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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. The initial eruptions of the volcano were preceded by repeated glaciations of much of the Mount St. Helens region, most recently during the next-to-last glaciation when the Hayden Creek Drift was formed. The initial eruptions of Mount St. Helens during the Ape Canyon eruptive stage, which began sometime before 40,000 years ago and lasted until about 35,000(?) years ago, produced lithic and pumiceous air-PRE-1980 PYROCLASTIC FLOWS AND LAHARS, MOUNT SAINT HELENS The early part of the Kalama eruptive period may be analogous to the dacitic eruptive activity that began at Mount St. Helens in 1980 with an explosive eruption followed by dome extrusion. If the current eruptive sequence repeats the events of Kalama time, future volcanic activity will include multiple eruptions of dacite in the form of domes, tephra, and pyroclastic flows, and andesite in the form of lava flows, tephra, and pyroclastic flows, and will continue intermittently for at least a century.

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