Volcano Hazards, Volcanic Activity Hazards

There are several kinds of events caused by volcanic activity that can be harmful to life and property. These include lava flows, lahars, ash falls, debris avalanches, and pyroclastic density currents.

Mitigation of hazards is an important goal of the volcanological community, including the U.S. Geological Survey. Volcanologists themselves require safety procedures for conducting hazardous scientific studies on volcanoes.

Many volcanoes around the world have been targeted for hazards research, with several of the most notorious volcanoes being designated Decade Volcanoes for concentrated hazards research, such as the Decade Volcano is Popocatepetl, which is within striking range of 30,000,000 people around it, including Mexico City.

Types of Hazard Make-up

Pyroclastic Flows

Pyroclastic flows and surges are high-speed avalanches of hot ash, rock fragments, and gas that move down the sides of a volcano during explosive eruptions or when the steep edge of a dome breaks apart and collapses. These pyroclastic flows, which can reach 1,500 degrees Fahrenheit and move at 100-150 miles per hour, are capable of knocking down and burning everything in their paths. A more energetic and dilute mixture of searing gas and rock fragments is called a pyroclastic surge. Surges move easily up and over ridges, while flows tend to follow valleys. For example, more than 2,000 people died from pyroclastic surges during the 1982 eruption of El Chichon in southern Mexico.

A blast can exhibit characteristics of both pyroclastic flows and surges. For instance, the May 18, 1980 eruption of Mount St. Helens, in the U.S., generated a horizontally directed series of explosions that formed a lateral blast. This blast destroyed a 230-square-mile area. Trees 6 feet in diameter were mowed down like blades of grass as far as 15 miles from the volcano.

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Lahars

Lahars are mixtures of water, rock, sand, and mud that rush down valleys leading away from a volcano. They can travel over 50 miles downstream, commonly reaching speeds between 20 and 40 miles per hour. Sometimes they contain so much rock debris (60-90% by weight), that they look like fast-moving rivers of wet concrete. Close to the volcano, they have the strength to rip huge boulders, trees, and houses from the ground and carry them down the valley. Further downstream, they simply entomb everything in mud. Historically, lahars have been one of the most deadly volcanic hazards.

Lahars can form in a variety of ways, either during an eruption or when a volcano is quiet. Some examples include: (1) rapid release of water from the breakout of a summit crater lake; (2) generation of water by melting snow and ice, especially when a pyroclastic flow erodes a glacier; (3) flooding following intense rainfall; and (4) transformation of a volcanic landslide into a lahar as it travels downstream.

Lahars have destroyed several villages on Indonesian volcanoes because most people live in the valleys where the lahars flow. In Armero, Colombia, 21,000 died when a lahar was formed during a small eruption of Nevado del Ruiz in 1985. This lahar was generated by meltwater, which resulted from the interaction of the pyroclastic surges with snow and ice. Downstream, lahars can transform into regular floods as they become increasingly diluted with water. This phenomenon was first discovered at Mount St. Helens, where hot pyroclastic surges transformed into lahars, which then transformed into hyperconcentrated streamflow and then into normal stream-flow turbulence (floods).

Landslides

A landslide is a rapid downslope movement of rock, snow, and ice. Landslides range in size from small movements of loose debris on the surface of a volcano, to massive failures of the entire summit or flanks of a volcano. Volcanic landslides are not always associated with eruptions; heavy rainfall or a large regional earthquake can trigger a landslide on steep slopes. Volcanoes are susceptible to landslides because they are composed of layers of weak, fragmented, volcanic rocks that tower above the surrounding terrain. Furthermore, some of these rocks have been altered to soft, slippery, clay minerals by hot, acidic ground water inside the volcano. At least five large landslides swept down the slopes of Mount Rainier during the past 6,000 years. The largest volcanic landslide recorded occurred at Mount St. Helens on May 18, 1980.

The hilly topography north of Mount Shasta in northern California is now known to be the result of a debris-flow avalanche. Some are known to extend up to 53 mile from their sources and cover tens to more than 386 square miles.

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Lava Flows

Molten rock, or magma, that pours onto the Earth's surface is called lava. The higher a lava's silica content, the more viscous it becomes. Low-silica basalt lava can form fast-moving (10-30 miles per hour), narrow lava streams or spread out into broad sheets up to several miles wide. Between 1983 and 1993, basalt lava flows erupted at Kilauea Volcano in Hawaii destroying nearly 200 houses and severing the coast highway along the volcano's south flank.

Major hazards of lava flows include, burying, crushing, covering, and burning everything in their path. Sometimes lava melts ice and snow to cause floods and lahars. Lava flows can dam rivers to form lakes that might overflow or break their dams causing floods. Methods for controlling the paths of lava flows include: constructing barriers and diversion channels, cooling the advancing front with water, or disrupting the of source or advancing front of the lava flow by explosives.

Tephra

When a volcano erupts it will sometimes eject material known as tephra, i.e. rock fragments, into the atmosphere. The largest pieces of tephra are called blocks and bombs. Blocks and bombs are normally shot ballistically from the volcano, and because they are so large, they tend to fall near their source. Blocks and bombs as large as 8-30 tons have fallen as far as .6 miles or 1 kilometer from their source. Small blocks and bombs have been known to travel as far away as 12-50 miles. Smaller ejecta, such as lapilli 0.5-16 inches in size and ash less than 0.5 inches, which are lifted upward by the heat of the eruption, will fall farther from the volcano. The smallest particles, which are less than 0.0025 inches, can stay in the atmosphere for two or three years after a volcanic eruption. Sometimes these particles produce fantastic sunsets, such as was seen after the eruptions of Krakatau in 1883 and Pinatubo in 1991. Some scientists believe that these particles may contribute to global warming.

The eruption of Krakatau deposited ash over an area of 800,000 square miles. People 43-50 miles away from the volcano had their clothes burnt by the ash. Fallout from the eruption of Tambora in 1815, which is the largest eruption in recent history, was deposited as far as 807 miles away.

Tephra produces a wide range of hazards. When the ejected material is in the atmosphere, it is electrically charged and often produces lightning, which has killed a number of people. Large ejecta shot ballistically from the volcano are also a hazard to those unfortunate enough to be near the volcano. Other hazards are produced when the ash is deposited on the ground. Ash can disrupt electricity, television, radio, and telephone communication lines, bury roads and other manmade structures, damage machinery, start fires, and clog drainage and sewage systems.

Ash can also produce poor visibility and cause respiratory problems. Often people living in areas affected by volcanic ash will wear filter masks or wet cloths over their nose and mouth to protect themselves from breathing in volcanic ash and fumes. If ash builds up on the tops of roofs, it will often cause structural collapse. This is especially common of flat-topped buildings. Most of the deaths resulting from the eruption of Mount Pinatubo in 1991 were due to collapsing roofs.

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Volcanic Gas

Volcanoes emit gases during eruptions. Even when a volcano is not erupting, cracks in the ground allow gases to vent to the surface. The most common volcanic gases are water vapor (90%), carbon dioxide, sulfur dioxide, hydrogen sulfide, and hydrogen. Sulfur dioxide gas can react downwind with water droplets in the atmosphere and fall as acid rain, causing corrosion, and adversely affecting vegetation. Carbon dioxide is heavier than air and tends to collect in depressions, where on occasion, it can accumulate in lethal concentrations and cause people and animals to suffocate.

Large eruptions inject sulfur dioxide gas into the stratosphere, where it combines with water to form an aerosol of sulfuric acid. By reflecting sunlight, the sulfur aerosols can lower Earth's average surface temperature by a few degrees Fahrenheit. These aerosols also hasten ozone destruction by altering chlorine and nitrogen chemical species in the stratosphere.

Tsunamis

A tsunami is a long-period seismic generated by a sudden displacement of water. Tsunamis travel at very high speeds through deep water as low broad waves, building to great heights as they approach the shallow bottom of shores. Most are caused by fault displacements on the sea floor, but many have been caused by volcanic action. The eruption of Krakatau in 1883 produced tsunamis that killed 36,000 people. The pyroclastic flow generated by this eruption, displaced the water that initiated the tsunamis.

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