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1.
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   

2.
The Scafell caldera-lake volcaniclastic succession is exceptionally well exposed. At the eastern margin of the caldera, a large andesitic explosive eruption (>5 km3) generated a high-mass-flux pyroclastic density current that flowed into the caldera lake for several hours and deposited the extensive Pavey Ark ignimbrite. The ignimbrite comprises a thick (≤125 m), proximal, spatter- and scoria-rich breccia that grades laterally and upwards into massive lapilli-tuff, which, in turn, is gradationally overlain by massive and normal-graded tuff showing evidence of soft-state disruption. The subaqueous pyroclastic current carried juvenile clasts ranging from fine ash to metre-scale blocks and from dense andesite through variably vesicular scoria to pumice (<103 kg m−3). Extreme ignimbrite lithofacies diversity resulted via particle segregation and selective deposition from the current. The lacustrine proximal ignimbrite breccia mainly comprises clast- to matrix-supported blocks and lapilli of vesicular andesite, but includes several layers rich in spatter (≤1.7 m diameter) that was emplaced in a ductile, hot state. In proximal locations, rapid deposition of the large and dense clasts caused displacement of interstitial fluid with elutriation of low-density lapilli and ash upwards, so that these particles were retained in the current and thus overpassed to medial and distal reaches. Medially, the lithofacies architecture records partial blocking, channelling and reflection of the depletive current by substantial basin-floor topography that included a lava dome and developing fault scarps. Diffuse layers reflect surging of the sustained current, and the overall normal grading reflects gradually waning flow with, finally, a transition to suspension sedimentation from an ash-choked water column. Fine to extremely fine tuff overlying the ignimbrite forms ∼25% of the whole and is the water-settled equivalent of co-ignimbrite ash; its great thickness (≤55 m) formed because the suspended ash was trapped within an enclosed basin and could not drift away. The ignimbrite architecture records widespread caldera subsidence during the eruption, involving volcanotectonic faulting of the lake floor. The eruption was partly driven by explosive disruption of a groundwater-hydrothermal system adjacent to the magma reservoir.  相似文献   

3.
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   

4.
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   

5.
 Coarse, co-ignimbrite lithic breccia, Ebx, occurs at the base of ignimbrite E, the most voluminous and widespread unit of the Kos Plateau Tuff (KPT) in Greece. Similar but generally less coarse-grained basal lithic breccias (Dbx) are also associated with the ignimbrites in the underlying D unit. Ebx shows considerable lateral variations in texture, geometry and contact relationships but is generally less than a few metres thick and comprises lithic clasts that are centimetres to a few metres in diameter in a matrix ranging from fines bearing (F2: 10 wt.%) to fines poor (F2: 0.1 wt.%). Lithic clasts are predominantly vent-derived andesite, although clasts derived locally from the underlying sedimentary formations are also present. There are no proximal exposures of KPT. There is a highly irregular lower erosional contact at the base of ignimbrite E at the closest exposures to the inferred vent, 10–14 km from the centre of the inferred source, but no Ebx was deposited. From 14 to <20 km from source, Ebx is present over a planar erosional contact. At 16 km Ebx is a 3-m-thick, coarse, fines-poor lithic breccia separated from the overlying fines-bearing, pumiceous ignimbrite by a sharp contact. This grades downcurrent into a lithic breccia that comprises a mixture of coarse lithic clasts, pumice and ash, or into a thinner one-clast-thick lithic breccia that grades upward into relatively lithic-poor, pumiceous ignimbrite. Distally, 27 to <36 km from source Ebx is a finer one-clast-thick lithic breccia that overlies a non-erosional base. A downcurrent change from strongly erosional to depositional basal contacts of Ebx dominantly reflects a depletive pyroclastic density current. Initially, the front of the flow was highly energetic and scoured tens of metres into the underlying deposits. Once deposition of the lithic clasts began, local topography influenced the geometry and distribution of Ebx, and in some cases Ebx was deposited only on topographic crests and slopes on the lee-side of ridges. The KPT ignimbrites also contain discontinuous lithic-rich layers within texturally uniform pumiceous ignimbrite. These intra-ignimbrite lithic breccias are finer grained and thinner than the basal lithic breccias and overlie non-erosional basal contacts. The proportion of fine ash within the KPT lithic breccias is heterogeneous and is attributed to a combination of fluidisation within the leading part of the flow, turbulence induced locally by interaction with topography, flushing by steam generated by passage of pyroclastic density currents over and deposition onto wet mud, and to self-fluidisation accompanying the settling of coarse, dense lithic clasts. There are problems in interpreting the KPT lithic breccias as conventional co-ignimbrite lithic breccias. These problems arise in part from the inherent assumption in conventional models that pyroclastic flows are highly concentrated, non-turbulent systems that deposit en masse. The KPT coarse basal lithic breccias are more readily interpreted in terms of aggradation from stratified, waning pyroclastic density currents and from variations in lithic clast supply from source. Received: 21 April 1997 / Accepted: 4 October 1997  相似文献   

6.
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   

7.
The young non-welded Taupo ignimbrite shows remarkable lateral variations which are documented by granulometric and component analyses, and studies of maximum clast size and density. The grain size spans practically the entire known ignimbrite field, the coarser proximal ignimbrite having a median diameter 100 times greater than the finest distal ignimbrite. The content and maximum size of lithic fragments decrease also by a factor of 100 between proximal and distal parts. The content of free crystals first rises to reach a peak, but thereafter decreases to attain a very low value in far-distal exposures. The pumice maximum size decreases by a factor of about 10, and the most conspicuously coarse pumice rocks occur in a girdle nearly halfway out from vent to distal limit. The pumice in each grain size class decreases in density to half of its near-source value in distal ignimbrite. The overall outward trend is towards an ignimbrite which consists wholly of fine vitric ash; some distal exposures closely approach this condition.These variations are accounted for by a combination of processes operating in the moving ash flow. One is a continuous fragmentation of pumice leading to a rounding of the clasts, a progressive decrease in maximum size, the generation of much vitric dust, and the liberation of crystals. Another is a continuous sedimentation of heavy constituents (lithics and crystals), and an antipathetic rise of lighter coarse pumice towards the top of the flow. These processes operated in a moving flow whose upper layers travelled progressively farther from source; it is the topmost layers, strongly depleted in heavy constituents and enriched in light pumice, which have travelled the farthest and constitute the far-distal parts of the ignimbrite.A number of ignimbrite facies are characterized: the ignimbrite proper, with its proximal, distal, and pumice concentration zone facies; the deposits which form in the head and are then over-ridden by the body of the flow, including the fines-depleted ignimbrite variant and the heavies-enriched ground layer; and the ignimbrite veneer deposits which are left behind by the flow, which differ little from the ignimbrite except in their landscape-mantling form and the occurrence in them of lee-side coarse pumice lenses.  相似文献   

8.
The Peperino Albano (approximately 19–36 ka old) is a phreatomagmatic pyroclastic flow deposit, cropping out along the slopes of the associated Albano maar (Colli Albani volcano, Italy). The deposit exhibits lateral and vertical transitions from valley pond to veneer facies, as well as intracrater facies. We present the results of a paleomagnetic study of thermal remanent magnetization (TRM) of the lithic clasts of the Peperino Albano ignimbrite that provide quantitative estimates of the range of emplacement temperatures across the different facies of the ignimbrite. Emplacement temperatures estimated for the Peperino Albano ignimbrite range between 240° and 350°C, with the temperatures defined in the intracrater facies being generally lower than in the valley pond and veneer facies. This is possibly due to the large size of the sampled clasts in the intracrater facies which, when coupled with low temperature at the vent, were not completely heated throughout their volume during emplacement. The emplacement temperatures derived from the paleomagnetic results are in good agreement with the presence of un-burnt plants at the base of the ignimbrite, indicating that the temperature of the pyroclastic flow was lower than the temperature of ignition of wood. Paleomagnetic results from the Peperino Albano confirm the reliability of the paleomagnetic approach in defining the thermal history of pyroclastic flow deposits.  相似文献   

9.
Palaeomagnetic data from lithic clasts collected at 46 sites within layers 1 and 2 of the 1.8-ka Taupo ignimbrite, New Zealand, have been used to determine the palaeotemperatures and thermal structure of the deposit on its emplacement. Equilibrium temperatures from sites less than 30–40 km from vent are 150–300 °C, whereas at greater distances site equilibrium temperatures increase up to 400–500 °C. This variation is seen in both layer 1 and 2 deposits, with values for layer 1 being somewhat cooler, and with its increase in temperature occurring at a greater distance from vent. A temperature maximum at ~50 km from vent coincides with a zone of pink thermal-oxidation colouration of pumices previously inferred to reflect higher emplacement temperatures. Additional palaeomagnetic data collected by us and others from pumice clasts show comparable temperature variations, but these temperature estimates are shown here to be due to a chemical remanence and unreliable for accurate temperature estimates. Cooler temperatures in proximal parts of the ignimbrite are consistent with admixture of >20% cold lithic clasts at source and interaction with the pre-eruption Lake Taupo. The similar, but offset, increases in equilibrium temperatures for medial and distal layers 1 and 2 are consistent with both layers being deposited from the same flow. However, any proximal deposits left by the later, hotter material must have been subsequently eroded, or be so thin that our collection failed to sample them. Radial asymmetries in equilibrium temperatures as well as other physical parameters suggest that the deposit emplacement temperature is primarily determined at source, rather than by interaction with air during transport. These data support previous interpretations that a concentrated basal flow played a dominant role in emplacement and deposition of the Taupo ignimbrite.Editorial responsibility: T. Druitt  相似文献   

10.
The 2.08-Ma Cerro Galán Ignimbrite (CGI) represents a >630-km3 dense rock equivalent (VEI 8) eruption from the long-lived Cerro Galán magma system (∼6 Ma). It is a crystal-rich (35–60%), pumice (<10% generally) and lithic-poor (<5% generally) rhyodacitic ignimbrite, lacking a preceding plinian fallout deposit. The CGI is preserved up to 80 km from the structural margins of the caldera, but almost certainly was deposited up to 100 km from the caldera in some places. Only one emplacement unit is preserved in proximal to medial settings and in most distal settings, suggesting constant flow conditions, but where the pyroclastic flow moved into a palaeotopography of substantial valleys and ridges, it interacted with valley walls, resulting in flow instabilities that generated multiple depositional units, often separated by pyroclastic surge deposits. The CGI preserves a widespread sub-horizontal fabric, defined by aligned elongate pumice and lithic clasts, and minerals (e.g. biotite). A sub-horizontal anisotropy of magnetic susceptibility fabric is defined by minute magnetic minerals in all localities where it has been analysed. The CGI is poor in both vent-derived (‘accessory’) lithics and locally derived lithics from the ground surface (‘accidental’) lithics. Locally derived lithics are small (<20 cm) and were not transported far from source points. All data suggest that the pyroclastic flow system producing the CGI was characterised throughout by high sedimentation rates, resulting from high particle concentration and suppressed turbulence at the depositional boundary layer, despite being a low aspect ratio ignimbrite. Based on these features, we question whether high velocity and momentum are necessary to account for extensive flow mobility. It is proposed that the CGI was deposited by a pyroclastic flow system that developed a substantial, high particle concentration granular under-flow, which flowed with suppressed turbulence. High particle concentration and fine-ash content hindered gas loss and maintained flow mobility. In order to explain the contemporaneous maintenance of high particle concentration, high sedimentation rate at the depositional boundary layer and a high level of mobility, it is also proposed that the flow(s) was continuously supplied at a high mass feeding rate. It is also proposed that internal gas pressure within the flow, directed downwards onto the substrate over which the flow was passing, reduced the friction between the flow and the substrate and also enhanced its mobility. The pervasive sub-horizontal fabric of aligned pumice, lithic and even biotite crystals indicates a consistent horizontal shear force existed during transport and deposition in the basal granular flow, consistent with the existence of a laminar, shearing, granular flow regime during the final stages of transport and deposition.  相似文献   

11.
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   

12.
The 0.196 Ma, lithic-rich Abrigo Ignimbrite on Tenerife, Canary Islands contains localised massive, coarse pumice-rich ignimbrite lobes (MPRILs). They typically form low ridges up to 2 m high with axes parallel to the flow direction, and, in cross-section, they range from symmetrical to asymmetrical and highly skewed lobate bedforms generally with flat bases. The major components are rounded pebble- to cobble-sized phonolitic pumice clasts within an ignimbritic matrix of ash, fine lithics and minor crystals, which varies from lithic-rich to lithic-poor. Commonly, there is a vertical increase in pumice concentration from matrix-supported texture at the base to clast-supported at the top, accompanied by an increase in pumice clast size. MPRILs often thin and grade laterally perpendicular to current flow into planar pumice concentration zones. They occur at one or more stratigraphic levels as either solitary lobes associated with flat topography or as multiple onlapping lobes or within a laterally complex stratified pumice-rich ignimbrite facies (LCSPIs) near palaeotopographic highs.MPRILs are original depositional features, not erosional in origin and are derived from a larger pyroclastic flow. It is likely that pumice was segregated to the upper and outer regions of the parent flow causing a significant rheological contrast with the lower lithic-rich zone. The more pumice-rich parts are interpreted to have detached from the parent flow as it decelerated onto gentler slopes or interacted with topographic highs and raced ahead as mobile derivative pyroclastic flows. The flow-parallel ridge shape of MPRILs may be a result of fingering within these flows or concentration of pumice within the intermediary clefts. Deposition occurred “en masse” at the termination of the flow front. The resultant lobate deposits were then overridden and mantled by normal ignimbrite facies from either a later flow pulse or the following main part of the parent flow.  相似文献   

13.
The term “ignimbrite veneer deposit” (IVD) is proposed for a new kind of pyroclastic deposit which is found associated with, and passes laterally into, Taupo ignimbrite of valley pond type in New Zealand. It forms a thin layer mantling the landscape over 15,000 km2, and is regarded as the deposit from the trailing “tail” of a pyroclastic flow, where a relaxation of shear stress favoured the deposition of the basal part of the flow. The IVD differs little in grain-size from the associated ignimbrite, but it shows a crude internal stratification attributed to the deposition of a succession of layers, one after the passage of each pulse of the pyroclastic flow. It locally contains laterally-discontinuous lenses of coarse pumice (“lee-side lenses”) on the far-vent side of topographic obstacles. In nearvent exposures the Taupo IVD shows lensoid and cross-stratified bed-forms even where it stands on a planar surface, attributed to deposition from a flow travelling at an exceedingly high velocity.An IVD can be distinguished from a poorly sorted pyroclastic fall deposit because the beds in it show more rapid lateral variations in thickness, it may show a low-angle cross-stratification, and it contains carbonised wood from trees not in the position of growth; from the deposit of a wet base surge because it lacks vesicles and strong antidune-like structures and contains carbonised vegetation, and from a hot and dry pyroclastic surge deposit because it possesses a high content of pumice and “fines”.The significance of an IVD is that it records the passage of a pyroclastic flow, where the flow itself has moved farther on.  相似文献   

14.
A geological, chemical and petrographical study of the Campanian ignimbrite, a pyroclastic flow deposit erupted about 30,000 years ago on the Neapolitan area (Italy), is reported. The ignimbrite covered an area of at least 7,000 km2; it consists of a single flow unit, and the lateral variations in both pumice and lithic fragments indicate that the source was located in the Phlegraean Fields area. Textural features, areal distribution and its morphological constraints suggests that the eruption was of the type of highly expanded low-temperature pyroclastic cloud. The original composition was strongly modified by post-depositional chemical changes involving most of the major and trace elements. No primary differences in the composition of the magma have been recognized. The Campanian ignimbrite is a nearly saturated potassic trachyte, similar to many other trachytes of the Quaternary volcanic province of Campania. Its chemistry indicates an affinity with the so-called «low-K association» of the Roman volcanic province.  相似文献   

15.
The late-seventeenth century BC Minoan eruption of Santorini discharged 30–60 km3 of magma, and caldera collapse deepened and widened the existing 22 ka caldera. A study of juvenile, cognate, and accidental components in the eruption products provides new constraints on vent development during the five eruptive phases, and on the processes that initiated the eruption. The eruption began with subplinian (phase 0) and plinian (phase 1) phases from a vent on a NE–SW fault line that bisects the volcanic field. During phase 1, the magma fragmentation level dropped from the surface to the level of subvolcanic basement and magmatic intrusions. The fragmentation level shallowed again, and the vent migrated northwards (during phase 2) into the flooded 22 ka caldera. The eruption then became strongly phreatomagmatic and discharged low-temperature ignimbrite containing abundant fragments of post-22 ka, pre-Minoan intracaldera lavas (phase 3). Phase 4 discharged hot, fluidized pyroclastic flows from subaerial vents and constructed three main ignimbrite fans (northwestern, eastern, and southern) around the volcano. The first phase-4 flows were discharged from a vent, or vents, in the northern half of the volcanic field, and laid down lithic-block-rich ignimbrite and lag breccias across much of the NW fan. About a tenth of the lithic debris in these flows was subvolcanic basement. New subaerial vents then opened up, probably across much of the volcanic field, and finer-grained ignimbrite was discharged to form the E and S fans. If major caldera collapse took place during the eruption, it probably occurred during phase 4. Three juvenile components were discharged during the eruption—a volumetrically dominant rhyodacitic pumice and two andesitic components: microphenocryst-rich andesitic pumices and quenched andesitic enclaves. The microphenocryst-rich pumices form a textural, mineralogical, chemical, and thermal continuum with co-erupted hornblende diorite nodules, and together they are interpreted as the contents of a small, variably crystallized intrusion that was fragmented and discharged during the eruption, mostly during phases 0 and 1. The microphenocryst-rich pumices, hornblende diorite, andesitic enclaves, and fragments of pre-Minoan intracaldera andesitic lava together form a chemically distinct suite of Ba-rich, Zr-poor andesites that is unique in the products of Santorini since 530 ka. Once the Minoan magma reservoir was primed for eruption by recharge-generated pressurization, the rhyodacite moved upwards by exploiting the plane of weakness offered by the pre-existing andesite–diorite intrusion, dragging some of the crystal-rich contents of the intrusion with it.  相似文献   

16.
The November 13, 1985 eruption of Nevado del Ruiz produced a series of pyroclastic flows and surges that eroded channels on the surface of the summit glacier and generated lahars which descended down most of the rivers that drain the volcano. The stratigraphy of the proximal pyroclastic deposits indicates that there were at least four episodes to the eruption. Episode I, deposited an unusual surge consisting of small pieces of ice mixed with ash and exhibiting planar stratification. Ballistically emplaced fragments are also intercalated with this unit. During Episode II, at least two pyroclastic flows were erupted. Their deposits contain the most evolved pumice of the entire eruption; SiO2 content of matrix glass ranges between 74.5 and 74.9%. Episode III is marked by the emplacement of a welded tuff with an average SiO2 content of about 66% in the matrix glass. The final Episode IV was characterized by the development of a high-altitude eruption column and the emplacement of several nonwelded pyroclastic flows. Banded pumice are common in the pyroclastic flow as well as in the pumice fall deposits. Co-existing dark and light pumice bands differ in SiO2 content by 3.5% and in general are similar to the composition of the welded pumice from Episode III.The compositional zonation of the pyroclastic deposits from Episode I to IV suggests that a nearsurface compositionally-stratified portion of the magma body was tapped during Episode II. During Episodes III and IV the main body of magma was involved although the coexistence of the compositionally distinct pumice clasts at similar stratigraphic levels argues for mixing of magma from different levels in the chamber during the eruptive process.  相似文献   

17.
A model is presented for the emplacement of intermediate volume ignimbrites based on a study of two 6 km3 volume ignimbrites on Roccamonfina Volcano, Italy. The model considers that the flows were slow moving, and quickly deflated from turbulent to non-turbulent conditions. Yield strength and density increased whereas fluidisation decreased with time and runout of the pyroclastic flows. In proximal locations, on the caldera rim, heterogeneous exposures including discontinuous lithic breccias, stratified and cross-stratified units interbedded with massive ignimbrite suggest deposition from turbulent flows. In medial locations thick, massive ignimbrite occurs associated with three types of co-ignimbrite lithic breccia which we interpret as being emplaced by non-turbulent flows. Multiple grading of different breccia/lithic concentration types within single flow units indicates that internal shear occurred producing overriding or overlapping of the rear of the flow onto the slower-moving front part. This overriding of different parts of non-turbulent pyroclastic flows could be caused by at least two different mechanisms: (1) changes in flow regime, such as hydraulic jumps that may occur at breaks in slope; and (2) periods of increased discharge rate, possibly associated with caldera collapse, producing fresh pulses of lithic-rich material that sheared onto the slower-moving part of the flow in front.We propose that ground surge deposits enriched in pumice compared with their associated ignimbrite probably formed by a flow separation mechanism from the top and front of the pyroclastic flow. These turbulent clouds moved ahead of the non-turbulent lower part of the flow to form stratified pumice-rich deposits. In distal regions well-developed coarse, often clast-supported, pumice concentrations zones and coarse intra-flow-unit lithic concentrations occur within the massive ignimbrite. We suggest that the flows were non-turbulent, possessed a relatively high yield strength and may have moved by plug flow prior to emplacement.  相似文献   

18.
Very thick units of massive pumice and lithic clast-rich breccia in the Early Permian Berserker beds at Mount Chalmers, Queensland, are deposits from cold, water-supported, volcaniclastic mass flows emplaced in a below-wave base submarine setting. Adjacent to syn-volcanic andesitic and rhyolitic sills and dykes, the pumice-lithic breccia shows a well-developed eutaxitic texture. The eutaxitic foliation is parallel to intrusive contacts and extends as far as a few metres away from the contact. At these sites, pumice clasts are strongly flattened and tube vesicles within the pumice clasts are compacted and aligned parallel to the direction of flattening. Some lenticular pumice clasts contain small (2 mm), round, quartz-filled amygdales and spherulites. Further away from the sills and dykes, the pumice clasts have randomly oriented, delicate tube vesicle structure and are blocky or lensoid in shape. Round amygdales were generated by re-vesiculation of the glass and the spherulites indicate devitrification of the glass at relatively high temperatures. The eutaxitic texture is therefore attributed to re-heating and welding compaction of glassy pumice-lithic breccia close to contacts with intrusions. In cases involving sills, secondary welding along the contacts formed extensive, conformable, eutaxitic zones in the pumice-lithic breccia that could be mistaken for primary welding compaction in a hot, primary pyroclastic deposit.  相似文献   

19.
The Tiribí Tuff covered much of the Valle Central of Costa Rica, currently the most densely populated area in the country (∼2.4 million inhabitants). Underlying the tuff, there is a related well-sorted pumice deposit, the Tibás Pumice Layer. Based on macroscopic characteristics of the rocks, we distinguish two main facies in the Tiribí Tuff in correlation to the differences in welding, devitrification, grain size, and abundance of pumice and lithic fragments. The Valle Central facies consists of an ignimbritic plateau of non-welded to welded deposits within the Valle Central basin and the Orotina facies is a gray to light-bluish gray, densely to partially welded rock, with yellowish and black pumice fragments cropping out mainly at the Grande de Tárcoles River Gorge and Orotina plain. This high-aspect ratio ignimbrite (1:920 or 1.1×10−3) covered an area of at least 820 km2 with a long runout of 80 km and a minimum volume outflow of 25 km3 (15 km3 DRE). Geochemically, the tuff shows a wide range of compositions from basaltic-andesites to rhyolites, but trachyandesites are predominant. Replicate new 40Ar/39Ar age determinations indicate that widespread exposures of this tuff represent a single ignimbrite that was erupted 322±2 ka. The inferred source is the Barva Caldera, as interpreted from isopach and isopleth maps, contours of the ignimbrite top and geochemical correlation (∼10 km in diameter). The Tiribí Tuff caldera-forming eruption is interpreted as having evolved from a plinian eruption, during which the widespread basal pumice fall was deposited, followed by fountaining pyroclastic flows. In the SW part of the Valle Central, the ignimbrite flowed into a narrow canyon, which might have acted as a pseudo-barrier, reflecting the flow back towards the source and thus thickening the deposits that were filling the Valle Central depression. The variable welding patterns are interpreted to be a result of the lithostatic load and the influence of the content and size of lithic fragments.  相似文献   

20.
By applying a number of analytical techniques across a spectrum of spatial scales (centimeter to micrometer) in juvenile components, we show that the Cerro Galán volcanic system has repeatedly erupted magmas with nearly identical geochemistries over >3.5 Myr. The Cerro Galán system produced nine ignimbrites (∼5.6 to 2 Ma) with a cumulative volume of >1,200 km3 (DRE; dense rock equivalent) of calc-alkaline, high-K rhyodacitic magmas (68–71 wt.% SiO2). The mineralogy is broadly constant throughout the eruptive sequence, comprising plagioclase, quartz, biotite, Fe–Ti oxides, apatite, and titanite. Early ignimbrite magmas also contained amphibole, while the final eruption, the most voluminous Cerro Galán ignimbrite (CGI; 2.08 ± 0.02 Ma) erupted a magma containing rare amphibole, but significant sanidine. Each ignimbrite contains two main juvenile clast types; dominant “white” pumice and ubiquitous but subordinate “grey” pumice. Fe–Ti oxide and amphibole-plagioclase thermometry coupled with amphibole barometry suggest that the grey pumice originated from potentially hotter and deeper magmas (800–840°C, 3–5 kbar) than the more voluminous white pumice (770–810°C, 1.5–2.5 kbar). The grey pumice is interpreted to represent the parental magmas to the Galán system emplaced into the upper crust from a deeper storage zone. Most inter-ignimbrite variations can be accounted for by differences in modal mineralogy and crystal contents that vary from 40 to 55 vol.% on a vesicle-free basis. Geochemical modeling shows that subtle bulk-rock variations in Ta, Y, Nb, Dy, and Yb between the Galán ignimbrites can be reconciled with differences in amounts of crystal fractionation from the “grey” parent magma. The amount of fractionation is inversely correlated with volume; the CGI (∼630 km3) and Real Grande Ignimbrite (∼390 km3) return higher F values (proportion of liquid remaining) than the older Toconquis Group ignimbrites (<50 km3), implying less crystal fractionation took place during the upper-crustal evolution of these larger volume magmas. We attribute this relationship to variations in magma chamber geometry; the younger, largest volume ignimbrites came from flat sill-like magma chambers, reducing the relative proportion of sidewall crystallization and fractionation compared to the older, smaller-volume ignimbrite eruptions. The grey pumice clasts also show evidence of silicic recharge throughout the history of the Cerro Galán system, and recharge days prior to eruption has previously been suggested based on reversely zoned (OH and Cl) apatite phenocrysts. A rare population of plagioclase phenocrysts with thin An-rich rims in juvenile clasts in many ignimbrites supports the importance of recharge in the evolution and potential triggering of eruptions. This study extends the notion that large volumes of nearly identical silicic magmas can be generated repeatedly, producing prolonged geochemical homogeneity from a long-lived magma source in a subduction zone volcanic setting. At Cerro Galán, we propose that there is a zone between mantle magma input and upper crustal chambers, where magmas are geochemically “buffered”, producing the underlying geochemical and isotopic signatures. This produces the same parental magmas that are delivered repeatedly to the upper crust. A lower-crustal MASH (melting, assimilation, storage, and homogenization) zone is proposed to act as this buffer zone. Subsequent upper crustal magmatic processes serve only to slightly modify the geochemistry of the magmas.  相似文献   

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