Oregon is home to numerous active or potentially active volcanoes, primarily located within the Cascade Range. These prominent peaks result from the Cascadia Subduction Zone, where the Juan de Fuca tectonic plate slides beneath the North American plate. This geological process creates magma that feeds the line of stratovolcanoes and calderas spanning the Pacific Northwest. While none of Oregon’s volcanoes are currently erupting, they are geologically young systems that scientists classify as potential future threats.
Defining Volcanic Activity in Oregon
Geologists classify a volcano as “active” based on its history and potential for future activity, not solely on whether it is currently erupting. The accepted scientific measure for an active volcano is one that has erupted within the Holocene epoch, which began approximately 11,700 years ago. This definition includes volcanoes with youthful magmatic systems capable of erupting again, even after centuries of silence.
Volcanoes that are not currently erupting are considered dormant or potentially active if they still show signs of a functioning magma system, such as seismic activity or gas emissions. Conversely, an extinct volcano is one whose magma source has been cut off or cooled, making future eruption unlikely. Oregon’s major Cascade volcanoes fall into the dormant or potentially active category, as their eruptive histories occurred within the Holocene time frame.
Oregon’s Principal Volcanic Centers
Oregon’s most significant volcanic centers are concentrated in the central and northern Cascade Range, each exhibiting evidence of recent activity.
Mount Hood
Mount Hood, located about 50 miles east-southeast of Portland, is the state’s highest peak and an active stratovolcano. Its most recent major eruptive period, known as the Old Maid period, occurred between the 1760s and early 1800s. This activity formed the prominent lava dome known as Crater Rock near the summit and involved dome growth, pyroclastic flows, and lahars (destructive volcanic mudflows).
Three Sisters Complex
The Three Sisters volcanic complex, situated near Bend, has shown signs of unrest in the modern era. Since the mid-1990s, an area approximately 12 miles wide, located west of South Sister, has experienced significant ground uplift. This deformation, sometimes called the Sisters Uplift, is attributed to magma accumulating about four miles beneath the surface, forcing the ground to bulge upward by several inches over two decades. Although the last eruption in the immediate area took place about 1,500 years ago near South Sister, the ongoing inflation confirms the presence of an active magmatic system.
Newberry Volcano
Newberry Volcano, a massive shield volcano and caldera system in central Oregon, is one of the largest and most active volcanoes in the contiguous United States. Its summit contains a large depression (caldera) that formed following a major explosive eruption about 75,000 years ago. Newberry’s most recent eruption created the Big Obsidian Flow about 1,300 years ago. The presence of hot springs and young lava flows within the caldera complex confirms that this system remains thermally and magmatically active.
Monitoring and Public Safety
The U.S. Geological Survey (USGS) and its Cascades Volcano Observatory (CVO) are responsible for tracking the potential activity of Oregon’s volcanoes. Scientists continuously monitor these systems using a sophisticated network of remote instruments.
The CVO uses several methods to detect unrest:
- Seismometers are deployed across the volcanoes to detect subtle earthquake swarms, which signal the movement of magma or volcanic fluids underground.
- Ground deformation is measured using Global Positioning System (GPS) receivers installed on the flanks of the volcanoes, detecting subtle changes in elevation and movement.
- Satellite radar technology (InSAR) provides a broad picture of ground movement, notably revealing the uplift near South Sister.
- Gas sensors analyze the chemistry of steam vents and fumaroles for changes in emissions that might precede an eruption.
This monitoring allows the CVO to classify Oregon’s high-threat volcanoes, including Mount Hood, Newberry, and Three Sisters. The goal is to detect early signs of volcanic unrest and provide timely warnings to the public and emergency management agencies. Hazard maps have been developed for all major centers, detailing the areas at risk from lava flows, ashfall, and lahars to inform preparedness and response planning.