Buried in mud: A look at the sedimentological and morphological characteristics of lahars on Pinatubo Volcano's eastern slopes.

Michael T Dolan (Geological Engineering, Michigan Technological University, Houghton MI 49931-1295; (906)487-2531; Fax (906)487-3371; Internet mtdolan@mtu.edu)

Although more than two years have passed since the catastrophic eruption of Mount Pinatubo, the communities surrounding the volcano are still coping with the threat to life and property from devastating lahars. Heavy rainfall generated by afternoon thunderstorms, typhoons, or monsoonal rains erode the hot and loosely consolidated pyroclastic flow deposits bringing forth thick steaming lahars that bury municipalities, damage bridges, and roads, and isolate or dislocate thousands of residents.

Visual observations of lahars show varying styles of flow with changes in gradient and flow density. In the proximal region for lahar generation, the flows are represented by thick debris flows having densities of 1.5-1.8 g/cc and 75-85 weight percent sediment. Although very clay and pumice rich these flows are capable of carrying dense lithics and can travel from a few to tens of kilometers depending on the intensity and duration of rainfall. The distal, or lahar-runout reaches, are characterized by flow densities of 1.1-1.2 g/cc and 35-45 weight percent sediment.

Deposition of sediment occurs along very shallow gradients or in areas where the flows spread laterally. Overbank deposits and channel levees tend to be pumice rich and represent the upper portion of the flow. In areas where the channel is aggrading, the characteristic deposit does not represent the sampled portions of active flows, and are pumice and clay poor while enriched in crystals and lithics.

Presented in this poster will be comparisons of the different active flow regimes, sedimentological characteristics of flows and deposits, and video from the 1992 and 1993 field seasons.