Although they occur less frequently than many other types of volcanic hazard, pumiceous pyroclastic flows are a devastating hazard. They are gravity-driven and will thus be influenecd, although not completely controlled, by the existing topography. These flows will tend to be channelized to existing drainages, but they could break across the topographic barriers. Pumiceous pyroclastic flows are common from many Central American and Caribbean volcanoes, but have occurred much in historic eruptions. They have been observed at Mount St. Helens and Mount Pelée.