Transport and deposition of tephra by pyroclastic flows and surges

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Pyroclastic surges are low-concentration, gas-solid dispersions that flow in a turbulent fashion. As a result of their more expanded nature, surges are less influenced than flows by underlying topography and produce radially distributed deposits around volcanic centers. Surge deposits are commonly cross-stratified and form dunes, although they can also occur as massive or planar beds. Cross-stratified dune structures indicate that particle support is dominantly by turbulence of the interstitial gases and that grain concentration is relatively low. Particles are also moved along near the base of a surge by saltation and rolling in a traction layer, where high, lateral, shear velocities are developed. -from Author

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Sedimentation in volcanic settings