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1.
The caldera of Santorini is a composite structure with a subsidence history extending over 100 ka or more. Geomorphological mapping shows that the present-day caldera wall is a complex assemblage of cliff surfaces of different ages, and that collapse at Santorini has repeatedly exhumed earlier caldera cliffs and unconformities. Cliffs bounding the southern, southeastern and northwestern rims of the caldera are morphologically fresh and probably formed during or soon after the Minoan eruption in the late Bronze Age. The well-scalloped shape of these cliffs is attributed to large-scale rotational landslip around the margins of the Minoan caldera. The deposit from one landslip is preserved subaerially. Minoan landslips in southeast santorini detached along the basement unconformity, exposing a cliff of the prevolcanic island. The caldera wall in the north, northeast and east preserves evidence for three generations of cliff: those of Minoan age and two earlier generations of caldera wall. The two early calderas can be dated relative to a well-established statigraphy of lavas and tuffs. The presence of in situ Minoan tephra plastered onto the present-day caldera wall provides evidence that these ancient caldera cliffs had already been exhumed prior to the Minoan eruption. Field relationships permit reconstruction of the physiography of Bronze-Age Santorini immediately before the Minoan eruption. The reconstruction differs from some previously published versions and is believed to be the most accurate to date. Bronze-Age Sa ntorini had a large flooded caldera formed 21 ka ago. This caldera must have acted as an excellent harbour for the Bronze-Age inhabitants of the island. The 3.6 ka Minoan eruption deepened and widened the extant caldera. The volume of Minoan collapse (25 km3) is in good agreement with published estimates for the volume of discharged magma if between 5 and 8 km3 of Minoan ignimbrite ponded as intracaldera tuff.  相似文献   

2.
Apoyo caldera, near Granada, Nicaragua, was formed by two phases of collapse following explosive eruptions of dacite pumice about 23,000 yr B.P. The caldera sits atop an older volcanic center consisting of lava flows, domes, and ignimbrite (ash-flow tuff). The earliest lavas erupted were compositionally homogeneous basalt flows, which were later intruded by small andesite and dacite flows along a well defined set of N—S-trending regional faults. Collapse of the roof of the magma chamber occurred along near-vertical ring faults during two widely separated eruptions. Field evidence suggests that the climactic eruption sequence opened with a powerful plinian blast, followed by eruption column collapse, which generated a complex sequence of pyroclastic surge and ignimbrite deposits and initiated caldera collapse. A period of quiescence was marked by the eruption of scoria-bearing tuff from the nearby Masaya caldera and the development of a soil horizon. Violent plinian eruptions then resumed from a vent located within the caldera. A second phase of caldera collapse followed, accompanied by the effusion of late-stage andesitic lavas, indicating the presence of an underlying zoned magma chamber. Detailed isopach and isopleth maps of the plinian deposits indicate moderate to great column heights and muzzle velocities compared to other eruptions of similar volume. Mapping of the Apoyo airfall and ignimbrite deposits gives a volume of 17.2 km3 within the 1-mm isopach. Crystal concentration studies show that the true erupted volume was 30.5 km3 (10.7 km3 Dense Rock Equivalent), approximately the volume necessary to fill the caldera. A vent area located in the northeast quadrant of the present caldera lake is deduced for all the silicic pyroclastic eruptions. This vent area is controlled by N—S-trending precaldera faults related to left-lateral motion along the adjacent volcanic segment break. Fractional crystallization of calc-alkaline basaltic magma was the primary differentiation process which led to the intermediate to silicic products erupted at Apoyo. Prior to caldera collapse, highly atypical tholeiitic magmas resembling low-K, high-Ca oceanic ridge basalts were erupted along tension faults peripheral to the magma chamber. The injection of tholeiitic magmas may have contributed to the paroxysmal caldera-forming eruptions.  相似文献   

3.
The 161 ka explosive eruption of the Kos Plateau Tuff (KPT) ejected a minimum of 60 km3 of rhyolitic magma, a minor amount of andesitic magma and incorporated more than 3 km3 of vent- and conduit-derived lithic debris. The source formed a caldera south of Kos, in the Aegean Sea, Greece. Textural and lithofacies characteristics of the KPT units are used to infer eruption dynamics and magma chamber processes, including the timing for the onset of catastrophic caldera collapse.The KPT consists of six units: (A) phreatoplinian fallout at the base; (B, C) stratified pyroclastic-density-current deposits; (D, E) volumetrically dominant, massive, non-welded ignimbrites; and (F) stratified pyroclastic-density-current deposits and ash fallout at the top. The ignimbrite units show increases in mass, grain size, abundance of vent- and conduit-derived lithic clasts, and runout of the pyroclastic density currents from source. Ignimbrite formation also corresponds to a change from phreatomagmatic to dry explosive activity. Textural and lithofacies characteristics of the KPT imply that the mass flux (i.e. eruption intensity) increased to the climax when major caldera collapse was initiated and the most voluminous, widespread, lithic-rich and coarsest ignimbrite was produced, followed by a waning period. During the eruption climax, deep basement lithic clasts were ejected, along with andesitic pumice and variably melted and vesiculated co-magmatic granitoid clasts from the magma chamber. Stratigraphic variations in pumice vesicularity and crystal content, provide evidence for variations in the distribution of crystal components and a subsidiary andesitic magma within the KPT magma chamber. The eruption climax culminated in tapping more coarsely crystal-rich magma. Increases in mass flux during the waxing phase is consistent with theoretical models for moderate-volume explosive eruptions that lead to caldera collapse.  相似文献   

4.
A new pyroclastic stratigraphy is presented for the island of Ischia, Italy, for the period ∼75–50 ka BP. The data indicate that this period bore witness to the largest eruptions recorded on the island and that it was considerably more volcanically active than previously thought. Numerous vents were probably active during this period. The deposits of at least 10 explosive phonolite to basaltic-trachyandesite eruptions are described and interpreted. They record a diverse range of explosive volcanic activity including voluminous fountain-fed ignimbrite eruptions, fallout from sustained eruption columns, block-and-ash flows, and phreatomagmatic eruptions. Previously unknown eruptions have been recognised for the first time on the island. Several of the eruptions produced pyroclastic density currents that covered the whole island as well as the neighbouring island of Procida and parts of the mainland. The morphology of Ischia was significantly different to that seen today, with edifices to the south and west and a submerged depression in the centre. The largest volcanic event, the Monte Epomeo Green Tuff (MEGT) resulted in caldera collapse across all or part of the island. It is shown to comprise at least two thick intracaldera ignimbrite flow-units, separated by volcaniclastic sediments that were deposited during a pause in the eruption. Extracaldera deposits of the MEGT include a pumice fall deposit emplaced during the opening phases of the eruption, a widespread lithic lag breccia outcropping across much of Ischia and Procida, and a distal ignimbrite in south-west Campi Flegrei. During this period the style and magnitude of volcanism was dictated by the dynamics of a large differentiated magma chamber, which was partially destroyed during the MEGT eruption. This contrasts with the small-volume Holocene and historical effusive and explosive activity on Ischia, the timing and distribution of which has been controlled by the resurgence of the Monte Epomeo block. The new data contribute to a clearer understanding of the long-term volcanic and magmatic evolution of Ischia.  相似文献   

5.
The 35 × 20 km Cerro Galán resurgent caldera is the largest post-Miocene caldera so far identified in the Andes. The Cerro Galán complex developed on a late pre-Cambrian to late Palaeozoic basement of gneisses, amphibolites, mica schists and deformed phyllites and quartzites. The basement was uplifted in the early Miocene along large north-south reverse faults, producing a horst-and-graben topography. Volcanism began in the area prior to 15 Ma with the formation of several andesite to dacite composite volcanoes. The Cerro Galán complex developed along two prominent north-south regional faults about 20 km apart. Dacitic to rhyodacitic magma ascended along these faults and caused at least nine ignimbrite eruptions in the period 7-4 Ma (K-Ar determinations). These ignimbrites are named the Toconquis Ignimbrite Formation. They are characterised by the presence of basal plinian deposits, many individual flow units and proximal co-ignimbrite lag breccias. The ignimbrites also have moderate to high macroscopic pumice and lithic contents and moderate to low crystal contents. Compositionally banded pumice occurs near the top of some units. Many of the Toconquis eruptions occurred from vents along a north-south line on the western rim of the young caldera. However, two of the ignimbrites erupted from vents on the eastern margin. Lava extrusions occurred contemporaneously along these north-south lines. The total D.R.E. volume of Toconquis ignimbrite exceeds 500 km3.Following a 2-Ma dormant period a single major eruption of rhyodacitic magma formed the 1000-km3 Cerro Galán ignimbrite and the caldera. The ignimbrite (age 2.1 Ma on Rb-Sr determination) forms a 30–200-m-thick outflow sheet extending up to 100 km in all directions from the caldera rim. At least 1.4 km of welded intracaldera ignimbrite also accumulated. The ignimbrite is a pumice-poor, crystal-rich deposit which contains few lithic clasts. No basal plinian deposit has been identified and proximal lag breccias are absent. The composition of pumice clasts is a very uniform rhyodacite which has a higher SiO2 content but a lower K2O content than the Toconquis ignimbrites. Preliminary data indicate no evidence for compositional zonation in the magma chamber. The eruption is considered to have been caused by the catastrophic foundering of a cauldron block into the magma chamber.Post-caldera extrusions occurred shortly after eruption along both the northern extension of the eastern boundary fault and the western caldera margin. Resurgence also occurred, doming up the intracaldera ignimbrite and sedimentary fill to form the central mountain range. Resurgent doming was centred along the eastern fault and resulted in radial tilting of the ignimbrite and overlying lake sediments.  相似文献   

6.
Although the oldest volcanic rocks exposed at Pantelleria (Strait of Sicily) are older than 300 ka, most of the island is covered by the 45–50 ka Green Tuff ignimbrite, thought to be related to the Cinque Denti caldera, and younger lavas and scoria cones. Pre-50 ka rocks (predominantly rheomorphic ignimbrites) are exposed at isolated sea cliffs, and their stratigraphy and chronology are not completely resolved. Based on volcanic stratigraphy and K/Ar dating, it has been proposed that the older La Vecchia caldera is related to ignimbrite Q (114 ka), and that ignimbrites F, D, and Z (106, 94, and 79 ka, respectively) were erupted after caldera formation. We report here the paleomagnetic directions obtained from 23 sites in ignimbrite P (133 ka) and four younger ignimbrites, and from an uncorrelated (and loosely dated) welded lithic breccia thought to record a caldera-forming eruption. The paleosecular variation of the geomagnetic field recorded by ignimbrites is used as correlative tool, with an estimated time resolution in the order of 100 years. We find that ignimbrites D and Z correspond, in good agreement with recent Ar/Ar ages constraining the D/Z eruption to 87 ka. The welded lithic breccia correlates with a thinner breccia lying just below ignimbrite P at another locality, implying that collapse of the La Vecchia caldera took place at ~130–160 ka. This caldera was subsequently buried by ignimbrites P, Q, F, and D/Z. Paleomagnetic data also show that the northern caldera margin underwent a ~10° west–northwest (outwards) tilting after emplacement of ignimbrite P, possibly recording magma resurgence in the crust.  相似文献   

7.
Studies of late Tertiary silicic volcanic centres in the Western and Eastern Cordilleras of the Central Andes show that three volcanic environments are appropriate sites for mineralization: (1) ring-fracture extrusions post-dating large calderas; (2) similar extrusions within ignimbrite shields; and (3) isolated, small silicic volcanoes. Subvolcanic tin mineralization in the Eastern Cordillera is located in silicic stocks and associated breccias of Miocene age. The Cerro Rico stock, Potosi, Bolivia, contains tin and silver mineralization and has an intrusion age apparently millions of years younger than that of the associated Kari Kari caldera. Similar age relationships between mineralization and caldera formation have been described from the San Juan province, Colorado. The vein deposits of Chocaya, southern Bolivia, were emplaced in the lower part of an ignimbrite shield, a type of volcanic edifice as yet unrecognized in comparable areas of silicic volcanism. The El Salvador porphyry copper deposit, Chile, is related to silicic stocks which may have been intruded along a caldera ring fracture. Cerro Bonete, Chile, provides a modern example of the volcanic superstructure which may have overlain isolated mineralized stocks and breccia pipes such as that of Salvadora at Llallagua, Bolivia.Existing models for the genesis of porphyry copper deposits suggest that they formed in granodioritic stocks located in the infrastructure of andesitic stratovolcanoes. Sites of porphyry-type subvolcanic tin mineralization in the Eastern Cordillera of Bolivia are distinguished by the absence of such andesitic structures. The surface expression of a typical subvolcanic porphyry tin deposit was probably an extrusive dome of quartz latite porphyry, sometimes related to a larger caldera structure. Evidence from the El Salvador porphyry copper deposit in the Eocene magmatic belt in Chile suggests that it too may be more closely related to a silicic volcanic structure than to an andesitic stratovolcano.The dome of La Soufriere, Guadeloupe is proposed as a modern analog for the surface expression of subvolcanic mineralization processes, the phreatic eruptions there suggesting the formation of hydrothermal breccia bodies in depth. Occurrence of mineralized porphyries, millions of years after caldera formation, does not necessarily indicate that intrusions and mineralization are not genetically related to the sub-caldera pluton, but may be a consequence of the long thermal histories (1–10 million years) of the lowermost parts of large plutons. Caldera formation can only inhibit mineralization by dispersal of ore metals when these are of magmatic origin, and ignimbrites should not be taken as being unlikely to be associated with porphyry mineralization. Whether ore metals are of wall rock or magmatic origin, the key to understanding the relationships between silicic volcanism and mineralization lies in the fractionation of trace elements within large zoned magma chambers during their igneous history, and their subsequent hydrothermal migration. Small, highly mineralized intrusions formed late in a caldera cycle (such as the Cerro Rico) may be due to the introduction of fresh supplies of mafic magma into the lower parts of the main pluton.  相似文献   

8.
Pyroclastic deposits exposed in the caldera walls of Santorini Volcano (Greece), contain several prominent horizons of coarse-grained andesitic spatter and cauliform volcanic bombs. These deposits can be traced around most of the caldera wall. They thicken in depressions and are intimately associated with ignimbrite and co-ignimbrite lithic lag breccias. They are interpreted as a proximal facies of pyroclastic flow deposits. Evidence for a flow origin includes the presence of a fine-grained pumiceous matrix, flow deformation of ductile spatter clasts, exceedingly coarse grain sizes several kilometres from any plausible vent, imbrication of flattened spatter clasts, intimate interbedding with normal pyroclastic flow deposits and the presence of inversely graded basal layers. The deposits contain hydrothermally altered, rounded lithic ejecta including gabbro nodules. The andesitic ejecta and the fine matrix are typically moderately to poorly vesicular indicating that magmatic gas had a subordinate role in the eruptive process. The andesitic clasts contain abundant angular lithic inclusions and some clasts are themselves formed of pre-existing agglutinate. We propose that these eruptions occurred when external water gained access to the vents, causing large-scale explosions which formed pyroclastic flows rich in coarse, semifluid but poorly vesicular ejecta. We postulate that large volumes of coarse pyroclastic ejecta and degassed lava accumulated in a deep crater prior to being disrupted by these large explosions to form pyroclastic flows.  相似文献   

9.
This petrologic analysis of the Negra Muerta Volcanic Complex (NMVC) contributes to understanding the magmatic evolution of eruptive centres associated with prominent NW-striking fault zones in the southern Central Andes. Specifically, the geochemical characteristics and magmatic evolution of the two eruptive episodes of this Complex are analysed. The first one occurred as an explosive eruption at 9 Ma and is represented by a strongly welded, fiamme-rich, andesitic to dacitic ignimbrite deposit. The second commenced with an eruption of a rhyolitic ignimbrite at 7.6 Ma followed by effusive discharge of hybrid lavas at 7.3 Ma and by emplacement of andesitic to rhyodacitic dykes and domes. Both explosive and effusive eruptions of the second episode occurred within a short time span, but geochemical interpretations permit consideration of the existence of different magmas interacting in the same magma chamber. Our model involves an andesitic recharge into a partially cooled rhyolitic magma chamber, pressurising the magmatic system and triggering explosive eruption of rhyolitic magma. Chemical or mechanical evidence for interaction between the rhyolitic and andesitic magma in the initial stages are not obvious because of their difference in composition, which could have been strong enough to inhibit the interaction between the two magmas. After the initial explosive stages of the eruption at 7.6 Ma, the magma chamber become more depressurised and the most mafic magma settled in compositional layers by fractional crystallisation. Restricted hybridisation occurred and was effective between adjacent and thermally equivalent layers close to the top of the magma chamber. At 7.3 Ma, increments of caldera formation were accompanied by effusive discharge of hybrid lavas through radially disposed dykes whereby andesitic magma gained in importance toward the end of this effusive episode in the central portion of the caldera. Assimilation during turbulent ascent (ATA) is invoked to explain a conspicuous reversed isotopic signature (87Sr/86Sr and 143Nd/144Nd) in the entire volcanic series. Therefore, the 7.6 to 7.3 Ma volcanic rocks of the NMVC resulted from synchronous and mutually interacting petrological processes such as recharge, fractional crystallization, hybridisation, and Assimilation during Turbulent Ascent (ATA).Geochemical characteristics of both volcanic episodes show diverse type and/or depth in the sources and variable influence of upper crustal processes, and indicate a recurrence in the magma-forming conditions. Similarly, other minor volcanic centres in the transversal volcanic belts of the Central Andes repeated their geochemical signatures throughout the Miocene.  相似文献   

10.
In explosive volcanic eruptions, vesicular magma droplets, produced by fragmentation, are propelled into the atmosphere where they are chilled to form pumices. The thermal history of droplets and the permeability of their internal bubble networks determine how much they are deformed in the eruption jet, and hence what information pumices record about the state of the magma at fragmentation. We study these aspects of the `Minoan' plinian eruption of Santorini Volcano by quantifying the rate of oxidation reactions that took place when air entered the hot magma fragments. In our experiments white Minoan pumices were heated for minutes to hours between 600 and 850°C, either in air, or in an atmosphere with an oxygen fugacity at the Ni–NiO buffer. Pumices were unchanged by heating at Ni–NiO. Those heated in air often became pink to dark pink, depending on heating time, and their Curie temperatures, as determined by magnetic susceptibility measurements, increased. We use oxidation rates deduced from these experiments, in conjunction with calculations of the rate of conductive cooling and of the rate at which air can enter a pumice, to constrain the conditions experienced by pumices during the eruption. Natural Minoan pumices less than about 5 cm in radius are white, whereas larger ones often have white rims and pink interiors with Curie temperatures higher than those of white material. We infer that small pumices were cooled before being oxidized, and that oxidation of the interiors of large clasts mostly took place during flight, at temperatures within a few tens of degrees of magmatic values. White rims of large pumices, despite being permeable, were cooled before oxidation could occur. Permeability developed in the liquid state, but did not develop early enough, with respect to cooling, or was not large enough to allow extreme oxidation. We give measurements of pumice permeabilities that should be close to magmatic values.  相似文献   

11.
The Latera caldera is a well-exposed volcano where more than 8 km3 of mafic silica-undersaturated potassic lavas, scoria and felsic ignimbrites were emplaced between 380 and 150 ka. Isotopic ages obtained by 40Ar/39Ar analysis of single sanidine crystals indicate at least four periods of explosive eruptions from the caldera. The initial period of caldera eruptions began at 232 ka with emplacement of trachytic pumice fallout and ignimbrite. They were closely followed by eruption of evolved phonolitic magma. After roughly 25 ky, several phonolitic ignimbrites were deposited, and they were followed by phreatomagmatic eruptions that produced trachytic ignimbrites and several smaller ash-flow units at 191 ka. Compositionally zoned magma then erupted from the northern caldera rim to produce widespread phonolitic tuffs, tephriphonolitic spatter, and scoria-bearing ignimbrites. After 40 ky of mafic surge deposit and scoria cone development around the caldera rim, a compositionally zoned pumice sequence was emplaced around a vent immediately northwest of the Latera caldera. This activity marks the end of large-scale explosive eruptions from the Latera volcano at 156 ka.  相似文献   

12.
Rabaul Caldera is the most recently active (1937–1943) of four adjoining volcanic centres aligned north-south through the northern extremity of eastern New Britain. Geological mapping after the 1983–1985 Rabaul seismic and deformation crisis has partially revealed a long and complex eruption history dominated by numerous explosive eruptions, the largest accompanied by caldera collapse. The oldest exposed eruptives are the basaltic pre-caldera cone Tovanumbatir Lavas K/Ar dated at 0.5 Ma. The dacitic Rabaul Quarry Lavas exposed in the caldera wall and K/Ar dated at 0.19 Ma, are overlain by a sequence of dacitic and andesitic pyroclastic flow and fall deposits. Uplifted coral reef limestones, interbedded within the pyroclastic sequence on the northeast coast, suggest that explosive eruptions in the Rabaul area had commenced prior to the 0.125 Ma last interglacial high sea level stand. The pyroclastic sequence includes the large Boroi Ignimbrites and Malaguna Pyroclastics both 40Ar/39Ar dated at about 0.1 Ma, and the Barge Tunnel Ignimbrite 40Ar/39Ar dated at around 0.04 Ma. Few reliable ages exist for the many younger eruptives. These include Holocene ignimbrites of the latest caldera-forming eruptions—the Raluan Pyroclastics variously dated (14C) at either about 3500 or 7000 yr B.P., and the ca. 1400 yr B.P. Rabaul Pyroclastics. At least eight intracaldera eruptions have occurred since the 1400 yr B.P. collapse, building small pyroclastic and lava cones within the caldera.A major erosional episode is evident as a widespread unconformity in the upper pyroclastic stratigraphy at Rabaul. Lacking relevant radiometric ages, this episode is assumed to have occurred during last glaciation low sea levels and is here arbitarily dated at ca. ?20 ka. At least five, possibly nine, significant ignimbrite eruptions have occurred at Rabaul during the last ?20 ka. The new eruptive history differs considerably from that previously published, which considered ignimbrite eruption and caldera collapse to have first occurred at 3500 yr B.P.Rabaul volcanism has been dominated by two main types: (a) basaltic and basaltic andesite cone building eruptions; and (b) dacitic, and rarely andesitic or rhyolitic, plinian/ignimbrite eruptions of both high- and low-aspect ratio types. The 1400 yr B.P. Rabaul Ignimbrite is a type example of a low-aspect ratio, high-energy, and potentially very damaging eruption. Fine vitric ash deposits, common in the Rabaul pyroclastic sequence, demonstrate the frequent modification of eruptions by external water probably related to early caldera lakes or bays. Interbedding of these fine ashes with plinian pumice lapilli beds suggests that many early eruptions occurred from multiple vents, located in both wet and dry areas.  相似文献   

13.
Mamaku Ignimbrite was deposited during the formation of Rotorua Caldera, Taupo Volcanic Zone, New Zealand, 220–230 ka. Its outflow sheet forms a fan north, northwest and southwest of Rotorua, capping the Mamaku–Kaimai Plateau. Mamaku Ignimbrite can be divided into a partly phreatomagmatic basal sequence, and a main sequence which comprises lower, middle, and upper ignimbrite. The internal stratigraphy indicates that it was emplaced progressively from a pyroclastic density current of varying energy that became less particulate away from source. Gradational contacts between lower, middle, and upper ignimbrite are consistent with it being deposited during one eruptive event from the same source. Variations in lithic clast content and coexistence of different pumice types through the ignimbrite sequence indicate that caldera collapse occurred throughout the eruption, but particularly when middle Mamaku Ignimbrite was deposited and in the final stages of deposition of upper Mamaku Ignimbrite. Maximum lithic data and the location of lithic lag breccias in upper Mamaku Ignimbrite confirm Rotorua Caldera as the source. At least 120 m of geothermally altered intra-caldera Mamaku Ignimbrite occurs inside Rotorua Caldera. Pumice clasts in the Mamaku Ignimbrite are dacite to high-silica rhyolite and can be chemically divided into three types: high–silica rhyolite (type 1), rhyolite (type 2), and dacite (type 3). All are petrogenetically related and types 1 and 2 may be derived by up to 20% crystal fractionation from the type 3 dacite. All three types probably resided in a single, gradationally zoned magma chamber. Andesitic juvenile fragments are found only in upper Mamaku Ignimbrite and inferred to represent a discrete magma that was injected into the silicic chamber and is considered to have accumulated as a sill at the base of the magma chamber. The contrast in density between the andesitic and silicic magmas did not allow eruption of the andesitic fragments during the deposition of lower and middle Mamaku Ignimbrite. The advanced stage of caldera collapse, late in the main eruptive phase, created withdrawal dynamics that allowed andesitic magma to reach the surface as fragments within upper Mamaku Ignimbrite.  相似文献   

14.
New investigations of the geology of Crater Lake National Park necessitate a reinterpretation of the eruptive history of Mount Mazama and of the formation of Crater Lake caldera. Mount Mazama consisted of a glaciated complex of overlapping shields and stratovolcanoes, each of which was probably active for a comparatively short interval. All the Mazama magmas apparently evolved within thermally and compositionally zoned crustal magma reservoirs, which reached their maximum volume and degree of differentiation in the climactic magma chamber 7000 yr B.P.The history displayed in the caldera walls begins with construction of the andesitic Phantom Cone 400,000 yr B.P. Subsequently, at least 6 major centers erupted combinations of mafic andesite, andesite, or dacite before initiation of the Wisconsin Glaciation 75,000 yr B.P. Eruption of andesitic and dacitic lavas from 5 or more discrete centers, as well as an episode of dacitic pyroclastic activity, occurred until 50,000 yr B.P.; by that time, intermediate lava had been erupted at several short-lived vents. Concurrently, and probably during much of the Pleistocene, basaltic to mafic andesitic monogenetic vents built cinder cones and erupted local lava flows low on the flanks of Mount Mazama. Basaltic magma from one of these vents, Forgotten Crater, intercepted the margin of the zoned intermediate to silicic magmatic system and caused eruption of commingled andesitic and dacitic lava along a radial trend sometime between 22,000 and 30,000 yr B.P. Dacitic deposits between 22,000 and 50,000 yr old appear to record emplacement of domes high on the south slope. A line of silicic domes that may be between 22,000 and 30,000 yr old, northeast of and radial to the caldera, and a single dome on the north wall were probably fed by the same developing magma chamber as the dacitic lavas of the Forgotten Crater complex. The dacitic Palisade flow on the northeast wall is 25,000 yr old. These relatively silicic lavas commonly contain traces of hornblende and record early stages in the development of the climatic magma chamber.Some 15,000 to 40,000 yr were apparently needed for development of the climactic magma chamber, which had begun to leak rhyodacitic magma by 7015 ± 45 yr B.P. Four rhyodacitic lava flows and associated tephras were emplaced from an arcuate array of vents north of the summit of Mount Mazama, during a period of 200 yr before the climactic eruption. The climactic eruption began 6845 ± 50 yr B.P. with voluminous airfall deposition from a high column, perhaps because ejection of 4−12 km3 of magma to form the lava flows and tephras depressurized the top of the system to the point where vesiculation at depth could sustain a Plinian column. Ejecta of this phase issued from a single vent north of the main Mazama edifice but within the area in which the caldera later formed. The Wineglass Welded Tuff of Williams (1942) is the proximal featheredge of thicker ash-flow deposits downslope to the north, northeast, and east of Mount Mazama and was deposited during the single-vent phase, after collapse of the high column, by ash flows that followed topographic depressions. Approximately 30 km3 of rhyodacitic magma were expelled before collapse of the roof of the magma chamber and inception of caldera formation ended the single-vent phase. Ash flows of the ensuing ring-vent phase erupted from multiple vents as the caldera collapsed. These ash flows surmounted virtually all topographic barriers, caused significant erosion, and produced voluminous deposits zoned from rhyodacite to mafic andesite. The entire climactic eruption and caldera formation were over before the youngest rhyodacitic lava flow had cooled completely, because all the climactic deposits are cut by fumaroles that originated within the underlying lava, and part of the flow oozed down the caldera wall.A total of 51−59 km3 of magma was ejected in the precursory and climactic eruptions, and 40−52 km3 of Mount Mazama was lost by caldera formation. The spectacular compositional zonation shown by the climactic ejecta — rhyodacite followed by subordinate andesite and mafic andesite — reflects partial emptying of a zoned system, halted when the crystal-rich magma became too viscous for explosive fragmentation. This zonation was probably brought about by convective separation of low-density, evolved magma from underlying mafic magma. Confinement of postclimactic eruptive activity to the caldera attests to continuing existence of the Mazama magmatic system.  相似文献   

15.
Taupo volcanic centre is one of two active rhyolite centres in the Taupo Volcanic Zone (TVZ), and has been sporadically active over the past ca. 300 ka. At least four large-scale ignimbrites have erupted from the centre, including the well documented 26.5 ka Oruanui ignimbrite and 1.8 ka Taupo ignimbrite. Because stratigraphy of earlier ignimbrites and their sources are masked by later volcanism, disrupted by regional tectonics and obscured by poor exposure, indirect methods must be applied in order to determine their source regions. In this paper detailed componentry, density and petrology of lithic fragments from three ignimbrites (Rangatira Point, Oruanui, Taupo) are used to reveal aspects of the sub-Taupo caldera geology, including the evolution of the Taupo volcanic centre, to assist in ignimbrite correlation and to evaluate structures within the Taupo caldera complex. Lithic fragments identify a complex subsurface geology. The Rangatira Point ignimbrite sampled dominantly rhyolite lavas, plus a variety of welded ignimbrites, rare high-silica dacites and a single dolerite. Most lithic fragments in the Oruanui ignimbrite are andesite with minor rhyolite, welded ignimbrite, dacite and rounded greywacke, while in the Taupo ignimbrite, rhyolite is again the dominant lithic component with subordinate welded ignimbrites, andesite, and greywacke. The densities of lithic fragments indicate similar ranges of values for all lava types, and thus density is a poor indicator of lithology. Care must, therefore, be taken before interpreting subcrustal stratigraphy using density as the sole criterion. The petrography and geochemistry of lithic types are more specific, and the variation can be used to identify sources for the ignimbrites. Both pumice chemistry and rhyolite lithic fragments from the Rangatira Point ignimbrite are comparable to domes exposed at the southern end of the Western Dome Complex and, combined with limited outcrop information, suggest the most likely source for this unit is in the northern part of the Taupo caldera complex. The dominance of andesite lithic fragments in the Oruanui ignimbrite suggests a major andesite cone existed beneath the source area, and the different lithic suites between Oruanui and Taupo ignimbrites suggest these ignimbrites came, at least in part, from mutually exclusive collapse structures. We believe that the Oruanui caldera is sited principally in the northwestern part of present-day Lake Taupo and the Taupo caldera in the northeastern part. Identification of abundant ignimbrite lithics in the Taupo ignimbrite, which are considered to represent an intracaldera facies of an earlier ignimbrite, that is not exposed at the surface, suggest there was a further (pre-Oruanui) ignimbrite caldera in the Taupo ignimbrite eruptive vent region.  相似文献   

16.
The eruption of Toba (75,000 years BP), Sumatra, is the largest magnitude eruption documented from the Quaternary. The eruption produced the largest-known caldera the dimensions of which are 100 × 30 km and which is surrounded by rhyolitic ignimbrite covering an area of over 20,000 km2. The associated deep-sea tephra layer is found in piston cores in the north-eastern Indian Ocean covering a minimum area of 5 × 106 km2. We have investigated the thickness, grain size and texture of the Toba deep-sea tephra layer in order to demonstrate the use of deep-sea tephra layers as a volcanological tool. The exceptional magnitude and intensity of the Toba eruption is demonstrated by comparison of these data with the deep-sea tephra layers associated with the eruptions of the Campanian ignimbrite, Italy and of Santorini, Greece in Minoan time. The volume of ignimbrite and distal tephra fall deposit produced in the Toba eruption are comparable, a total of at least 1000 km3 of dense rhyolitic magma. In contrast the volume of dense magma produced by the Campanian and Santorini eruptions are approximately 70 and 13 km3 respectively. Thickness versus distance data on the three deep-sea tephra layers show that eruptions of smaller magnitude than Santorini are unlikely to be preserved as distinct tephra layers in most deep-sea cores. In proximal cores all three tephra layers show two distinct units: a lower coarse-grained unit and an upper fine-grained unit. We interpret the lower unit as a plinian deposit and the upper unit as a co-ignimbrite ash-fall deposit, indicating two major eruptive phases. The Toba tephra layer is coarser both in maximum and median grain size than the Campanian and Santorini layers at a given distance from source. These data are interpreted to indicate a very high cruption column, estimated to be at least 45 km. We have applied a method for estimating the duration of the Toba eruption from the style of graded-bedding in deep-sea tephra layers. Studies of two cores yield estimates of 9 and 14 days. The eruption column height and duration estimates both indicate an average volume discharge rate of approximately 106 m3/sec. The Toba eruption therefore was not only of exceptional magnitude, but also of exceptional intensity.  相似文献   

17.
The small- to moderate-volume, Quaternary, Siwi pyroclastic sequence was erupted during formation of a 4 km-wide caldera on the eastern margin of Tanna, an island arc volcano in southern Vanuatu. This high-potassium, andesitic eruption followed a period of effusive basaltic andesite volcanism and represents the most felsic magma erupted from the volcano. The sequence is up to 13 m thick and can be traced in near-continuous outcrop over 11 km. Facies grade laterally from lithic-rich, partly welded spatter agglomerate along the caldera rim to two medial, pumiceous, non-welded ignimbrites that are separated by a layer of lithic-rich, spatter agglomerate. Juvenile clasts comprise a wide range of densities and grain sizes. They vary between black, incipiently vesicular, highly elongate spatter clasts that have breadcrusted pumiceous rinds and reach several metres across to silky, grey pumice lapilli. The pumice lapilli range from highly vesicular clasts with tube or coalesced spherical vesicles to denser finely vesicular clasts that include lithic fragments.Textural and lithofacies characteristics of the Siwi pyroclastic sequence suggest that the first phase of the eruption produced a base surge deposit and spatter-poor pumiceous ignimbrite. A voluminous eruption of spatter and lithic pyroclasts coincided with a relatively deep withdrawal of magma presumably driven by a catastrophic collapse of the magma chamber roof. During this phase, spatter clasts rapidly accumulated in the proximal zone largely as fallout, creating a variably welded and lithic-rich agglomerate. This phase was followed by the eruption of moderately to highly vesiculated magma that generated the most widespread, upper pumiceous ignimbrite. The combination of spatter and pumice in pyroclastic deposits from a single eruption appears to be related to highly explosive, magmatic eruptions involving low-viscosity magmas. The combination also indicates the coexistence of a spatter fountain and explosive eruption plume for much of the eruption.Editorial responsibility: R. Cioni  相似文献   

18.
Geology of the peralkaline volcano at Pantelleria,Strait of Sicily   总被引：1，自引：1，他引：1  
Situated in a submerged continental rift, Pantelleria is a volcanic island with a subaerial eruptive history longer than 300 Ka. Its eruptive behavior, edifice morphologies, and complex, multiunit geologic history are representative of strongly peralkaline centers. It is dominated by the 6-km-wide Cinque Denti caldera, which formed ca. 45 Ka ago during eruption of the Green Tuff, a strongly rheomorphic unit zoned from pantellerite to trachyte and consisting of falls, surges, and pyroclastic flows. Soon after collapse, trachyte lava flows from an intracaldera central vent built a broad cone that compensated isostatically for the volume of the caldera and nearly filled it. Progressive chemical evolution of the chamber between 45 and 18 Ka ago is recorded in the increasing peralkalinity of the youngest lava of the intracaldera trachyte cone and the few lavas erupted northwest of the caldera. Beginning about 18 Ka ago, inflation of the chamber opened old ring fractures and new radial fractures, along which recently differentiated pantellerite constructed more than 25 pumice cones and shields. Continued uplift raised the northwest half of the intracaldera trachyte cone 275 m, creating the island's present summit, Montagna Grande, by trapdoor uplift. Pantellerite erupted along the trapdoor faults and their hingeline, forming numerous pumice cones and agglutinate sheets as well as five lava domes. Degassing and drawdown of the upper pantelleritic part of a compositionally and thermally stratified magma chamber during this 18-3-Ka episode led to entrainment of subjacent, crystal-rich, pantelleritic trachyte magma as crenulate inclusions. Progressive mixing between host and inclusions resulted in a secular decrease in the degree of evolution of the 0.82 km3 of magma erupted during the episode.The 45-Ka-old caldera is nested within the La Vecchia caldera, which is thought to have formed around 114 Ka ago. This older caldera was filled by three widespread welded units erupted 106, 94, and 79 Ka ago. Reactivation of the ring fracture ca. 67 Ka ago is indicated by venting of a large pantellerite centero and a chain of small shields along the ring fault. For each of the two nested calderas, the onset of postcaldera ring-fracture volcanism coincides with a low stand of sea level.Rates of chemical regeneration within the chamber are rapid, the 3% crystallization/Ka of the post-Green Tuff period being typical. Highly evolved pantellerites are rare, however, because intervals between major eruptions (averaging 13–6 Ka during the last 190 Ka) are short. Benmoreites and mugearites are entirely lacking. Fe-Ti-rich alkalic basalts have erupted peripherally along NW-trending lineaments parallel to the enclosing rift but not within the nested calderas, suggesting that felsic magma persists beneath them. The most recent basaltic eruption (in 1891) took place 4 km northwest of Pantelleria, manifesting the long-term northwestward migration of the volcanic focus. These strongly differentiated basalts reflect low-pressure fractional crystallization of partial melts of garnet peridotite that coalesce in small magma reservoirs replenished only infrequently in this continental rift environment.  相似文献   

19.
Large silicic explosive eruptions are the most catastrophic volcanic events. Yet, the intratelluric mechanisms underlying are not fully understood. Here we report a field and laboratory study of the Kos Plateau Tuff (KPT, 161 ka, Aegean Volcanic Arc), which provides an excellent geological example of conduit processes that control magma vesiculation and fragmentation during intermediate- to large-scale caldera-forming eruptions. A prominent feature of the KPT is the occurrence of quite unusual platy-shaped tube pumice clasts in pyroclastic fall and current deposits from the early eruption phases preceding caldera collapse. On macroscopic and SEM observations, flat clast faces are elongated parallel to tube vesicles, while transverse surfaces often occur at ~ 45° to vesicle elongation. This peculiar pumice texture provides evidence of high shear stresses related to strong velocity gradients normal to conduit walls, which induced vesiculation and fragmentation of the ascending magma. Either an increasing mass discharge rate without adequate enlargement of a narrow central feeder conduit or a developing fissure-like feeder system related to incipient caldera collapse provided suitable conditions for the generation of plate tube pumice within magma volumes under high shear during the pre-climactic KPT eruption phases. This mechanism implies that the closer to the conduit walls (where the stronger are the velocity gradients) the larger was the proportion of plate vs. conventional (lensoid) juvenile fragments in the ascending gas–pyroclast mixture. Consequently, plate pumice clasts were mainly entrained in the outer portions of the jet and convecting regions of a sustained, Plinian-type, eruption column, as well as in occasional lateral blast currents generated at the vent. As a whole, plate pumice clasts in the peripheral portions of the column were transported at lower altitudes and deposited by fallout or partial collapse closer to the vent relative to lensoid ones that dominated in the inner column portions. The plate tube pumice proportion decreased abruptly up to disappearance during the emplacement of the main pyroclastic currents and lithic-rich breccias related to extensive caldera collapse at the eruption climax, as a consequence of an overall widening of the magma feeder system through the opening of multiple conduits and eruptive vents, along with fissure erosion, concomitant to the disruption of the collapsing block.  相似文献   

20.
During the past 1.2 m.y., a magma chamber of batholithic proportions has developed under the 100 by 30 km Toba Caldera Complex. Four separate eruptions have occurred from vents within the present collapse structure, which formed from eruption of the 2800 km3 Youngest Toba Tuff (YTT) at 74 ka. Eruption of the three older Toba Tuffs alternated from calderas situated in northern and southern portions of the present caldera. The northern caldera apparently developed upon a large andesitic stratovolcano. The calderas associated with the three older tuffs are obscured by caldera collapse and resurgence resulting from eruption of the YTT. Samosir Island and the Uluan Block are two sides of a single resurgent dome that has resurged since eruption of the YTT. Samosir Island is composed of thick YTT caldera fill, whereas the Uluan Block consists mainly of the Oldest Toba Tuff (OTT). In the past 74000 years lava domes have been extruded on Samosir Island and along the caldera's western ring fracture. This part of the ring fracture is the site of the only current activity at Toba: updoming and fumarolic activity. The Toba eruptions document the growth of the laterally continuous magma body which eventually erupted the YTT. Repose periods between the four Toba Tuffs range between 0.34 and 0.43 m.y. and give insights into pluton emplacement and magmatic evolution at Toba.  相似文献   

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