Mount St. Helens is a prominent and active stratovolcano located in the Cascade Range of Washington State. Its history of explosive activity makes it one of the most closely studied volcanoes in the world. As part of the Pacific Ring of Fire, the mountain is constantly scrutinized by scientists seeking to understand its internal processes and assess its current status.
Answering the Query: The 2004 to 2008 Dome Building Events
The most recent period of eruption for Mount St. Helens began in October 2004 and concluded in late January 2008. This multi-year event involved the gradual, non-explosive extrusion of highly viscous magma onto the crater floor. Activity started with a swarm of shallow earthquakes and a few small steam and ash explosions in late September 2004.
The primary mechanism was the slow growth of a new lava dome, which formed a large, solidified mass of rock within the horseshoe-shaped crater created in 1980. The dome-building continued for over three years. This type of eruption is characterized by degassed and relatively non-fragmented magma, contrasting sharply with the mountain’s earlier catastrophic blast. By the time the activity ceased in early 2008, the new dome contained approximately 125 million cubic yards of material.
The Catastrophic Eruption of 1980
The most destructive eruption occurred on Sunday, May 18, 1980, at 8:32 a.m. This event followed two months of precursor activity, including hundreds of earthquakes and the formation of a bulge on the volcano’s north flank. The growing bulge resulted from magma pushing upward into the cone, causing the north face to deform outward by over 300 feet.
The eruption was initiated by a magnitude 5.1 earthquake that triggered the largest recorded subaerial landslide in history. The collapse of the north flank removed support for the high-pressure magma system beneath, leading to immediate depressurization. This sudden release resulted in a powerful, directed lateral blast that exploded sideways across the landscape. The blast devastated over 150 square miles, leveling forests and depositing hot debris up to 19 miles away.
The explosive event ejected approximately one cubic mile of material and sent an ash column up to 80,000 feet into the atmosphere. The loss of material and the subsequent collapse of the summit reduced the mountain’s elevation by about 1,300 feet. This eruption transformed the mountain from a symmetrical peak into a hollowed-out cone with a breach opening to the north. Subsequent, smaller, non-explosive events following 1980 built an earlier lava dome within the crater.
Current Monitoring and Seismic Status
Mount St. Helens is one of the most intensively monitored volcanoes in the Cascade Range by the U.S. Geological Survey’s Cascades Volcano Observatory (CVO). Scientists use a network of instruments, including seismometers, GPS receivers, and gas emission sensors, to track changes in its behavior. Seismometers continuously record small earthquakes beneath the mountain, which can indicate the movement of magma or fluids.
The volcano is currently designated at a Volcano Alert Level NORMAL and an Aviation Color Code GREEN, which are the lowest levels on the four-tiered scale. This status indicates the mountain is at its typical, non-eruptive background level of activity. While scientists routinely detect minor earthquake swarms, these events are generally small—often less than magnitude 1.0—and are attributed to the normal process of magma “recharge” deep within the crust. These small, deep earthquakes do not currently suggest a high probability of an imminent, large-scale eruption.