Your Local Risks · Hazard Guide
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.
Understanding the hazard
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
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.
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.
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.
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.
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.
Before it happens
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.
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.
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.
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.
The key skill
Ashfall is the volcanic hazard most people will actually experience. It can happen hundreds of miles from the eruption. Here's what to do.
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.
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.
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.
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
Next step
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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