Your Local Risks · Hazard Guide
Volcanic Eruption.
The mountain remembers what we forget.
The U.S. has 161 active volcanoes. Eruptions produce ashfall that grounds aircraft and damages engines, lahars that bury river valleys, and pyroclastic flows that move faster than any evacuation. The hazards vary by distance and terrain.
FROM THE FIELD
Most people affected by the 1980 eruption of Mount St. Helens were not near the mountain. They were in Spokane, Yakima, and communities across eastern Washington and beyond, watching the sky go dark in the middle of the day. Accounts describe the same disbelief: residents who had never thought about volcanic risk scrambling to deal with abrasive ash they could not identify, in communities with no cleanup plan. Ashfall is the volcanic hazard most households will face, and the ones farthest from the volcano are typically the least prepared for it.
Understanding the hazard
Four hazards, not one
Volcanic eruptions are not a single hazard. They produce at least four distinct threats, each dangerous at different distances and timescales. Understanding which ones apply to your location is the foundation of volcanic preparedness.
Pyroclastic flows are superheated clouds of gas and rock that race downhill at highway speeds. Nothing survives in their path, but they follow valleys and terrain. Lahars are volcanic mudflows that travel down river valleys, sometimes triggered hours or days after an eruption by rain or snowmelt. They can reach communities 50 miles or more from the volcano.
Ashfall is the most widespread hazard. Fine volcanic glass particles can travel hundreds of miles, damaging vehicle and aircraft engines, collapsing roofs under accumulation, contaminating water supplies, and causing respiratory problems. Volcanic gas emissions, particularly sulfur dioxide, can create acid rain and air quality emergencies downwind of the eruption.
Know your exposure
U.S. volcanic risk regions
Volcanic risk in the U.S. is concentrated in specific regions. Your distance from the volcano and your position relative to river valleys determines which hazards apply.
Cascade Range
Washington, Oregon, and Northern California. Mount Rainier is considered the most dangerous volcano in the contiguous U.S. due to its proximity to the Seattle-Tacoma metro area and extensive lahar hazard zones in river valleys.
Hawaii
Kilauea is one of the most active volcanoes on Earth. Hawaiian eruptions typically produce lava flows and volcanic gas (vog) rather than explosive eruptions. Lava flow zones are mapped and communities in those zones face recurring risk.
Alaska
More active volcanoes than any other state. Alaska eruptions primarily threaten air travel and remote communities. Ash clouds from Alaskan volcanoes have disrupted Pacific air routes multiple times.
Ashfall zones
A major eruption in the Cascades could deposit ash across the Pacific Northwest, Northern Rockies, and northern Great Plains depending on wind direction. Even trace amounts of ash affect air quality, vehicle engines, and water supplies.
From the field · Mount St. Helens gave two months of earthquakes and small eruptions before May 18, 1980. Accounts describe people photographing the activity from nearby hills. The prolonged warning became spectacle rather than a signal to prepare, and the scale of the actual event exceeded anything the warning period had suggested.
Before it happens
Preparation for volcanic eruptions
Know your hazard zones
The USGS publishes hazard maps for every monitored volcano. Lahar zones follow river valleys. Pyroclastic flow zones radiate from the summit. Check whether your home, workplace, or commute route falls within a mapped hazard zone.
Respiratory protection
N95 respirators filter volcanic ash particles. Stock a supply of N95 masks for every household member. Volcanic ash is fine glass, not soft dust. It damages lungs, irritates eyes, and aggravates asthma and COPD. Goggles (not sunglasses) protect eyes.
Vehicle and engine protection
Volcanic ash destroys vehicle air filters and engine internals. If ashfall is expected, park vehicles in a garage or cover them. Change air filters frequently during and after ashfall. Avoid driving through heavy ash, which reduces visibility and damages engines.
Evacuation routes
If you live in a lahar zone, know your evacuation route and practice it. Lahar warning sirens are installed in some communities around Mount Rainier and other Cascade volcanoes. Move to high ground perpendicular to the river valley. Do not try to outrun a lahar downstream.
From the field · In high-risk volcanic zones, few insurers will write homeowner's policies at all. On the mainland, volcanic damage may or may not fall under standard coverage depending on the policy. Households in both settings describe discovering their exposure only after the event. Review your coverage before you need it.
The key skill
Responding to ashfall
Ashfall is the volcanic hazard most people will actually experience. It can happen hundreds of miles from the eruption. Here's what to do.
Go inside and seal up
Close all windows and doors. Turn off HVAC systems that draw outside air. Seal gaps with damp towels. Ash entering your home contaminates surfaces and is difficult to remove completely.
Protect your breathing
Wear an N95 mask if you must go outside. A damp cloth over the nose and mouth is better than nothing but not adequate protection. People with asthma, COPD, or other respiratory conditions should stay indoors and keep rescue inhalers accessible.
Protect water supplies
Cover rain barrels and outdoor water collection. Volcanic ash contaminates surface water. If you rely on a cistern or open water source, switch to stored water until your source is tested. Municipal water treatment plants generally handle ash contamination, but follow local advisories.
Clear ash carefully
Dampen ash before sweeping to prevent it from becoming airborne again. Heavy ash accumulation on roofs can cause collapse. Remove it carefully. Ash is abrasive and heavy when wet. Do not wash it into storm drains, which can clog drainage systems.
Official resources
Where to learn more
Next step
Build your air quality plan
Ashfall is primarily an air quality event. N95 masks, sealed windows, and a plan for protecting engines and water are the foundations. The Local Risks hub connects you to air quality monitoring and alert systems for your area.
Your local risks guide"History doesn't repeat itself, but it often rhymes."
— Mark Twain
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