排序方式： 共有10条查询结果，搜索用时 15 毫秒
1
1.
The Franz Josef Glacier, Westland, New Zealand, has a history of catastrophic sediment‐laden outburst ?oods associated with extreme rainfall events when the glacier toe is advanced over its own sediments. Consideration of these events and inspection of recent sediment deposits suggest that there are three distinct modes of outburst. The ?rst is associated with fans fed by over?ow along the glacier margin. As the glacier has advanced across its own fore?eld gravels, it is inferred that the primary drainage conduit has developed a reach of negative slope. In high ?ows massive boulders can block the conduit, trapping lesser clasts. The resulting backup of water causes over?ows through marginal moulins, producing the fan type of deposit. The second type of outburst deposits massive imbricated boulders at a greater or lesser distance from the glacier portal. In this case, pressure buildup drives the blockage out of the portal where the boulders deposit. Smaller materials are generally carried away. The third type consists of very shallow ?ows, and produces massive gravel deposits of uncertain provenance. In this condition, the excess pressure in the conduit results in slight uplift of the glacier and widespread discharge of water and sediment below the glacier snout; gravels and smaller sediments are laid down in a massive deposit across the fore?eld. The massive, boulder‐veneered deposit from the December 1995 outburst is interpreted in the light of the above mechanisms as a hyperconcentrated ?ow deposit from hydraulic jacking, overlain by boulders emplaced by a subsequent conduit outburst. A possible association of outbursts with the present advanced position of the glacier is suggested. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
2.
Bowser Lake, a fiord lake in the northern Coast Mountains of British Columbia, contains a thick Holocene fill consisting mainly of silt and clay varves. These sediments were carried into the lake by proglacial Bowser River which drains a high-energy, heavily glacierized basin. Sedimentation in the lake is controlled by seasonal snow and ice melt, by autumn rainstorms, and by rare, but very large jökulhlaups from glacier-dammed lakes in the upper Bowser River basin which complicate environmental inferences from the sedimentary record. Sediment is dispersed through the deep western part of the lake by energetic turbidity currents. The turbidity currents apparently do not overtop a sill that separates the western basin from much shallower areas to the east. Large amounts of silt and clay are deposited from suspension in the eastern part of the lake, but sediment accumulation rates there are much lower than to the west. Several strong acoustic reflectors punctuate the varved fill in the western basin; these may be thick or relatively coarse beds deposited during jökulhlaups or exceptionally large storms. The contemporary sediment yield to Bowser Lake, estimated from sediments in the lake, is about 360 t km-2a-1. This is a relatively high value, but it is less than yields insome other, similar montane basins with extensive snow and ice cover.The most likely explanation for the difference is that large amounts of sediment have been, and continue to be, stored on the Bowser delta andin small proglacial lakes. 相似文献
3.
Behaviour of particle-laden flows into the ocean: experimental simulation and geological implications 总被引：3，自引：0，他引：3
The behaviour of subaerial particle-laden gravity currents (e.g. pyroclastic flows, lahars, debris flows, sediment-bearing floods and jökulhlaups) flowing into the sea has been simulated with analogue experiments. Flows of either saline solution, simple suspensions of silicon carbide (SiC) in water or complex suspensions of SiC and plastic particles in methanol were released down a slope into a tank of water. The excess momentum between subaerial and subaqueous flow is dissipated by a surface wave. At relatively low density contrasts between the tank water and the saline or simple suspensions, the flow mixture enters the water and forms a turbulent cloud involving extensive entrainment of water. The cloud then collapses gravitationally to form an underwater gravity current, which progresses along the tank floor. At higher density contrasts, the subaerial flow develops directly into a subaqueous flow. The flow slows and thickens in response to the reduced density contrast, which is driving motion, and then continues in the typical gravity current manner. Complex suspensions become dense flows along the tank floor or buoyant flows along the water surface, if the mixtures are sufficiently denser or lighter than water respectively. Flows of initially intermediate density are strongly influenced by the internal stratification of the subaerial flow. Material from the particulate-depleted upper sections of the subaerial flow becomes a buoyant gravity current along the water surface, whereas material from the particulate-enriched lower sections forms a dense flow along the tank floor. Sedimentation from the dense flow results in a reduction in bulk density until the mixture attains buoyancy, lifts off and becomes a secondary buoyant flow along the water surface. Jökulhlaups, lahars and debris flows are typically much denser than seawater and, thus, will usually form dense flows along the seabed. After sufficient sedimentation, the freshwater particulate mixture can lift off to form a buoyant flow at the sea surface, leading to a decoupling of the fine and coarse particles. Flood waters with low particulate concentrations (<2%) may form buoyant flows immediately upon entering the ocean. Subaerial pyroclastic flows develop a pronounced internal stratification during subaerial run-out and, thus, a flow-splitting behaviour is probable, which agrees with evidence for sea surface and underwater flows from historic eruptions of Krakatau and Mont Pelée. A pyroclastic flow with a bulk density closer to that of sea water may form a turbulent cloud, resulting in the deposition of much of the pyroclasts close to the shore. Dense subaqueous pyroclastic flows will eventually lift off and form secondary buoyant flows, either before or after the transformation to a water-supported nature. 相似文献
4.
5.
6.
Buried channels cut to a depth of over 100m into the underlying substratum of North Germany are almost exclusively of Elsterian age. Evaluation of geophysical well logs in the Hamburg area for the first time allows the construction of detailed longitudinal and transverse profiles. It can be shown that an apparently chaotic infill known from borehole records in some cases is actually a regular sedimentary sequence, fining from proximal to distal. Channel formation seems to have been the result of catastrophic meltwater release in the marginal parts of the ice sheet. The channel infill reflects these episodic outbursts of meltwater. During deglaciation, the channel remnants became infilled with fine sediments. The channels ceased to exist as morphologically detectable forms at the end of the Holsteinian Stage. 相似文献
7.
8.
9.
Armann Höskuldsson Robert S. J. Sparks Michael R. Carroll 《Bulletin of Volcanology》2006,68(7-8):689-701
The Kverkfjöll area, NE Iceland is characterised by subglacial basalt pillow lavas erupted under thick ice during the last major glaciation in Iceland. The water contents of slightly vesiculated glassy rims of pillows in six localities range from 0.85±0.03 to 1.04±0.03 wt %. The water content measurements allow the ice thickness to be estimated at between 1.2 and 1.6 km, with the range reflecting the uncertainty in the CO2 and water contents of the melt. The upper estimates agree with other observations and models that the ice thickness in the centre of Iceland was 1.5–2.0 km at the time of the last glacial maximum. Many of the pillows in the Kverkfjöll area are characterised by vesiculated cores (40–60% vesicles) surrounded by a thick outer zone of moderately vesicular basalt (15–20% vesicles). The core contains ～1 mm diameter spherical vesicles distributed uniformly. This observation suggests a sudden decompression and vesiculation of the still molten core followed by rapid cooling. The cores are attributed to a jökulhlaup in which melt water created by the eruption is suddenly released reducing the environmental pressure. Mass balance and solubility relationships for water allow a pressure decrease to be calculated from the observed change of vesicularity of between 4.4 and 4.7 MPa depressurization equivalent to a drop in the water level in the range 440–470 m. Consideration of the thickness of solid crust around the molten cores at the time of the jökulhlaup indicates an interval of 1–3 days between pillow emplacement and the jökulhlaup. Upper limits for ice melting rates of order 10?3 m/s are indicated. This interpretation suggests that jökulhlaups can reactivate eruptions. 相似文献
10.
1