EROSION BY PYROCLASTIC FLOWS ON LASCAR VOLCANO, CHILE

Pyroclastic flows generated in the 19-20 April 1993 eruption of Lascar Volcano, Chile, produced spectacular erosion features. Scree and talu s were stripped from the channels and steep slopes on the flanks of th e volcano. Exposed bedrock and boulders suffered severe abrasion, prod ucing smoothed surfaces on coarse breccias and striations and percussi on marks on bedrock and large boulders. Erosional furrows developed wi th wavelengths of 0.5-2 m and depths of 0.1-0.3 m. Furrows commonly nu cleated downstream of large boulders or blocks, which are striated on the upstream side, and thereby produced crag-and-tail structures. Eros ive features were produced where flows accelerated through topographic restrictions or where they moved over steep slopes. The pyroclastic f lows are inferred to have segregated during movement into lithic-rich and pumice-rich parts. Lithic-rich deposits occur on slopes up to 14 d egrees, whereas put mice-rich deposits occur only on slopes less than 4 degrees, and mainly at the margins and distal parts of the 1993 fan. The lithic-rich deposits contain large (up to 1 m) lithic clasts erod ed from the substrate and transported from the vent, whereas pumice-ri ch deposits contain only small (typically < 2 cm) lithic clasts. These observations suggest that lithic clasts segregated to the base of the flows and were responsible for much of the erosive phenomena. The ero sive features, distribution of lithic clasts and deposit morphology in dicate that the 1993 flows were highly concentrated avalanches dominat ed by particle interactions. In some places the flows slid over the be drock causing abrasion and long striations which imply that large bloc ks were locked in fixed positions for periods of about 1 s. However, s horter striae at different angles, impact marks, segregation of the de posits into pumice- and lithic-rich parts, and mixing of bedrock-deriv ed lithic clasts throughout the deposits indicate that clasts often ha d some freedom of movement and that jostling of particles allowed inte rnal mixing and density segregation to occur within the flows.