IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1A1_3G-O1Room A4

Date/Time: July 21 8:45-9:15

Inverting pyroclastic flow deposits from the characteristics of substrate-derived clasts:
an experimental investigation
Olivier Roche1 , Anne Mangeney2 , Yarko Niño3 , Brittany Brand4 , Nick Pollock5 , Greg Valentine6
1

Laboratoire Magmas and Volcans, Universite Blaise Pascal-CNRS-IRD, Clermont-Ferrand, France,
2Institut de Physique du Globe de Paris, Sorbonne Paris Cite, Universite Paris Diderot, France,
3
Department of Civil Engineering, University of Chile, Chile, 4 Department of Geosciences, Boise State
University, USA, 5 Department of Earth and Space Sciences, University of Washington, USA,
6
Department of Geology and Center for Geohazards Studies, University at Buffalo, USA
E-mail: o.roche@opgc.univ-bpclermont.fr

Pyroclastic flows commonly propagate on granular substrates formed by earlier geological events and whose
properties may influence the emplacement dynamics.

We addressed this issue through scaled laboratory

experiments on granular flows propagating on a granular substrate. The flows were generated from the releaseof columns of fine (80 microns) glass beads into a 3 m-long horizontal channel whose base consisted of an
unconsolidated granular layer. The initial column was either non-fluidized or gas-fluidized in order to investigate
flows having a range of properties. Pore pressure measurements revealed an upward pressure gradient at theflow-substrate interface because a dynamic underpressure (relative to that of atmosphere) proportional to thesquare of the front velocity was generated there while (atmospheric) pressure within the substrate was unchanged.
A smooth substrate of fine (80 microns) particles was sheared by the sliding head of the flow, leading to theformation of small-height (1 mm) bedforms. These structures stretched horizontally as the flow propagated sothat there was almost no evidence of entrainment in the deposit. In contrast, in the case of a rough substrate ofcoarse (1.6 mm) beads, most of the uppermost particles were first dragged horizontally individually. Then, manyof these particles were uplifted to a height that increased up to 6-8 mm at flow velocities up to 2.5-3 m s-1. They
were entrained over significant distances of several tens of cm, and then fell down so that the basal layer of theresulting deposit consisted of a mixture of fine and coarse particles. Assuming particle uplift was caused by theupward pressure gradient evidenced by our measurements, we did a theoretical analysis to determine the criticalpressure gradient and the corresponding flow velocity at which uplift can occur, and found that this depended
basically on the mass of individual particles entrained and on the flow bulk density. These contrasting modes ofsubstrate entrainment are counterintuitive but nonetheless consistent with field observations. As shown by thePeach Springs tuff, an unconsolidated substrate of relatively fine (typically less than 1-2 mm) particles can bepreserved at the base of a pyroclastic flow deposit whereas a substrate made of large particles (pebbles, boulders)is commonly reworked. The latter phenomenon is also observed at Mt St-Helens, where the 1980 pyroclastic flows
emplaced on a debris avalanche deposit and entrained dense lava boulders of size up to several tens of cm onsubhorizontal slopes. Using our model in order to invert the deposits, assuming a bulk flow density of 875-1400 kg
m-3, we obtain flow velocities of 9-13 m s-1 in agreement with field observations.

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