Mount St. Helens, located in Washington State, is classified as an active volcano and is the most recently and frequently active volcano in the contiguous United States. Its powerful 1980 eruption drastically reshaped its northern flank and brought global attention to the area. While the volcano is currently quiet, a live magmatic system beneath the surface means it remains capable of future eruptions.
What Active Means for Mount St. Helens
The classification of a volcano as “active” means it has erupted within the last 10,000 years (the Holocene epoch), not that it is erupting continuously. This definition distinguishes it from extinct volcanoes. Mount St. Helens has had two eruptive episodes in the past three decades—the major event of 1980–1986 and a dome-building event from 2004–2008—placing it firmly in the active category. The mountain’s activity is driven by the Cascadia Subduction Zone, where the Juan de Fuca tectonic plate slides beneath the North American plate. This process creates the magma that feeds the entire Cascade Arc, and Mount St. Helens is the most productive volcano in the arc. The volcano’s current quiet phase is a period of dormancy, common for active volcanoes, where the magma system rests before the next cycle begins. This resting phase involves magma recharge, the slow influx of new molten rock into the plumbing system deep beneath the volcano.
Current Monitoring and Observed Activity
Scientists from the U.S. Geological Survey (USGS) Cascades Volcano Observatory employ an extensive network of instruments to track the volcano’s subtle subterranean movements. This monitoring system provides continuous, real-time data on the three primary indicators of volcanic unrest: seismicity, ground deformation, and gas emissions.
Seismicity
Seismicity, or earthquake activity, is the most common sign of magma movement, and elevated rates of small earthquakes are frequently observed. These events, typically below magnitude 1.0, occur several miles beneath the mountain, often signaling a “recharge” phase as new magma enters the system. Scientists track the depth and frequency of these tremors, as an increase in magnitude or a shallowing of the quake locations would indicate magma ascending toward the surface.
Ground Deformation
Ground deformation is measured using Global Positioning System (GPS) receivers and tiltmeters placed around the volcano. GPS stations track minute changes in the volcano’s position, detecting swelling or bulging that may indicate pressure building from below. Tiltmeters measure changes in ground slope with exceptional precision. While Mount St. Helens is currently stable, significant deformation was a major precursor to both the 1980 and 2004 eruptions.
Volcanic Gas Emissions
Volcanic gas emissions are also a focus of continuous monitoring, particularly the levels of carbon dioxide (CO2) and sulfur dioxide (SO2). CO2 separates from magma at deeper levels, making an increase in its emission a potential indicator of a new magma pulse. Conversely, a sharp rise in SO2, which degasses closer to the surface, suggests a magma body is near the surface. A multi-GAS station named SNIF, located on the lava dome within the crater, provides real-time data, allowing scientists to quickly identify significant changes in gas composition.
Assessing Future Eruption Potential
Future eruptions at Mount St. Helens are considered a certainty, though the exact timing is unknown. Based on its geological history, the volcano has an average recurrence interval of roughly 100 to 300 years between major events. Expected hazards include pyroclastic flows (fast-moving currents of hot gas and rock) and lahars (destructive volcanic mudflows that travel down river valleys). The massive, open crater created by the 1980 eruption makes a repeat of the devastating lateral blast less likely, as the crater now acts as a pressure release point. Future activity is more likely to involve dome-building eruptions within the crater, accompanied by ash fall and pyroclastic flows confined primarily to the immediate area. The USGS utilizes a four-tiered warning system, consisting of Volcano Alert Levels (Normal, Advisory, Watch, Warning) and Aviation Color Codes (Green, Yellow, Orange, Red). This system communicates the volcano’s status to the public and aviation sector. Crucially, the extensive monitoring network is expected to provide scientists with days to months of advance notice before any major eruptive event occurs.