Mount St. Helens, a stratovolcano in the Cascade Range of Washington state, is the most active volcano in the region over the last 10,000 years. Its history is often defined by the explosive event of May 1980, which drastically reshaped the mountain and brought volcanic hazards into sharp focus across the United States. While this remains the most destructive eruption in North American history, the volcano has experienced multiple, less-violent periods of magmatic activity since then. The most recent instance of magma reaching the surface concluded in 2008.
The Catastrophic Eruption of May 1980
The catastrophic event on May 18, 1980, was preceded by two months of intense geological unrest. Beginning on March 20, 1980, a magnitude 4.2 earthquake signaled the volcano’s reawakening after 123 years of dormancy, followed by thousands of smaller quakes and phreatic, or steam-blast, eruptions. The intrusion of magma into the volcano’s core caused the entire north flank to bulge outward by more than 260 feet, creating an unstable slope over the course of two months.
The eruption was ultimately triggered at 8:32 a.m. PDT by a magnitude 5.1 earthquake. This seismic shock caused the massive, oversteepened north face of the volcano to slide away in the largest landslide ever recorded on Earth. This sudden removal of the overlying rock depressurized the magma system, leading to a powerful, near-supersonic lateral blast of hot gases, steam, and rock debris that swept northward.
The sheer force of the blast devastated a 230-square-mile area, flattening forests up to 19 miles away from the summit. Following the lateral explosion, a vertical column of ash and gas rose over 15 miles into the atmosphere within minutes, eventually depositing 520 million tons of ash across 11 U.S. states. The mountain’s elevation was reduced from 9,677 feet to 8,363 feet, leaving a massive, horseshoe-shaped crater nearly a mile wide open to the north.
Defining the Last Activity Since 1980
The explosive eruption of 1980 was followed by a prolonged period of dome-building activity. From 1980 to 1986, the volcano experienced numerous smaller explosive events and continuous extrusion of thick, viscous dacite lava, which began to construct a new lava dome within the crater. These eruptions were fundamentally different from the 1980 blast, as they involved magma slowly oozing out rather than violently exploding.
After a quiet period, the volcano reawakened in late 2004, beginning a four-year episode of sustained growth. This activity involved the non-explosive extrusion of new dacite lava, which pushed the existing dome upward and formed a new dome of rock. This period was characterized by the slow, steady growth of a lava spine and dome within the 1980 crater, a process driven by pressure from magma rising from below the surface.
This sustained eruption of new lava continued until January 2008, when the extrusion of material effectively ceased. Since that time, the mountain has not shown any evidence of fresh magma reaching the surface. Therefore, the latest magmatic eruption of Mount St. Helens concluded in early 2008, marking the most recent time the volcano was actively growing.
Current Monitoring and Future Risk Assessment
Mount St. Helens is currently in a period of relative quiet, but it remains an active and closely monitored volcano. The U.S. Geological Survey (USGS) and the Cascades Volcano Observatory (CVO) maintain a robust network of instruments to detect any changes that may signal renewed unrest. This extensive monitoring system includes seismometers to detect ground shaking caused by moving magma and GPS receivers that measure minute changes in the volcano’s shape as magma pressure builds.
Scientists also track the output of volcanic gases, such as sulfur dioxide, which can indicate the depth and movement of magma beneath the surface. Based on its history of frequent activity, Mount St. Helens is considered the Cascade volcano most likely to erupt again in the future. Future activity is expected to involve either the resumption of lava-dome growth or small to large explosive eruptions that produce ash.
The massive crater formed in 1980 makes a repeat of the devastating lateral blast highly improbable, as there is no longer a large, unstable flank to slide away and depressurize the system. While the current alert level is low, constant monitoring ensures that scientists can provide timely warnings of any significant changes. Risk assessment focuses on the potential for ashfall, pyroclastic flows within the crater, and mudflows in the surrounding river valleys.