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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.
Julie M. Palais Philip R. Kyle William C. McIntosh Diane Seward 《Journal of Volcanology and Geothermal Research》1988,35(4)
The morphology, grain size characteristics and composition of ash particles in 30 ka to 150 ka tephra layers from the Byrd ice core were examined to characterize the eruptions which produced them and to test the suggestion that they were erupted from Mt. Takahe, a shield volcano in Marie Byrd Land, West Antarctica. Volcanic deposits at Mt. Takahe were examined for evidence of recent activity which could correlate with the tephra layers in the ice core.Coarse- and fine-ash layers have been recognized in the Byrd ice core. The coarse-ash layers have a higher mass concentration than the fine-ash layers and are characterized by fresh glass shards > 50 μm diameter, many containing elongate pipe vesicles. The fine-ash layers have a lower mass concentration and contain a greater variety of particles, typically < 20 μm diameter. Many of these particles are aggregate grains composed of glass and crystal fragments showing S and Cl surface alteration. The grain-size distributions of the coarse and fine-ash layers overlap, in part because of the aggregate nature of grains in the fine-ash layers. The coarse-ash layers are interpreted as having formed by magmatic eruption whereas the fine-ash layers are believed to be hydrovolcanic in origin.Mt. Takahe is the favored source for the tephra because: (a) chemical analyses of samples from the volcano are distinctive, being peralkaline trachyte, and similar in composition to the analyzed tephra; (b) Mt. Takahe is a young volcano (< 0.3 Ma); (c) pyroclastic deposits on Mt. Takahe indicate styles of eruption similar to that inferred for the ice core tephra; and (d) Mt. Takahe is only about 350 km from the calculated site of tephra deposition.A speculative eruptive history for Mt. Takahe is established by combining observations from Mt. Takahe and the Byrd ice core tephra. Initial eruptions at Mt. Takahe were subglacial and then graded into alternating subaerial and subglacial activity. The tephra suggest alternating subaerial magmatic and hydrovolcanic eruptions from 30 to 20 ka B.P., followed by a sustained period of hydrovolcanic eruptions from 20 to 14 ka B.P., which peaked at 18 ka B.P. 相似文献
3.
The Azores archipelago is one of the most active volcanic areas in the North Atlantic region, with approximately 30 eruptions during the last 600 years. The geochemical composition of associated tephra-derived glass is, however, not well characterized. This study presents major element compositions of glass shards from five major eruptives on the Azores: a trachybasaltic eruptive on the island of Faial (Capelinhos AD, 1957) and four explosive trachytic eruptives on the island of São Miguel (Fogo A c. 5600 cal yrs. BP, Sete Cidades c. AD 1440, Fogo AD 1563 and Furnas AD 1630). The major element compositions suggest that tephras from three active stratovolcanoes on São Miguel, Sete Cidades, Fogo and Furnas, can be distinguished from one another using bi-plots of FeOtot vs. TiO2 and FeOtot vs. CaO. Late Holocene tephras found on Ireland have previously been attributed to eruptions occurring on Jan Mayen but possess a strong geochemical similarity to proximal tephras from the Azores, especially those from the Furnas volcano. The similarity of the proximal tephras on São Miguel, especially Furnas AD 1563 and Furnas AD 1630 and distal tephras in Ireland is demonstrated by strong similarity coefficients (>0.95) and the closeness of major element composition. The dominant wind direction over the Azores is favourable for tephra dispersal to western Europe and we suggest that at least three tephras found in Ireland were erupted from the Furnas volcano, and that trachytic tephras erupted from explosive eruptions on São Miguel have a potential to contribute to the construction of a European-wide tephrostratigraphic framework. 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.
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.
M. Nakagawa K. Wada T. Thordarson C. P. Wood J. A. Gamble 《Bulletin of Volcanology》1999,61(1-2):15-31
Ruapehu volcano erupted intermittently between September and November 1995, and June and July 1996, producing juvenile andesitic
scoria and bombs. The volcanic activity was characterized by small, sequential phreatomagmatic and strombolian eruptions.
The petrography and geochemistry of dated samples from 1995 (initial magmatic eruption of 18 September 1995, and two larger
events on 23 September and 11 October), and from 1996 (initial and larger eruptions on 17–18 June) suggest that episodes of
magma mixing occurred in separate magma pockets within the upper part of the magma plumbing system, producing juvenile andesitic
magma by mixing between relatively high (1000–1200 °C)- and low (∼1000 °C)- temperature (T) end members. Oscillatory zoning
in pyroxene phenocrysts suggests that repeated mixing events occurred prior to and during the 1995 and 1996 eruptions. Although
the 1995 and 1996 andesitic magmas are products of similar mixing processes, they display chronological variations in phenocryst
clinopyroxene, matrix glass, and whole-rock compositions. A comparison of the chemistry of magnesian clinopyroxene in the
four tephras indicates that, from 18 September through June 1996, the tephras were derived from at least two discrete high-temperature
(high-T) batches of magma. Crystals of magnesian clinopyroxene in the 23 September and 11 October tephras appear to be derived
from different high-T magma batches. Whole-rock and matrix-glass compositions of all tephras are consistent with their derivation
from distinct mixed melts. We propose that, prior to 1995 there was a shallow low-temperature (low-T) magma storage system
comprising crystal-rich mush and remnant magma from preceding eruptive episodes. Crystal clots and gabbroic inclusions in
the tephras attest to the existence of relict crystal mush. At least two discrete high-T magmas were then repeatedly injected
into the mush zone, forming discrete and mixed magma pockets within the shallow system. The intermittent 1995 and 1996 eruptions
sequentially tapped these magma pockets.
Received: 1 April 1998 / Accepted: 22 December 1998 相似文献
7.
Field, geochronologic, and geochemical evidence from proximal fine-grained tephras, and from limited exposures of Holocene
lava flows and a small pyroclastic flow document ten–12 eruptions of Mount Rainier over the last 2,600 years, contrasting
with previously published evidence for only 11–12 eruptions of the volcano for all of the Holocene. Except for the pumiceous
subplinian C event of 2,200 cal year BP, the late-Holocene eruptions were weakly explosive, involving lava effusions and at
least two block-and-ash pyroclastic flows. Eruptions were clustered from ∼2,600 to ∼2,200 cal year BP, an interval referred
to as the Summerland eruptive period that includes the youngest lava effusion from the volcano. Thin, fine-grained tephras
are the only known primary volcanic products from eruptions near 1,500 and 1,000 cal year BP, but these and earlier eruptions
were penecontemporaneous with far-traveled lahars, probably created from newly erupted materials melting snow and glacial
ice. The most recent magmatic eruption of Mount Rainier, documented geochemically, was the 1,000 cal year BP event. Products
from a proposed eruption of Mount Rainier between AD 1820 and 1854 (X tephra of Mullineaux (US Geol Surv Bull 1326:1–83, 1974))
are redeposited C tephra, probably transported onto young moraines by snow avalanches, and do not record a nineteenth century
eruption. We found no conclusive evidence for an eruption associated with the clay-rich Electron Mudflow of ∼500 cal year
BP, and though rare, non-eruptive collapse of unstable edifice flanks remains as a potential hazard from Mount Rainier.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
T. W. Sisson and J. W. Vallance contributed equally to this study. 相似文献
8.
Contemporaneous Plinian eruptions of rhyolite pumice from Glass Mountain and Little Glass Mountain during the last 1100 years B.P. were followed by extrusion of lava flows. 1.2 km of material was erupted and 10% by volume is tephra. All of the tephra deposits consist of very poorly sorted coarse ash and lapilli that are mostly pumice pyroclasts.Eruptive sequences, chemical composition and petrographic character of the rhyolites at Little Glass Mountain and Glass Mountain suggest that they came from the same magma body. The 1:9 ratio of tephra to lavas is typical of small silicic magma chambers. Eruption from a small chamber, 4–6 km deep, at vents 15 km apart is possible if magma rose along cone sheets with dips of 45–60°. The caldera rim and arcuate lines of vents near it may represent the surface expression of several concentric cone sheets.Pumice pyroclasts erupted at Glass Mountain and Little Glass Mountain may have formed in the following manner: (1) vesicle growth and coalescence beginning at 1–2 km depths; (2) elongation of the vesicles by flow within the cone sheets; (3) disruption of the vesiculated magma when it reached the surface by an expansion wave passing down through it; and (4) eruption of comminution products as pumice pyroclasts. Plinian activity at Little Glass Mountain and Glass Mountain continued until the volatile-rich top of the magma chamber had been depleted. 相似文献
9.
Numerous volcanoes in the Afar Triangle and adjacent Ethiopian Rift Valley have erupted during the Quaternary, depositing volcanic ash (tephra) horizons that have provided crucial chronology for archaeological sites in eastern Africa. However, late Pleistocene and Holocene tephras have hitherto been largely unstudied and the more recent volcanic history of Ethiopia remains poorly constrained. Here, we use sediments from lakes Ashenge and Hayk (Ethiopian Highlands) to construct the first <17 cal ka BP tephrostratigraphy for the Afar Triangle. The tephra record reveals 21 visible and crypto-tephra layers, and our new database of major and trace element glass compositions will aid the future identification of these tephra layers from proximal to distal locations. Tephra compositions include comendites, pantellerites and minor peraluminous and metaluminous rhyolites. Variable and distinct glass compositions of the tephra layers indicate they may have been erupted from as many as seven volcanoes, most likely located in the Afar Triangle. Between 15.3−1.6 cal. ka BP, explosive eruptions occurred at a return period of <1000 years. The majority of tephras are dated at 7.5−1.6 cal. ka BP, possibly reflecting a peak in regional volcanic activity. These findings demonstrate the potential and necessity for further study to construct a comprehensive tephra framework. Such tephrostratigraphic work will support the understanding of volcanic hazards in this rapidly developing region. 相似文献
10.
Numerous volcanoes in the Afar Triangle and adjacent Ethiopian Rift Valley have erupted during the Quaternary, depositing volcanic ash (tephra) horizons that have provided crucial chronology for archaeological sites in eastern Africa. However, late Pleistocene and Holocene tephras have hitherto been largely unstudied and the more recent volcanic history of Ethiopia remains poorly constrained. Here, we use sediments from lakes Ashenge and Hayk (Ethiopian Highlands) to construct the first <17 cal ka BP tephrostratigraphy for the Afar Triangle. The tephra record reveals 21 visible and crypto-tephra layers, and our new database of major and trace element glass compositions will aid the future identification of these tephra layers from proximal to distal locations. Tephra compositions include comendites, pantellerites and minor peraluminous and metaluminous rhyolites. Variable and distinct glass compositions of the tephra layers indicate they may have been erupted from as many as seven volcanoes, most likely located in the Afar Triangle. Between 15.3−1.6 cal. ka BP, explosive eruptions occurred at a return period of <1000 years. The majority of tephras are dated at 7.5−1.6 cal. ka BP, possibly reflecting a peak in regional volcanic activity. These findings demonstrate the potential and necessity for further study to construct a comprehensive tephra framework. Such tephrostratigraphic work will support the understanding of volcanic hazards in this rapidly developing region. 相似文献
11.
12.
Canonical discriminant function analysis was employed to discriminate between electron microprobe-determined titanomagnetite
and hornblende analyses from Egmont volcano and Tongariro Volcanic Centre. Data sets of 436 titanomagnetite and 206 hornblende
analyses from the two sources were used for the study. Titanomagnetite chemistry provided the best discrimination between
these two sources with classification efficiencies of 99% for sample averages and 95% for individual analyses. The difference
between sources for hornblende chemistry was less marked, but classification efficiencies of 100% for sample averages and
87% for individual analyses were achieved. Using the same methods a preliminary discrimination of individual Egmont volcano-sourced
tephras was attempted. Titanomagnetite chemistry enabled the discrimination of several individual tephras or at least pairs
of tephra units, but hornblende chemistry provided little discrimination. This technique provides an improvement on previous
methods for chemically distinguishing distal tephra from the two sources as well as potentially identifying individual tephras
from a particular source. A major advantage over previous discrimination techniques is that individual analyses can be classified
with a known probability of group membership (with groups such as volcano source or an individual tephra unit). Tephras in
a depositional environment where mixing is common such as within soil, loess and marine sequences, can be sourced or identified
more easily with classification of individual grains.
Received: 19 July 1995 / Accepted: 13 February 1996 相似文献
13.
David A. Clague Jennifer B. Paduan Alice S. Davis 《Journal of Volcanology and Geothermal Research》2009,180(2-4):171
Glassy lava fragments were collected in pushcores or using a small suction-sampler from over 450 sites along the Juan de Fuca Ridge, Blanco Transform Fault, Gorda Ridge, northern East Pacific Rise, southern East Pacific Rise, Fiji back-arc basin, and near-ridge seamounts in the Vance, President Jackson, Taney, and a seamount off southern California. The samples consist of angular glass fragments, limu o Pele, Pele's hair, and other fluidal fragments formed during pyroclastic eruptions. Since many of the sites are deeper than the critical point of seawater, fragmentation cannot be hydrovolcanic and caused by expansion of seawater to steam. The glass fragments have a wide range of MORB compositions, ranging from fractionated to primitive and from depleted to enriched. Enriched magmas, which have higher volatile contents, may form more abundant pyroclasts than depleted magmas. Eruptions with high effusion rates produce sheet flows and abundant pyroclasts whereas those with low effusion rates produce pillow ridges and few pyroclasts. This relation suggests that high effusion and conduit rise rates are coupled to high magmatic gas contents. The eruptions are mainly effusive with a minor strombolian bubble burst component. We propose that the gas phase is an added component of variable amounts of magmatic foam from the top of the magma reservoir. As the mixture of resident magma and foam rises in the conduit, the larger bubbles in the foam rise more quickly and sweep up the smaller bubbles nucleating and growing from the resident magma. On eruption, the process of bubble coalescence is more complete for the slower rising, gas-poor lavas that erupt as pillow lavas whereas the limu o Pele associated with sheet flow eruptions commonly contain several percent vesicles that avoided coalescence during ascent. The spatter erupted at the vent is quench granulated in seawater above the vent, reducing the pyroclast grainsize. The granulated spatter and limu o Pele fragments are then entrained in a rising plume of seawater heated by the eruption, which disperses them to distances as great as 5 km from the vent. 相似文献
14.
Charles R. Stern 《Bulletin of Volcanology》2008,70(4):435-454
For regionally widespread Holocene tephra layers in southernmost Patagonia, correlations based on both chemical and chronological
data indicate their derivation from five large-volume (>1 km3) explosive eruptions of four different volcanoes in the southernmost Andes. Bulk-tephra and tephra-glass major and trace-element
chemistry and Sr isotopic ratios unambiguously distinguish different source volcanoes, and imply that two of the regionally
widespread tephra (MB1 and MB2) were derived from Mt. Burney (52°S), one (R1) from Reclus (51°S), one (A1) from Aguilera (50°S) and one (H1) from Hudson volcano (46°S). The H1 tephra derived from the Hudson volcano, which is located at the southern end of the Andean Southern Volcanic Zone (SVZ; 33–46°S),
contains distinctive greenish andesitic glass with FeO > 4.5 wt.% and TiO2 > 1.2 wt.%. In contrast, rhyolitic glass in tephra derived from the eruptions of Mt. Burney, Reclus and Aguilera volcanoes,
which are located in the Andean Austral Volcanic Zone (AVZ; 49–55°S), is clear and transparent and has significantly lower
FeO and TiO2. Tephra derived from these three AVZ volcanoes all contain plagioclase, orthopyroxene, minor clinopyroxene and amphibole.
Biotite occurs only in the Aguilera A1 tephra, which also has the highest bulk-tephra and tephra-glass K2O and Rb contents. Averages of new and published 14C ages determined on organic material in soil and sediment samples above and below these tephra constrain the uncalibrated
14C age of the R1 eruption of Reclus volcano to 12,685 ± 260 years BP, the MB1 and MB2 eruptions of Mt. Burney to 8,425 ± 500 and 3,830 ± 390 years BP, the Hudson H1 eruption to 6,850 ± 160 years BP, and the A1 eruption of Aguilera volcano to 3,000 ± 100 years BP. The volume of the largest of these eruptions, H1 of the Hudson volcano, is estimated as >18 km3. The volume of the Reclus R1 eruption is estimated at >10 km3, the Aguilera A1 eruption at between 4 and 9 km3, and the younger Mt. Burney MB2 eruption at ≥2.8 km3. The volume of the older MB1 Mt. Burney eruption is the least well constrained, but must have been larger than the younger MB2 eruption. The data indicate that the frequency of explosive activity of volcanic centers in the AVZ is lower than in the
southern SVZ. 相似文献
15.
Judy Fierstein 《Bulletin of Volcanology》2007,69(5):469-509
At least 15 explosive eruptions from the Katmai cluster of volcanoes and another nine from other volcanoes on the Alaska Peninsula
are preserved as tephra layers in syn- and post-glacial (Last Glacial Maximum) loess and soil sections in Katmai National
Park, AK. About 400 tephra samples from 150 measured sections have been collected between Kaguyak volcano and Mount Martin
and from Shelikof Strait to Bristol Bay (∼8,500 km2). Five tephra layers are distinctive and widespread enough to be used as marker horizons in the Valley of Ten Thousand Smokes
area, and 140 radiocarbon dates on enclosing soils have established a time framework for entire soil–tephra sections to 10 ka;
the white rhyolitic ash from the 1912 plinian eruption of Novarupta caps almost all sections. Stratigraphy, distribution and
tephra characteristics have been combined with microprobe analyses of glass and Fe–Ti oxide minerals to correlate ash layers
with their source vents. Microprobe analyses (typically 20–50 analyses per glass or oxide sample) commonly show oxide compositions
to be more definitive than glass in distinguishing one tephra from another; oxides from the Kaguyak caldera-forming event
are so compositionally coherent that they have been used as internal standards throughout this study. Other than the Novarupta
and Trident eruptions of the last century, the youngest locally derived tephra is associated with emplacement of the Snowy
Mountain summit dome (<250 14C years B.P.). East Mageik has erupted most frequently during Holocene time with seven explosive events (9,400 to 2,400 14C years B.P.) preserved as tephra layers. Mount Martin erupted entirely during the Holocene, with lava coulees (>6 ka), two
tephras (∼3,700 and ∼2,700 14C years B.P.), and a summit scoria cone with a crater still steaming today. Mount Katmai has three times produced very large
explosive plinian to sub-plinian events (in 1912; 12–16 ka; and 23 ka) and many smaller pyroclastic deposits show that explosive
activity has long been common there. Mount Griggs, fumarolically active and moderately productive during postglacial time
(mostly andesitic lavas), has three nested summit craters, two of which are on top of a Holocene central cone. Only one ash
has been found that is (tentatively) correlated with the most recent eruptive activity on Griggs (<3,460 14C years B.P.). Eruptions from other volcanoes NE and SW beyond the Katmai cluster represented in this area include: (1) coignimbrite
ash from Kaguyak’s caldera-forming event (5,800 14C years B.P.); (2) the climactic event from Fisher caldera (∼9,100 14C years B.P.—tentatively correlated); (3) at least three eruptions most likely from Mount Peulik (∼700, ∼7,700 and ∼8,500
14C years B.P.); and (4) a phreatic fallout most likely from the Gas Rocks (∼2,300 14C years B.P.). Most of the radiocarbon dating has been done on loess, soil and peat enclosing this tephra. Ash correlations
supported by stratigraphy and microprobe data are combined with radiocarbon dating to show that variably organics-bearing
substrates can provide reliable limiting ages for ash layers, especially when data for several sites is available. 相似文献
16.
We investigated a late Quaternary terrestrial sedimentary sequence (Uwa Formation) in core IC2, from a site adjacent to that of the reported core IC on NW Shikoku Island, SW Japan, and developed its tephra and pollen stratigraphy to refine the age model of the formation. First, we identified 19 horizons with high glass shard concentrations in the IC2 core sediments as possible tephras or cryptotephras, and correlated them with reported tephras on the basis of the major- and trace-element compositions of their glass shards. All correlated widespread tephras and cryptotephras were products of volcanoes in the Kyushu volcanic zone (Aso, Kakuto, Aira, Ata, and Kikai calderas). Second, we confirmed the presence in core IC2 of two pollen zones dominated by Quercus subgen. Cyclobalanopsis, which is an indicator of very warm interglacial vegetation. In the Japanese Islands, these two vegetation zones have usually been considered to characterize marine isotopic stages (MISs) 1 and 11. A previous study of the Uwa Formation correlated the upper pollen zone to MIS 1, but the lower zone was not correlated to MIS 11; rather, it was inferred to be older than MIS 12 because it was stratigraphically below the “Oda” tephra (equivalent to a distal Kasamori 5 [Ks5] tephra [MIS 12]). In this study, however, noting that the Naruohama-IV tephra (Nh-IV; MIS 10d) and Ks5 cannot be distinguished by their shard chemistries, we inferred that the suggested “Oda” tephra actually correlates to Nh-IV, rather than to the Ks5 tephra. By re-assigning the “Oda” tephra to Nh-IV, we could correlate the underlying Quercus subgen. Cyclobalanopsis-abundant zone to MIS 11 and, consequently, a pair of pollen zones indicating cool and warm conditions below the MIS 11 pollen zone to MISs 12 and 13, respectively. The resulting age model whereby tephra and pollen constraints are integrated showed a roughly constant sedimentation rate from MIS 13, without any long-term gaps; further, our MIS 13 horizon in core IC2 corresponds to the reported 1 Ma tephra horizon in core IC. Therefore, these findings represent a dramatic change in the Uwa Formation age model and validate the Uwa Formation as one of the most useful terrestrial archives of Quaternary tephrostratigraphy and paleoclimatic fluctuation in SW Japan. 相似文献
17.
Michael B. Turner Mark S. Bebbington Shane J. Cronin Robert B. Stewart 《Bulletin of Volcanology》2009,71(8):903-918
Acquiring detailed eruption frequency datasets for a volcano system is essential for realistic eruption forecasts. However,
accurate datasets are inherently difficult to compile, even if one or more well-dated eruption records are available. A single
record typically under-represents the eruption frequency, while combining two or more records may result in an overrepresentation.
Although glass compositions have proven to be successful in tephrochronological studies of dominantly rhyolitic tephras; microlitic
growth and thin glass shards inhibit their application to andesitic tephras. A method consisting of a combination of two techniques
for correlating syn-eruptive deposits is demonstrated on data from the typical andesitic stratovolcano of Mt. Taranaki, New
Zealand. Firstly, tentative matches are identified using the radiocarbon age and associated error of each event. Secondly,
the compositions of titanomagnetite micro-phenocrysts are used as an independent check, and shown to be a useful correlation
tool where age data is available. Using two lake-core records containing tephra layers in an overlapping time-frame, the radiocarbon
age-correlation procedure suggested 31 tephra matches. Geochemistry data were available for 15 of these pairs. In three of
these cases, the titanomagnetite compositions did not match. Hence, these “paired” tephras were from compositionally distinct
magmas and therefore likely represent separate events. An additional three matches were reassigned within the temporal uncertainty
limits of the dating procedure, based on better geochemical pairing. The final combined dataset suggests that there have been
at least 138 separate ash fall-producing eruptions between 96 and 10 150 years B.P. from Taranaki. Using the combined dataset
the mixture of Weibulls renewal model forecasts a probability of 0.52 for an eruption occurring in the next 50 years at this
volcano. The present annual eruption probability is estimated at 1.6%. This likelihood is almost double that obtained when
relying on a single stratigraphic record. 相似文献
18.
Shane J. Cronin Vincent E. Neall Robert B. Stewart Alan S. Palmer 《Journal of Volcanology and Geothermal Research》1996,72(3-4)
A multi-parameter approach was used to correlate andesitic tephras in a complex tephra sequence ranging in age from ca. 23 to ca. 75 ka on the eastern ring plain of Ruapehu volcano, North Island. Field properties, combined with ferromagnesian mineral assemblages and mineral compositions, were required to map and correlate this sequence. Three tephra units could be identified based on their unique physical appearance, but other tephras could not be correlated on this basis alone. Hornblende and olivine proved to be valuable marker minerals enabling further distinction of two of the marker units recognised by field properties, as well as defining two further marker tephras. Unweathered titanomagnetite crystals, present in all of the tephras, were subjected to major-element analysis by electron microprobe. Canonical discriminant function analysis (DFA) of these analyses enabled the grouping and discrimination of tephra units, further aiding the identification of defined marker units, as well as defining new marker units. The titanomagnetite chemistry showed a strong relationship to the ferromagnesian mineralogy, showing that the ferromagnesian phenocrysts formed from the same melt or under the same melt conditions prior to eruption of each tephra. Canonical DFA was also applied to hornblende and olivine mineral analyses to identify further marker beds and to confirm identifications of previously defined units. This statistical analysis was found to be invaluable in reducing the large amount of compositional data from this study into a useable form for andesitic tephra correlation and mapping. 相似文献
19.
Taveuni is a Fijian ocean-island volcano that sporadically erupted throughout the Holocene. The 437-km2 island is an active monogenetic volcanic field with a constantly shifting locus of activity along a single apparent rift
axis. Although the eruptions were not large ( ≤VEI 2), unexpected shifts in Taveuni volcanism had the potential to affect
habitation sites. Since known human settlement of the Fiji Group (ca. 950–750 BC), there have been at least 58 eruptions on
Taveuni. Up to 25 of these eruptions potentially affected pre-European inhabitants of the island and at least four former
occupation sites are known to have been affected by volcanic products. Despite apparent earliest settlement of Taveuni post-dating
other nearby islands by up to 600 years, volcanism probably did not hinder or stall settlement of Taveuni compared with neighbouring
islands. However, a period of voluminous eruptions between 300 and 500 AD covered much of south Taveuni with lava and/or thick
tephra, apparently causing abandonment of at least this portion of Taveuni until approximately 1100 AD. Most eruptions were
not of catastrophic proportions and, due to their localised effects, re-settlement was rapid in marginal unaffected areas.
Localised stories and a relict place name survive to describe former eruption locations and effects since approximately 120–320 AD.
Knowledge of the impacts on Taveuni's past inhabitants forms the basis of volcanic disaster-mitigation strategies to minimise
future effects on the current 14,500 residents.
Received: 9 September 1999 / Accepted: 21 February 1999 相似文献
20.
The 1995–1996 eruption of Mt. Ruapehu has provided a number of insights into the geochemical processes operating within the magmatic-hydrothermal system of this volcano. Both pre-eruption degassing of the rising magma and its eventual intrusion into the convective zone of the hydrothermal system beneath the lake were clearly reflected in lake water compositions. The eruptions of September–October 1995 expelled the lake, and provided the first-ever opportunity to characterise gas discharges from this volcano. The fumarolic discharges revealed compositions typical of andesite volcanoes and strong interaction with the enclosing meteoric and hydrothermal system fluids. Some 1.1 MT of SO2 gas was released from the volcano between September 1995 and December 1996, whereas ca. twice this amount (2.2 MT equivalent SO2) was erupted as soluble (i.e. leachable) oxyanions of sulphur. Significantly more sulphur was released from the volcano over this period than can be accounted for from the magma volume actually erupted. The evidence suggests that a sizable component of the evolved sulphur was remobilised from the long-lived hydrothermal system within the volcano during the 1995–1996 activity. 相似文献