Mount Hood, Oregon’s highest peak, is a prominent stratovolcano in the Pacific Northwest. Recognized as potentially active, its volcanic nature is important for nearby communities and infrastructure.
Mount Hood’s Volcanic Past
Mount Hood has erupted episodically for over one million years, with at least four significant eruptive periods in the last 15,000 years. Its history is marked by recurrent eruptions, primarily involving lava dome growth and collapse, pyroclastic flows, lahars, and ashfall.
Within the past 1,500 years, Mount Hood has undergone two notable eruptive phases. The most recent, the Old Maid period (1781-1800s), involved dacitic lava domes that periodically collapsed. This generated pyroclastic flows and mudflows down river drainages, with effects still evident to the Lewis and Clark Expedition in 1805.
Prior to this, the Timberline eruption (around 1500 AD) also involved lava dome formation and a large summit collapse, producing debris flows and lahars. Mount Hood’s historical record demonstrates its capacity for repeated, often non-explosive, activity.
Monitoring Mount Hood’s Activity
Scientists continuously monitor Mount Hood for unrest using a network of instruments operated by the U.S. Geological Survey (USGS) Cascades Volcano Observatory (CVO) and the Pacific Northwest Seismic Network (PNSN). This system detects subtle changes signaling increased volcanic activity.
Seismometers detect earthquakes, common beneath Mount Hood, with one to two events typically occurring weekly within 10 kilometers. Scientists analyze these ground vibrations, as increased frequency or magnitude can indicate magma or fluid movement.
GPS stations measure ground deformation, recording subtle swelling or shrinking of the volcano’s flanks, which reflects magma accumulation or withdrawal. Gas sensors monitor the composition and emission rates of volcanic gases, like carbon dioxide and sulfur dioxide, released from fumaroles. Changes in gas emissions indicate shifts in the volcano’s internal plumbing.
Remote sensing technologies, including satellite imagery and thermal cameras, also contribute to monitoring. These tools provide broad views, detecting changes in ground temperature or surface features from a distance. Data from these instruments transmit in real-time, offering a comprehensive picture of Mount Hood’s subsurface activity.
Assessing Future Eruption Likelihood
Predicting the exact timing of a volcanic eruption remains a complex scientific challenge. Scientists assess the likelihood of future activity by analyzing monitoring data and the volcano’s past behavior. The USGS designates Mount Hood as a “very high threat volcano” due to its eruptive style and proximity to populated areas.
Current scientific estimates place the probability of an eruption at Mount Hood in the next 30 to 50 years between 3% and 7%. While this indicates a relatively low short-term probability, its history of recent activity means an eruption is possible within our lifetimes.
The volcano currently exhibits low levels of unrest, characterized by frequent, small earthquakes and ongoing gas emissions from fumaroles. These are typical background activities for Mount Hood.
Volcanic alert levels, established by the USGS, serve as a framework for communicating the status of a volcano to the public and emergency managers. These levels provide a standardized way to indicate whether a volcano is in a normal, advisory, watch, or warning state, based on observed monitoring data. The purpose of these alert levels is to provide timely information and warnings, allowing for appropriate preparedness and response actions.
Scientists continuously evaluate the collected data to determine if there are any significant deviations from background activity. An increase in earthquake frequency or size, noticeable ground deformation, or changes in gas emissions would be indicators of potential unrest. While an exact date cannot be given, the ongoing monitoring efforts aim to provide as much advance warning as possible should Mount Hood show signs of reawakening.
What an Eruption Could Entail
Based on its geological history, a future eruption from Mount Hood would likely involve specific types of hazards. The primary eruptive style has historically included the growth and subsequent collapse of lava domes, which are masses of viscous lava that pile up over a vent. These domes, composed of andesite and dacite lavas, can become unstable and collapse.
The collapse of a lava dome can generate pyroclastic flows, which are fast-moving, hot avalanches of volcanic gases, ash, and rock fragments. These flows can travel at high speeds down the volcano’s flanks, posing a direct hazard to areas in their path. The intense heat of pyroclastic flows can also rapidly melt snow and ice on the mountain.
The melting of snow and ice by pyroclastic flows, or by landslides of hydrothermally altered rock, can lead to the formation of lahars. Lahars are destructive volcanic mudflows, mixtures of water, ash, and rock debris, that can sweep down river valleys far from the volcano. For Mount Hood, the Sandy and White River valleys are particularly susceptible to lahars, which are considered the most widespread and hazardous consequence of an eruption.
Ashfall, or tephra, is another potential hazard, though past eruptions from Mount Hood have typically produced only modest amounts. This fine volcanic material can drift downwind, affecting air quality, visibility, and potentially disrupting transportation and infrastructure over a broader region. The specific nature and extent of hazards during a future eruption would depend on the eruption’s location, volume, and explosivity.