Pyroclastic flows can form either from the collapse of an energetic eruption cloud (not relevant here) or by the collapse of unstable lava domes that have oozed out of stratovolcanoes. The hot rock tumbles downhill as an avalanche then rapidly transforms into an expanding cloud of hot rock, "ash" and gas as it entrains ambient air and heats it, and as the hot volatiles within the lava violently decompress. The result is a self-supporting dense cloud of hot debris and gas that flows downhill as a fluid body. A cloud of "ash" rises above the flow, and it variously scours its path and/or deposits distinctive linear buildups of rock and debris, depending on the flow conditions. Pyroclastic flows are one of the most destructive and dangerous aspects of volcanic eruptions and provide graphic evidence that rocks can be transported on a cushion of gas.

The puzzling deposits of the outburst "flood" channels, with multiple layers of sediment, longitudinal grooving, and teardrop shaped islands are, in fact, classic features of pyroclastic flows, except on a far larger scale than is seen on Earth. The White Mars model of cold gas-supported flows ("cryoclastic flows") explains how these occured on Cold dry Mars by collapse of thick layered regolith containing solid CO2 which literally exploded into the near-vacuum of Mars thin atmosphere.

USGS Web pages:-

Rocks Transported by Hot Air

More images

Learn about Pyroclastic Flows

Mount Unzen in an eruptive episode from 1991 to 1994 over 10,000 individual pyroclastic flows were recorded. Note the broad paths of the pyroclastic flows across flat cultivated land.
 
 

      Created: May 2002
      Last modified: May 2002
      Authorised by:  Head, Earth Sciences

      Maintained by: Nick Hoffman
      Email: nhoffman@unimelb.edu.au