Rheologie, Transportmechanismen und Separationsprozesse pyroklastischer Ströme
Final Report Abstract
Small-volume pyroclastic flows are some of the most hazardous volcanic phenomena because they form frequently during explosive eruptions as well as by repeated lava-dome collapse, they occur suddenly, and they spread at high velocities. This project investigated the mechanisms of such granular flows by two approaches: (1) fundamental experimental studies on granular flows formed by collapse of granular materials, and functional description of the resulting motion history; (2) field work on the pristine flow deposits of Ngauruhoe 1975, New Zealand, to produce a comprehensive data set for comparison with experimental results. (1) The experiments involved the sudden collapse of a column of granular material of initial aspect ratio a=height/width into a 2-D chute. Experimental variables were size, shape and composition of grains, resulting in different static friction angles, and the inclination and roughness of the chute floor. High-speed videos recorded flow parameters at high temporal resolution. The major result was that spreading behavior of all rapidly collapsing materials follows a common pattern regardless of material composition or other variables; runout length and runout time are simply a function of the initial aspect ratio. This is due to the inertial forces largely dominating flow resisting forces during runout; frictional forces - considered essential in earlier models - play no important role except at the very final stage before flows arrest. Bottom friction played no role in these flows because the boundary between static and flowing material was not at the chute floor but was a moving internal boundary inside the collapsing mass. (2) The concept of the internal boundary between static and moving material proved very helpful in the interpretation of deposit structures measured in detail at Ngauruhoe, New Zealand. The 1975 Ngauruhoe pyroclastic flows moved over loose, erodable pyroclastic ground down the steep flank of the volcano but also spread onto lower land where slope angles were lower than the static angle of internal friction. There was no deposition on the steep upper slopes, a proximal facies was emplaced on middle slopes, and the bulk of the deposit forms a levee-and-channel facies on the lower slopes. We recorded in detail the changes in internal structure of the deposits with distance from vent, granulometrically analyzed profiles through the deposits, and could estimate flow velocities from local deposit geometries. The resulting model for these flows centers on the downstream evolution of the boundary between static and moving material, which proximally lies in the loose ground material that is eroded but with distance rises upward through the flow and when it reaches its top flowage terminates.
Publications
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Collapses of two-dimensional granular columns. Phys Rev E 72: 041301
Lube G, Huppert HE, Sparks RSJ, Freundt A
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The flow and depositional mechanisms of granular matter: experimental and field studies with implications for pyroclastic flows. Dissertation, Univ Kiel, 2006, 160 S.
Lube G
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Flow and deposition of pyroclastic granular flows: a type example from the 1975 Ngauruhoe eruption, New Zealand. J Volcanol Geotherm Res 161: 165-186
Lube G, Cronin SJ, Platz T, Freundt A, Procter J, Henderson C, Sheridan MF
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Static and flowing regions in granular collapses down channels. Phys Fluids 19: 043301
Lube G, Huppert HE, Sparks RSJ, Freundt A
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Granular column collapses down rough, inclined channels. Journal of Fluid Mechanics, Vol. 675. 2011, pp. 347-368.
Lube G, Huppert HE, Sparks RSJ, Freundt A
