The United States sits atop more than 160 potentially active volcanoes, and federal scientists rank them by how much damage an eruption could cause. Seismometers, GPS stations and gas sensors feed a round-the-clock watch on the highest-threat peaks, most of them clustered in Hawaii, the Cascades and Alaska. Here are eleven American volcanoes that scientists monitor most closely.
1. Kilauea: The Most Closely Watched Volcano In America
Kilauea, on the Big Island of Hawaii, holds the top threat ranking among American volcanoes and sits under the densest instrument network the country operates. The Hawaiian Volcano Observatory tracks its summit through seismometers, tiltmeters, gas sensors and webcams, publishing USGS volcano updates that shift between advisory and watch as summit activity rises and pauses. Recent behavior has centered on episodic lava fountaining inside Halemaumau crater at the summit caldera.
Most of that activity stays within a closed area of Hawaii Volcanoes National Park, so the practical hazards are volcanic gas, glass fibers carried downwind and sudden closures rather than threats to towns.
2. Mauna Loa: The Largest Active Volcano On Earth
Rising beside Kilauea, Mauna Loa is the largest active volcano on the planet by volume and covers roughly half of Hawaii’s Big Island. Its 2022 eruption, the first in 38 years, sent lava toward the Daniel K. Inouye Highway and reminded emergency planners how quickly flows can travel down its long, gentle flanks. Instruments feeding USGS hazard science now watch its summit for the earthquake swarms and inflation that preceded that event.
Communities in Kona and Hilo lie within reach of lava paths that Mauna Loa has used before, which is why hazard maps divide the island into numbered flow zones.
3. Mount St. Helens: Restless Decades After The 1980 Blast
Washington’s Mount St. Helens killed 57 people on May 18, 1980, when its north flank collapsed and a lateral blast flattened forest up to 19 miles away. The volcano rebuilt a lava dome inside its horseshoe crater through 1986 and again from 2004 to 2008, and it remains one of the most heavily instrumented peaks tracked by federal natural-hazard science programs. Small earthquake swarms beneath the crater are recorded regularly.
Such swarms generally indicate a recharging magma system rather than an eruption underway; scientists treat renewed dome growth as the most likely future scenario, with ash and lahars down the Toutle River the main hazards.
4. Mount Rainier: The Mudflow Threat Aimed At Tacoma
Mount Rainier carries the highest threat score of any Cascade volcano, and the reason is water rather than lava. Roughly 26 glaciers cap the 14,411-foot peak, and collapsing, water-saturated rock can turn into a lahar, a fast concrete-like mudflow that follows river valleys toward Puyallup, Orting and the Tacoma area. Acoustic-flow monitors and seismic stations tied to current volcano status reports exist specifically to trigger sirens in those valleys.
An estimated 80,000 people live on old lahar deposits, which means the danger arrives on a timeline of minutes even though Rainier has not erupted significantly in more than a century.
5. Yellowstone: The Supervolcano That Breathes Every Year
Beneath Yellowstone National Park sits a caldera roughly 30 by 45 miles wide, formed by three enormous eruptions over the past 2.1 million years. Ground inside it rises and falls by inches annually as hydrothermal fluids and magma shift, a signal captured by GPS stations and the seismic network described in federal earthquake hazard research. Thousands of small quakes strike the park each year, most of them undetectable without instruments.
Scientists put the odds of another caldera-forming eruption at vanishingly low; the realistic hazards are hydrothermal explosions and lava flows, and no eruption is considered likely in the foreseeable future.
6. Mount Hood: Oregon’s Peak Above A Metro Area
Oregon’s tallest mountain stands about 50 miles east of Portland and last erupted in the 1780s, shortly before Lewis and Clark passed nearby. Mount Hood tends toward slow lava-dome growth rather than violent blasts, but dome collapse generates pyroclastic flows and lahars down the Sandy and Hood River drainages. Seismic and deformation instruments feed the monitoring effort documented by government hazard research agencies watching the Cascade arc.
Ski areas, highways and reservoirs sit close to those valleys, so even a modest eruption would disrupt water supply and travel across the Portland metro region.
7. Mount Shasta: A Fourteen-Thousand-Foot Cascade Giant
Northern California’s Mount Shasta rises 14,179 feet and has erupted roughly every 600 to 800 years across the past 10,000, with its most recent activity a few centuries ago. Its bulk, four overlapping cones and heavy glacier cover give it a high threat ranking within the volcano hazards program. Debris avalanches off its flanks have travelled more than 25 miles, across ground now occupied by ranches and the town of Weed.
Interstate 5 and the Sacramento River headwaters run at its base, so pyroclastic flows or lahars would cut a major transport corridor and California water supply.
8. Long Valley Caldera: The California Basin That Swells
A catastrophic eruption 767,000 years ago blew out a 20-mile-long basin in eastern California and buried much of the West in Bishop Tuff ash. Long Valley Caldera has been restless since 1980, when four magnitude-6 earthquakes struck and the resurgent dome began lifting; the floor has risen more than half a meter since. Deformation and swarm data flow into earthquake hazards monitoring run at the caldera.
Carbon dioxide seeping through soil near Mammoth Mountain has killed stands of trees and poses a real risk in snow pits and basements, a quieter hazard than eruption but the one residents actually encounter.
9. Glacier Peak: The Cascades’ Hidden Explosive Threat
Hidden behind closer ridges about 70 miles northeast of Seattle, Glacier Peak is the least visible of Washington’s volcanoes and among the most explosive. It has produced at least five large eruptions in the past 15,000 years, some of them bigger than the 1980 Mount St. Helens event, a record catalogued through national science monitoring of the Cascade Range. Only a handful of instruments sit on the mountain.
Lahars from past eruptions reached the Skagit and Stillaguamish valleys, where Darrington, Arlington and thousands of homes now stand, which keeps the peak high on federal priority lists for added sensors.
10. Mount Baker: The Steaming Peak Above Bellingham
Mount Baker vents steam continuously from Sherman Crater, just south of its summit, and heat output there jumped sharply in 1975 before settling back without an eruption. The northern Washington volcano holds more snow and ice than every other Cascade peak except Rainier, which makes meltwater-driven lahars its defining hazard in daily volcano activity notices. Its last confirmed eruptive activity came in the mid-1800s.
Baker Lake and Lake Shannon sit directly downslope behind hydroelectric dams, so a large flank collapse could displace reservoir water toward the Skagit Valley and the communities along it.
11. Redoubt: The Alaskan Volcano That Grounds Airliners
Redoubt stands across Cook Inlet from Anchorage and has erupted four times in the past century, most recently in 2009. Its 1989 eruption stalled all four engines of a KLM Boeing 747 full of passengers, which fell more than 13,000 feet before the crew restarted them, the incident that reshaped aviation ash policy and the alert framework tied to the federal hazards program.
Ash clouds from the volcano drift into transpacific flight corridors, and mudflows down the Drift River have repeatedly threatened an oil terminal at its mouth, making Redoubt a commercial as much as a geological concern.
