Living With Volcano Hazards The grandeur of volcanoes, whether The eruption and erosion processes olcanic eruptions are among capped by snow, covered by forests, that shape volcanoes are an ever-present VEarth’s most dramatic and or rocky and barren, often fails to hint threat. This fact sheet describes volcanic powerful agents of change. Ash, at activity deep underground. Beneath hazards and their impacts at two common mudflows, and lava flows can devastate an active volcano, rising magma types of volcanoes—stratovolcanoes and communities near volcanoes and (underground molten rock) generates shield volcanoes. Volcano terms, shown in cause havoc in areas far downwind, earthquakes as it exerts pressure on and bold, are explained in two large graphics. downstream, and downslope. Even fractures surrounding rock. These fractures Using the information in this fact sheet when a volcano is quiet, steep volcanic create pathways for rising heat, volcanic and the internet resources on page 6, you slopes can collapse to become gases, and magma. Just below the surface, can learn more about volcanoes and how landslides, and large rocks can be groundwater boils to steam. When magma to prepare for future activity. hurled by powerful steam blasts. reaches the surface a variety of eruption Hazardous volcanic conditions might processes can occur. At some volcanoes, last for a day or decades, all the while tiny shards of rock can be blasted skyward threatening people’s health and safety. in columns of ash and steam. Lava can be Volcano Hazards Are Scientists with the U.S. Geological pushed out of the ground as thick flows or Location Dependent— Survey and partner agencies assess pile up into steep-sided mounds. At other hazards and closely monitor activity at volcanoes, more-fluid lava might fountain Heed Emergency Advice the Nation’s volcanoes. They provide into the air in a spectacular incandescent Volcano hazards have the potential volcano updates and warnings of display and then flow quickly across to cause great harm and destruction, hazardous situations, as well as the ground. With each new series of but whether or not you will be affected guidance on actions to take. You can eruptions, the shape of a volcano changes, depends on the type of volcano prepare your family and community by growing more massive or catastrophically (stratovolcano or shield volcano), the familiarizing yourself with the types of collapsing. Even when activity stops, specific hazard, and your location hazards at volcanoes near where you potentially hazardous change continues relative to it. In all instances, heed the live and visit. as water scours loose volcanic debris and advice of emergency officials! transports it far down river valleys. In this nightime view, Mount Shasta, a stratovolcano of the Cascade Range, rises above the town of Weed in northern California—one of four small towns around the base of the volcano. This region is in the center of major interstate, rail, and air transportation corridors on the West Coast. (Copyrighted photograph by Mark Stensaas, used with permission.) U.S. Department of the Interior Fact Sheet 2018–3075, April 2019 U.S. Geological Survey Supercedes USGS Fact Sheet 002–97 Stratovolcanoes Prevailing wind Eruption cloud Ash (tephra) fall Eruption column Acid rain Landslide (debris avalanche) Lava dome collapse Pyroclastic density current Lava dome Vent Pyroclastic density current Fumarole Hydrothermal explosion Lahar (mud or debris flow) Lava flow Groundwater Magma Cracks Stratovolcanoes are tall, cone-shaped, and often glaciated Avoid Hazards and Heed Official Advice volcanoes that tend to erupt explosively. In the Western United States, they are located in Alaska, Washington, Oregon, and northern • Lava flows, pyroclastic density currents, landslides, and California. Eruptions can proceed in the following sequence, but these rockfalls seldom travel far beyond a stratovolcano’s base. At events may occur independently. Magma (underground molten rock) the volcano, tephra fall will include large hot blocks of rock, rises from deep below the volcano, and (1) explosive eruptions blast and dangerous concentrations of volcanic gases can collect volcanic debris (tephra) into the sky, forming an eruption column and in caves. At actively outgassing volcanoes, avoid low and cloud. (2) The ash cloud cools and drifts downwind, and ash falls to confined areas where dangerous gases might accumulate. the ground at local air temperature. (3) Lava flows move downslope or form lava domes at the erupting vent. (4) Eruption columns, lava flows, • Volcanic ash can fall tens to even thousands of miles away or lava domes collapse, creating hot pyroclastic density currents. from an eruption. Avoid travel and reduce exposure by staying (5) These can melt snow and ice or enter rivers to create lahars. Even indoors. When exposed to intense falling or windblown ash, use when a volcano is not erupting, landslides or debris avalanches can protective breathing devices, such as masks. occur due to gravity. At stratovolcanoes and caldera systems that host hot springs, fumaroles, and other thermal features, hot rock and • Lahars are a hazard on volcanic slopes and to downstream steam blasts called hydrothermal explosions can occur at any time. communities tens of miles from a volcano. If you receive a Caldera systems are large volcanic centers usually characterized by public emergency notification of a lahar, or otherwise recognize a massive central crater, like at Long Valley in California and in the its approach, get off valley floors to higher ground as quickly Wyoming part of Yellowstone National Park. and safely as possible. 2 Eruption Clouds and Ashfall—The machinery and is a health hazard to humans such as fires, methane explosions, and Most Widespread Volcano Hazards and animals. dangerous conditions where they enter bodies of water. Fortunately, lava flows Explosive eruptions originate at Lava Flows Consume Everything in typically travel slowly enough that people volcanic vents and blast tephra (rock and can avoid contact. volcanic glass fragments) and volcanic Their Way gases into the air with tremendous force. Lava flows are molten rock and ooze Pyroclastic Density Currents Are An eruption column can reach more onto the Earth’s surface from a volcanic Hot, Fast, and Destructive than 12 miles above a volcano in less vent. Stratovolcanoes, such as the large than 30 minutes. The column rises and cone-shaped volcanoes in the Pacific The term pyroclastic density current spreads to form an eruption cloud. Northwest and Alaska, often erupt viscous refers to a family of ground-hugging Volcanic ash particles (those smaller lava that flows like toothpaste. Thick and avalanches of hot gases and dry boulder- than about 0.08 inch in diameter) within sluggish lava may travel a mile or more to-ash-size rock fragments that rush down the column and cloud are highly charged downslope. When flows of extremely volcanic slopes at speeds that can exceed and generate lightning and thunder. The viscous lava pile up around a vent, they 220 mi/hr. They originate as viscous eruption cloud can extend thousands of build a lava dome. The thick and rubbly lava flows, domes, and eruption columns miles downwind and rain gritty ash over edges of new lava flows and domes that collapse and then roar downward. vast regions. The largest tephra pieces fall can collapse to form hot, fast-moving Rock-rich pyroclastic flows typically near the vent, whereas the wind blows pyroclastic density currents. follow channels and valleys. Gas-rich cooled volcanic ash great distances— Shield volcanoes, like those in the pyroclastic surges are more mobile and even around the world. State of Hawaii, erupt less-viscous lava that spread unpredictably over valleys and hills. Volcanic ash is a threat to people, flows like warm honey. These lava flows Flows may reach more than 10 miles from agriculture, aviation, ground transportation, travel as glowing orange rivers or creep the source and leave deposits that range and communications and utility systems. across terrain as shimmering silver masses in thickness from a fraction of an inch on For both ground and air transportation, ash that sprout lobes of red liquid rock. Lava hillslopes to more than 700 feet in deep reduces visibility, short-circuits electronic can flow as fast as 30 miles per hour (mi/hr) valleys. The extreme temperatures of such equipment, and damages engines—jet on steep slopes but at less than 1 mi/hr on flows (as high as 1,500 °F) can cause fires aircraft are particularly vulnerable. gentle slopes. Lava is erupted directly from and melt snow and ice, which may mix to Flight delays and cancellations can open vents as flows or lava fountains (jets generate lahars (see page 5). interrupt delivery of goods and supplies. of low-viscosity lava sprayed into the air). As pyroclastic density currents spread, Accumulation of ash several inches thick, Anything in the way of an advancing they knock down, shatter, burn, bury, and especially when wet, can collapse roofs, lava flow will be surrounded, buried, or (or) carry away nearly everything in their clog wastewater systems, and reduce the burned by extreme heat (as high as paths. People and animals may be killed or quality of drinking water. Even minor 2,100 degrees Fahrenheit, °F). Lava seriously injured from burns and inhalation ashfall can harm crops, electronics, and flows also create secondary hazards of hot ash and gases. Ashfall from the May 18, 1980, eruption of Lava flows have destroyed more than 900 structures On August 7, 1980, a small explosive eruption Mount St. Helens, Washington, blocked the sun on the Island of Hawaiʻi since 1983. In May 2018, occurred in the crater of Mount St. Helens, causing street lights to turn on at 1:30 p.m. along erupting fissures opened along Kīlauea Volcano’s Washington. The eruption generated a Yakima Avenue in Yakima, Washington.