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1.
Susan L. Donoghue Alan S. Palmer Elizabeth McClelland Kate Hobson Robert B. Stewart Vincent E. Neall Jèrôme Lecointre Richard Price 《Bulletin of Volcanology》1999,61(4):223-240
The ca. 10,500 years B.P. eruptions at Ruapehu volcano deposited 0.2–0.3 km3 of tephra on the flanks of Ruapehu and the surrounding ring plain and generated the only known pyroclastic flows from this
volcano in the late Quaternary. Evidence of the eruptions is recorded in the stratigraphy of the volcanic ring plain and cone,
where pyroclastic flow deposits and several lithologically similar tephra deposits are identified. These deposits are grouped
into the newly defined Taurewa Formation and two members, Okupata Member (tephra-fall deposits) and Pourahu Member (pyroclastic
flow deposits). These eruptions identify a brief (<ca. 2000-year) but explosive period of volcanism at Ruapehu, which we define
as the Taurewa Eruptive Episode. This Episode represents the largest event within Ruapehu's ca. 22,500-year eruptive history
and also marks its culmination in activity ca. 10,000 years B.P. Following this episode, Ruapehu volcano entered a ca. 8000-year
period of relative quiescence. We propose that the episode began with the eruption of small-volume pyroclastic flows triggered
by a magma-mingling event. Flows from this event travelled down valleys east and west of Ruapehu onto the upper volcanic ring
plain, where their distal remnants are preserved. The genesis of these deposits is inferred from the remanent magnetisation
of pumice and lithic clasts. We envisage contemporaneous eruption and emplacement of distal pumice-rich tephras and proximal
welded tuff deposits. The potential for generation of pyroclastic flows during plinian eruptions at Ruapehu has not been previously
considered in hazard assessments at this volcano. Recognition of these events in the volcanological record is thus an important
new factor in future risk assessments and mitigation of volcanic risk at Tongariro Volcanic Centre.
Received: 5 July 1998 / Accepted: 12 March 1999 相似文献
2.
Guido Giordano 《Bulletin of Volcanology》1998,60(1):10-26
The Quaternary White Trachytic Tuffs Formation from Roccamonfina Volcano (southern Italy) comprises four non-welded, trachytic,
pyroclastic sequences bounded by paleosols, each of which corresponds to small- to intermediate-volume explosive eruptions
from central vents. From oldest to youngest they are: White Trachytic Tuff (WTT) Cupa, WTT Aulpi, WTT S. Clemente, and WTT
Galluccio. The WTT Galluccio eruption was the largest and emplaced ∼ 4 km3 of magma. The internal stratigraphy of all four WTT eruptive units is a complex association of fallout, surge, and pyroclastic
flow deposits. Each eruptive unit is organized into two facies associations, Facies Association A below Facies Association
B. The emplacement of the two facies associations may have been separated by short time breaks allowing for limited reworking
and erosion. Facies Association A consists of interbedded fallout deposits, surge deposits, and subordinate ignimbrites. This
facies association involved the eruption of the most evolved trachytic magma, and pumice clasts are white and well vesiculated.
The grain size coarsens upward in Facies Association A, with upward increases of dune bedform wavelengths and a decrease in
the proportion of fine ash. These trends could reflect an increase in eruption column height from the onset of the eruption
and possibly also in mass eruption rate. Facies Association B comprises massive ignimbrites that are progressively richer
in lithic clast content. This association involved the eruption of more mafic magma, and pumice clasts are gray and poorly
vesiculated. Facies Association B is interpreted to record the climax of the eruption. Phreatomagmatic deposits occur at different
stratigraphic levels in the four WTT and have different facies characteristics. The deposits reflect the style and degree
of magma–water interaction and the local hydrogeology. Very fine-grained, lithic-poor phreatomagmatic surge deposits found
at the base of WTT Cupa and WTT Galluccio could record the interaction of the erupting magma with a lake that occupied the
Roccamonfina summit depression. Renewed magma–water interaction later in the WTT Galluccio eruption is indicated by fine grained,
lithic-bearing phreatomagmatic fall and surge deposits occurring at the top of Facies Association A. They could be interpreted
to reflect shifts of the magma fragmentation level to highly transmissive, regional aquifers located beneath the Roccamonfina
edifice, possibly heralding a caldera collapse event.
Received: 26 August 1996 / Accepted: 27 February 1998 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.
New insights into Late Pleistocene explosive volcanic activity and caldera formation on Ischia (southern Italy) 总被引：2，自引：1，他引：1
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.
Natalia Pardo Shane J. Cronin Alan S. Palmer Karoly Németh 《Bulletin of Volcanology》2012,74(3):617-640
We analysed the tephra record of Mt. Ruapehu for the period 27,097 ± 957 to ~10,000 cal. years BP to determine the largest-scale
explosive eruptions expected from the most active New Zealand andesitic volcano. From the lithostratigraphic analysis, a systematic
change in the explosive behaviour is identified from older deposits suggesting dry magmatic eruptions and steady eruptive
columns, characterised by frothy to expanded pumice fabrics, to younger deposits that are products of unsteady conditions
and collapsing columns, characterised by microvesicular, fibrous, and colour-banded pumice fabrics. The end-members were separated
by eruptions with steady columns linked to water–magma interaction and highly unstable conduit walls. Dry magmatic eruptions
producing steady plinian columns were most common between 27,097 ± 957 and shortly after 13,635 + 165 cal. years BP. Following
this time, activity continued with eruptions that produced dominantly oscillating unsteady columns, which engendered pyroclastic
density currents, until ~10 ka when there was an abrupt transition at Mt. Ruapehu since which eruptions have been an order
of magnitude lower in intensity and volume. These data demonstrate long-period transitions in eruption behaviour at an andesitic
stratovolcano, which is critical to understand if realistic time-variable hazard forecasts are to be developed. 相似文献
6.
The chronology of deposits of the 1976 eruption of Augustine volcano, which produced pyroclastic falls, pyroclastic flows, and lava domes, is determined by correlating the stratigraphy with published records of seismicity, plume observations, and distant ash falls. Three thin air-fall ash beds (unit A1, A2 and A3) correlate with events near the beginning of the 1976 eruption on 22 and 23 January. On 24 January a small-volume, ash-cloud-surge deposit (unit S) accumulated over the north half of Augustine Island. A series of pumiceous pyroclastic flows represented by the lobate pumiceous deposits (unit F) occurred on 24 January and locally melted the snowpack to cause small pumice-laden floods. A thin ash bed (unit A4) was deposited on 24 January, and the main plinian eruption (unit P) occurred on 25 January. In middle to late February and again in mid April, lava domes were extruded at the summit accompanied by incandescent block-and-ash flows down the north flank. A hut near the north coast of the island was mechanically and thermally damaged by the small-volume ash-cloud surge of unit S before the eruption of the pumice flow of unit F; the metal roof was then penetrated by lithic fragments of the plinian fall of 25 January. Explosive eruptions in the early stage of an eruption-like that which deposited unit S — are important hazards at Augustine Island, as are infrequent debris avalanches and attendant tsunamis.deceased on 18 May 1980 相似文献
7.
A distinctive type of fine-grained air-fall ash is found intimately associated with many ignimbrites. They have crystal/glass ratios systematically lower than artificially crushed pumice from the same ignimbrites. The crystal enrichment found in crystal-bearing ignimbrites indicates substantial losses of the vitric component, amounting to an average of at least 35% by weight of the original juvenile material, and this lost material is believed to occur in the ash-fall deposits. These ashes thus complement ignimbrite, and are here called “co-ignimbrite ashes”. The loss is believed to take place during ignimbrite eruptions as a result of: (1) the escape of fine ash and gas above a collapsing eruptive column; (2) the preferential entry of fine vitric ash into an upper turbulent cloud when (immediately following column collapse) the segregation of a dense pyroclastic flow from an initially highly turbulent, low-concentation density flow takes place; (3) the elutriation of fine vitric ash (generated in part within the pyroclastic flow) from the fluidised flow. Ash from all three mechanisms would be expected to rise to a great height in convective plumes and be dispersed by winds to produce extensive, vitric-enriched ash-fall deposits.The data indicate that the co-ignimbrite ashes must have volumes comparable with those of ignimbrites, and examples are given of particularly large ash-fall deposits (including some found in deep-sea cores) associated with large ignimbrites which may be of this type rather than fall-out from a preceding plinian phase as hitherto thought. 相似文献
8.
The Pliocene Roque Nublo Series, the second of three major magmatic series on Gran Canaria (Canary Islands), consists of a lower sequence (200 m) of alkalic lavas (basanite to phonolite) and a thicker upper section (600 m) of interlayered lava and widespread breccia sheets: encompassing pyroclastic flow deposits, lahars and reworked epiclastic rocks. Components in the poorly sorted block — and ash — flow deposits are (unwelded) pumice, rock fragments, crystals, glass shards and, locally, bread-crust bombs. Some flow units are graded with fine-grained basal zones and lithic-rich lower and pumice-rich upper parts. Some have strongly grooved the underlying rocks, directions of these striations being independent of preexisting topography and are constant in direction for more than 5 km. The flows are thought to have been emplaced below minimum welding temperatures by collapse of eruption columns. They are similar in many respects to coarse-grained pyroclastic flow deposits found in andesite volcanoes. Glass of tephritic to phonolitic composition of clasts of the breccias is generally altered to «palagonite» and is partly replaced by clay minerals and zeolites (mainly chabazite and phillipsite). Palagonitization was a low temperature diagenetic process, resulting in the hydration of glass accompanied and followed by precipitation of zeolites and clay minerals. Electron-microprobe data suggest the following decreasing order of mobility of selected elements during palagonitization: Na, K, Al, Si, Ca, Mg, and Fe; Ti was assumed to be inert. 相似文献
9.
Kenichiro Tani Richard S. Fiske Yoshihiko Tamura Yukari Kido Jiro Naka Hiroshi Shukuno Rika Takeuchi 《Bulletin of Volcanology》2008,70(5):547-562
Sumisu volcano was the site of an eruption during 30–60 ka that introduced ∼48–50 km3 of rhyolite tephra into the open-ocean environment at the front of the Izu-Bonin arc. The resulting caldera is 8 × 10 km
in diameter, has steep inner walls 550–780 m high, and a floor averaging 900 m below sea level. In the course of five research
cruises to the Sumisu area, a manned submersible, two ROVs, a Deep-Tow camera sled, and dredge samples were used to study
the caldera and surrounding areas. These studies were augmented by newly acquired single-channel seismic profiles and multi-beam
seafloor swath-mapping. Caldera-wall traverses show that pre-caldera eruptions built a complex of overlapping dacitic and
basaltic edifices, that eventually grew above sea level to form an island about 200 m high. The caldera-forming eruption began
on the island and probably produced a large eruption column. We interpret that prodigious rates of tephra fallback overwhelmed
the Sumisu area, forming huge rafts of floating pumice, choking the nearby water column with hyperconcentrations of slowly
settling tephra, and generating pyroclastic gravity currents of water-saturated pumice that traveled downslope along the sea
floor. Thick, compositionally similar pumice deposits encountered in ODP Leg 126 cores 70 km to the south could have been
deposited by these gravity currents. The caldera-rim, presently at ocean depths of 100–400 m, is mantled by an extensive layer
of coarse dense lithic clasts, but syn-caldera pumice deposits are only thin and locally preserved. The paucity of syn-caldera
pumice could be due to the combined effects of proximal non-deposition and later erosion by strong ocean currents. Post-caldera
edifice instability resulted in the collapse of a 15° sector of the eastern caldera rim and the formation of bathymetrically
conspicuous wavy slump structures that disturb much of the volcano’s surface. 相似文献
10.
Batur volcanic field (BVF) in Bali, Indonesia, underwent two successive caldera-forming eruptions, CI and CII (29,300 and 20,150 years b.p., respectively) that resulted in the deposition of dacitic ignimbrites. The respective ignimbrites show contrasted stratigraphies, exemplify the variability of dynamics associated with caldera-forming eruptions and provide insights into the possible controls exerted by caldera collapse mechanisms. The Ubud Ignimbrite is widespread and covers most of southern Bali. The deposits consist dominantly of pyroclastic flow with minor pumice fall deposits. The intra-caldera succession comprises three distinct, partially to densely welded cooling units separated by non-welded pyroclastic flow and fall deposits. The three cooling units consist of pyroclastic flow deposits only and together represent up to 16 distinct flow units, each including a thin, basal, lithic-rich breccia. This eruption was related to a 13.5×10 km caldera (CI) with a minimum collapsed volume of 62 km3. The floor of caldera CI is inferred to have a piecemeal geometry. The Ubud Ignimbrite is interpreted as the product of a relatively long-lasting, pulsating, collapsing fountain that underwent at least two time breaks. A stable column developed during the second time break. Discharge rate was high overall, but oscillatory, and increased toward the end of the eruption. These dynamics are thought to reflect sequential collapse of the CI structure. The Gunungkawi Ignimbrite is of more limited extent outside the source caldera and occurs only in central southern Bali. The Gunungkawi Ignimbrite proximal deposits consist of interbedded accretionary lapilli-bearing ash surge, ash fall, pumice lapilli fall and thin pyroclastic flow deposits, overlain by a thick and massive pyroclastic flow deposit with a thick basal lag breccia. The caldera (CII) is 7.5×6 km in size, with a minimum collapsed volume of 9 km3. The CII eruption included two distinct phases. During the first, eruption intensity was low to moderate and an unstable, essentially phreatomagmatic column developed. During the second phase, the onset of caldera collapse drastically increased the eruption intensity, resulting in column collapse. The caldera floor is believed to have subsided rapidly, producing a single, short-lived burst of high eruption intensity that resulted in the deposition of the uppermost massive pyroclastic flow.Editorial responsibility: T. Druitt 相似文献
11.
Gerardo J. Aguirre-Díaz 《Bulletin of Volcanology》2001,63(4):238-251
The Amealco Tuff is a widespread (>2880 km2), trachyandesitic to rhyolitic pyroclastic deposit in the central Mexican Volcanic Belt that was erupted from the Amealco caldera at 4.7ǂ.1 Ma. It includes three major ignimbrites, each showing complex mingling of pumice fragments and matrix glass with andesitic to rhyolitic compositions. The different glasses are well mingled throughout each of the pyroclastic-flow deposits. Mingling of glasses may have occurred just before and during the explosive eruptions that produced the pyroclastic flows, as the distinct melts had insufficient time to homogenize. Mingling of glasses is evident in each of the three separate major ignimbrites of the Amealco Tuff; thus, the processes that caused it were repetitive. It is infered that the repetitive mingling of melts was due to repeated mafic magma inputs to an evolved magma chamber. 相似文献
12.
The volcanic history of Ruapehu during the past 2 millennia based on the record of Tufa Trig tephras
Tufa Trig Formation comprises a sequence of at least 19 andesitic tephras erupted from Mt. Ruapehu (Tongariro Volcanic Centre,
New Zealand). Tephras of Tufa Trig Formation are the most recent eruptives from Ruapehu, dated between ca. 1850 years B.P.
and the present. Members of the Formation show restricted dispersals, principally to the east of Mt. Ruapehu. Volumes calculated
for the most widespread members are all less than 0.1 km3. Compared with other Mt. Ruapehu eruptives, Tufa Trig Formation tephras represent small eruptions that have contributed little
tephra to the ring plain. They do, however, show a greater frequency of eruption with one event occurring on average every
100 years. Tufa Trig Formation members Tf3–Tf18 are black to dark grey, vitric, coarse-ash and lapilli-grade tephras which
mantle the relief. They contain juvenile vitric particles which exhibit varying degrees of vesicularity, together with free
crystals of pyroxene and feldspar, and few lithic fragments. Several morphological types of vitric pyroclasts are recognised
in these tephras, the dominant type being of equant blocky morphology with fracture-bound surfaces (type-1 morphology). Field
characteristics, tephra distributions, and the morphologies and textures of constituent pyroclasts suggest that these members
(Tf3–Tf18) are the products of small-volume hydrovolcanic eruptions resulting from the interaction of fresh magma and meteoric
water. We propose that a source of this water was an ancestral crater lake which formed within the late Holocene ca. 3000
years B.P. The morphological, compositional, and chemical (major-element) characteristics of three Tufa Trig Formation Tephras
are compared with those of two new tephras erupted from Ruapehu Volcano during the October 1995 eruptions which comprise part
of a newly defined member (Tf19) of Tufa Trig Formation. The comparisons support our interpretation that the majority of the
Tufa Trig Formation tephras are primarily the products of hydrovolcanic eruptions. Other members of the Formation (Tf1 and
Tf2) are coarse-grained scoriaceous tephras and are interpreted to be the products of strombolian events.
Received: 14 September 1996 / Accepted: 6 June 1997 相似文献
13.
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
14.
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
15.
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
16.
Evdokia Tema Despina Kondopoulou Spyros Pavlides 《Studia Geophysica et Geodaetica》2013,57(4):627-646
Thermal remanent magnetization analyses were carried out on ceramic fragments and lithic clasts embedded in the first pumice fall deposits of the Minoan eruption. The aim of this study is to estimate the equilibrium temperature after deposition of these pyroclastic fall deposits and their thermal effect on the pre-Minoan surface. A total of 30 samples from 22 independent ceramic fragments and 20 samples from 14 lithic clasts have been studied. Samples were collected from the Megalochori Quarry, located at the southern part of Santorini island. Stepwise thermal demagnetization reveals that the ceramics were mostly re-heated at temperatures around 140–180°C; in few ceramics a higher temperature component is also present, probably related to the original heating or the use of the ceramics before the eruption. Thermal demagnetization of the lithic clasts shows similar results with slightly higher re-heating temperatures, around 180–240°C. The estimated temperatures represent the equilibrium temperatures obtained after the deposition of the pumice fall and show that the pyroclastic fall deposits at a distance of around 6 km from the eruption vent maintained a temperature high enough to re-heat the buried ceramics at temperatures around 140–180°C. 相似文献
17.
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
18.
19.
Basal layered deposits of the large-volume Peach Springs Tuff occur beneath the main pyroclastic flow deposit over a minimum lateral distance of 70 km in northwestern Arizona (USA). The basal deposits are interpreted to record initial blasting and pyroclastic surge events at the beginning of the eruption; the pyroclastic surges traveled a minimum of 100 km from the (as yet unknown) source. Changes in bedding structures with increasing flow distance are related to the decreasing sediment load of the surges. Some bed forms in the most proximal part of the study area (Kingman, Arizona) can be interpreted as being shock induced, reflecting a blast origin for the surges. Component analyses support a hydrovolcanic origin for some of the blasting and subsequent pyroclastic surges. The eruption apparently began with magmatic blasts, which were replaced by hydrovolcanic blasts. Hydrovolcanic activity may be partially related to failure of the conduit walls that temporarily plugged the vent. A single large-volume pyroclastic flow immediately followed the blast phase, and no evidence has been observed for a Plinian eruption column. The stratigraphic sequence indicates that powerful hydrovolcanic blasting rapidly widened the vent, thus bypassing a Plinian fallout phase and causing rapid evolution to a collapsing eruption column. Similar processes may occur in other large-volume ignimbrite eruptions, which commonly lack significant Plinian fallout deposits. 相似文献
20.
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