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101.
Nature and significance of small volume fall deposits at composite volcanoes: Insights from the October 14, 1974 Fuego eruption,Guatemala 总被引:3,自引:2,他引:1
W. I. Rose S. Self P. J. Murrow C. Bonadonna A. J. Durant G. G. J. Ernst 《Bulletin of Volcanology》2008,70(9):1043-1067
The first of four successive pulses of the 1974 explosive eruption of Fuego volcano, Guatemala, produced a small volume (∼0.02 km3 DRE) basaltic sub-plinian tephra fall and flow deposit. Samples collected within 48 h after deposition over much of the dispersal
area (7–80 km from the volcano) have been size analyzed down to 8 φ (4 μm). Tephra along the dispersal axis were all well-sorted
(σ
φ = 0.25–1.00), and sorting increased whereas thickness and median grain size decreased systematically downwind. Skewness varied
from slightly positive near the vent to slightly negative in distal regions and is consistent with decoupling between coarse
ejecta falling off the rising eruption column and fine ash falling off the windblown volcanic cloud advecting at the final
level of rise. Less dense, vesicular coarse particles form a log normal sub-population when separated from the smaller (Mdφ < 3φ or < 0.125 mm), denser shard and crystal sub-population. A unimodal, relatively coarse (Mdφ = 0.58φ or 0.7 mm σ
φ = 1.2) initial grain size population is estimated for the whole (fall and flow) deposit. Only a small part of the fine-grained,
thin 1974 Fuego tephra deposit has survived erosion to the present day. The initial October 14 pulse, with an estimated column
height of 15 km above sea level, was a primary cause of a detectable perturbation in the northern hemisphere stratospheric
aerosol layer in late 1974 to early 1975. Such small, sulfur-rich, explosive eruptions may substantially contribute to the
overall stratospheric sulfur budget, yet leave only transient deposits, which have little chance of survival even in the recent
geologic record. The fraction of finest particles (Mdφ = 4–8φ or 4–63 μm) in the Fuego tephra makes up a separate but minor size mode in the size distribution of samples around
the margin of the deposit. A previously undocumented bimodal–unimodal–bimodal change in grain size distribution across the
dispersal axis at 20 km downwind from the vent is best accounted for as the result of fallout dispersal of ash from a higher
subplinian column and a lower “co-pf” cloud resulting from pyroclastic flows. In addition, there is a degree of asymmetry
in the documented grain-size fallout pattern which is attributed to vertically veering wind direction and changing windspeeds,
especially across the tropopause. The distribution of fine particles (<8 μm diameter) in the tephra deposit is asymmetrical,
mainly along the N edge, with a small enrichment along the S edge. This pattern has hazard significance. 相似文献
102.
103.
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
104.
Michael Marden Colin Mazengarb Alan Palmer Kelvin Berryman Donna Rowan 《Geomorphology》2008,99(1-4):404-419
The sediment flux generated by postglacial channel incision has been calculated for the 2150 km2, non-glacial, Waipaoa catchment located on the tectonically active Hikurangi Margin, eastern North Island, New Zealand. Sediment production both at a sub-catchment scale and for the Waipaoa catchment as a whole was calculated by first using the tensioned spline method within ARC MAP to create an approximation of the aggradational Waipaoa-1 surface (contemporaneous with the Last Glacial Maximum), and second using grid calculator functions in the GIS to subtract the modern day surface from the Waipaoa-1 surface. The Waipaoa-1 surface was mapped using stereo aerial photography, and global positioning technology fixed the position of individual terrace remnants in the landscape. The recent discovery of Kawakawa Tephra within Waipaoa-1 aggradation gravels in this catchment demonstrates that aggradation was coincidental with or began before the deposition of this 22 600 14C-year-old tephra and, using the stratigraphic relationship of Rerewhakaaitu Tephra, the end of aggradation is dated at ca 15 000 14C years (ca 18 000 cal. years BP). The construction of the Waipaoa-1 terrace is considered to be synchronous and broadly correlated with aggradation elsewhere in the North Island and northern South Island, indicating that aggradation ended at the same time over a wide area. Subsequent downcutting, a manifestation of base-level lowering following a switch to postglacial incision at the end of glacial-age aggradation, points to a significant Southern Hemisphere climatic warming occurring soon after ca 15 000 14C years (ca 18 000 cal. years BP) during the Older Dryas interval. Elevation differences between the Waipaoa-1 (c.15 ka) terrace and the level of maximum channel incision (i.e. before aggradation since the turn of the 20th century) suggest about 50% of the topographic relief within headwater reaches of the Waipaoa catchment has been formed in postglacial times. The postglacial sediment flux generated by channel incision from Waipaoa catchment is of the order of 9.5 km3, of which ~ 6.6 km3 is stored within the confines of the Poverty Bay floodplain. Thus, although the postglacial period represented a time of high terrigenous sediment generation and delivery, only ~ 30% of the sediment generated by channel incision from Waipaoa catchment probably reached the marine shelf and slope of the Hikurangi Margin during this time. The smaller adjacent Waimata catchment probably contributed an additional 2.6 km3 to the same depocentre to give a total postglacial sediment contribution to the shelf and beyond of ~ 5.5 km3. Sediment generated by postglacial channel incision represents only ~ 25% of the total sediment yield from this landscape with ~ 75% of the estimated volume of the postglacial storage offshore probably derived from hillslope erosion processes following base-level fall at times when sediment yield from these catchments exceeded storage. 相似文献
105.
Kroly Nmeth Shane J. Cronin Robert B. Stewart Douglas Charley 《Sedimentary Geology》2009,220(3-4):256
Ambrym is one of the most voluminous active volcanoes in the Melanesian arc. It consists of a 35 by 50 km island elongated east–west, parallel with an active fissure zone. The central part of Ambrym, about 800 m above sea level, contains a 12 kilometre-wide caldera, with two active intra-caldera cone-complexes, Marum and Benbow. These frequently erupting complexes provide large volumes of tephra (lapilli and ash) to fill the surrounding caldera and create an exceptionally large devegetated plateau “ash plain”, as well as sediment-choked fluvial systems leading outward from the summit caldera. Deposits from fall, subordinate base surge and small-volume pyroclastic (scoria) flows dominate the volcaniclastic sequences in near vent regions. Frequent and high-intensity rainfall results in rapid erosion of freshly deposited tephra, forming small-scale debris flow- and modified grain flow-dominated deposits. Box-shaped channel systems are initially deep and narrow on the upper flanks of the composite cones and are filled bank-to-bank with lapilli-dominated debris flow deposits. These units spill out into larger channel systems forming debris aprons of thousands of overlapping and anastomosing long, narrow lobes of poorly sorted lapilli-dominated deposits. These deposits are typically remobilised by hyperconcentrated flows, debris-rich stream flows and rare debris flows that pass down increasingly shallower and broader box-shaped valleys. Lenses and lags of fines and primary fall deposits occur interbedded between the dominantly tabular hyperconcentrated flow deposits of these reaches. Aeolian sedimentation forms elongated sand dunes flanking the western rim of the ash-plain. Outside the caldera, initially steep-sided immature box-canyons are formed again, conveying dominantly hyperconcentrated flow deposits. These gradually pass into broad channels on lesser gradients in coastal areas and terminate at the coast in the form of prograding fans of ash-dominated deposits. The extra-caldera deposits are typically better sorted and contain other bedding features characteristic of more dilute fluvial flows and transitional hyperconcentrated flows. These outer flank volcaniclastics fill valleys to modify restricted portions of the dominantly constructional landscape (lava flows, and satellite cones) of Ambrym. Apparent maturity of the volcanic system has resulted in the subsidence of the present summit caldera at a similar rate to its infill by volcaniclastic deposits. 相似文献
106.
Agathe Fourmont Jean-Jacques Macaire Jean-Gabriel Brhret Jacqueline Argant Batrice Prat Grard Vernet 《Comptes Rendus Geoscience》2006,338(16):1141-1149
The Sarliève marsh sediments (Massif Central, France) contain two tephras. The first tephra [, ca. 12 000 BP], regionally well known, enables to date the beginning of lacustrine infill to the Lateglacial. The second tephra, the ‘tephra de Sarliève’, the emitting volcano of which is unknown, would be dated to around the Early Subboreal from pollen data. This occurrence, after the discovery of the ‘tephra de Beaunit’, emphasizes that volcanic eruption(s) occurred in the ‘Chaîne des Puys’ or in the volcanic Cézallier more than 1000 years after the last known eruption (Pavin) in the ‘Chaîne des Puys’ at around 6.6/6.7 ka (5800/5900 BP). In the Sarliève piles, these tephras, well preserved in thick and more silicated deposits of deltas, were not observed in carbonated basin sediments where they were altered. The abundance of authigenic zeolites formed during the Lateglacial in restricted depocentre lacustrine waters allows us to detect initial CF1 tephra occurrence. To cite this article: A. Fourmont et al., C. R. Geoscience 338 (2006). 相似文献
107.
基于弹性理论的顺层边坡失稳破坏力学模型分析 总被引:4,自引:1,他引:3
在分析顺层边坡受力状况的基础上,当边坡的坡脚未被破坏时,将顺层边坡视为弹性地基上的悬臂梁,由此提出其失稳破坏的力学模型。应用弹性理论求出在力作用下悬臂梁的应力状态,并根据摩尔-库伦准则,导出可判定其稳定性的安全系数计算公式,得出了顺层边坡处于极限平衡状态极限长度。同时通过实例计算,将文中方法与应用突变理论所得结果对比,验证了本文方法的可行性。 相似文献
108.
In regions with limited knowledge of the historical volcanic record, like remote areas in the Andean Southern Volcanic Zone, the definition of reliable age-depth models for lake sequences represents a valuable tool for tephra layers dating. In Lake Futalaufquen (42.8°S), Northern Patagonia, a short sedimentary sequence was extracted after the AD 2008 Chaitén eruption with the purpose to analyze the records of volcanic eruptions at these poorly studied latitudes. The sequence was dated by 210Pb, 137Cs, and 14C techniques. Five tephras were identified for the last 1600 years, restricted to the last 5 centuries. Sedimentology, morphology, and geochemical properties allowed the characterization of the tephras and their correlation with tephras recently identified proximal to the sources, mainly from Chaitén and Huequi volcanoes, and Michinmahuida accessory cones, representing the first distal records reported of these tephras. Furthermore, tephras modeled ages obtained by the sequence age-depth model shrink the ages for the volcanic events, like a potential cycle of activity from Michinmauida accessory cones during AD 1530 ± 55, one eruption from Huequi volcano at AD 1695 ± 50, and a possible recent eruption from Chaitén at AD 1775 ± 40. Additionally, the work contributes to improve the regional volcanic records knowledge, basic for volcanic hazard assessment. 相似文献
109.
Evidence is presented to show that two measurable concentrations of microtephra particles can be detected in deposits of Late Devensian Late-glacial age in three sites in Scotland. One layer is attributed to the Vedde Ash, a marker horizon within the Younger Dryas chronozone. The second is a new tephra reported for the first time, which we name the Borrobol Tephra. This occurs consistently near the base of the Late-glacial Interstadial organic sediments at each site, and is thought to date to around 12.5 14C ka BP. Geochemical determinations using an electron microprobe confirm the identification of the Vedde Ash, suggest the Borrobol Tephra to have an Icelandic origin, and demonstrate the consistency of the geochemical signals at all three sites. © 1997 John Wiley & Sons, Ltd. 相似文献
110.