Mount Hood, the highest peak in Oregon, is an active stratovolcano located in the northern portion of the Cascade Range, about 50 miles east-southeast of Portland. This prominent mountain is part of the Cascade Volcanic Arc, a chain of volcanoes formed by the subduction of the Juan de Fuca tectonic plate beneath the North American plate. Mount Hood is classified as a stratovolcano, also commonly called a composite cone volcano. This type of volcano is known for its steep profile and is recognized as one of the most common and potentially hazardous types globally.
Mt. Hood’s Classification as a Stratovolcano
A stratovolcano earns its name from the alternating layers, or strata, of materials that build its structure over time. These volcanoes are characterized by their steeply sloping sides and symmetrical, conical shape, which is the result of the type of magma they erupt. The magma associated with stratovolcanoes, such as andesite and dacite at Mount Hood, has a high to intermediate silica content. This composition makes the magma highly viscous, or sticky, which prevents it from flowing easily or traveling far from the vent.
Because the lava is thick and slow-moving, it solidifies quickly, piling up around the central vent and creating the mountain’s steep flanks, typically with slopes between 30 and 35 degrees. This contrasts with shield volcanoes, which erupt low-viscosity, fluid lava that spreads out over vast distances. Stratovolcanoes tend to have more explosive eruptions because the viscous magma traps volcanic gases, leading to a buildup of internal pressure. When this pressure is released, it often results in pyroclastic flows and ash fallout rather than gentle lava flows.
Defining the Stratovolcano Structure
The physical architecture of Mount Hood is a classic example of a composite cone, built from a sequence of eruptions over hundreds of thousands of years. The structure is a layered mix of hardened lava flows, fragmented rock, and volcanic ash, creating the distinct strata. The cone has been built episodically, with periods of frequent eruptions separated by much longer quiet periods. Eruptions have typically alternated between producing lava flows and the growth of thick lava domes that pile up over the vents.
A prominent feature near Mount Hood’s summit is the presence of lava domes, which form when very viscous magma solidifies without flowing far. Crater Rock, a large pinnacle just below the summit, is hypothesized to be the remains of one of the most recent lava domes, formed during an eruptive period that ended around 170 to 220 years ago. The repeated growth and subsequent gravitational collapse of these lava domes on the steep upper slopes have been a mechanism for generating hot, fast-moving pyroclastic flows. These flows can melt snow and ice to produce lahars, which are destructive volcanic mudflows that travel far down the river valleys surrounding the mountain.
History of Activity and Current Monitoring
Mount Hood has experienced recurrent volcanic activity for at least the past 500,000 years. In the last 1,800 years, the volcano has had two significant eruptive periods, the most recent being the Old Maid period, which began around 1781 and continued intermittently into the mid-19th century. This latest activity primarily involved the growth and collapse of lava domes, producing mudflows and pyroclastic flows without large, explosive ash eruptions. Mount Hood is currently considered active but dormant, meaning it is expected to erupt again in the future.
The U.S. Geological Survey (USGS) Cascades Volcano Observatory maintains a comprehensive monitoring system to detect any signs of renewed activity. This network includes a regional array of seismometers positioned within 12 miles of the volcano to continuously track earthquake activity. An increase in the number or magnitude of earthquakes, especially if they become shallower, is often the first indicator that magma is moving toward the surface.
Scientists also use GPS deformation sensors to monitor minute changes in the volcano’s shape, as the flanks of the mountain may swell or tilt if magma begins to accumulate beneath the surface. Gas sensors are also in place to measure the composition and volume of gases emitted from fumaroles, which are steam vents, particularly those near Crater Rock. Collectively, these monitoring efforts allow scientists to assess the volcano’s status and provide an early warning of a potential eruption.