Mount Hood is a prominent, snow-capped stratovolcano located approximately 50 miles east of the heavily populated Portland metropolitan area. As the highest peak in Oregon, it stands as a significant geographic feature within the Cascade Range and is an active member of the Cascade volcanic arc. While currently quiet, its proximity to major cities, transportation corridors, and recreational areas means that any renewed activity poses a serious regional concern.
The Eruptive History and Likelihood
Mount Hood has experienced two major eruptive periods in the geologically recent past, one about 1,500 years ago and another about 200 years ago. The most recent major activity, known as the Old Maid eruptive period, occurred in the 1780s. The volcano’s typical eruptive style involves the slow growth and periodic collapse of lava domes near the summit, which generates pyroclastic flows and subsequent mudflows. This is different from the massive, explosive events seen at Mount St. Helens in 1980.
Although the mountain is currently considered dormant, it is one of the most seismically active volcanoes in the Oregon Cascades, producing frequent minor earthquakes and emitting steam and gases. The U.S. Geological Survey (USGS) characterizes Mount Hood as “potentially active” and has designated it a very high threat volcano due to its history and location. Scientists estimate the probability of an eruption occurring in the next 30 years to be between 3 and 7 percent.
Primary Hazards: Ashfall and Pyroclastic Flows
A future eruption would produce two main immediate hazards: pyroclastic flows and ashfall. Pyroclastic flows are extremely hot, fast-moving mixtures of gas, ash, and rock fragments that travel down the mountain’s flanks. These flows are devastatingly destructive but are generally limited to the steep upper valleys and typically do not travel more than about eight miles from the summit. On Mount Hood, these flows are most often generated by the collapse of an unstable lava dome.
The most widespread primary hazard is ashfall, or tephra, which could affect the Portland area and other communities depending on wind direction. Airborne ash clouds could reach altitudes of 3,000 to 50,000 feet. This poses a severe threat to aviation by causing jet engine failure and grounding air traffic at Portland International Airport.
Closer to the ground, fine volcanic ash can cause respiratory problems and disrupt regional infrastructure. Wet ash is particularly problematic as it becomes heavy, leading to the short-circuiting of electric transformers and power lines. Ashfall would also increase the wear on vehicle engines and clog filters, creating significant cleanup issues. The fallout could also compromise municipal water supplies by increasing turbidity in vulnerable watersheds.
Secondary Hazards: Lahars and Water Impacts
The most far-reaching hazard associated with a Mount Hood eruption is the generation of lahars, or volcanic mudflows. Lahars are fast-moving mixtures of water, volcanic ash, rock debris, and sediment that sweep down river valleys at high speed. They are generated when hot pyroclastic flows rapidly melt the mountain’s extensive snow and ice cover, which includes eleven named glaciers. This volume of water and debris allows these flows to travel for tens of miles away from the mountain.
Past lahars have completely buried valley floors and reached as far as the Columbia River. The Sandy and White River drainages are the most susceptible to large lahars, which could severely impact communities like Welches, Brightwood, and Government Camp. Structures and homes near river channels are at the greatest risk of being destroyed, as lahars can affect areas over 100 vertical feet above the riverbed.
A lahar event would seriously disrupt major transportation routes, including U.S. Highway 26 and Oregon Highway 35, which could be severed and potentially closed for years. The sediment delivered to the Columbia River could narrow the shipping channel, negatively affecting commerce and navigation. The long-term adjustment of river channels to this volcanic sediment can continue to cause flooding and erosion for years after the eruption ends.
Monitoring and Preparedness
The U.S. Geological Survey’s Cascades Volcano Observatory (CVO) maintains a continuous monitoring network on Mount Hood to detect any signs of renewed volcanic activity. This network includes seismic stations to track earthquakes, as well as instruments to measure ground deformation using GPS and volcanic gas emissions. Scientists use this real-time data to look for unrest signals, such as an increase in earthquake swarms or changes in the mountain’s shape, which typically precede an eruption by weeks or more.
The CVO uses a tiered alert system to communicate potential hazards to emergency management authorities and the public. A “Volcano Advisory” is issued when monitoring indicates processes that could lead to a hazardous event. A “Volcano Alert” is declared if a life- or property-threatening event is imminent or underway. Local and regional preparedness plans focus on establishing clear evacuation routes and educating the public, particularly in communities located along the high-risk river valleys.