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1.
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
2.
The Campanian Ignimbrite (36000 years B.P.) was produced by the explosive eruption of at least 80 km3 DRE of trachytic ash and pumice which covered most of the southern Italian peninsula and the eastern Mediterranean region. The eruption has been related to the 12-x15-km-diameter caldera located in the Phlegraean Fields, west of Naples. Proximal deposits on the periphery of the Phlegraean Fields comprise the following pyroclastic sequence from base to top: densely welded ignimbrite and lithic-rich breccias (unit A); sintered ignimbrite, low-grade ignimbrite and lithic-rich breccia (unit B); lithic-rich breccia and spatter agglutinate (unit C); and low-grade ignimbrite (unit D). Stratigraphic and componentry data, as well as distribution of accidental lithic types and the composition of pumice clasts of different units, indicate that coarse, lithic-rich breccias were emplaced at different stages during the eruption. Lower breccias are associated with fines-rich ignimbrites and are interpreted as co-ignimbrite lag breccia deposits. The main breccia unit (C) does not grade into a fines-rich ignimbrite, and therefore is interpreted as formed from a distinct lithic-rich flow. Units A and B exhibit a similar pattern of accidental lithic types, indicating that they were erupted from the same area, probably in the E of the caldera. Units C and D display a distinct pattern of lithics indicating expulsion from vent(s) that cut different areas. We suggest that unit C was ejected from several vents during the main stage of caldera collapse. Field relationships between spatter agglutinate and the breccia support the possibility that these deposits were erupted contemporaneously from vents with different eruptive style. The breccia may have resulted from a combination of magmatic and hydrothermal explosive activity that accompanied extensive fracturing and subsidence of the magma-chamber roof. The spatter rags probably derived from sustained and vigorous pyroclastic fountains. We propose that the association lithic-rich breccia and spatter agglutinate records the occurrence of catastrophic piecemeal collapse. 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.
Non-welded, lithic-rich ignimbrites, hereintermed the Roque Nublo ignimbrites, are the most distinctive deposits of the Pliocene
Roque Nublo group, which forms the products of second magmatic cycle on Gran Canaria. They are very heterogeneous, with 35–55%
volume lithic fragments, 15-30% mildly vesiculated pumice, 5–7% crystals and 20–30% ash matrix. The vitric components (pumice
fragments and ash matrix) are largely altered and transformed into zeolites and subordinate smectites. The Roque Nublo ignimbrites
originated from hydrovolcanic eruptions that caused rapid and significant erosion of vents thus incorporating a high proportion
of lithic clasts into the eruption columns. These columns rapidly became too dense to be sustained as vertical eruption columns
and were transformed into tephra fountains which fed high-density pyroclastic flows. The deposits from these flows were mainly
confined to palaeovalleys and topographic depressions. In distal areas close to the coast line, where these palaeovalleys
widened, most of the pyroclastic flows expanded laterally and formed numerous thin flow units. The combined effect of the
magma–water interaction and the high content of lithic fragments is sufficient to explain the characteristic low emplacement
temperature of the Roque Nublo ignimbrites. This fact also explains the transition from pyroclastic flows into lahar deposits
observed in distal facies of the Roque Nublo ignimbrites. The existence of hydrovolcanic eruptions generating high-density
pyroclastic flows, unable to efficiently separate the water vapour from the vitric components during transport, also accounts
for the intense zeolitic alteration in these deposits.
Received: 5 November 1996 / Accepted: 3 March 1997 相似文献
5.
The Ohakune Craters form one of several parasitic centres surrounding Ruapehu volcano, at the southern end of the Taupo Volcanic Zone. An inner scoria cone and an outer, probably older, tuff ring are the principal structures in a nested cluster of four vents.The scoria cone consists of alternating lava flows and coarse, welded and unwelded, strombolian block and bomb beds. The strombolian beds consist of principally two discrete types of essential clast, vesicular bombs and dense angular blocks. Rare finer-grained beds are unusually block-rich. The tuff ring consists of alternating strombolian and phreatomagmatic units. Strombolian beds have similar grain size characteristics to scoria cone units, but contain more highly vesicular unoxidised bombs and few blocks. Phreatomagmatic deposits, which contain clasts with variable degrees of palagonitisation, consist of less well-sorted airfall deposits and very poorly sorted, crystal-rich pyroclastic surge deposits.Disruption by expanding magmatic gas bubbles was a major but relatively constant influence on both strombolian and phreatomagmatic eruptions at Ohakune. Instead, the nature of deposits was principally controlled by two other variables, vent geometry and the relative influence of external water during volcanism. During tuff-ring construction, magma is considered to have risen rapidly to the surface, and to have been ejected without sufficient residence time in the vent for non-explosive degassing. Availability of external water principally governed the eruption mechanism and hence the nature of the deposits. Essentials clasts of the scoria cone are, by comparison, dense, degassed and oxidised. It is suggested that a change in vent geometry, possibly the construction of the tuff ring itself, permitted lava ponding and degassing during scoria cone growth. During strombolian eruptions, magma remaining in the vent probably became depleted in gas, leading to the formation of an inert zone, or crust, above actively degassing magma. Subsequent explosions had therefore to disrupt both this passive crust and underlying, vesiculating magma “driving” the eruption. Cycles of strombolian eruption are thought to have stopped when the thickness of the inert crust precluded explosive eruption and only recommenced when some of this material was removed, either as a lava flow or during phreatomagmatic explosions when external water entered the vent. Such explosions probably formed the unusually fine-grained and block-rich beds in the strombolian sequence.The Ohakune deposits are an excellent example of the products of explosive eruption of fluid, gas-rich basic magma vesiculating under very near-surface conditions. A complex interplay of rate of magma rise, rate and depth of formation of gas bubbles, vent geometry, abundance of shallow external water, wind velocity and accumulation rate of ejecta determines the nature of deposits of such eruptions. 相似文献
6.
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
7.
This study investigates the types of subaqueous deposits that occur when hot pyroclastic flows turbulently mix with water
at the shoreline through field studies of the Znp marine tephra in Japan and flume experiments where hot tephra sample interacted
with water. The Znp is a very thick, pumice-rich density current deposit that was sourced from subaerial pyroclastic flows
entering the Japan Sea in the Pliocene. Notable characteristics are well-developed grain size and density grading (lithic-rich
base, pumice-rich middle, and ash-rich top), preponderance of sedimentary lithic clasts picked up from the seafloor during
transport, fine ash depletion in coarse facies, and presence of curviplanar pumice clasts. Flume experiments provide a framework
for interpreting the origin and proximity to source of the Znp tephra. On contact of hot tephra sample with water, steam explosions
produced a gas-supported pyroclastic density current that advanced over the water while a water-supported density current
was produced on the tank floor from the base of a turbulent mixing zone. Experimental deposits comprise proximal lithic breccia,
medial pumice breccia, and distal fine ash. Experiments undertaken with cold, water-saturated slurries of tephra sample and
water did not produce proximal lithic breccias but a medial basal lithic breccia beneath an upper pumice breccia. Results
suggest the characteristics and variations in Znp facies were strongly controlled by turbulent mixing and quenching, proximity
to the shoreline, and depositional setting within the basin. Presence of abundant curviplanar pumice clasts in submarine breccias
reflects brittle fracture and dismembering that can occur during fragmentation at the vent or during quenching. Subsequent
transport in water-supported pumiceous density currents preserves the fragmental textures. Careful study is needed to distinguish
the products of subaerial versus subaqueous eruptions. 相似文献
8.
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
9.
Bernardino Giannetti 《Journal of Volcanology and Geothermal Research》1996,71(1):53-72
The 227 ka Yellow Trachytic Tuff (YTT) of the Roccamonfina volcano is a multiunit ash-, pumice-, scoria- and lithic-ignimbrite with a proximal sandwave surge deposit. The YTT has an estimated volume of 0.42 km3. It erupted in the northern, subsided sector of the volcano from Gli Stagli caldera, and was channelled down ravines northward between the limestone range of M.Cesima and M. Camino that bounds the depression. Up to 5 YTT units occur close to the outer part of the northern rim of Gli Stagli. The basal four units are separated by lithic-rich marker layers which are inferred to result from gravity segregation followed by shearing. The first three units are consolidated by chabazite cementation, the fourth one is not consolidated. The uppermost unit is altered. One or two units characterize the YTT deposits in medial to distal zones. Here, the unconsolidated unit underlies the consolidated one. Absence of markers precludes correlation with proximal stratigraphy. The YTT is poorly sorted and, except the surge deposit and the altered faciés which are very fine-grained, has moderate median diameter typical of pyroclastic flows. Matrix, pumice, and scoria clasts are poorly vesicular. Matrix shards are equant, blocky-shaped, hydrated, and range from non-vesicular to vesicular. These features suggest that magma-water interaction played a role in the YTT eruption process, with some magmatic fragmentation.The complex near-Gli Stagli-rim YTT sequence could record the arrival of successive flows from the source vent, or also form by interaction of one or two flows with the caldera rim. In both cases, the absence of basal Plinian deposits in YTT units suggests that the eruptions were low pyroclastic fountains. The YTT distribution was controlled by interaction with the northern rim of Gli Stagli caldera and with the limestone range that bounds the northern depression. The near-rim stratigraphy shows the complete record of the eruption, whereas the medial to distal sequences provide only the initial pyroclastic flow possibly with the final flow spilling over the caldera rim. The proximal surge episode probably resulted from higher velocity of a later pyroclastic flow due to steeper slope of the volcano in that locality. 相似文献
10.
A devastating pyroclastic surge and resultant lahars at Mount St. Helens on 18 May 1980 produced several catastrophic flowages into tributaries on the northeast volcano flank. The tributaries channeled the flows to Smith Creek valley, which lies within the area devastated by the surge but was unaffected by the great debris avalanche on the north flank. Stratigraphy shows that the pyroclastic surge preceded the lahars; there is no notable “wet” character to the surge deposits. Therefore the lahars must have originated as snowmelt, not as ejected water-saturated debris that segregated from the pyroclastic surge as has been inferred for other flanks of the volcano. In stratigraphic order the Smith Creek valley-floor materials comprise (1) a complex valley-bottom facies of the pyroclastic surge and a related pyroclastic flow, (2) an unusual hummocky diamict caused by complex mixing of lahars with the dry pyroclastic debris, and (3) deposits of secondary pyroclastic flows. These units are capped by silt containing accretionary lapilli, which began falling from a rapidly expanding mushroom-shaped cloud 20 minutes after the eruption's onset. The Smith Creek valley-bottom pyroclastic facies consists of (a) a weakly graded basal bed of fines-poor granular sand, the deposit of a low-concentration lithic pyroclastic surge, and (b) a bed of very poorly sorted pebble to cobble gravel inversely graded near its base, the deposit of a high-concentration lithic pyroclastic flow. The surge apparently segregated while crossing the steep headwater tributaries of Smith Creek; large fragments that settled from the turbulent surge formed a dense pyroclastic flow along the valley floor that lagged behind the front of the overland surge. The unusual hummocky diamict as thick as 15 m contains large lithic clasts supported by a tough, brown muddy sand matrix like that of lahar deposits upvalley. This unit contains irregular friable lenses and pods meters in diameter, blocks incorporated from the underlying dry and hot pyroclastic material that had been deposited only moments earlier. The hummocky unit is the deposit of a high-viscosity debris flow which formed when lahars mingled with the pyroclastic materials on Smith Creek valley floor. Overlying the debris flow are voluminous pyroclastic deposits of pebbly sand cut by fines-poor gas-escape pipes and containing charred wood. The deposits are thickest in topographic lows along margins of the hummocky diamict. Emplaced several minutes after the hot surge had passed, this is the deposit of numerous secondary pyroclastic flows derived from surge material deposited unstably on steep valley sides. 相似文献
11.
We describe the stratigraphy, chronology, and grain size characteristics of the white trachytic tuff (WTT) of Roccamonfina Volcano (Italy). The pyroclastic rock was emplaced between 317 and 230 Ma BP during seven major eruptive events (units A to G) and three minor events (units BC, CD, and DE). These units are separated by paleosol layers and compositionally well-differentiated pyroclastic successions. Stratigraphic control is favored by the occurrence at the base of major units of marker layers. Four WTT units (1 to 4) occur within the central caldera. These are not positively correlated with specific extracaldera units.The source of most of the WTT units was the central caldera. Units B and C were controlled by the western wall of the caldera, whereas units D and E were able to overcome this barrier, spreading symmetrically along the flanks of MC. The maximum pumice size (MP) of units increases with distance from the caldera, whereas the maximum lithic size (ML) decreases. MP and ML of the marker layer of unit D (MKDa–MKDp) do not show any systematic variations with respect to the central caldera. In contrast, the thickness of surge MKDa decreases with distance from the source, and MKDp accumulates to the north of MC probably controlled, respectively, by mobility-transport power and by wind blowing northwards.The grain size characteristics of the WTT deposits are used for classifying the units. There is no systematic variation of the grain size as a function of stratigraphic height either among units or within single units. Large variation of components in subunit E1, with repetitive alternation of pyroclastic flow to surge through fallout vs. surge deposits, suggests that the process of eruption took place in a complex or piecemeal fashion.Pumice concentration zones (PCZ) occur at all WTT levels on the volcano, but they are much thicker and pumice clasts are much larger within the central caldera. These were probably originated by the disruption of lava (flow or dome) to pumice fragments and fine ash due to sudden depressurization and interaction with lake waters of the molten lava. Local basal PCZ are, in some cases, similar to the lapilli-rich “layer 1P” that has been described elsewhere, and may have been deposited from currents transitional between pyroclastic surge and flow. Other basal PCZ formed in response to small undulations in the substrate, or can be originated by fallout. Lenticular PCZ within ignimbrite interiors and tops are interpreted to record marginal pumice levees and pumice rafts, some of which were buried by subsequant pyroclastic flows.Lithic concentration zones (LCZ) also occur at various stratigraphic height within the extracaldera ignimbrites, whereas intracaldera LCZ are absent, probably due to the fact that ignimbrite currents are strongly energetic and erosive near vent. LCZ at the top of basal inversely graded layers are formed by mechanical sieving or dispersive pressure in response to variable velocity gradients and particle concentration gradients (a segregation process). Coarse LCZ and coarse lithic breccias (LB), that reside in the interior or tops of pyroclastic flows and that occur in medial to distal areas, are interpreted to be the result of slugs of lithic-rich debris introduced by vent collapse or rockslides into the moving pyroclastic flows along their flow paths. These LCZ become mixed to varying degrees due to differential densities and velocities relative to the pyroclastic flows (desegregation processes). 相似文献
12.
R. S. J. Sparks M. C. Gardeweg E. S. Calder S. J. Matthews 《Bulletin of Volcanology》1997,58(7):557-565
Pyroclastic flows generated in the 19–20 April 1993 eruption of Lascar Volcano, Chile, produced spectacular erosion features.
Scree and talus were stripped from the channels and steep slopes on the flanks of the volcano. Exposed bedrock and boulders
suffered severe abrasion, producing smoothed surfaces on coarse breccias and striations and percussion marks on bedrock and
large boulders. Erosional furrows developed with wavelengths of 0.5–2 m and depths of 0.1–0.3 m. Furrows commonly nucleated
downstream of large boulders or blocks, which are striated on the upstream side, and thereby produced crag-and-tail structures.
Erosive features were produced where flows accelerated through topographic restrictions or where they moved over steep slopes.
The pyroclastic flows are inferred to have segregated during movement into lithic-rich and pumice-rich parts. Lithic-rich
deposits occur on slopes up to 14°, whereas pumice-rich deposits occur only on slopes less than 4°, and mainly at the margins
and distal parts of the 1993 fan. The lithic-rich deposits contain large (up to 1 m) lithic clasts eroded from the substrate
and transported from the vent, whereas pumice-rich deposits contain only small (typically <2 cm) lithic clasts. These observations
suggest that lithic clasts segregated to the base of the flows and were responsible for much of the erosive phenomena. The
erosive features, distribution of lithic clasts and deposit morphology indicate that the 1993 flows were highly concentrated
avalanches dominated by particle interactions. In some places the flows slid over the bedrock causing abrasion and long striations
which imply that large blocks were locked in fixed positions for periods of about 1 s. However, shorter striae at different
angles, impact marks, segregation of the deposits into pumice- and lithic-rich parts, and mixing of bedrock-derived lithic
clasts throughout the deposits indicate that clasts often had some freedom of movement and that jostling of particles allowed
internal mixing and density segregation to occur within the flows.
Received: 15 July 1996 / Accepted: 15 January 1997 相似文献
13.
Fuji volcano is the largest active volcano in Japan, and consists of Ko-Fuji and Shin-Fuji volcanoes. Although basaltic in composition, small-volume pyroclastic flows have been repeatedly generated during the Younger stage of Shin-Fuji volcano. Deposits of those pyroclastic flows have been identified along multiple drainage valleys on the western flanks between 1,300 and 2,000 m a.s.l., and have been stratigraphically divided into the Shin-Fuji Younger pyroclastic flows (SYP) 1 to 4. Downstream debris flow deposits are found which contain abundant material derived from the pyroclastic flow deposits. The new14C ages for SYP1 to SYP4 are 3.2, 3.0, 2.9, and 2.5 ka, respectively, and correspond to a period where explosive summit eruptions generated many scoria fall deposits mostly toward the east. The SYP1 to SYP4 deposits consist of two facies: the massive facies is about 2 m thick and contains basaltic bombs of less than 50 cm in size, scoria lapilli, and fresh lithic basalt fragments supported in an ash matrix; the surge facies is represented by beds 1 to 15 cm thick, consisting mainly of ash with minor amount of fine lapilli. The bombs and scoria are 15 to 30% in volume within the massive facies. The ashes within the SYP deposits consist largely of comminuted basalt lithics and crystals that are derived from the Middle-stage lava flows exposed at the western flanks. SYP1 to SYP4 were only dispersed down the western flanks. The reason for this one-sided distribution is the asymmetric topography of the edifice; the western slopes of the volcano are the steepest (over 34 degrees). Most pyroclastic materials cannot rest stably on the slopes steeper than 33 degrees. Therefore, ejecta from the explosive summit eruptions that fell on the steep slopes tumbled down the slopes and were remobilized as high-temperature granular flows. These flows consisted of large pyroclastics and moved as granular avalanches along the valley bottom. Furthermore, the avalanching flows increased in volume by abrasion from the edifice and generated abundant ashes by the collision of clasts. The large amount of the fine material was presumably available within the transport system as the basal avalanches propagated below the angle of repose. Taking the typical kinetic friction coefficient of small pyroclastic flows, such flows could descend the western flanks where scattered houses are below 1,000 m a.s.l. A similar type of pyroclastic flow could result if explosive summit eruptions occur in the future.Editorial responsibility: R Cioni 相似文献
14.
The 274 ka “Basalt-Trachytic Tuff of Tuoripunzoli” (TBTT) from Roccamonfina volcano (Roman Region, Italy) consists of a basaltic scoria lapilli fall (Unit A) overlain by a trachytic sequence formed by a surge (Unit B), repetitive pumice lapilli and ash-rich layers both of fallout origin (Unit C) and a pyroclastic flow deposit (Unit D). The TBTT is widespread (40 km2) in the northern sector of the volcano, but limited to a small area on the southern slopes of the main cone. Interpolation between the northern deposits and the latter one yields a minimum depositional area of 123 km2, and an approximate bulk volume of 0.2-0.3 km3. Isopach and isopleth maps are consistent with a source vent within the main caldera of Roccamonfina.Unit A shows a fairly good sorting and a moderate grain size; glass fragments are cuspate and vesicular. Unit B is fine grained and poorly sorted; shards are blocky and nonvesicular. Pumice lapilli of Unit C are moderately sorted and moderately coarse grained. Glass shards are equant and vesicular. Lithic clasts are strongly comminuted to submillimetric sizes. By contrast, the ash-rich internal divisions are very fine grained and poorly sorted. They consist of a mixture of equant shards which are prevailingly blocky and poorly vesicular. Unit D is a massive, poorly sorted, moderately coarse-grained deposit. Glass fragments are nearly equant and slightly or nonvesicular.The TBTT is interpreted as due to eruption of a basaltic magma followed in rapid succession by one trachyte magma. Unit A formed by Subplinian fallout of a moderate, purely magmatic column. Interaction between a trachyte magma and water resulted in eruption of surge Unit B. A high-standing eruption column erupted alternating fallout pumice lapilli and fallout ashes. Pumice lapilli originated prevailingly from the inner part of the eruption column, whereas magma-water interaction on the external parts of the column resulted in ash fallout. The uppermost pyroclastic flow Unit D is interpreted as due to final collapse of the eruption column. 相似文献
15.
Nature and origin of cone-forming volcanic breccias in the Te Herenga Formation, Ruapehu, New Zealand 总被引：1，自引：1，他引：0
Volcanic breccias form large parts of composite volcanoes and are commonly viewed as containing pyroclastic fragments emplaced
by pyroclastic processes or redistributed as laharic deposits. Field study of cone-forming breccias of the andesitic middle
Pleistocene Te Herenga Formation on Ruapehu volcano, New Zealand, was complemented by paleomagnetic laboratory investigation
permitting estimation of emplacement temperatures of constituent breccia clasts. The observations and data collected suggest
that most breccias are autoclastic deposits. Five breccia types and subordinate, coherent lava-flow cores constitute nine,
unconformity-bounded constructional units. Two types of breccia are gradational with lava-flow cores. Red breccias gradational
with irregularly shaped lava-flow cores were emplaced at temperatures in excess of 580 °C and are interpreted as aa flow
breccias. Clasts in gray breccia gradational with tabular lava-flow cores, and in some places forming down-slope-dipping avalanche
bedding beneath flows, were emplaced at varying temperatures between 200 and 550 °C and are interpreted as forming part of
block lava flows. Three textural types of breccia are found in less intimate association with lava-flow cores. Matrix-poor,
well-sorted breccia can be traced upslope to lava-flow cores encased in autoclastic breccia. Unsorted boulder breccia comprises
constructional units lacking significant exposed lava-flow cores. Clasts in both of these breccia types have paleomagnetic
properties generally similar to those of the gray breccias gradational with lava-flow cores; they indicate reorientation after
acquisition of some, or all, magnetization and ultimate emplacement over a range of temperatures between 100 and 550 °C.
These breccias are interpreted as autoclastic breccias associated with block lava flows. Matrix-poor, well-sorted breccia
formed by disintegration of lava flows on steep slopes and unsorted boulder breccia is interpreted to represent channel-floor
and levee breccias for block lava flows that continued down slope. Less common, matrix-rich, stratified tuff breccias consisting
of angular blocks, minor scoria, and a conspicuously well-sorted ash matrix were generally emplaced at ambient temperature,
although some deposits contain clasts possibly emplaced at temperatures as high as 525 °C. These breccias are interpreted
as debris-flow and sheetwash deposits with a dominant pyroclastic matrix and containing clasts likely of mixed autoclastic
and pyroclastic origin. Pyroclastic deposits have limited preservation potential on the steep, proximal slopes of composite
volcanoes. Likewise, these steep slopes are more likely sites of erosion and transport by channeled or unconfined runoff rather
than depositional sites for reworked volcaniclastic debris. Autoclastic breccias need not be intimately associated with coherent
lava flows in single outcrops, and fine matrix can be of autoclastic rather than pyroclastic origin. In these cases, and likely
many other cases, the alternation of coherent lava flows and fragmental deposits defining composite volcanoes is better described
as interlayered lava-flow cores and cogenetic autoclastic breccias, rather than as interlayered lava flows and pyroclastic
beds. Reworked deposits are probably insignificant components of most proximal cone-forming sequences.
Received: 1 October 1998 / Accepted: 28 December 1998 相似文献
16.
The majority of tephra generated during the paroxysmal 1883 eruption of Krakatau volcano, Indonesia, was deposited in the sea within a 15-km radius of the caldera. Two syneruptive pyroclastic facies have been recovered in SCUBA cores which sampled the 1883 subaqueous pyroclastic deposit. The most commonly recovered facies is a massive textured, poorly sorted mixture of pumice and lithic lapilli-to-block-sized fragments set in a silty to sandy ash matrix. This facies is indistinguishable from the 1883 subaerial pyroclastic flow deposits preserved on the Krakatau islands on the basis of grain size and component abundances. A less common facies consists of well-sorted, planarlaminated to low-angle cross-bedded, vitric-enriched silty ash. Entrance of subaerial pyroclastic flows into the sea resulted in subaqueous deposition of the massive facies primarily by deceleration and sinking of highly concentrated, deflated components of pyroclastic flows as they traveled over water. The basal component of the deposit suggests no mixing with seawater as inferred from retention of the fine ash fraction, high temperature of emplacement, and lack of traction structures, and no significant hydraulic sorting of components. The laminated facies was most likely deposited from low-concentration pyroclastic density currents generated by shear along the boundary between the submarine pyroclastic flows and seawater. The Krakatau deposits are the first well-documented example of true submarine pyroclastic flow deposition from a modern eruption, and thus constitute an important analog for the interpretation of ancient sequences where subaqueous deposition has been inferred based on the facies characteristics of encapsulating sedimentary sequences. 相似文献
17.
Pyroclastic deposits as a guide for reconstructing the multi-stage evolution of the Somma-Vesuvius Caldera 总被引：1，自引：0，他引：1
The evolution of the Somma-Vesuvius caldera has been reconstructed based on geomorphic observations, detailed stratigraphic
studies, and the distribution and facies variations of pyroclastic and epiclastic deposits produced by the past 20,000 years
of volcanic activity. The present caldera is a multicyclic, nested structure related to the emptying of large, shallow reservoirs
during Plinian eruptions. The caldera cuts a stratovolcano whose original summit was at 1600–1900 m elevation, approximately
500 m north of the present crater. Four caldera-forming events have been recognized, each occurring during major Plinian eruptions
(18,300 BP "Pomici di Base", 8000 BP "Mercato Pumice", 3400 BP "Avellino Pumice" and AD 79 "Pompeii Pumice"). The timing of
each caldera collapse is defined by peculiar "collapse-marking" deposits, characterized by large amounts of lithic clasts
from the outer margins of the magma chamber and its apophysis as well as from the shallow volcanic and sedimentary units.
In proximal sites the deposits consist of coarse breccias resulting from emplacement of either dense pyroclastic flows (Pomici
di Base and Pompeii eruptions) or fall layers (Avellino eruption). During each caldera collapse, the destabilization of the
shallow magmatic system induced decompression of hydrothermal–magmatic and hydrothermal fluids hosted in the wall rocks. This
process, and the magma–ground water interaction triggered by the fracturing of the thick Mesozoic carbonate basement hosting
the aquifer system, strongly enhanced the explosivity of the eruptions.
Received: 24 November 1997 / Accepted: 23 March 1999 相似文献
18.
Two large (106–107 m3 erupted volume) hydrothermal explosions occurred from craters on the eastern margin of Kawerau Geothermal Field at c. 14,500 and 9,000 yrs B.P. Explosion products are interbedded within C14 dated pyroclastic fall deposits and contain clasts of hydrothermally altered ignimbrite, rhyolite and tuff, in a silty hydrothermal clay matrix. No magmatic ejecta are found. Some ejected blocks record earlier pre-eruption episodes of shallow hydraulic fracturing and silica cementation. Drillhole stratigraphy indicates that explosion extended to about 190 m below present ground level. The explosion is analysed as a rock/water interaction with eruptive energy provided by flashing of about half the available water. Although surface heat flow and shallow temperatures are now low at eastern Kawerau, the hydrothermal explosions demonstrate the previous existence of a high temperature shallow geothermal system, probably related to a major fault feeding water up through the basement. 相似文献
19.
The Yampa and Elkhead Mountains volcanic fields were erupted into sediment-filled fault basins during Miocene crustal extension in NW Colorado. Post-Miocene uplift and erosion has exposed alkali basalt lavas, pyroclastic deposits, volcanic necks and dykes which record hydrovolcanic and strombolian phenomena at different erosion depths. The occurrence of these different phenomena was related to the degree of lithification of the rocks through which the magmas rose. Hydrovolcanic interactions only occurred where rising basaltic magma encountered wet, porous, non-lithified sediments of the 600 m thick Miocene Brown's Park Formation. The interactions were fuelled by groundwater in these sediments: there was probably no standing surface water. Dykes intruded into the sediments have pillowed sides, and local swirled inclusions of sediment that were injected while fluidized in steam from heated pore water. Volcanic necks in the sediments consist of basaltic tuff, sediment blocks and separated grains derived from the sediments, lithic blocks (mostly derived from a conglomerate forming the local base of the Brown's Park Formation), and dykes composed of disaggregated sediment. The necks are cut by contemporaneous basalt dykes. Hydrovolcanic pyroclastic deposits formed tuff cones up to 100 m thick consisting of bedded air-fall, pyroclastic surge, and massive, poorly sorted deposits (MPSDs). All these contain sub-equal volumes of basaltic tuff and disaggregated sediment grains from the Brown's Park Formation. Possible explosive and effusive modes of formation for the MPSDs are discussed. Contemporaneous strombolian scoria deposits overlie lithified Cretaceous sedimentary rocks or thick basalt lavas. Volcanic necks intruded into the Cretaceous rocks consist of basalt clasts (some with spindle-shape), lithic clasts, and megacrysts derived from the magma, and are cut by basalt dykes. Rarely, strombolian deposits are interbedded with hydrovolcanic pyroclastic deposits, recording changes in eruption behaviour during one eruption. The hydrovolcanic eruptions occurred by interaction of magma with groundwater in the Brown's Park sediments. The explosive interactions disaggregated the sediment. Such direct digestion of sediment by the magma in the vents would probably not have released enough water to maintain a water/magma mass ratio sufficient for hydrovolcanic explosions to produce the tuff cones. Probably, additional water (perhaps 76% of the total) was derived by flow through the permeable sediments (especially the basal conglomerate to the formation), and into the vents. 相似文献
20.
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