A volcano is considered active if it has erupted within the last 10,000 years and is likely to erupt again. Washington State is located within the 700-mile-long Cascade Volcanic Arc and hosts five major volcanic centers that meet this criterion. These towering peaks are the surface expression of deep, ongoing geological processes that define the Pacific Northwest.
The Cascadia Subduction Zone
The existence of Washington’s volcanoes is directly tied to the Cascadia Subduction Zone, a major plate boundary located offshore. Here, the dense, oceanic Juan de Fuca plate is slowly sliding eastward and plunging beneath the lighter North American continental plate.
As the oceanic plate descends deeper into the Earth’s mantle, it heats up, releasing trapped water into the overlying mantle rock. This water lowers the melting point of the rock, causing it to partially melt and generate magma. The buoyant magma then rises through the North American plate, collecting in reservoirs beneath the surface until it eventually erupts to form the chain of stratovolcanoes known as the Cascades.
Washington’s Currently Active Volcanoes
Washington is home to five active volcanoes: Mount St. Helens, Mount Rainier, Mount Baker, Glacier Peak, and Mount Adams. Mount St. Helens is the most frequently active of the group, known globally for its cataclysmic 1980 eruption. Following the initial blast, the volcano continued with periods of dome-building and lava extrusions between 1980 and 1986, and again from 2004 to 2008. During this later period, a new lava dome, called the “Whaleback,” grew above the crater floor.
Mount Rainier presents the greatest risk due to its immense size and proximity to major population centers like Seattle and Tacoma. The volcano is covered by more than one cubic mile of glacial ice, which is more than all other Cascade volcanoes combined. This massive ice cap makes lahars the primary hazard, as they can travel at speeds up to 45 to 50 miles per hour. Current estimates indicate that over 150,000 people reside on deposits from past lahars, with approximately 80,000 living in mapped lahar-hazard zones.
Mount Baker and Glacier Peak complete the northern section of the state’s active peaks. Mount Baker, near Bellingham, is characterized by a strong hydrothermal system evidenced by persistent steam plumes rising from Sherman Crater. The volcano experienced a period of unrest and reheating in 1975. Glacier Peak is a more remote volcano, but its history includes a massive eruption about 13,000 years ago that was ten times larger than the 1980 event at Mount St. Helens.
Mount Adams is Washington’s largest volcano by volume, though it is typically less explosive than its neighbors. Its past eruptions were mostly effusive, producing slow-moving lava flows. The mountain is generally quiet, though it recently experienced a notable increase in seismic activity compared to its typical background rate.
Tracking Volcanic Activity and Associated Hazards
The Cascades Volcano Observatory (CVO) and the Pacific Northwest Seismic Network (PNSN) continuously monitor the five active volcanoes for signs of unrest. Monitoring relies on a network of sensitive instruments designed to detect subtle changes that may precede an eruption.
Seismometers record the frequency and location of small earthquakes, which can signal magma or gas movement beneath the surface. Ground deformation is tracked using GPS receivers, which measure changes in the volcano’s shape, such as bulging caused by rising magma. Gas sensors measure the composition and output of volcanic gases, such as sulfur dioxide and carbon dioxide, from fumaroles and vents. This constant 24/7 observation allows scientists to identify the transition from normal background activity to a state of volcanic unrest.
The two most relevant hazards for communities are lahars and ash fall. Lahars are fast-moving mixtures of rock, water, and mud that look and behave like flowing concrete, posing a threat to all river valleys draining the major peaks. Ash fall can travel hundreds of miles downwind, disrupting air travel, damaging infrastructure, and impacting public health and agriculture. While all of Washington’s active volcanoes are currently at a normal alert level, the monitoring systems are in place to provide critical early warnings if conditions change.