Volcanic cones of this internal structure are called composite cones or stratovolcanoes . Mount St. Helens includes layers of basalt and andesite through which several domes of dacite lava have erupted. September 3-6, Cispus Center, Washington. Mount St. Helens is a young volcano that developed over the last 40,000 years within a highly dissected terrain of Tertiary volcanic and metavolcanic rocks. Major valleys surrounding the volcano were extensively glaciated during Pleistocene glacial advances, but mass wasting, fluvial, and other erosion processes have carved complex landforms that are not dominated by the signature of any single process.
Most major floods result from prolonged rainfall or from warm rain on wet snow.
The pre-eruption landscape was dominated by dense coniferous forests and clear streams and lakes. Clearcuts up to about 30 years in age were scattered throughout the area, especially on private lands northwest of the volcano. Streams and lakes were cold and contained low levels of nutrients and organisms. Productive resident and anadromous fisheries made these fast-flowing streams and clear lakes popular recreational areas. The streams are fed by abundant rain and snow that dump an average of about 140 inches of water on Mount St. Helens a year, according to National Weather Service data.