from The New Book of Knowledge®
While millions of years are needed for natural forces to build mountain ranges and erode canyons, an erupting volcano can build or destroy large, sometimes massive, structures in a matter of minutes or days. A volcano is a place where molten (melted) rock, pieces of hot solid rock, and hot gases formed inside the Earth erupt through its surface. Volcanic activity is the most dramatic and rapid of all the geologic processes that shape the surface of the Earth. For example, on May 18, 1980, the eruption of Mount St. Helens, in the northwestern United States, destroyed much of the mountain and killed about sixty people.
How Volcanoes are Formed
The Earth's interior is very hot, and the heat left over from its formation continually escapes toward the surface. The escape of this heat combined with the heat released by decaying radioactive elements in the interior provides the energy for volcanic activity. As the heat rises, it warms rock in the Earth's interior, and this heated rock begins rising slowly toward the surface. Much of this molten rock, or magma, gradually cools within the Earth. Sometimes, however, magma continues to push upward.
While heat is what causes magma to rise to the surface, escaping gases provide the driving force for volcanic eruptions. The most common volcanic gas is steam. This is formed when water from within the Earth or rainwater soaking into the ground is absorbed by magma and then heated.
When gas-filled magma rises from the Earth's interior, it collects in large pools called magma chambers that are several miles below the surface. As the amount of gases increases, the magma becomes lighter, causing it to rise through a system of conduits, or channels. When the magma reaches the surface, it spews out of vents, or openings, at the tops of the magma conduits. Magma that erupts from a vent is known as lava. Lava and other material, such as ash and rock fragments, ultimately build up to form a volcanic cone.
Types of Volcanoes
Some volcanoes are large mountains with summits, or peaks, reaching 20,000 feet (6,000 meters) or more above sea level. Others are small domes of ash or lava that on average are only about 100 feet (30 meters) high. Scientists recognize four main types of volcanoes: stratovolcanoes, shield volcanoes, cinder cones, and ashflow calderas.
Stratovolcanoes, or composite volcanoes, get their name from the different layers, or strata, of ash and lava that form them. Most stratovolcanoes are irregularly shaped mountains formed by material ejected from both a central vent and other vents on the sides of the mountain. But some stratovolcanoes, such as Mount Fuji in Japan, are almost perfectly cone-shaped. These shapes occur because volcanic materials erupt from one central vent at the top.
Stratovolcanoes usually have large, circular depressions at their summits. When they are less than 1 mile (1.6 kilometers) across, they are called craters. Larger depressions are called calderas. Craters are simply eruption vents--the tops of magma conduits. Calderas are more complicated. Those found on stratovolcanoes typically are surrounded by thick layers of ash and other material ejected during eruptions. Others are formed when the summit of a volcano collapses into an empty magma cavern that had stored the magma prior to eruption.
Shield volcanoes, formed when large flows of lava spread rapidly from central vents or rows of vents, are the largest volcanoes. Because the lava moves rapidly and spreads out in a broad area, it does not form a tall, steep cone. Instead, shield volcanoes are dome-shaped mountains with broad bases and gentle slopes. Shield volcanoes commonly occur in overlapping groups. Mauna Loa, one of the largest shield volcanoes, is one of five overlapping volcanoes that make up the large island of Hawaii.
Shield volcanoes also have craters or calderas at their summits. The calderas on shield volcanoes are almost always due to collapse rather than from the eruption of ash layers.
Not all volcanoes are huge. Many small ones called cinder cones dot the landscape in volcanic regions. In fact, cinder cones are the most common type of volcano on the Earth's surface. They are formed as a result of repeated small explosions of gas-rich magma, which eject small chunks of lava and ash onto a small area around the vent. The average cinder cone is about 2,600 feet (800 meters) wide at the base and 325 feet (100 meters) high.
The most explosive eruptions on Earth are produced by ashflow calderas. These volcanoes have a distinctive form that is essentially a large caldera surrounded by low hills of ash deposits. Their powerful eruptions tear magma into ash and dust-size fragments and blow them so far from the volcanic vent that almost no mountain is built up. The ash is carried by hurricane-force winds that propel it over the ground like a violent sandstorm. An ashflow eruption created Yellowstone caldera about 600,000 years ago, spreading ash over most of the western United States.
Kinds of Volcanic Eruptions
Scientists use special terms to describe the different kinds of volcanic eruptions. The gentlest eruptions are nonexplosive Hawaiian eruptions, named after the volcanoes found in Hawaii. In Hawaiian eruptions, lava erupts relatively quietly from one or more vents, producing quick-moving lava flows and lava fountains. Strombolian eruptions, named after a volcano in Italy, are characterized by many small but relatively weak explosive eruptions. The eruptions occur in fairly short, regular bursts. More explosive than a Strombolian eruption is a Vulcanian eruption, which produces large amounts of ash but few lava flows. It takes its name from the island of Vulcano in Italy. A Peléean eruption, named after Mount Pelée in Martinique, is a violent explosion that is often accompanied by rapidly moving flows of ash, rock fragments, and gases. Plinian eruptions are among the most powerful eruptions known. Named after Pliny the Elder, a famous Roman scholar killed during the eruption of Mount Vesuvius in A.D. 79, this type of eruption hurls plumes of ash tens of miles into the sky.
In addition to the kind of eruption, scientists classify volcanoes according to how often they erupt. Active volcanoes erupt either constantly or they have erupted sometime in the last 10,000 years. Dormant volcanoes are ones that have been inactive for long periods of time but may become active again at any time. Extinct volcanoes have been inactive for so long, perhaps tens of thousands of years or more, that they are unlikely to erupt again. In some instances, extinct or dormant volcanoes are so old that erosion has made them unrecognizable as volcanoes.
Products of Volcanoes
An erupting volcano produces a number of volcanic materials; among the most important are lava, rock, and heat and gas.
Lava and Lava Flows
During most volcano eruptions, lava flows out of vents as a red stream of hot, molten rock. Its temperature is about 2000°F (1100°C). As it cools, the top of the lava becomes a silvery blue color, while the inside remains a brilliant red. Some lavas are fluid and flow freely. Others are very viscous (thick and sticky) and flow slowly. The most significant factor in determining the degree of viscosity is the composition of the magma.
Rhyolite lavas are sticky, slow-moving lavas containing large amounts of silica, aluminum, potassium, and sodium. These lavas are sometimes so thick that they hardly flow at all. Instead, they pile up over a vent, forming a steep-sided hill called a volcanic dome. Sometimes rhyolite lava squeezes through breaks in the solid shell of a volcanic dome and hardens into sharp spines. Most of these spines are less than 100 feet (30 meters) high.
Basalt lavas are fluid, fast-moving lavas containing about 50 percent silica along with aluminum, iron, magnesium, and calcium. Because they are fluid and move rapidly, basaltic lava flows may travel great distances. Eruptions that occur again and again over millions of years form immense plateaus when they harden. Such massive floods of basaltic lavas have created enormous plateaus in Ethiopia, Siberia, India, South Africa, Brazil, and the northwestern United States.
Depending on how rapidly lava flows, its surface can appear quite different. Two common types of lava flows are pahoehoe and aa, which are named after typical Hawaiian lava flows. Pahoehoe lava has a smooth or gently rolling surface that is sometimes wrinkled into forms that look like coils of rope. When pahoehoe lava erupts from an underwater volcano, it often forms what is known as pillow lava because it resembles a pile of pillows. Aa lava has a very rough surface made up of large numbers of jagged fragments. When aa lava is flowing, it looks like jagged blocks tumbling over one another above a massive pasty interior. Another type, block lava, resembles huge piles of smooth-sided blocks, some up to 3 feet (0.9 meter) wide. Block lava forms in stiff, slowly moving lava flows.
Rocks formed when magma becomes solid are known as igneous (fire-formed) rocks. One type of igneous rock is granite, which is formed from magma that never reaches the Earth's surface. This magma slowly cools within the Earth. Large crystals of minerals grow within the magma as it gradually hardens. Mountain-building processes or erosion may bring this granite rock to the surface. Another type of igneous rock is obsidian. Obsidian is formed from magma that erupts onto the surface and cools so quickly it does not have a chance to form mineral crystals.
During explosive eruptions, lava is hurled into the air and torn into small fragments. Exposed to the air, the fragments cool and harden. Such solid materials are known as pyroclastics, which means "fire-broken" in Greek. Scoria is a type of rock fragment commonly produced in this manner. When scoria is full of holes, formed as a result of gas bubbles in the lava, it is called pumice. Pumice is sometimes light enough to float on water.
Explosions of fluid lava produce a variety of rock fragments that can be classified according to size. Most of the material thrown out in an eruption is volcanic ash, fragments smaller than 2/10 inch (5 millimeters) in size. Ash sometimes falls on the surrounding countryside, blanketing hundreds of square miles. After it falls, loose volcanic ash often becomes cemented together, forming a hard rock called tuff. Large, irregularly shaped solid fragments are called blocks. Some blocks may be as large as automobiles. Small fragments of material that are bigger than ash yet smaller than blocks are called lapilli, meaning "little stones" in Italian.
Ash Flows and Mudflows
In large volcanic eruptions, huge flows of hot ash, pumice, and gases mixed together can be propelled outward by hurricane-force winds. Known as ash flows, they can move along the ground at hundreds of miles per hour, sweeping down slopes, through valleys, and over hills. Sometimes the flows glow red hot and are known as glowing avalanches. Ash flow eruptions are among the most dangerous types of volcanic activity. The combination of high temperatures, high speeds, and gases makes them very destructive and deadly.
The summits of some volcanoes are covered with snow and ice. Other volcanoes are located in rainy regions. When these erupt, they sometimes produce large mudflows composed of pyroclastic material and water. Mudflows can be either hot or cold. Hot mudflows generally form when heat from an eruption melts snow or ice and the water mixes with volcanic debris. Cold mudflows can occur when heavy rain loosens cold cinder and ash, sending it racing down the mountainside.
Heat and Gas
Heat and gas are two important products of a volcanic eruption. Enormous amounts of heat are released during a volcano's lifetime. Typically, most of the heat escapes into the air. However, in some volcanic regions, much of the heat remains in the ground. The captive heat produces a natural form of energy known as geothermal energy. These geothermal regions often contain hot springs and geysers, natural springs that erupt at intervals, throwing up fountains of hot water and steam.
Gas also rushes out of a volcano's vents during an eruption, often carrying ash and fragments of rock. Most of the gas produced during an eruption is steam and carbon dioxide. Much of the water now on the surface of the Earth is the product of steam given off during 4.5 billion years of volcanic activity. Another common gas produced during eruptions is sulfur dioxide, which has a strong, suffocating odor. Some gases produced, such as chlorine and fluorine, can be very deadly.
Where Volcanoes Occur
Volcanoes have always been a part of Earth's long history. Volcanoes or the evidence of volcanoes has been located almost everywhere on the Earth's surface. Ancient lava flows have been found in many places where there are no volcanoes today, and most of the deep ocean floors are made up of thick layers of lava underneath much thinner layers of sediment. Today, however, volcanoes do not occur everywhere. Most are located near the edges of continents. The geologic theory of plate tectonics helps explain this.
According to the theory of plate tectonics, the Earth's crust consists of several huge plates, or blocks, that move slowly around the planet. As these crustal plates move, they frequently collide, forcing one plate to slide beneath the other. When this happens, the rocks in the plate are pushed toward the Earth's interior where they become hot and melt. This melted rock then rises as magma, forming volcanoes near where the plates meet.
Along the edges of the Pacific Ocean, the huge Pacific plate is sliding under a number of continental plates. The area along this large arc is called the Ring of Fire because many active volcanoes are found along it. Other arcs of active volcanoes run through Italy, Greece, Turkey, Armenia, and Indonesia.
Volcanoes also are located where plates are spreading apart. When this happens, magma pushes up between the plates and creates volcanic activity. Scientists have discovered this type of volcanic activity in the Atlantic Ocean along the Mid-Atlantic Ridge, an undersea mountain range that marks the boundary between huge spreading plates. Iceland's volcanoes are found along this volcanic ridge. A number of volcanoes also occur along the Rift Valley of eastern Africa, another region where crustal plates are spreading apart.
While most volcanoes form where crustal plates are colliding or spreading apart, some are located in the middle of plates. Geologists believe that these midplate volcanoes form over stationary hot spots, where huge columns of magma rise from deep inside the Earth and break through the plate. As the plate above a hot spot moves, the fixed column of magma continues to form new volcanoes. The Hawaiian islands, the most famous midplate volcanoes, are a good example of this process. Each island was formed one at a time over a hot spot. As the plate above the hot spot moved, a new volcano (and new island) was formed. The youngest island, the big island of Hawaii, is now above the hot spot, and this is where Hawaii's active volcanoes are located.
Today most volcanologists, the people who study volcanoes, teach in universities or work for national geologic organizations, such as the U.S. Geological Survey. A few hundred, however, work at special observatories around the world where they monitor active volcanoes. The main task at these volcano observatories is to watch for any activity that may signal an impending eruption: earthquakes, the tilting of the ground caused by rising magma, and the emission of gases. In making these observations, scientists hope they can warn people whose lives may be threatened if a volcano were to erupt. The monitoring of both active and dormant volcanoes also helps scientists learn more about the Earth and its history.
In addition to ground-based monitoring, scientists now use satellites and satellite technology to track volcanoes. Satellites provide information that traditional methods cannot. For example, sensors on satellites can detect the warming of volcanoes, sometimes even before they erupt. Sensors can also measure the amount of gases given off during eruptions. For volcanoes in remote areas, satellite monitoring may be the only method available.
—Charles A. Wood