USGS/Cascades Volcano Observatory

Mt. St. Helens, in Washington State, has been undergoing a slow eruption since October 2004. So far, this has been a relatively quiet eruption, compared to the explosive eruption of May 18, 1980. In the past year and a half, there have been a few minor eruptions that have ejected ash into the atmosphere, but the primary mode of eruption has been the slow extrusion of thick, sticky dacitic lava. This lava has a lower silica (SiO₂) and higher water content than the more fluid basaltic lava that characterizes Hawaiian volcanoes, and it tends to pile up in a dome rather than flowing for any distance.

A good description of how a lava dome forms is from the Cascade Volcano Observatory:

The dome at Mount St. Helens is termed a composite dome by scientists, because it represents the net result of many eruptive events, not just one event. The dome-building process may be pictured as the periodic squeezing of an upward-pointing tube of toothpaste or caulking compound. The process is dynamic, involving the upward movement of new material, cracking and pushing aside of old material, sloughing of material from steep surfaces of the dome, and occasional, small but violent explosions that blast out pieces of the dome.

The “fin” in the picture above is being pushed up from beneath at a rate of 1-2 m per day by magma that is slowly rising from a magma chamber, perhaps 1 km beneath the crater. It is now about 100 m (300 ft) tall. It is quite hot at its base, and cools as it rises. The fin will likely collapse into a pile of boulders in the future.

What will happen long term at Mt. St. Helens? Will the volcano be in a dome-building phase for decades, eventually filling in the horseshoe-shaped scar from the 1980 eruption? Will it stop its present eruptive stage tomorrow? Will it have another explosive phase, wiping out the domes that have formed in the crater? No one knows for sure.

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The following image from the USGS shows the lava dome that formed inside the crater of Mt. St. Helens during the 1980-1986 dome-building eruption.

The new dome (2004-present) is forming higher up in the horseshoe-shaped crater and is shown below. The new lava dome has steam rising from it; the 1980-1986 dome is the rocky area downhill from the new dome.

Grace and Peace