AVERY, Ellen R.; JOHNSSON, Mark J., Department of Geology, Bryn Mawr College, Bryn Mawr, PA 19010, eavery@brynmawr.edu and mjohnsso@brynmawr.edu

Sediment grain size distributions are affected by the type of source rock, erosional processes, and transportational processes such as hydrodynamic sorting. The Hawaiian islands provide a controlled natural laboratory to study the effect of chemical weathering on grain size distributions because of the narrow range of bedrock types and the wide range of weathering environments. In order to decrease the influence of hydrodynamic sorting, streams with similar hydrologic properties can be examined. The wide range of chemical weathering environments in Hawaii results from orographic precipitation, which creates a wet climate on the windward side of each volcano, and a dry climate on the leeward side. The duration of weathering is related to the age of the geomorphic surface exposed to erosion. The grain size distributions of stream sediments from six volcanoes were analyzed in order to determine if grain size distribution can be an indicator of the relative intensity and duration of chemical weathering in the drainage basin under erosion.

Results show that mean grain size of sediments from streams draining the leeward sides of East Maui, West Maui, and Kauai are smaller than those of streams draining the windward sides. On these three volcanoes, the drainage basins under erosion on the leeward side drain older geomorphic surfaces, which implies a longer duration of weathering. On the other hand, the mean grain size of sediments on Kohala are greater on the leeward side, which could suggest that intensity of chemical weathering plays a greater role on young surfaces exposed to weathering. The data collected from East Molokai and Koolau volcano show no such differences in mean grain size on leeward vs. windward side of the volcano. This probably reflects the fact that on these two volcanoes streams on the leeward side drain areas that receive a large amount of rainfall, as do streams on the windward side. Overall, these results imply that initially mean grain size is smaller on the windward sides of the volcanoes due to the high intens ity of weathering caused by higher precipitation, but that extended weathering in a low intensity environment on leeward exposures can reduce grain sizes more efficiently than this briefer exposure to a more intense weathering environment.