Mount Hood is an imposing stratovolcano located about 50 miles east-southeast of Portland, Oregon. Rising to 11,249 feet, it is the highest point in the state and a striking feature of the Pacific Northwest landscape. It is an active member of the Cascade Volcanic Arc, formed by the subduction of the Juan de Fuca Plate beneath the North American Plate. Mount Hood is classified as a potentially active volcano with a long history of intermittent eruptive activity.
The Most Recent Eruptive Period
The last significant eruptive sequence occurred during the Old Maid eruptive period, spanning the late 18th century, primarily between about 1781 and 1793. Tree-ring dating confirms these major events fell within this timeframe, making it the most recent activity before the arrival of American settlers.
This eruptive period was characterized by the growth and collapse of a dacite lava dome near the summit, rather than a single massive explosion. The remnant of this dome-building phase is Crater Rock, a prominent formation on the mountain’s south side. The dome’s periodic instability led to small explosive events and the generation of hot, fast-moving pyroclastic flows down the mountain’s flanks.
The most widespread hazard was the creation of massive volcanic mudflows, known as lahars. Lahars formed when hot volcanic material mixed with the mountain’s abundant glacial ice and water, sending torrents of debris and sediment down major river valleys. Deposits traveled over 30 miles down the Sandy River and 9 miles down the White River, altering the surrounding lowlands. The Lewis and Clark Expedition, passing in 1805, noted the Sandy River was choked with sediment from these recent events.
Minor Activity Since the Last Major Event
Since the Old Maid period ended in the late 1790s, Mount Hood has remained largely quiet, with no confirmed magmatic eruption. However, the mountain is not dormant and shows continuous signs of a persistent internal heat source. The most visible evidence of this activity is the presence of fumaroles, or steam vents, clustered near Crater Rock.
These vents release gases primarily composed of water vapor, carbon dioxide (CO2), and hydrogen sulfide (H2S). Steam temperatures generally range between 50 and \(85^\circ\text{C}\), indicating the volcano maintains an active hydrothermal system. Scientists routinely sample these gases to monitor for changes in chemical composition, which could signal new magma rising deep below.
The volcano also experiences frequent, small earthquake swarms, typically occurring one to three times annually. These swarms usually last from a few hours to a few days, located several miles south of the summit at depths around 4 to 7 kilometers. Analysis indicates these seismic events are tectonic in origin, caused by regional stresses on faults rather than magma movement. However, some burst-like swarms are likely caused by the movement of superheated hydrothermal fluids within the mountain’s plumbing system.
Current Status and Future Eruption Potential
Mount Hood is closely monitored by the U.S. Geological Survey (USGS) Cascades Volcano Observatory (CVO). It is currently at a Volcano Alert Level of NORMAL and an Aviation Color Code of GREEN, indicating background, non-eruptive activity. The monitoring network includes an array of instruments to detect the earliest signs of unrest. Broadband seismometers detect small earthquakes, while sensitive GPS receivers measure minute changes in the volcano’s shape, which would indicate ground inflation from rising magma. Gas sensors and annual sampling track the temperature and composition of the fumarole gases near Crater Rock.
Any sudden, persistent increase in seismicity, a significant change in gas chemistry, or measurable ground swelling would prompt an immediate elevation of the alert level. Given its history, Mount Hood is certain to erupt again, though the timing remains unpredictable. The next eruption is expected to be similar to the Old Maid events: slow growth and collapse of a lava dome, accompanied by pyroclastic flows and large lahars. Because of the volume of snow and ice on the mountain, future lahars pose the primary near-term hazard, threatening communities and infrastructure along the Sandy and White River drainages.