Unit 5: The Dynamic Earth Chapter 18: Volcanic Activity     Volcanoes and the Environment One glance at a world map shows that many major population centers are located near active volcanoes. From Naples, Italy to Seattle, Washington, millions of people live and work on or near the slopes of volcanoes despite the toll that volcanoes take on life and property. Why do people live and work near volcanoes? There are benefits, as well as risks, to living in the shadow of danger. Danger Zones The force of a volcanic eruption is only one of the factors that determines the danger it poses to life. The more explosive an eruption, the greater the risk to humans and the environment. This risk is measured on a qualitative scale called the volcanic explosivity index (VEI), a modified version of which is shown in the table below. The volcanic explosivity index rates volcanic risk based on the volume of material ejected, the height to which the material rises, and the duration of the eruption. The VEI ranks eruptions from 0 to 8, with 8 representing the most violent eruptions. The 1980 eruption of Mount St. Helens rated 5 on the VEI scale. Only one other eruption in history has been ranked higher-the 1815 eruption of Tambora in Indonesia ranked 7 on the VEI scale. Modified Volcanic Explosivity Index (VEI) VEI General Description Cloud Height (km) Total Number of Historical Eruptions 0 Nonexplosive Less than 0.1 487 1 Small 0.1-1 623 2 Moderate 1-5 3176 3 Moderately large 3-15 733 4 Large 10-25 119 5 Very large 25 19 6 Highly explosive Unknown 5 7 Very destructive Unknown 1 8 Catastrophic Unknown 0 Living on the Edge Volcanoes are the third-most dangerous natural hazard, after coastal flooding and earthquakes. Why, then, do people live near them? One reason is that the world's population has grown tremendously during the last three centuries. Since the 1940s, for example, the population of the United States has doubled. Millions of people now live in the Seattle-Tacoma metropolitan area in Washington state. They spend their lives in the shadow of the Cascade Range, which contains 13 potentially active volcanoes. With the exception of Mount St. Helens, however, none of these volcanoes has erupted since 1950. Study the data in the table below. Violent, explosive eruptions are quite dangerous; fortunately, they are rare events. Hundreds or even thousands of years often separate the violent eruptions of a single volcano. In terms of a human life span, such eruptions are very unusual and uncommon geologic events. Most people don't plan their lives around a natural hazard that may happen 100 years from now. They know that there is a chance that an eruption may happen sooner, but many people are willing to take that risk because of the benefits they gain from volcanoes. Some Notable Volcanic Eruptions Year Volcano and Location VEI Comments 1620 B.C. Santorini (Thera), Greece 6 Late Minoan civilization is devastated. 79 Vesuvius, Italy 5 Pompeii and Herculaneum are buried; at least 3000 people are killed. 1792 Unzen, Japan 2 A debris avalanche and tsunami kill 14,500. 1815 Tambora, Indonesia 7 Most explosive eruption in recorded history; 92,000 people die-80,000 deaths as a result of famine from crop failures. Weather changes due to eruption. 1883 Krakatau, Indonesia 6 Caldera collapses, killing 36,000 people. 1902 Mount Pelée, Martinique 4 St. Pierre is destroyed; 30,000-40,000 people die. 1912 Katmai, Alaska 6 May have been the largest 20th century eruption; 33 km3 of tephra were ejected. 1914-1917 Lassen Peak, California 3 California's last historic eruption 1980 Mount St. Helens, Washington state 5 Six hundred square kilometers of land devastated; at least 60 people perish. 1982 El Chichon, Mexico 4 Ash flow killed 1877 people. Eruption affected climate. 1985 Nevado del Ruiz, Columbia 3 Eruption caused damaging mudflows; 23,000 people are killed. 1986 Lake Nyos, Cameroon, Africa ? Carbon dioxide gas cloud erupted from beneath lake; 1746 people are killed. 1991 Mt. Pinatubo, Philippines 5 Second largest eruption in the 20th century; 932 people are killed. 1991 Unzen, Japan 4 Pyroclastic flow kills 43 people. 1983-present Kilauea, Hawaii ? Longest known continuous volcanic eruption LINK-UP: Find out more information about volcanic eruptions. Volcanic Soils Many people risk living near volcanoes for economic reasons. Soils formed from volcanic debris and rock are rich in calcium, phosphate, boron, and potash, all of which make them very fertile. Also, the porous nature of volcanic debris promotes good drainage and enhances the decay of organic material. These two factors also contribute to the soils' high productivity. Volcanically active regions are some of the most valuable agricultural regions in the world. Other Useful Volcanic Materials Volcanic rocks and minerals have important industrial uses. Basalt, for example, is used to build roads. Pumice is commonly used in soap and as an abrasive for polishing. Native sulfur, which forms near volcanic vents, is used to make sulfuric acid, which is used to manufacture dyes, paints, paper, textiles, and industrial chemicals. Sulfur is also used in the production of metals and in petroleum refining. It is also an ingredient in fertilizers, insecticides, fungicides, rubber, shampoos, storage batteries, and many medicines. Geothermal Energy Another valuable resource associated with volcanic areas is geothermal energy. Large magma chambers situated near Earth's surface hold tremendous amounts of thermal energy, which can be tapped as a source of heat or converted to electricity. Because Iceland sits atop the Mid-Atlantic Ridge, many homes in this country are heated directly by volcanic hot springs. In volcanically active areas in the United States, Mexico, Italy, and New Zealand, geothermal energy is also used to produce electricity. Currently, two kinds of reservoirs, dry-steam reservoirs and reservoirs of high-temperature water that can be changed into steam, are used to generate electricity. In both processes, steam from the reservoir is used to spin turbines and generators to produce electricity. Geothermal energy has many advantages over other sources of energy. First, it produces almost no air pollution. Second, it is highly reliable. Third, geothermal energy is one of the cleanest sources of electric power currently available. Why, then, isn't it used more extensively to meet the ever-growing energy demands of Earth's increasing human population? Relatively few other places on Earth have magma chambers close to the surface, and thus, the use of geothermal energy is limited. To generate geothermal power elsewhere, deep wells must be drilled. This process is very expensive and can disrupt ecosystems near the wells. Another drawback to the use of geothermal energy is the large volume of dissolved minerals in geothermal fluids that precipitate when the fluids cool. These minerals can create solid-waste problems for geothermal power plants because the minerals often contain heavy metals that are dangerous to humans and other living things. The minerals can also clog and corrode pipes in the power plants. If these problems can be solved, geothermal energy could well be the energy source of the future. Scientists estimate that the magma chambers situated within 10 km beneath Earth's surface hold roughly 5000 times as much energy as people in the United States alone use in one year! This essentially untapped source of energy could decrease the world's dependence on other sources of fuel and electricity. Activity Research one of the historical volcanic eruptions listed in the table entitled Some Notable Volcanic Eruptions. Write an account of the eruption, as if you had been an eyewitness. Include in your account some background as to why you were living near the volcano.