| Mount St. Helens in southern Washington has been one of the most active and most explosive volcanoes in the Cascade Range since its birth about 40,000 years ago. In this report its history is divided into four eruptive stages, the most recent of which is subdivided into six eruptive periods. Prior to 2,500 years ago the volcano erupted tephra, domes, and pyroclastic flows, but only a few lava flows that were large enough to extend beyond the base of the volcano. Most products of volcanism were dacite or silicic andesite. Eruptive behavior changed about 2,500 years ago, and since then the volcano has produced more mafic rock types as well as dacite. -- Crandell, 1987 |
