The most dangerous volcanoes in the U.S., according to scientists

A volcano's place on the U.S. Geological Survey's threat list says little about when it will next erupt. It says a great deal about what happens if it does. The USGS National Volcanic Threat Assessment, updated in 2018, scores 161 active volcanoes on two separate axes. The first is hazard — the kinds of destruction a volcano can produce, including pyroclastic flows, lahars, ashfall and lava flows. The second is exposure — the people, aircraft, water systems, highways and power lines sitting within reach. Multiply the two and you get a threat score. The very high threat category holds 18 volcanoes.

That method produces some counterintuitive results. A mountain silent for a thousand years can outrank one that erupts every few years, because tens of thousands of homes now sit on the debris of its last collapse. A remote Aleutian peak can rank near the top while endangering almost no one on the ground. Its ash plumes rise into the flight corridors linking North America and Asia. The most dangerous volcanoes in the U.S. cluster in three regions — the Cascade Range of the Pacific Northwest, Alaska and Hawaii — and each poses a different kind of risk.

Volcanoes differ from earthquakes in one respect that shapes how they are managed. Magma moving toward the surface usually announces itself weeks or months ahead, through earthquake swarms, measurable swelling of the ground and changes in gas chemistry. That warning window justifies the seismometers, GPS stations, gas sensors and satellite radar the USGS concentrates on these mountains. It also justifies the evacuation drills and warning sirens in the valleys below.

The seven volcanoes that follow rank at or near the top of the federal assessment, each for a different reason. The list runs through relentless activity, a catastrophic explosive record, mudflow exposure, closeness to a metro area, aviation disruption and the raw scale of what a caldera system can do. Together they amount to a working map of volcanic risk in the country.

Illustration: Quartz

Kilauea holds the highest threat score of any volcano in the U.S. Geological Survey's national threat assessment, and the reason is frequency rather than explosive force. The Big Island volcano erupted almost continuously from 1983 to 2018, and has erupted repeatedly since, with homes standing within a few miles of where lava reaches the surface.

The 2018 eruption in the lower East Rift Zone showed what that proximity costs. Fissures opened in and around the Leilani Estates subdivision in the Puna district — more than 20 of them over the course of the episode — and lava destroyed more than 700 homes and other structures before entering the ocean. Roads, water lines and power poles went with them. Residents evacuated as the fissures opened, which is characteristic of Kilauea: basaltic lava moves slowly enough that it seldom kills, but it buries whatever lies in its path.

Kilauea is a shield volcano, built from fluid lava rather than the sticky, gas-rich magma that drives the explosive eruptions of Cascade stratovolcanoes. That limits the scale of its blasts without ruling them out. When the summit lava lake drained in 2018 and groundwater reached the hot conduit, Halemaumau crater produced a sequence of explosive events and the caldera floor collapsed inward. A comparable explosive episode occurred at the summit in 1924.

Sulfur dioxide is the hazard that travels farthest. Gas emissions react in the atmosphere to form vog, a haze that drifts downwind across the island and degrades air quality far from the eruption site, with the sharpest effect on people who have asthma or heart conditions.

The USGS Hawaiian Volcano Observatory tracks the volcano with seismometers, GPS receivers, tiltmeters and gas sensors, and its accessibility has made Kilauea one of the most thoroughly studied volcanoes anywhere.

The risk stays concentrated on the island's southeastern flank rather than spread across Hawaii. Honolulu sits on Oahu, well outside the hazard zone, and the communities closest to Kilauea — Pahoa, Volcano and the lower Puna subdivisions — are small. Near-constant eruption beside settled ground is what puts Kilauea first in the federal ranking.

Illustration: Quartz

Mount St. Helens in Washington state carries the highest threat score of any volcano in the contiguous U.S. Its May 18, 1980, eruption killed 57 people, the deadliest volcanic event in American history.

That morning began with a magnitude 5.1 earthquake that collapsed the mountain's bulging north flank, producing the largest landslide in recorded history. The collapse released the pressure holding magma and superheated groundwater in place, and the resulting lateral blast flattened forest across roughly 230 square miles.

The ash column climbed about 15 miles into the atmosphere. Ash fell across 11 states, drifting as far east as Montana. The summit lost roughly 1,300 feet of elevation, leaving the horseshoe-shaped crater that still defines the peak.

Mudflows carried the destruction farthest. Debris flows ran down the North Fork Toutle River into the Cowlitz, wrecking bridges, homes and sections of highway tens of miles from the crater. They also pushed enough sediment into the Columbia River to shoal the shipping channel serving Portland, Oregon, stranding vessels until dredging reopened it.

The volcano has stayed active. A dome-building episode between 2004 and 2008 pushed fresh lava into the crater without threatening anyone outside the closed zone, evidence that the magmatic system is still charged. The USGS Cascades Volcano Observatory watches it with seismometers, tiltmeters, GPS receivers and satellite radar capable of detecting the ground swelling that usually precedes an eruption.

The mountain stands about 50 miles northeast of Portland, with Washington state towns including Castle Rock and Toutle sitting along its drainage corridors. The U.S. Army Corps of Engineers operates a sediment retention structure on the North Fork Toutle built to hold back the loose 1980 debris still working its way downstream decades later.

Cowlitz County emergency planners rehearse evacuations of those river valleys for a reason. Renewed explosive activity would route lahars down the same channels, and a flow moving at highway speeds leaves little time for anyone below to react on short notice.

Illustration: Quartz

Mount Rainier puts more people at risk than any other volcano in the country, and the danger is not mainly the eruption itself. The threat is the lahar — a fast-moving slurry of melted ice, rock and mud that can travel down river valleys where tens of thousands of Washington state residents live. The peak last erupted roughly 1,000 years ago.

Two conditions make it unusual. Rainier holds more glacial ice than any other mountain in the contiguous U.S., a water supply that eruptive heat can release in minutes. Large sections of its upper cone have also been altered by hydrothermal fluids, acidic water circulating through the volcano over thousands of years that turns hard rock into soft clay. Clay-rich slopes fail easily, with or without magma involved.

The deposits ringing the mountain record the result. The Osceola Mudflow, about 5,600 years ago, carried away Rainier's summit and spread across the Puget Sound lowlands, covering ground now built over in the Kent and Auburn area. The Electron Mudflow, roughly 500 years ago, ran down the Puyallup River valley with no confirmed eruptive trigger, leaving sediment beneath present-day Orting.

That second case shapes emergency planning. A flank collapse can begin without rising magma, so the familiar precursors — earthquake swarms, ground swelling, shifts in gas output — may never appear. Inundation modeling gives valley communities on the order of 30 to 60 minutes between a flow starting high on the mountain and reaching town.

Pierce County runs a lahar detection and siren network built for exactly that window, one of the few civilian systems of its kind in the U.S. The U.S. Geological Survey's Cascades Volcano Observatory supports it with acoustic flow monitors, sensors tuned to the ground vibration of moving debris rather than to seismic waves from earthquakes. Schools inside mapped hazard zones rehearse evacuation on foot, since vehicles on valley roads would not clear the zone in time.

Illustration: Quartz

Mount Shasta sits in the very high threat tier of the USGS assessment because a single eruption could produce pyroclastic flows, lava dome collapses and glacier-fed mudflows at once, with towns, a railroad and Interstate 5 built within a few miles of its base. The 14,179-foot stratovolcano has erupted on average every 600 to 800 years over the past 10,000 years, and its most recent activity is generally placed around 1250 CE.

Shasta is four overlapping cones rather than a single peak, built and rebuilt over roughly 600,000 years. The youngest, Shastina, formed about 9,500 years ago and sent pyroclastic flows down the western flank, across the ground the city of Weed now occupies. Those currents of hot ash and gas travel at highway speeds, which leaves no realistic margin for evacuation once one begins.

The mountain holds the largest concentration of glacial ice in California, including Whitney Glacier, the longest in the state. That ice shapes hazard planning even when no magma is involved. During the hot summer of 1924, meltwater mobilized loose debris and sent mudflows down Mud Creek on the south flank. Eruptive heat would melt snow and ice far faster, pushing debris flows into the Sacramento, Shasta and McCloud river drainages, the corridors USGS hazard maps treat as most exposed.

Interstate 5 runs along the western base of the volcano, carrying freight and passenger traffic between California and Oregon, and the Union Pacific $UNP main line follows the same corridor. An eruption would threaten both at once, along with the towns strung along those valleys — Weed, Mount Shasta, Dunsmuir and McCloud.

Shasta generates little background seismicity, which leaves scientists a thin baseline against which to judge any new signal. The California Volcano Observatory, created in 2012, has added seismic and ground-deformation instruments on and around the mountain to close that gap, since a quiet volcano can give less obvious warning than one that rumbles regularly.

Illustration: Quartz

Mount Hood ranks in the very high threat tier of the USGS assessment less for how violently it erupts than for what sits downstream and downwind. The peak stands about 50 miles east of downtown Portland, Oregon, above valleys that carry highways, ski resorts and year-round residential communities.

Hood's last major eruptive period came in the late 1700s, when magma built the lava dome now called Crater Rock just below the summit on the south side. Collapsing material from that dome sent debris flows down the Sandy River and loaded the channel with so much sediment that Lewis and Clark found the lower river clogged with sand when they reached its mouth in 1805. Observers reported minor steam and ash events in the mid-1800s and again in 1907.

The glaciers on Hood hold far less ice than those on Rainier, which caps the volume of meltwater available to drive a mudflow but does not remove the hazard. Debris flows have traveled the Sandy, Zigzag and White river drainages repeatedly, and those corridors now hold Government Camp, Rhododendron and long stretches of U.S. 26 and Oregon 35. USGS hazard assessments map the inundation zones along them to guide emergency planning and land use decisions.

Unrest at Hood is modest today. Steam and gas escape from fumaroles near Crater Rock, and small earthquake swarms cluster a few miles south of the summit. Neither signals an eruption. The Cascades Volcano Observatory maintains seismic stations on the flanks because even a moderate event would carry consequences for a metropolitan area of more than two million people.

Ashfall is the hazard most likely to reach Portland itself. Volcanic ash halts air traffic, clogs engine filters and storm drains, and fouls water treatment systems. Hood's eruptive style leans toward dome growth and localized pyroclastic flows rather than a lateral blast, but proximity, not magnitude, drives its ranking.

Illustration: Quartz

Mount Spurr and Redoubt Volcano rank near the top of Alaska's federal threat list mainly because of aviation and infrastructure risk rather than the number of people living nearby. Both stand in thinly settled country west of Anchorage, and both can loft ash into the altitudes where jets cruise.

Redoubt gave the clearest demonstration in December 1989, when it erupted and KLM Flight 867, a Boeing $BA 747 carrying more than 200 passengers and crew, flew into the ash cloud at night. All four engines quit. The pilots restarted them after the aircraft had fallen more than 10,000 feet, then landed at Anchorage. Damage was heavy enough that all four engines had to be replaced.

Volcanic ash is pulverized rock and glass, not smoke. It melts in the hot section of a jet engine, builds up on turbine blades and sandblasts windscreens until crews can barely see through them. Onboard weather radar does not reliably separate it from ordinary cloud, so avoiding ash depends on ground-based volcano monitoring and forecast plume tracks rather than on what a pilot can spot ahead.

Redoubt erupted again in 2009. Spurr's Crater Peak vent erupted three times in 1992, dusting Anchorage with ash and closing the city's international airport. Many flights between North America and Asia cross this stretch of the Aleutian Arc each day, so one sustained plume can force reroutes far beyond Alaska.

Redoubt carries a hazard on the ground as well. Its eruptions melt summit ice and send lahars down the Drift River valley toward the Drift River Terminal, an oil storage facility near the river mouth. During the 2009 eruption, operators drained stored crude and raised protective berms as mudflows reached the site.

The Alaska Volcano Observatory watches both volcanoes with seismic networks, satellite imagery and infrasound sensors, since much of the arc lies too far from roads for routine ground visits. It publishes color-coded aviation alerts — green, yellow, orange and red — that airlines and the National Weather Service use to shift routes before ash spreads downwind. More than 50 Alaska volcanoes have erupted since about 1760.

Illustration: Quartz

Long Valley Caldera earns its place among the highest-threat volcanoes in the U.S. not for how often it erupts but for the scale of what it can produce. The eastern California basin next to Mammoth Lakes formed roughly 760,000 years ago in an eruption that blanketed the region in ash and sent fallout as far east as present-day Nebraska.

There is no summit to point at. The caldera is a depression about 20 miles long and 11 miles wide, left behind when the roof of a magma chamber dropped into the void the eruption had emptied. A resurgent dome sits inside it, a block of the caldera floor shoved upward by pressure from below, and that dome is the feature geologists track most carefully.

The modern record of unrest starts in May 1980, when four magnitude 6 earthquakes hit near the caldera's southern margin. Surveys afterward found the resurgent dome had risen, and swarms and uplift returned in episodes through the 1980s and 1990s without producing an eruption. That sequence pushed the USGS to install one of its densest volcano monitoring networks anywhere in the country, pairing GPS receivers, borehole strainmeters, seismometers and gas sensors.

Mammoth Mountain, on the caldera's southwest rim, carries a hazard that requires no eruption at all. After an earthquake swarm in 1989, carbon dioxide rising from depth began seeping through the soil at concentrations high enough to kill stands of trees near Horseshoe Lake. The gas is heavier than air and can collect in snow wells, pits, basements and other depressions, and the U.S. Forest Service posts warnings in the affected areas.

A repeat of the caldera-forming event is not what the USGS treats as the likely scenario. The more probable outcome is a smaller eruption along the Mono-Inyo Craters chain, which extends north from the caldera toward Mono Lake and last erupted roughly 600 years ago. Those eruptions built lava domes and generated ash and pyroclastic flows on a local scale — dangerous close to the vent, but far short of the Bishop Tuff.