Geology and History

Mount St. Helens, located in Washington State, is the most active volcano in the Cascade Range, and it is the most likely of the contiguous U.S. volcanoes to erupt in the future.

Volcanologists have recognized and named four stages of volcanic activity—Ape Canyon, Cougar, Swift Creek, and Spirit Lake—separated by dormant intervals.

Summary of Holocene activity prior to the May 18, 1980 eruption.

Magma began intruding into the Mount St. Helens edifice in the late winter and early spring of 1980. By May 18, the cryptodome (bulge) on the north flank had likely reached the point of instability, and was creeping more rapidly toward failure.

Explosions and Dome Growth, 1980-1986 and 1989-1991.

The sudden reawakening of Mount St. Helens in late September 2004 was surprising because the preceding four years had seen the fewest earthquakes since the 1980-86 eruption ended.

Extensive glaciers covered Mount St. Helens and the surrounding vicinity during major Ice Ages of the past few hundred thousand years. Much of the geologic record of these events is sparse and incomplete, having been removed by erosion or buried by volcanic deposits.

We know that Mount St. Helens is the volcano in the Cascades most likely to erupt again in our lifetimes. It is likely that the types, frequencies, and magnitudes of past activity will be repeated in the future. However, neither a large debris avalanche nor a major lateral blast like those of May 18, 1980 is likely now that a deep crater has formed.

The Mount St. Helens National Volcanic Monument is within the Gifford Pinchot National Forest and managed by the USDA Forest Service.

Streams that head on the volcano enter three main river systems -- the Toutle River on the north and northwest, the Kalama River on the west, and the Lewis River on the south and east. Lahars can form on any of these.

The first documented observation of Mount St. Helens by Europeans was by George Vancouver on May 19, 1792, as he was charting the inlets of Puget Sound at Point Lawton, near present-day Seattle. Vancouver did not name the mountain until October 20, 1792, when it came into view as his ship passed the mouth of the Columbia River.

Information about the 2004-2008 eruption.