , a t e r m o r i g i n a l l y u s e d f o r glassy v o l c a n i c l a s t i c r o c k s d e r i v e d f r o m p i l l o w lava ( R i t t m a n n , 1 9 6 0 ) , is a c o m m o n c o m p o n e n t o f s u b a q u a t i c b a s a l t s e q u e n c e s o f all ages. P i c h l e r ( 1 9 6 5 ) e x t e n d e d t h e t e r m t o c o v e r a c i d i c v o l c a n i c l a s t i c r o c k s o f s i m i l a r o r i g i n , a t e r m w h i c h is a c c e p t e d a n d u s e d f o r t h e r o c k s d e a l t w i t h i n t h i s a c c o u n t . A c i d h y a l o c l a s t i t e s are r a t h e r rare. T h i s m a y b e d u e t o t h e f a c t t h a t r h y o l i t i c v o l c a n i c r o c k s ( e x c e p t i g n i m b r i t e s ) are less c o m m o n t h a n b a s a l t s , a n d t h a t t h e y are m o s t c o m m o n l y s u b a e r i a l . H o w ever, t h e s t r i k i n g a b s e n c e o f a n y d e s c r i p t i o n s o f acid h y a l o c l a s t i t e s in t h e l i t e r a t u r e u n t i l q u i t e r e c e n t l y , m a y also b e d u e t o m i s i n t e r p r e t a t i o n . O n e ex0377-0273/80/0000--0000/$02.25 © 1980 Elsevier Scientific Publishing Company

96

ample seems to be provided by the rhyolitic volcaniclastic rocks on the island of Ponza in the Tyrrhenian Sea. Prior to their interpretation as hyaloclastites by Pichler (1965), they were referred to as conglomerates and tuffs (Abicl% 1841), as pumiceous conglomerates (Judd, 1875), as a subaerial breccia (Doelter, 1876), while Sabatini (1893) and Bieber (1924) found evidence for a submarine origin. It has recently become apparent that subglacial acid hyaloclastites are common in the Quaternary central volcanic complexes in Iceland (Fridleifsson, 1970, 1973; GrSnvold, 1972; Saemundsson, 1972; Saemundsson and Noll, 1973). The present account is the result of a project to study the structural relationships of the various components of selected Icelandic subglacial volcanic rocks of the tholeiitic series. Since the rocks of dacitic and rhyolitic compositions show very different characteristics from the basaltic end of the series, the features of subglacial basalts and andesites will be described separately (Fridleifsson et al., in preparation). The areas and deposits studied (Fig. 1) are believed to illustrate general and characteristic features of gentle to violent dacitic/rhyolitic eruptions in a subglacial environment.

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LEGEND

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MMosfell, SSiqalda, St Stapafell

Fig. 1. Simplified geological m a p of Iceland, showing geographical location of the studied acid subglacial hyaloclastites.

97 DESCRIPTION OF ACID SUBGLACIAL DEPOSITS Subglacial hyaloclastite sequences of all compositions, typically occur in Iceland as either steep-sided ridges or steep circular mountains depending on the shape of the volcanic edifice. Compared with fresh basaltic hyaloclastite sequences which consist of pillow lavas, pillow breccias and sideromelane fragments, acid hyaloclastites show a greater lithological diversity, typically consisting of glassy, structureless to strongly flow-banded and flow-folded fragments of pumice and obsidian. This fragmental material is c o m m o n l y in intimate association with large, irregular to subspherical bodies of vesicular to non-vesicular rhyolite, enclosed within one or more obsidian layers. Such bodies were referred to as lobes by GrSnvold (1972) and Saemundsson (1972), a term t h a t will be retained here. The thickness of the ice, into which the dacites/rhyolites of this account have been erupted is not known, but most probably it was in the order of a few hundred meters. GrSnvold (1972) described an example in KerlingarfjSll (Fig. 1) where the erupting rhyolite lava emerged through the meltwater lake to form a table mountain; from the geological map in that account the water depth can be estimated to have been around 200--300 m. Saemundsson (1972) described a similar formation in Kirkjufell near Bl~hn~kur (Fig. 1); there the water depth appears also to have been about 200 m. The maximum thickness of the deposits dealt with here is 400 m (in KerlingarfjSll, Fig. 1), which is a minimum thickness of the icecap under which the eruption took place. In the following, general and detailed descriptions of the acid hyaloclastites and associated lobes will be given.

Hyaloclastite The individual acid hyaloclastite deposits c o m m o n l y comprise volumes of the order of 0.01~0.1 km 3. The hyaloclastite may, according to its field appearance, be subdivided into two types, which, as will be discussed later, also differ greatly in their genesis: Type 1 consists of fragments of grey pumice and glass; Type 2 consists of fragments of obsidian, flow-banded/flow-folded pumice and rhyolite. Pumice fragments are the major component of the type 1 hyaloclastite. Their sizes range from 0.1 mm to 50 cm (Figs. 2, 3). Their shape varies from nearly spherical to elongate, and the edges are usually ragged, though smoothsurfaced fragments, usually less than 1 mm, occur (Fig. 3). Sometimes a perlitic texture is developed. The vesicles may be spherical or drawn out (Fig. 3). Where individual hyaloclastite units can be seen, as for example on the southeastern side of Bl~hnfikur (Fig. 1), their thickness may reach 40 m (Fig. 4). These thick units, which are poorly sorted (Fig. 2), are not divisible into thinner units. The nearly aphyric to slightly porphyritic pumice is charged with needle-like crystals most c o m m o n l y of plagioclase, about 0.1--0.3 mm long.

98

Fig. 2. Acid type 1 hyaloclastite consisting of pumice fragments of various sizes

The d o m i n a n t phenoeryst phase is plagioclase (in KerlingarfjStl also anorthoclase) and some clinopyroxene, usually in a glomeroporphyritic arrangement. The feldspar phenocrysts {about 1 2 mm long} show oscillatory to patchy zoning. In the type 2 hyaloclastites, the angular fragments of obsidian, flowbanded/flow-folded pumiceous rhyolite and lithic rhyolite range in size from less than 1 mm to at least 40 cm (Fig. 5). These components are identical to material from various zones of the lobes, and it will be argued later that they are genetically related to the formation of the lobes. The proportion of these components increases upon approaching areas where lobes are abundant. Obsidian is the most frequent component, whereas lithic fragments occur in only minor quantities. Some of the fragments show the truncation of flowbanding and flow folds {Fig. 3b). As in the case with the type 1, the type 2 hyaloclastites define thick, poorly sorted units (Fig. 5). Lobes

In the Icelandic subglacial deposits of dacite to rhyolitic composition, lobes are always present, and in fact characterize the deposits (Fig. 7a). Since it is mostly erosional remnants of lobes that can be studied, it is difficult to determine their exact morphology. They appear, however, to range from ellipsoidal to irregularly shaped bodies, and may have small offshoots (Fig. 6). The size of the lobes varies considerably. GrSnvold (1972) reported diameters

99

Fig. 3. Photomicrographs of acid hyaloclastite from Bl~hnfikur. (a) Pumice fragments in type 1 hyaloclastite. (b) Truncated flow-banding in obsidian fragments of type 2 hyaloclastite.

100

Fig. 4. Southern slope of Bl~ihnfikur, showing separate layers of lobe complexes/associated type 2 hyaloclastite in alternation with layers of pumiceous type 1 hyaloclastite.

Fig. 5. Type 2 acid hyaloclastite consisting of angular fragments of obsidian (black), grey to nearly white pumice (some are flow-banded), and a very few fragments of lithic rhyolite. Upper part of photograph shows bottom of a lobe. Strutur (Hfisafell). f r o m a b o u t 40 c m t o m o r e t h a n 40 m in Kerlingarfj511, a n d S a e m u n d s s o n a n d Noll ( 1 9 7 4 ) m e n t i o n e d l o b e s u p t o 10 m a c r o s s in Hfisafell. T h e long axes of t h e l o b e s o f t h e p r e s e n t s t u d y c o m m o n l y range f r o m a b o u t 2 m t o 70 m, w i t h an average of 7 m. L o b e s of e x c e p t i o n a l l y large d i m e n s i o n s o c c u r in t h e subg l a c i a l l y e r u p t e d r h y o l i t e at P r e s t a h n f i k u r in t h e H u s a f e l l a r e a ( E i n a r s s o n , 1 9 7 5 ) (Fig. 1), w h e r e o n e m e a s u r e s 6 0 0 × 9 0 0 m.

101

Fig. 6. Irregular lobe, indicated by the locally fanning columnar jointing of massive rhyolite Bl~hnflkur.

The lobes, which mineralogically and chemically are similar to the pumice described above, consist in general of three physically and structurally different components. These are: (1) an outer obsidian zone (zone 1); (2) a zone of flow-banded and flow-folded vesiculated and glassy rhyolite adjacent to the obsidian (zone 2); (3) lithic rhyolite in the central part of lobes (zone 3). Z o n e 1. The thickness of the outermost black obsidian layer ranges from about 20 cm to more than 5 m, and generally increases as the size of the lobe increases. The obsidian is mostly non-vesicular; only occasionally does it contain abundant, usually strongly stretched vesicles. Normally only one obsidian layer is developed, though multiple layers do occur. C o m m o n l y there is a transitional b o u n d a r y between the outer obsidian margin of a lobe and the grey pumiceous hyaloclastite (Fig. 5). Z o n e 2. Adjacent to, and within zone 1, there is usually a zone up to I m thick where flow-banding and flow-folding occur (Fig. 7b, c). Unlike the obsidian zone, there is no apparent relationship between the thickness of this zone and the size of the lobe. The material of this zone is invariably highly vesicular and glassy. At the junction between 1 and 2 all stages from incipient to complete break-up (on both a macroscopic and microscopic scale) of the obsidian has occurred, and obsidian £ragments can be found in zone 2. There is usually a single zone 2, although multiple zones, separated by obsidian, do occur in some cases. The flow-banding is mostly concentric with the obsidian zone. Columnar jointing is occasionally present.

102

Fig. 7. (a) Well-exposed lobe, s h o w i n g t h e c h a r a c t e r i s t i c d e v e l o p m e n t o n an o u t e r zone oi' black o b s i d i a n ( z o n e 1), an a d j a c e n t z o n e of light-coloured f l o w - b a n d e d / f l o w - f o l d e d pumiceous material ( z o n e 2), and a central p a r t of massive, c o l u m n a r - j o i n t e d r h y o l i l e ( z o n e 3). This lobe is typical for isolated lobes w h i c h are d e t a c h e d f r o m t h e i r feeder d y k e or flow. In t h e b a c k g r o u n d several o t h e r isolated lobes. N o r t h e r n side of BlAhnfikur. (b) Close-up showing flow-folding from z o n e 2. (c) Close-up showing f l o w - b a n d i n g from zone 2

103

Zone 3. Inside z o n e 2, the r o c k ranges f r o m slightly to c o m p l e t e l y crystalline r h y o l i t e and crystallinity increases t o w a r d s t h e c e n t r e o f the lobe. Vesicularity is highly variable. Characteristically, the massive r h y o l i t e has well-developed c o l u m n a r jointing, and c u r v a t u r e o f c o l u m n s to give a fanning a r r a n g e m e n t is c o m m o n (Fig. 6). Where b o t h zones 2 and 3 are preserved, the c o l u m n s are o r i e n t a t e d p e r p e n d i c u l a r t o the f l o w - b a n d i n g o f zone 2. Since c o l u m n a r jointing is d e v e l o p e d n o r m a l to a cooling surface {Macdonald, 1972), the overall a r r a n g e m e n t o f c o l u m n s t h e r e f o r e gives i n f o r m a t i o n a b o u t the g e o m e t r y o f the lobe. In m o s t cases z o n e 3 f o r m s m o r e t h a n 80% o f each lobe. P o r o s i t y d e t e r m i n a t i o n s * o f samples f r o m the various layers across a number o f lobes (Table 1) show t h a t the p o r o s i t y o f the obsidian {zone 1) is sign i f i c a n t l y lower t h a n t h a t o f zone 2, whereas the p o r o s i t y o f zone 3 m a y be higher or l o w e r t h a n t h a t o f zone 2, t h o u g h m o s t l y l o w e r (Fig. 8). The b o u n d ary b e t w e e n zones 1 and 2 m a y be sharp or gradational, whereas the b o u n d ary b e t w e e n zones 2 and 3 is always gradational. 'FABLE 1 Porosity measurements from the different zones of the rhyolitic lobes (~ = mean, s = standard deviation, n = number of measurements)

Maximum porosity Minimum porosity x s n

Zone 1: obsidian

Zone 2: pumiceous rhyolite

Zone 3 : lithic rhyolite

0.26 0.02 0.09 0.07 17

0.57 0.16 0.27 0.12 10

0.23 0.16 0.19 0.03 10

Arrangement o f lobes On t h e basis o f the Icelandic o c c u r r e n c e s w h i c h have b e e n e x a m i n e d , acid lobes a p p e a r to be spatially related to one a n o t h e r and t o enclosing hyaloclastite in o n e o f three ways, a l t h o u g h it m a y be t h a t gradational e x a m p l e s await discovery. (1) A p p a r e n t l y u n c o n n e c t e d and discrete lobes a n d / o r i n t e r c o n n e c t e d lobe c o m p l e x e s are m o r e or less u n i f o r m l y d i s t r i b u t e d within a single identifiable stratigraphic bed o f hyaloclastite. This style is particularly well seen on the s o u t h e a s t e r n slopes o f Blhhnfikur w h e r e a caldera fault has e x p o s e d a section a p p r o x i m a t e l y at right angles t o the c r u d e stratification of the subglacial *The porosity measurements were made both on whole-rock samples and on samples crushed to less than 1 mm grain size. Only values for the crushed samples are quoted in this paper. The volume of the samples was measured by immersion in distilled water under vacuum, and density measurements were made using a pycnometer.

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Fig. 8. Porosity profiles across the obsidian, flow-banded/flow-folded pumiceous zone, and massive, columnar jointed rhyolite of lobes. From Blfihnfikur. deposits. Here two subhorizontal planar units o f hyaloclastite, each 2 0 - - 4 0 m thick, and containing a high concentration of individual lobes and lobe complexes are separated by weakly bedded pumiceous hyaloclastite (type 1) units devoid of lobes (Fig. 4). The bases of the large lobes to the left and right of Fig. 4, defined b y thick obsidian layers, are clearly visible. (2) Large lobes are greatly elongated in a vertical direction and grossly discordant to the bedding of the enclosing hyaloclastite. These discordant lobes, examples of which occur at KerlingarfjSll and Bl~hnhkur, tend to be larger than the first type, and may be very much larger since their lower terminations are not seen. Despite differences in m o r p h o l o g y and size, these lobes

105 are in other respects typical, having zones of obsidian and pumice around cores of columnar-jointed lithic cores. Some of these lobes appear to have acted as feeders, supplying magma to overlying parts of the volcanic pile. (3) Apparently isolated lobes occur randomly and sporadically and without any special association one with another or with the surrounding hyaloclastite. This type is probably the most abundant, and there are reasons, discussed below, for suggesting that they may be typical of an advanced stage of lobe/ hyaloclastite formation during an acid subglacial eruption.

Relative amounts o f lobes and hyaloclastite The volume proportions of lobes to hyaloclastite vary considerably. For example, at Storagil in Hdsafell (Fig. 1), a complicated network of interconnected lobes occurs, with only minor interstitial hyaloclastite, whereas in parts of KerlingarfjSll and Bl~hndkur (Fig. 1), lobes are absent over large areas of outcrop, which thus consist entirely of hyaloclastite. On the southeast slopes of Bl~hndkur (100% exposure), approximately 13% of the outcrop, as estimated from field studies and photographs, consists of lobes. FORMATION OF ACID SUBGLACIAL DEPOSITS There is no sure knowledge of, and little published speculations about the mechanisms of eruption of acid magma beneath an ice sheet or glaciers. Direct observation of such an event is, of course~ impossible, and few of the resulting rock types and structures have been identified and described. In a subglacial eruption, the physical environment is totally different from the subaerial in its restriction in space, and in water being the chilling medium instead of air. From the alternation between thick layers of pumiceous hyaloclastite (type 1) only, and the more heterogeneous type 2 hyaloclastite with lobe complexes (Fig. 4), it seems evident that pronounced changes in the erupting style, from the quiet effusion of lava to the explosive ejection of pumice, must have occurred. It seems probable, by analogy with subaerial acid eruptions, that the initial phase of the eruption of rhyolitic magma beneath ice will be explosive. The sudden release of hydrostatic pressure on initial extrusion normally results in enormous outgassing of dissolved volatiles present in high concentrations in siliceous magmas. It is proposed that the pumiceous type 1 hyaloclastite is the product of such powerful explosive events {Fig. 9). The realization that the shapes and sizes of pyroclastic fragments are related to their mode of origin, and may thus give information on the style of eruption (Walker and Croasdale, 1971, 1972; Walker, 1973) may have some relevance to the present problem. For example, the fragments that result from a strombolian/hawaiian-type eruption are partly ragged and partly bounded by smooth or rounded surfaces moulded by surface tension, whereas surtseyan fragments are bounded only by fracture surfaces and the inner walls of broken

106

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107 vesicles (Walker, 1973). According to Walker and Croasdale (1971) fragments produced in plinian-type eruptions are rather coarse-grained and have ragged shapes indicative of tearing apart of viscous magma. An ash deposit of trachytic composition on Sao Miguel, Azores, which is ascribed to a subaquatic eruption, and thus represents a salic equivalent of the basic surtseyan-type eruption, is much finer-grained than the subaerial plinian-type deposit (Booth et al., 1978). It is therefore tentatively suggested that the great variety in size and shape of the fragments in the subglacial t y p e 1 hyaloclastites may indicate changes in the style of explosive eruption. This could happen if water produced by melting of ice came into contact with the magma. Such an event would change a strombolian-type eruption into a surtseyan one, or a subplinian-type eruption into the salic t y p e of the surtseyan-type eruption, which would result in a higher degree of fragmentation (Walker, 1973). In the following we present an evolutionary model for the formation and destruction of lobes, which we believe is of fundamental importance in the understanding of how the type 2 hyaloclastite may form. It is proposed that the pumiceous type 1 hyaloclastite is the product of violently explosive events, probably at the initial stage of eruption (Fig. 9 A). Further supplies of magma now relatively poorer in dissolved volatiles may be expected to flow along the hyaloclastite/meltwater interface (Fig. 9, B1) and/or (perhaps more c o m m o n l y ) to intrude the structureless, water-saturated hyaloclastite pile (Fig. 9, B~), in either case non-explosively at first. These flows, whether extrusive or intrusive, will suffer rapid chilling and an outer obsidian skin will form. At this stage the magma must have been relatively degassed, since the obsidian is mostly non-vesicular. With further supply of more volatile-rich magma to the flows, a rapid buildup of internal pressure (magma pressure + volatile pressure) is to be expected since the flows are effectively sealed within their obsidian envelopes. Rising gas bubbles will accumulate beneath internal concavities on the roofs of the flows (Fig. 9, B~, B2). Magma viscosity will decrease (Macdonald, 1972) and flow will be directed parallel to the obsidian envelope, producing flow-banded and flow-folded fabrics and the streaking o u t of vesicles; thus an incipient lobe will develop (Fig. 9, C1). With further build-up of pressure, the strength of the obsidian layer will eventually be exceeded and it will crack or explode (Fig. 9, C2, Ca). At this stage, as schematically indicated in Fig. 9, C:, type 2 hyaloclastite consisting of violently disrupted fragments of obsidian, flowbanded pumice and, more rarely, lithic rhyolite will form and be pushed aside by fresh magma emerging in lobe-shaped masses from rifts in the parent flow unit. Rapid chilling of this magma will produce new obsidian envelopes, and the process will be repeated periodically so long as fresh supplies of magma are available or until the eruptive style changes to a continuously explosive phase producing t y p e 1 hyaloclastite only. When the stage is reached that a lobe loses its supply of lava and/or gases, the build-up/break-down process will stop, and the lobe is no longer a poten-

108 tial hyaloclastite {type 2) supplier. This will result when the lobe has become semidetached from its parent, so that lava no longer can be channeled into it {Fig. 9, C4). At this stage, complete detachment of a fully developed lobe may take place, depending on whether or not it is carried along by its parent. The predominance of isolated lobes in the field suggests that detachment commonly takes place. The remaining magma now crystallizes, to give the columnar jointed rhyolite in the central part of the lobe (Fig. 9, C4). Field evidence suggests that lobes that reach this advanced stage seldom collapse (or explode). A b o u t 80% of such a lobe consists of lithic rhyolite, but lithic fragments occur normally only in minor quantities in the hyaloclastite {type 2). The above model gains some support from the porosity (= vesicularity) measurements on samples from various zones within a number of lobes (Table 1, Fig. 8). The obsidian zones are almost invariably the least vesicular, suggesting that the magma was relatively degassed at this stage. The adjacent flow-banded/flow-folded zones are usually, but not always, more vesicular than the lithic rhyolite cores, indicating a build-up of gas pressure within a sealed lobe. These porosity measurements have been made on samples from surviving lobes. Lobes destroyed as a result of excess internal gas pressure would be expected to form fragments of hyaloclastite (type 2) more vesicular than pumice of the surviving lobes. Field evidence accords with this expectation. An extreme example of the dimensions a lobe may attain occurs in the subglacial sequence at Prestahn6kur (Husafell area, Fig. 1), where minimum lengths of the two measurable axes are 600 and 900 m. It is not clear h o w such a lobe, which consists of several successive layers of perlite, obsidian, flow-banded/flowfolded pumiceous material and rather coarsely crystalline rhyolite can form according to the proposed model. One possible explanation involves the continuous escape of magmatic volatites during growth of the b o d y , so that internal pressures are never sufficient to cause fragmentation. This could perhaps be analogous with the formation of rhyolite domes under subaerial conditions. The concentrically layered lava pods, up to ca. 200 m high and 300 m wide, of calc-alkaline andesite and dacite, intrusive into peperite or mud in Unalaska Island, Alaska (Snyder and Fraser, 1963} also seem to share some features with the large lobes of the subglacial Icelandic examples dealt with in this account. SUMMARY

AND CONCLUSIONS

Different processes must be invoked in order to explain the formation o f the various c o m p o n e n t s of the Icelandic subglacial acid volcanics o f this study. T w o major c o m p o n e n t s can immediately be distinguished, i.e., a volcaniclastic rock {hyaioclastite) consisting predominantly of pumice fragments, and lobes which consist of obsidian, strongly vesiculated and flow-banded/flowfolded rhyolite, and columnar jointed rhyolite. The hyaloclastite comprises four different c o m p o n e n t s and can be subdivided into two genetically different types:

109 (1) H y a l o c l a s t i t e consisting e n t i r e l y o f f r a g m e n t s o f p u m i c e ( t y p e 1 h y a l o clastite). (2) H y a l o c l a s t i t e consisting o f f r a g m e n t s o f obsidian, f l o w - b a n d e d / f l o w folded p u m i c e a n d lithic r h y o l i t e ( t y p e 2 hyaloclastite}. T h e large q u a n t i t i e s o f p u m i c e - r i c h t y p e 1 h y a l o c l a s t i t e , w h i c h is b y far t h e m o s t c o m m o n , a n d its large-scale v a r i a t i o n s in size and f r a g m e n t shapes, are t h o u g h t to be t h e result o f v i o l e n t l y e x p l o s i v e events. T h e s e c o u l d be o f e i t h e r s u b p l i n i a n or plinian t y p e , i n t e r s p e r s e d b y pulses o f r h y o l i t i c S u r t s e y a n a c t i v i t y when meltwater contacted magma. T y p e 2 h y a l o c l a s t i t e is i n v a r i a b l y associated w i t h , a n d consists o f f r a g m e n t s o f t h e s a m e m a t e r i a l as f o u n d in t h e lobes. It is t h e r e f o r e p r o p o s e d t h a t these f r a g m e n t s w e r e derived d i r e c t l y f r o m t h e lobes d u r i n g t h e i r d e v e l o p m e n t , f r o m t h e b r e a k i n g u p o f a lava f l o w o r intrusive m a g m a b o d y b y a c o n t i n u o u s process o f a l t e r n a t i n g c o n s t r u c t i o n and d e s t r u c t i o n o f lobes. ACKNOWLEDGEMENTS T h e a u t h o r s are greatly in d e b t to K. G r 6 n v o l d a n d K. S a e m u n d s s o n f o r t h e i r g u i d a n c e in t h e selection o f specific places f o r s t u d y in the KerlingarfjSll and H6safell areas respectively. T h e p o r o s i t y m e a s u r e m e n t s w e r e m a d e b y G.T. H a r a l d s s o n o f t h e N a t i o n a l E n e r g y A u t h o r i t y in R e y k j a v i k . T h e field w o r k was s p o n s o r e d b y the N a t i o n a l E n e r g y A u t h o r i t y o f Iceland, t h e N a n s e n F u n d and t h e Meltzer H 6 y s k o l e f o n d , all of w h i c h g r a t e f u l l y are a c k n o w l e d g e d . G.P.L. Walker, B. R o b i n s and R. S u t h r e n m a d e c o m m e n t s and suggested alterations to an early d r a f t o f t h e m a n u s c r i p t w h i c h i m p r o v e d t h e c o n t e n t , a n d E. Irgens, J. Lien and M. A d a c h i m a d e the illustrations. T h e m a n u s c r i p t was t y p e d b y E. Wallace L i n d e b r a e k k e . We express o u r t h a n k s t o all these persons. REFERENCES Abich, H., 1841. Geologische Beobachtungen fiber die vulkanisehen Erscheinungen und Bildungen in Unter- und Mittel Italien. Braunschweig, 134 pp. Bieber, O., 1924. Die Ponza-Inseln im Tyrrhenischen Meer. Z. Vulkanol., Erg~nz-Bd., 5: 168 pp. Booth, B., Croasdale, R. and Walker, G.P.L., 1978. A quantitative study of five thousand years of volcanism on Sao Miguel, Azores. Philos. Trans. R. Soc. London, 288:271 319. Doelter, C., 1876. Die Vulkangruppe der Pontinischen Inseln. Denkschr. K. Akad. Wiss. Wien, Math.-Nat. Kl., 36/2: 141--184. Einarsson, G.Th., 1975. Jardfraedi Prestahnuks (The geology of Prestahnt~kur). B .S. Thesis, University of Iceland, 45 pp. Fridleifsson, I.B., 1970. The Stora-Laxa igneous complex, S. Iceland. B.Sc. Thesis, University of St. Andrews, 88 pp. Fridleifsson, I.B., 1973. Petrology and structure of the Esja Quaternary volcanic region, southwest Iceland. D. Phil. Thesis, Oxford University, 208 pp. GrSnvold, K., 1972. Structural and petrochemical studies in the KerlingarfjSll region, central Iceland. D. Phil. Thesis, Oxford University, 237 pp. Judd, J.W., 1875. Contributions to the study of volcanoes, VII. The Ponza Islands. Geol. Mag., 12: 298--308.

110 Macdonald, G.A., 1972. Volcanoes. Prentice-Hall, Englewood Cliffs, N.J., 510 pp. Pichler, H., 1965. Acid hyaloclastites. Bull. Volcanol., 28: 293--310. Rittmann, A., 1960. Vulkane und ihre T~itigkeit. Ferdinand Enke, Stuttgart, 336 pp. Sabatini, V., 1893. Descriptizione geologica delle Isole Pontine. Boll. R. Comit. Geol. Ital., 24: 228 -267. Saemundsson, K., 1972. Notes on the geology of the TorfajSkull central volcano. Natturufraedingnum, 42: 81--99. Saemundsson, K. and Noll, H., 1974. K/Ar ages of rocks from Husafell, western Iceland, and the development of the Husafe|l central volcano, dOkull, 24: 40--58. Snyder, G.L. and Fraser, G.D., 1963. Pillowed lavas, I. Intrusive layered lava pods and pillowed lavas, Unalaska Islands, Alaska. U.S. Geol. Surv, Prof. Paper, 454-B: 1--23. Walker, G.P.L., 1973. Explosive volcanic e r u p t i o n s - - a new classification scheme. Geol. R u n d s c h . , 6 2 : 4 3 1 446. Walker, G.P.L. and Croasdale, R., 1971. Two plinian-type eruptions in the Azores. J. Geol. Soc. London, 127:17--55. Walker, G.P.L. and Croasdale, R., 1972. Characteristics of some basaltic pyroclastics. Bull. Volcanol., 3 5 : 3 0 3 317.