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Hazardous Phenomena at Volcanoes I

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Hazardous Phenomena at Volcanoes I VOLCANO HAZARDS FACT SHEET ;«verit of the U. S. Geological Survey Hazardous Phenomena at Volcanoes eruption column ra)fal landslide ^ prevailing wind ___ (debris avalanche) bombs ^/W^^-^ e dome collapse ^ dome pyroclastic flow pyroclastic flow lahar {debris flow) lava flow ^F ......,... ......,-.«' :L^^ * :::'" ~.:"- ^^^^E I I I i Volcanoes generate a wide variety i of phenomena that can alter the !Ei i Earth's surface and atmosphere and s endanger people and property. While 1 ii most of the natural hazards ,, ? illustrated and described in this fact i sheet are associated with eruptions, I some, like landslides, can occur even . J when a volcano is quiet. Small < events may pose a hazard only within 1 a few miles of a volcano, while large j 1 j events can directly or indirectly 1 [ endanger people and property tens to SET 8 5 hundreds of miles away. Simplified sketch of a i volcano and associated i~ hazardous phenomena. m . , M. ... ERUPTION COLUMNS AND CLOUliS LAVA FLOWS AND DOMES always associated with eruptions; heavy An explosive eruption blasts molten and Molten rock (magma) that pours or oozes rainfall or a large regional earthquake solid rock fragments (tephra) into the air onto the Earth's surface is called lava. can trigger a landslide on steep slopes. with tremendous force. The largest The higher a lava's silica content, the Volcanoes are susceptible to landslides fragments (bombs) fall back to the ground more viscous it becomes. For example, because they are composed of layers near the vent, usually within 2 miles. The low-silica basalt lava can form of weak, fragmented, volcanic rocks smallest rock fragments (ash) continue fast-moving (10-30 miles per hour), that tower above the surrounding rising into the air, forming a huge, narrow lava streams or spread out in broad terrane. Furthermore, some of these rocks billowing eruption column. Volcanic sheets up to several miles wide. Between have been altered to soft, slippery, clay ash is composed of fragments of rock, 1983 and 1993, basalt lava flows erupted minerals by hot, acidic ground water minerals, and glass that are less than 2 at Kilauea Volcano in Hawaii destroyed inside the volcano. At least five large millimeters (0.08 inch) in diameter. nearly 200 houses and severed the coast landslides swept down the slopes of Eruption columns can be enormous in highway along the volcano's south flank. Mount Rainier during the past 6,000 size and grow rapidly, reaching more In contrast, higher-silica andesite and years. The largest volcanic landslide in than 12 miles above a volcano in less than dacite lava flows tend to be thick, move historical time occurred at Mount St. 30 minutes. Once in the air, the volcanic slowly, and travel short distances from a Helens on May 18, 1980. ash and gas form an eruption cloud. vent. Dacite and rhyolite lava flows often VOLCANIC GASES Volcanoes Eruption clouds pose a serious hazard to form mound-shaped features called aviation. During the past 15 years about domes. Between 1980 and 1986, Mount emit gases during eruptions. Even when a 80 commercial jets have been damaged by St. Helens built a lava dome about 1,000 volcano is not erupting, cracks in the ground allow gases to vent to the surface inadvertently flying into ash, and several feet high and 3,500 feet in diameter. have nearly crashed. Large eruption through fumaroles. The most common clouds can travel hundreds of miles LAHARS (DEBRIS FLOWS OK volcanic gases are water vapor (90%), downwind from a volcano, resulting in MUDFLOWS) Lahars are mixtures carbon dioxide, sulfur dioxide, hydrogen ash fall over enormous areas. Ash from of water, rock, sand, and mud that rush sulfide, and hydrogen. Sulfur dioxide gas the May 18, 1980, eruption of Mount St. down valleys leading away from a can react with water droplets in the Helens was deposited over 22,000 square volcano. They can travel over 50 miles atmosphere downwind and fall as acid rain, miles of the western United States. With downstream, commonly reaching speeds causing corrosion and adversely affecting increasing distance downwind from a between 20 and 40 miles per hour. vegetation. Carbon dioxide is heavier than volcano, the ash particles become smaller Sometimes they contain so much rock air and tends to collect in depressions, where and the thickness of the resulting layer debris (60-90% by weight) that they look on occasion it can accumulate in lethal decreases. Minor ashfall can be a like fast-moving rivers of wet concrete. concentrations and cause people and animals nuisance to people and damage crops, Close to the volcano they have the to suffocate. Sometimes, toxic concentrations electronics, and machinery; heavy ashfall strength to rip huge boulders, trees, and of fluorine are adsorbed onto ash and can collapse buildings. houses from the ground and carry them ingested by livestock or leached into downvalley. Further downstream they domestic water supplies. PYROCLASTIC FLOWS High-speed simply entomb everything in mud. Large eruptions inject sulfur dioxide avalanches of hot ash, rock fragments, and Historically, lahars have been one of the gas into the stratosphere, where it gas move down the sides of a volcano most deadly volcanic hazards. combines with water to form an aerosol of during explosive eruptions or when the Lahars can form in a variety of ways, sulfuric acid. By reflecting solar radiation, steep edge of a dome breaks apart and either during an eruption or when a the sulfur aerosols can lower Earth's collapses. These pyroclastic flows, which volcano is quiet. Some examples include average surface temperature by a few can reach 1500° F and move at 100-150 the following: (1) rapid release of water degrees Fahrenheit. These aerosols also miles per hour, are capable of knocking from the breakout of a summit crater lake; hasten ozone destruction by altering down and burning everything in their (2) generation of water by melting snow chlorine and nitrogen chemical species in paths. A more energetic and dilute and ice, especially when a pyroclastic the stratosphere. mixture of searing gas and rock fragments flow erodes a glacier; (3) flooding is called a pyroclastic surge. Surges move following intense rainfall; and (4) Bobbie Myers and Steven R. Brantley, 1995 easily up and over ridges; flows tend to transformation of a volcanic landslide into follow valleys. a lahar as it travels downstream. The May 18, 1980 eruption of Mount For more information, contact: St. Helens generated a horizontally VOLCANIC LANDSLIDES iDEDRIS U.S. Geological Survey directed series of explosions that formed a AVALANCHES^ A landslide is a Cascades Volcano Observatory lateral blast. This blast destroyed an area rapid downslope movement of rock, 5400 MacArthur Blvd. Vancouver, Washington 98661 of 230 square miles. Trees 6 feet in snow, and ice. Landslides range in size diameter were mowed down like blades of from small movements of loose debris on Tel:(360) 696-7693 Email: [email protected] grass as far as 15 miles from the volcano. the surface of a volcano to massive Fax: (360) 696-7866 The blast exhibited characteristics of both failures of the entire summit or flanks of a URL: http://vulcan.wr.usgs.gov/ pyroclastic flows and surges. volcano. Volcanic landslides are not April 1995 USGS Open-File Report 95-231.
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