GEOLOGIC SETTING
Mount Shasta (fig. 1) is a composite of at least four overlapping volcanic cones (Christiansen and others, 1977). Eruptive rocks are exposed on all sides of the mountain, but most areas are covered by varying thicknesses of till of Wisconsin or neoglacial age, pyroclastic flows, and debris-flow deposits. The widespread occurrences of interbedded pyroclastic and debris-flow deposits on Mount Shasta indicate that debris flows have been an important geomorphic process in the History of the mountain (Osterkamp and others, 1986).
Wisconsin-aged glacial deposits (older than about 10,000 years) generally are within 6 km of the Mount Shasta summit. Most neoglacial moraines, presumably less than 4,500 years in age, are restricted to upper slopes within a few kilometers of the present glacial termini. A few neoglacial-aged rock glaciers are within 6 km of the summit.
A summary of eruptive and noneruptive activities on Mount Shasta that includes pyroclastic flows, block-and-ash flows, ash and cinder fall, and debris flows (Miller, 1980) is given in table 1. On the basis of hydrologic factors, the area around Mount Shasta has been separated into four areas: Sacramento River, McCloud River, The Whaleback-Ash Creek Butte depression, and Shasta Valley (table 1, fig. 2) (Blodgett and others, 1988). The areas with the most eruptive activity around Mount Shasta are the McCloud River and The Whaleback-Ash Creek Butte depression (fig. 2). Significantly, and certainly advantageous to the populated areas around the city of Mount Shasta, the Sacramento River area is least active.
Many of the historic eruptive and noneruptive flows had sufficient volume and mobility to develop terraces and fans on the lower flanks of the mountain (fig. 3); these areas of deposition are now being developed and populated. New debris flows of noneruptive origin, comparable in size to historic flows, could be disastrous and could create a potential for loss of life and the disruption of communities and major transportation routes.
