Depositional processes and gas pore pressure in pyroclastic flows: an experimental perspective

被引:82
|
作者
Roche, Olivier [1 ,2 ,3 ]
机构
[1] IRD, LMV, R 163, F-63038 Clermont Ferrand, France
[2] CNRS, UMR 6524, LMV, F-63038 Clermont Ferrand, France
[3] Univ Blaise Pascal, Clermont Univ, Lab Magmas & Volcans, F-63000 Clermont Ferrand, France
关键词
Pyroclastic flow; Density current; Pore pressure; Deposition; Experiments; Dam-break; FLUIDIZED GRANULAR MASSES; IGNIMBRITE; EMPLACEMENT; PROPAGATION; AVALANCHES; DYNAMICS;
D O I
10.1007/s00445-012-0639-4
中图分类号
P [天文学、地球科学];
学科分类号
07 ;
摘要
The depositional processes and gas pore pressure in pyroclastic flows are investigated through scaled experiments on transient, initially fluidized granular flows. The flow structure consists of a sliding head whose basal velocity decreases backwards from the front velocity (U (f)) until onset of deposition occurs, which marks transition to the flow body where the basal deposit grows continuously. The flows propagate in a fluid-inertial regime despite formation of the deposit. Their head generates underpressure proportional to U (f) (2) whereas their body generates overpressure whose values suggest that pore pressure diffuses during emplacement. Complementary experiments on defluidizing static columns prove that the concept of pore pressure diffusion is relevant for gas-particle mixtures and allow characterization of the diffusion timescale (t (d)) as a function of the material properties. Initial material expansion increases the diffusion time compared with the nonexpanded state, suggesting that pore pressure is self-generated during compaction. Application to pyroclastic flows gives minimum diffusion timescales of seconds to tens of minutes, depending principally on the flow height and permeability. This study also helps to reconcile the concepts of en masse and progressive deposition of pyroclastic flow units or discrete pulses. Onset of deposition, whose causes deserve further investigation, is the most critical parameter for determining the structure of the deposits. Even if sedimentation is fundamentally continuous, it is proposed that late onset of deposition and rapid aggradation in relatively thin flows can generate deposits that are almost snapshots of the flow structure. In this context, deposition can be considered as occurring en masse, though not strictly instantaneously.
引用
收藏
页码:1807 / 1820
页数:14
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