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1.
We consider the thermodynamic and fluid dynamic processes that occur during subglacial effusive eruptions. Subglacial eruptions typically generate catastrophic floods (jökulhlaups) due to melting of ice by lava and generation of a large water cavity. We consider the heat transfer from basaltic and rhyolitic lava eruptions to the ice for typical ranges of magma discharge and geometry of subglacial lavas in Iceland. Our analysis shows that the heat flux out of cooling lava is large enough to sustain vigorous natural convection in the surrounding meltwater. In subglacial eruptions the temperature difference driving convection is in the range 10–100??°C. Average temperature of the meltwater must exceed 4??°C and is usually substantially greater. We calculate melting rates of the walls of the ice cavity in the range 1–40?m/day, indicating that large subglacial lakes can form rapidly as observed in the 1918 eruption of Katla and the 1996 eruption of Gjálp fissure in Vatnajökull. The volume changes associated with subglacial eruptions can cause large pressure changes in the developing ice cavity. These pressure changes can be much larger than those associated with variation of bedrock and glacier surface topography. Previous models of water-cavity stability based on hydrostatic and equilibrium conditions may not be applicable to water cavities produced rapidly in volcanic eruptions. Energy released by cooling of basaltic lava at the temperature of 1200??°C results in a volume deficiency due to volume difference between ice and water, provided that heat exchange efficiency is greater than approximately 80%. A negative pressure change inhibits escape of water, allowing large cavities to build up. Rhyolitic eruptions and basaltic eruptions, with less than approximately 80% heat exchange efficiency, cause positive pressure changes promoting continual escape of meltwater. The pressure changes in the water cavity can cause surface deformation of the ice. Laboratory experiments were carried out to investigate the development of a water cavity by melting ice from a finite source area at its base. The results confirm that the water cavity develops by convective heat transfer.  相似文献   

2.
We present field observations from Bláhnúkur, a small volume (<0.1 km3) subglacial rhyolite edifice at the Torfajökull central volcano, south-central Iceland. Bláhnúkur was probably emplaced during the last glacial period (ca. 115–11 ka). The characteristics of the deposits suggest that they were formed by an effusive eruption in an exclusively subglacial environment, beneath a glacier >400 m thick. Lithofacies associations attest to complex patterns of volcano-ice interaction. Erosive channels at the base of the subglacial sequence are filled by both eruption-derived material and subglacial till, which show evidence for deposition by flowing meltwater. This suggests that meltwater was able to drain away from the vent area during the eruption. Much of the subglacial volcanic deposits consist of conical-to-irregularly shaped lava lobes typically 5–10 m long, set in poorly sorted breccias with an ash-grade matrix. A gradational lavabreccia contact at the base of lava lobes represents a fossilised fragmentation interface, driven by magma-water interaction as the lava flowed over poorly consolidated, waterlogged debris. Sets of columnar joints on the upper surfaces of lobes are interpreted as ice-contact features. The morphology of the lobes suggests that they chilled within conically shaped subglacial cavities 2–5 m high. Avalanche deposits mantling the flanks of Bláhnúkur appear to have been generated by the collapse of lava lobes and surrounding breccia. A variety of deposit characteristics suggests that this occurred both prior to and after quenching of the lava lobes. Collapse events may have occurred when the supporting ice walls were melted back from around the cooling lava lobes and breccias. Much larger lava flows were emplaced in the latter stages of the eruption. Columnar joint patterns suggest that these flowed and chilled within subglacial cavities 20 m high and 100–200 m in length. There is little evidence for magma-water interaction at lava flow margins which suggests that these larger cavities were drained of meltwater. As rhyolite magma rose to the base of the glacier, the nature of the subglacial cavity system played an important role in governing the style of eruption and the volcanic facies generated. We present evidence that the cavity system evolved during the eruption, reflecting variations in both melting rate and edifice growth that are best explained by a fluctuating eruption rate.  相似文献   

3.
Historical eruptions have produced lahars and floods by perturbing snow and ice at more than 40 volcanoes worldwide. Most of these volcanoes are located at latitudes higher than 35°; those at lower latitudes reach altitudes generally above 4000 m. Volcanic events can perturb mantles of snow and ice in at least five ways: (1) scouring and melting by flowing pyroclastic debris or blasts of hot gases and pyroclastic debris, (2) surficial melting by lava flows, (3) basal melting of glacial ice or snow by subglacial eruptions or geothermal activity, (4) ejection of water by eruptions through a crater lake, and (5) deposition of tephra fall. Historical records of volcanic eruptions at snow-clad volcanoes show the following: (1) Flowing pyroclastic debris (pyroclastic flows and surges) and blasts of hot gases and pyroclastic debris are the most common volcanic events that generate lahars and floods; (2) Surficial lava flows generally cannot melt snow and ice rapidly enough to form large lahars or floods; (3) Heating the base of a glacier or snowpack by subglacial eruptions or by geothermal activity can induce basal melting that may result in ponding of water and lead to sudden outpourings of water or sediment-rich debris flows; (4) Tephra falls usually alter ablation rates of snow and ice but generally produce little meltwater that results in the formation of lahars and floods; (5) Lahars and floods generated by flowing pyroclastic debris, blasts of hot gases and pyroclastic debris, or basal melting of snow and ice commonly have volumes that exceed 105 m3.The glowing lava (pyroclastic flow) which flowed with force over ravines and ridges...gathered in the basin quickly and then forced downwards. As a result, tremendously wide and deep pathways in the ice and snow were made and produced great streams of water (Wolf 1878).  相似文献   

4.
The 13-day-long Gjálp eruption within the Vatnajökull ice cap in October 1996 provided important data on ice–volcano interaction in a thick temperate glacier. The eruption produced 0.8 km3 of mainly volcanic glass with a basaltic icelandite composition (equivalent to 0.45 km3 of magma). Ice thickness above the 6-km-long volcanic fissure was initially 550–750 m. The eruption was mainly subglacial forming a 150–500 m high ridge; only 2–4% of the volcanic material was erupted subaerially. Monitoring of the formation of ice cauldrons above the vents provided data on ice melting, heat flux and indirectly on eruption rate. The heat flux was 5–6×105 W m-2 in the first 4 days. This high heat flux can only be explained by fragmentation of magma into volcanic glass. The pattern of ice melting during and after the eruption indicates that the efficiency of instantaneous heat exchange between magma and ice at the eruption site was 50–60%. If this is characteristic for magma fragmentation in subglacial eruptions, volcanic material and meltwater will in most cases take up more space than the ice melted in the eruption. Water accumulation would therefore cause buildup of basal water pressure and lead to rapid release of the meltwater. Continuous drainage of meltwater is therefore the most likely scenario in subglacial eruptions under temperate glaciers. Deformation and fracturing of ice played a significant role in the eruption and modified the subglacial water pressure. It is found that water pressure at a vent under a subsiding cauldron is substantially less than it would be during static loading by the overlying ice, since the load is partly compensated for by shear forces in the rapidly deforming ice. In addition to intensive crevassing due to subsidence at Gjálp, a long and straight crevasse formed over the southernmost part of the volcanic fissure on the first day of the eruption. It is suggested that the feeder dyke may have overshot the bedrock–ice interface, caused high deformation rates and fractured the ice up to the surface. The crevasse later modified the flow of meltwater, explaining surface flow of water past the highest part of the edifice. The dominance of magma fragmentation in the Gjálp eruption suggests that initial ice thickness greater than 600–700 m is required if effusive eruption of pillow lava is to be the main style of activity, at least in similar eruptions of high initial magma discharge.Editorial responsibility: J. Donnelly-Nolan  相似文献   

5.
 Pliocene–Recent volcanic outcrops at Seal Nunataks and Beethoven Peninsula (Antarctic Peninsula) are remnants of several monogenetic volcanoes formed by eruption of vesiculating basaltic magma into shallow water, in an englacial environment. The diversity of sedimentary and volcanic lithofacies present in the Antarctic Peninsula outcrops provides a clear illustration of the wide range of eruptive, transportational and depositional processes which are associated with englacial Surtseyan volcanism. Early-formed pillow lava and glassy breccia, representing a pillow volcano stage of construction, are draped by tephra erupted explosively during a tuff cone stage. The tephra was resedimented around the volcano flanks, mainly by coarse-grained sediment gravity flows. Fine-grained lithofacies are rare, and fine material probably bypassed the main volcanic edifice, accumulating in the surrounding englacial basin. The pattern of sedimentation records variations in eruption dynamics. Products of continuous-uprush eruptions are thought to be represented by stacks of poorly bedded gravelly sandstone, whereas better bedded, lithologically more diverse sequences accumulated during periods of quiescence or effusive activity. Evidence for volcano flank failure is common. In Seal Nunataks, subaerial lithofacies (mainly lavas and cinder cone deposits) are volumetrically minor and occur at a similar stratigraphical position to pillow lava, suggesting that glacial lake drainage may have occurred prior to or during deposition of the subaerial lithofacies. By contrast, voluminous subaerial effusion in Beethoven Peninsula led to the development of laterally extensive stratified glassy breccias representing progradation of hyaloclastite deltas. Received: 5 February 1996 / Accepted: 17 January 1997  相似文献   

6.
Late Miocene (c. 13–5 Ma) volcanic sequences of the Hallett Volcanic Province (HVP) crop out along >250 km of western Ross Sea coast in northern Victoria Land. Eight primary volcanic and six sedimentary lithofacies have been identified, and they are organised into at least five different sequence architectures as a consequence of different combinations of eruptive and/or depositional conditions. The volcanoes were erupted in association with a Miocene glacial cover and the sequences are overwhelmingly glaciovolcanic. The commonest and most representative are products of mafic aa lava-fed deltas, a type of glaciovolcanic sequence that has not been described before. It is distinguished by (1) a subaerially emplaced relatively thin caprock of aa lavas lying on and passing down-dip into (2) a thicker association of chaotic to crudely bedded hyaloclastite breccias, water-chilled lava sheets and irregular lava masses, collectively called lobe-hyaloclastite. A second distinctive sequence type present is characterised by water-cooled lavas and associated sedimentary lithofacies (diamictite (probably glacigenic) and fluvial sands and gravels) similar to some mafic glaciovolcanic sheet-like sequences (see Smellie, Earth-Science Reviews, 74, 241–268, 2008), but including (for the first time) examples of likely sheet-like sequences with felsic compositions. Other sequence types in the HVP are minor and include tuff cones, cinder cones and a single ice-marginal lacustrine sequence. The glacial thermal regime varied from polar, characterised by sequences lacking glacial erosion, glacigenic sediments or evidence for free water, to temperate or sub-polar for sequences in which all of these features are conspicuously developed.  相似文献   

7.
Hlöðufell is a familiar 1186 m high landmark, located about 80 km northeast of Reykjavík, and 9 km south of the Langkjökull ice-cap in south-west Iceland. This is the first detailed study of this well-exposed and easily accessible subglacial to emergent basaltic volcano. Eight coherent and eleven volcaniclastic lithofacies are described and interpreted, and its evolution subdivided into four growth stages (I–IV) on the basis of facies architecture. Vents for stages I, II, and IV lie along the same fissure zone, which trends parallel to the dominant NNE–SSW volcano-tectonic axis of the Western Volcanic Zone in this part of Iceland, but the stage III vent lies to the north, and is probably responsible for the present N–S elongation of the volcano. The basal stage (I) is dominated by subglacially erupted lava mounds and ridges, which are of 240 m maximum thickness, were fed from short fissures and locally display lava tubes. Some of the stage I lavas preserve laterally extensive flat to bulbous, steep, glassy surfaces that are interpreted to have formed by direct contact with surrounding ice, and are termed ice-contact lava confinement surfaces. These surfaces preserve several distinctive structures, such as lava shelves, pillows that have one flat surface and mini-pillow (< 10 cm across) breakouts, which are interpreted to have formed by the interplay of lava chilling and confinement against ice, ice melting and ice fracture. The ice-contact lava confinement surfaces are also associated with zones of distinctive open cavities in the lavas that range from about 1 m to several metres across. The cavities are interpreted as having arisen by lava engulfing blocks of ice, that had become trapped in a narrow zone of meltwater between the lava and the surrounding ice, and are termed ice-block meltout cavities. The same areas of the lavas also display included and sometimes clearly rotated blocks of massive to planar to cross-stratified hyaloclastite lapilli tuffs and tuff–breccias, termed hyaloclastite inclusions, which are interpreted as engulfed blocks of hyaloclastite/pillow breccia carapace and talus, or their equivalents reworked by meltwater. Some of the stage I lavas are mantled at the southern end of the mountain by up to 35 m thickness of well-bedded vitric lapilli tuffs (stage II), of phreatomagmatic origin, which were erupted from a now dissected cone, preserved in this area. The tephra was deposited dominantly by subaqueous sediment gravity flows (density currents) in an ice-bound lake (or less likely a sub-ice water vault), and was also transported to the south by sub-ice meltwater traction currents. This cone is onlapped by a subaerial pahoehoe lava-fed delta sequence, formed during stage III, and which was most likely fed from a now buried vent(s), located somewhere in the north-central part of the mountain. A 150 m rise in lake level submerged the capping lavas, and was associated with progradation of a new pahoehoe lava-fed delta sequence, produced during stage IV, and which was fed from the present summit cone vent. The water level rise and onset of stage IV eruptions were not associated with any obviously exposed phreatomagmatic deposits, but they are most likely buried beneath stage IV delta deposits. Stage IV lava-fed deltas display steep benches, which do not appear to be due to syn- or post-depositional mass wasting, but were probably generated during later erosion by ice. The possibility that they are due to shorter progradation distances than the underlying stage III deltas, due to ice-confinement or lower volumes of supplied lava is also considered.  相似文献   

8.
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9.
Bulk runoff and meteorological data suggest the occurrence of two meltwater outburst events at Finsterwalderbreen, Svalbard, during the 1995 and 1999 melt seasons. Increased bulk meltwater concentrations of Cl? during the outbursts indicate the release of snowmelt from storage. Bulk meltwater hydrochemical data and suspended sediment concentrations suggest that this snowmelt accessed a chemical weathering environment characterized by high rock:water ratios and long rock–water contact times. This is consistent with a subglacial origin. The trigger for both the 1995 and 1999 outbursts is believed to be high rates of surface meltwater production and the oversupply of meltwater to areas of the glacier bed that were at the pressure melting point, but which were unconnected to the main subglacial drainage network. An increase in subglacial water pressure to above the overburden pressure lead to the forcing of a hydrological connection between the expanding subglacial reservoir and the ice‐marginal channelized system. The purging of ice blocks from the glacier during the outbursts may indicate the breach of an ice dam during connection. Although subglacial meltwater issued continually from the glacier terminus via a subglacial upwelling during both melt seasons, field observations showed outburst meltwaters were released solely via an ice‐marginal channel. It is possible that outburst events are a seasonal phenomenon at this glacier and reflect the periodic drainage of meltwaters from the same subglacial reservoir from year to year. However, the location of this reservoir is uncertain. A 100 m high bedrock ridge traverses the glacier 6·5 km from its terminus. The overdeepened area up‐glacier from this is the most probable site for subglacial meltwater accumulation. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

10.
Till deposition by glacier submarginal,incremental thickening   总被引：1，自引：0，他引：1  
Macro‐ and micro‐scale sedimentological analyses of recently deposited tills and complex push/squeeze moraines on the forelands of Icelandic glaciers and in a stacked till sequence at the former Younger Dryas margin of the Loch Lomond glacier lobe in Scotland are used to assess the depositional processes involved in glacier submarginal emplacement of sediment. Where subglacial meltwater is unable to flush out subglacial sediment or construct thick debris‐rich basal ice by cumulative freeze‐on processes, glacier submarginal processes are dictated by seasonal cycles of refreezing and melt‐out of tills advected from up‐ice by a combination of lodgement, deformation and ice keel and clast ploughing. Although individual till layers may display typical A and B horizon deformation characteristics, the spatially and temporally variable mosaic of subglacial processes will overprint sedimentary and structural signatures on till sequences to the extent that they would be almost impossible to classify genetically in the ancient sediment record. At the macro‐scale, Icelandic tills display moderately strong clast fabrics that conform to the ice flow directions documented by surface flutings; very strong fabrics typify unequivocally lodged clasts. Despite previous interpretations of these tills as subglacial deforming layers, micro‐morphological analysis reveals that shearing played only a partial role in the emplacement of till matrixes, and water escape and sediment flowage features are widespread. A model of submarginal incremental thickening is presented as an explanation of these data, involving till slab emplacement over several seasonal cycles. Each cycle involves: (1) late summer subglacial lodgement, bedrock and sediment plucking, subglacial deformation and ice keel ploughing; (2) early winter freeze‐on of subglacial sediment to the thin outer snout; (3) late winter readvance and failure along a decollement plane within the till, resulting in the carriage of till onto the proximal side of the previous year's push moraine; (4) early summer melt‐out of the till slab, initiating porewater migration, water escape and sediment flow and extrusion. Repeated reworking of the thin end of submarginal till wedges produces overprinted strain signatures and clast pavements. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

11.
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12.
Detailed facies analysis of hyaloclastites and associated lavas from eight table mountains and similar "hyaloclastite volcanoes" in the Icelandic rift zone contradict a rapid and continuous, "monogenetic", entirely subglacial evolution of most volcanoes studied. The majority of the exposed hyaloclastite deposits formed in large, stable lakes as indicated by widespread, up to 300-m-thick, continuous sections of deep water, shallow water and emergent facies. Salient features include extensively layered or bedded successions comprising mainly debris flow deposits, turbidites, base surge and fallout deposits consisting of texturally and compositionally variable, slightly altered hyaloclastites, as well as sheet and pillow lavas. In contrast, chaotic assemblages of coarser-grained, more poorly sorted and more strongly palagonitized hyaloclastite tuffs and breccias, as well as scoria and lava are interpreted to have formed under sub- or englacial conditions in small, chimney-like ice cavities or ice-bound lakes. Irregularly shaped and erratically arranged hyaloclastite bodies produced at variable water levels appear to have resulted mainly from rapid changes of the eruptive environment due to repeated build-up and drainage of ice-bound lakes as well as the restricted space between the ice walls. We distinguish a "deep water" facies formed during high water levels of the lake, a hydroclastic shallow water and emergent facies (leakage of the lake or growth of the volcano above the water surface). Our model implies the temporary existence of large, stable lakes in Iceland probably formed by climatically induced ice melting. The highly complex edifices of many table mountains and similar volcanoes were constructed during several eruptive periods in changing environments characterized by contrasting volcanic and sedimentary processes. Received: 10 June 1997 / Accepted: 28 July 1998  相似文献   

13.
This paper describes unusual rhyolitic deposits at Dalakvísl, Torfajökull, Iceland that were emplaced during a Quaternary subglacial eruption. Despite its small volume (<0.2 km3), the eruption mechanisms were highly variable and involved both explosive and intrusive phases. The explosive phase involved vesiculation-driven magma fragmentation at the glacier base and generated a pumiceous pyroclastic deposit containing deformed sheets of dense obsidian. Textures suggest that the obsidian was generated by the collapse of partly fragmented foam that was intruding the deposit and water contents indicate quenching at elevated pressures. In contrast, the intrusive phase of the eruption generated vesicle-poor quench hyaloclastites associated with a variety of peperitic lava bodies. The presence of juvenile-rich fluvio-lacustrine sediments is the first documented evidence that meltwater may pond close to the vent during subglacial rhyolite eruptions if the bedrock topography is favourable. In order to explain the variable eruption mechanisms, a conceptual model is presented in which the transition from an explosive to an intrusive eruption was controlled by the space available for fragmentation within the subglacial cavity melted above the vent. When the cavity became completely filled by volcanic deposits, the vent became blocked and rising magma was forced to intrude through poorly consolidated debris. This led to arrested fragmentation and welding of foam domains to form vesicle-poor obsidian lava; the transition to an intrusive eruption has taken place. Although this vent-blocking mechanism is particularly relevant to subglacial eruptions, it may also apply to subaerial rhyolitic eruptions, where patterns of explosive and effusive activity cannot be explained by shallow degassing processes alone. Meanwhile, the variable style of a small-volume subglacial rhyolite eruption further highlights the complex processes that mediate volcano-ice interactions.  相似文献   

14.
Within the neovolcanic zones of Iceland many volcanoes grew upward through icecaps that have subsequently melted. These steep-walled and flat-topped basaltic subglacial volcanoes, called tuyas, are composed of a lower sequence of subaqueously erupted, pillowed lavas overlain by breccias and hyaloclastites produced by phreatomagmatic explosions in shallow water, capped by a subaerially erupted lava plateau. Glass and whole-rock analyses of samples collected from six tuyas indicate systematic variations in major elements showing that the individual volcanoes are monogenetic, and that commonly the tholeiitic magmas differentiated and became more evolved through the course of the eruption that built the tuya. At Herdubreid, the most extensively studies tuya, the upward change in composition indicates that more than 50 wt.% of the first erupted lavas need crystallize over a range of 60°C to produce the last erupted lavas. The S content of glass commonly decreases upward in the tuyas from an average of about 0.08 wt.% at the base to < 0.02 wt.% in the subaerially erupted lava at the top, and is a measure of the depth of water (or ice) above the eruptive vent. The extensive subsurface crystallization that generates the more evolved, lower-temperature melts during the growth of the tuyas, apparently results from cooling and degassing of magma contained in shallow magma chambers and feeders beneath the volcanoes. Cooling may result from percolation of meltwater down cracks, vaporization, and cycling in a hydrothermal circulation. Degassing occurs when progressively lower pressure eruption (as the volcanic vent grows above the ice/water surface) lowers the volatile vapour pressure of subsurface melt, thus elevating the temperature of the liquidus and hastening liquid-crystal differentiation.  相似文献   

15.
The volcanic island of Milos, Greece, comprises an Upper Pliocene –Pleistocene, thick (up to 700 m), compositionally and texturally diverse succession of calc-alkaline, volcanic, and sedimentary rocks that record a transition from a relatively shallow but dominantly below-wave-base submarine setting to a subaerial one. The volcanic activity began at 2.66±0.07 Ma and has been more or less continuous since then. Subaerial emergence probably occurred at 1.44±0.08 Ma, in response to a combination of volcanic constructional processes and fault-controlled volcano-tectonic uplift. The architecture of the dominantly felsic-intermediate volcanic succession reflects contrasts in eruption style, proximity to source, depositional environment and emplacement processes. The juxtaposition of submarine and subaerial facies indicates that for part of the volcanic history, below-wave base to above-wave base, and shoaling to subaerial depositional environments coexisted in most areas. The volcanic facies architecture comprises interfingering proximal (near vent), medial and distal facies associations related to five main volcano types: (1) submarine felsic cryptodome-pumice cone volcanoes; (2) submarine dacitic and andesitic lava domes; (3) submarine-to-subaerial scoria cones; (4) submarine-to-subaerial dacitic and andesitic lava domes and (5) subaerial lava-pumice cone volcanoes. The volcanic facies are interbedded with a sedimentary facies association comprising sandstone and/or fossiliferous mudstone mainly derived from erosion of pre-existing volcanic deposits. The main facies associations are interpreted to have conformable, disconformable, and interfingering contacts, and there are no mappable angular unconformities or disconformities within the volcanic succession.Electronic Supplementary Material Supplementary material is available for this article at  相似文献   

16.
Contrary to general belief, not all large igneous provinces (LIPs) are characterised by rocks of basaltic composition. Silicic-dominated LIPs, such as the Whitsunday Volcanic Province of NE Australia, are being increasingly recognised in the rock record. These silicic LIPs are consistent in being: (1) volumetrically dominated by ignimbrite; (2) active over prolonged periods (40–50 m.y.), based on available age data; and (3) spatially and temporally associated with plate break-up. This silicic-dominated LIP, related to the break-up of eastern continental Gondwana, is also significant for being the source of >1.4×106 km3 of coeval volcanogenic sediment preserved in adjacent sedimentary basins of eastern Australia.The Whitsunday Volcanic Province is volumetrically dominated by medium- to high-grade, dacitic to rhyolitic lithic ignimbrites. Individual ignimbrite units are commonly between 10 and 100 m thick, and the ignimbrite-dominated sequences exceed 1 km in thickness. Coarse lithic lag breccias containing clasts up to 6 m diameter are associated with the ignimbrites in proximal sections. Pyroclastic surge and fallout deposits, subordinate basaltic to rhyolitic lavas, phreatomagmatic deposits, and locally significant thicknesses of coarse-grained volcanogenic conglomerate and sandstone are interbedded with the ignimbrites. The volcanic sequences are intruded by gabbro/dolerite to rhyolite dykes (up to 50 m in width), sills and comagmatic granite. Dyke orientations are primarily from NW to NNE.The volcanic sequences are characterised by the interstratification of proximal/near-vent lithofacies such as rhyolite domes and lavas, and basaltic agglomerate, with medial to distal facies of ignimbrite. The burial of these near-vent lithofacies by ignimbrites, coupled with the paucity of mass wastage products such as debris-flow deposits indicates a low-relief depositional environment. Furthermore, the volcanic succession records a temporal change in: (1) eruptive styles; (2) the nature of source vents; and (3) erupted compositions. An early explosive dacitic pyroclastic phase was succeeded by a later mixed pyroclastic-effusive phase producing an essentially bimodal suite of lavas and rhyolitic ignimbrite. From the nature and distribution of volcanic lithofacies, the volcanic sequences are interpreted to record the evolution of a multiple vent, low-relief volcanic region, dominated by several large caldera centres.  相似文献   

17.
Our understanding of Lake Vostok, the huge subglacial lake beneath the East Antarctic Ice Sheet, has improved recently through the identification of key physical and chemical interactions between the ice sheet and the lake. The north of the lake, where the overlying ice sheet is thickest, is characterized by subglacial melting, whereas freezing of lake water occurs in the south, resulting in ～210 m of ice accretion to the underside of the ice sheet. The accreted ice contains lower concentrations of the impurities normally found in glacier ice, suggesting a net transfer of material from meltwater into the lake. The small numbers of microbes found so far within the accreted ice have DNA profiles similar to those of contemporary surface microbes. Microbiologists expect, however, that Lake Vostok, and other subglacial lakes, will harbour unique species, particularly within the deeper waters and associated sediments. The extreme environments of subglacial lakes are characterized by high pressures, low temperatures, permanent darkness, limited nutrient availability, and oxygen concentrations that are derived from the ice that provides the meltwater. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

18.
Studies of the eruptive products from volcanoes with variable ice and snow cover and a long history of activity enable reconstruction of erupted palaeoenvironments, as well as highlighting the hazards associated with meltwater production, such as jökulhlaups and magma-water interaction. Existing difficulties include estimation of ice/snow thicknesses and discrimination between ice- and snow-contact lithofacies. We present field evidence from the Cerro Blanco subcomplex of Nevados de Chillán stratovolcano, central Chile, which has erupted numerous times in glacial and non-glacial periods and most recently produced andesitic lava flows in the 1861–1865 eruption from the Santa Gertrudis cone on the northwest flank of the volcano. The main period of lava effusion occurred during the winter of 1861 when the upper flanks of the volcano were reportedly covered in snow and ice. The bases and margins of the first lava flows produced are cut by arcuate fractures, which are interpreted as snow-contact features formed when steam generated from the melting of snow entered tensional fractures at the flow base. In contrast, the interiors and upper parts of these flows, as well as the overlying flow units, have autobrecciated and blocky textures typical of subaerial conditions, due to insulation by the underlying lava. Similar textures found in a lava flow dated at 90.0±0.6 ka that was emplaced on the northwest flank of Cerro Blanco, are also inferred to be ice and snow-contact features. These textures have been used to infer that a small valley glacier, overlain by snow, existed in the Santa Gertrudis Valley at the time of the eruption. Such reconstructions are important for determining the long-term evolution of the volcano as well as assessing future hazards at seasonally snow-covered volcanoes.  相似文献   

19.
An eruption along a 2.5 km-long rhyolitic dyke at Krafla volcano, northern Iceland during the last glacial period formed a ridge of obsidian (Hrafntinnuhryggur). The ridge rises up to 80 m above the surrounding land and is composed of a number of small-volume lava bodies with minor fragmental material. The total volume is < 0.05 km3. The lava bodies are flow- or dome-like in morphology and many display columnar-jointed sides typical of magma–ice interaction, quench-fragmented lower margins indicative of interaction with meltwater and pumiceous upper surfaces typical of subaerial obsidian flows. The fragmental material compromises poorly-sorted perlitic quench hyaloclastites and poorly-exposed pumiceous tuffs. Lava bodies on the western ridge flanks are columnar jointed and extensively hydrothermally altered. At the southern end of the ridge the feeder dyke is exposed at an elevation  95 m beneath the ridge crest and flares upwards into a lava body.Using the distribution of lithofacies, we interpret that the eruption melted through ice only 35–55 m thick, which is likely to have been dominated by firn. Hrafntinnuhryggur is therefore the first documented example of a rhyolitic fissure eruption beneath thin ice/firn. The eruption breached the ice, leading to subaerial but ice/firn-contact lava effusion, and only minor explosive activity occurred. The ridge appears to have been well-drained during the eruption, aided by the high permeability of the thin ice/firn, which appears not to have greatly affected the eruption mechanisms. We estimate that the eruption lasted between 2 and 20 months and would not have generated a significant jökulhlaup (< 70 m3 s− 1).  相似文献   

20.
A Pleistocene subaqueous, volcanic sequence in South Iceland consists of flows of basaltic hyaloclastite and lava with interbedded sedimentary diamictite units. Emplacement occurred on a distal submarine shelf in drowned valleys along the southern coast of Iceland. The higher sea level was caused by eustatic sea-level change, probably towards the end of a glaciation. This sequence, nearly 700 m thick, rests unconformably on eroded flatlying lavas and sedimentary rocks of likely Tertiary age. A Standard Depositional Unit, describing the flows of hyaloclastite, starts with compact columnar-jointed basalt overlain by cubejointed basalt, and/or pillow lava. This in turn is overlain by thick unstructured hyaloclastite containing aligned basalt lobes, and bedded hyaloclastite at the top. A similar lithofacies succession is valid for proximal to distal locations. The flows were produced by repeated voluminous extrusions of basaltic lava from subaquatic fissures on the Eastern Rift Zone of Iceland. The fissures are assumed to lie in the same general area as the 1783 Laki fissure which produced 12 km3 of basaltic lava. Due to very high extrusion rates, the effective water/melt ratio was low, preventing optimal fragmentation of the melt. The result was a heterogeneous mass of hyaloclastite and fluid melt which moved en masse downslope with the melt at the bottom of the flow and increasingly vesicular hyaloclastite fragments above. The upper and distal parts of the flow moved as low-concentration turbulent suspensions that deposited bedded hyaloclastite.  相似文献   

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