Overview of the tectonic setting and recent studies of eruptions of Mount St. Helens, Washington

Abstract

Mount St. Helens lies along the western front of the Cascade Range between Mount Rainier, Washington, and Mount Hood, Oregon. This is an area of transition, both geologically and geophysically. North of Mount Rainier, late Cenozoic volcanism is limited to the major stratovolcanoes, whereas south of Mount Hood the volcanic cover from both stratovolcanoes and monogenetic vents is nearly continuous for 300 km. The dominant stratovolcanoes in the southern Washington Cascade Range are physio‐graphically similar to those in the North Cascades, but the volcanic cover is similar to the Oregon Cascades. A saddle occurs in the Bouguer gravity data over the Columbia River, interrupting a west‐east gravity gradient observed along the rest of the Oregon Cascade Range. Heat flow values are intermediate, being lower than those observed in Oregon but higher than those observed north of Mount Rainier. Seismicity is mostly concentrated along the St. Helens zone (SHZ), and all crustal earthquakes greater than magnitude 5 since 1960 in Washington and northern Oregon have occurred between Mount Rainier and Mount Hood. Little seismicity is known within the Cascade Range south of Mount Hood, and few earthquakes occur within the North Cascades of Washington. Magnetotelluric studies indicate a conductor within the area bounded by Mount Rainier, Mount Adams, and Mount St. Helens, and the conductor is interpreted as an east dipping, compressed marine terrain in the upper plate. Mount St. Helens is located at a dextral offset in the SHZ where the SHZ intersects a set of older, mapped fractures that are preferentially aligned with the contemporary regional principal stress direction. Aeromagnetic and gravity data suggest the presence of an intrusive body beneath the volcano; petrogenesis studies favor multiple intrusions of small batches of magma into the shallow volcanic system. Studies of pyroclastic flows indicate that turbulence within the head of pyroclastic flows may be important for the generation of ash clouds and in forming basal deposits. Observations of the May 18, 1980, eruption and subsequent erosion of pyroclastic flow deposits has allowed a revision of the chronology of the eruption. A numerical study of the slope failure of May 18 suggests that gravity alone was sufficient to trigger the avalanche that initiated the cataclysmic eruption. Hundreds of similar earthquakes recorded during eruptions in 1984 and 1985 suggest that repeated failure or slip within a small volume occurs around the magma supply conduit because of very high strain rates.