Volcano

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Volcano

A volcano is an opening in Earth's surface through which molten rock, hot gases, and rocks are ejected. Volcanoes create new land and islands. They can also produce economically important mineral deposits, fertile soils, and beautiful landscapes. However, volcanoes can also destroy lives and property. Therefore, they constitute significant geologic hazards in many parts of the world.

With regard to the hazard that they present, volcanoes can be classified as active, dormant, or extinct. Active volcanoes are those that have erupted within recorded history. Dormant volcanoes are those that have not erupted during recorded history but may erupt again, whereas extinct volcanoes are those for which there is little or no chance of future eruptions.

Where volcanoes develop

Most of the volcanoes on Earth are located along the boundaries between lithospheric plates, which can be convergent (subduction zones) or divergent (mid ocean ridges). The chain of volcanoes along the Pacific Rim, often referred to as the Ring of Fire, is an example of subduction zone volcanism. Iceland, in contrast, is a volcanic island straddling the Mid-Atlantic Ridge (a divergent plate boundary). Although they are not as numerous as plate boundary volcanoes, intraplate volcanoes can occur where plates pass over mantle hot spots or along continental rift zones where plates are being pulled apart. The Hawaiian Islands, for example, were formed as the Pacific Plate slowly passed over a magma-generating hot spot within the mantle.

Oceanic ridges are chains of volcanoes located along the boundary between two diverging oceanic plates. New oceanic crust is formed along the ridges as two oceanic plates move apart. Where the rate of plate formation is rapid, older crust is quickly pushed out of the way and only small volcanic vents form. Where the spreading rate is slower, volcanic eruptions may form large volcanoes.

Subduction zones are areas in which oceanic plates are overridden by continental plates, forcing the heavier oceanic crust deep enough to be melted and recycled as second generation magma . Because the size of Earth is constant, the amount of oceanic crust consumed by subduction zones must be approximately equal the amount produced along mid-ocean ridges. The volcanoes of the western coast of North and South America , Indonesia, the Philippines, Japan, Kamchatka, and Alaska all sit atop subduction zones and collectively comprise the Ring of Fire.

Some volcanoes form above hot spots far from the edges of tectonic plates, and are known as intraplate volcanoes. A hot spot is an upwelling of magma from far beneath the Earth's crust, caused by a disturbance at the boundary between the solid mantle and the liquid outer core of Earth's interior . Hot spots are, compared to mid-ocean ridges or subduction zones, relatively small and isolated features. Well known intraplate hot spot volcanoes include the Hawaiian Islands, the Jemez Mountains and Capulin volcanic field of New Mexico, and the volcanoes that produced volcanic rocks throughout the Yellowstone region of Wyoming.

Hot spots provide information about the rates and directions of movement of tectonic plates over geologic time scales. When magma from a hot spot rises from the lower mantle and through the lithospheric plate, a volcano forms on the surface. The plate continues to move over the stationary hot spot and eventually produces a chain of volcanoes that increase in age away from the hot spot and show the direction of plate movement. The Hawaiian Islands mark the track of a hot spot, over which the Pacific Plate has moved in a northwesterly direction, toward Japan. Likewise, the volcanic rocks of the Snake River Plain in southern Idaho recorded the progress of the North American plate as it moved over the Yellowstone hot spots.

The origin of magma

All magma forms through melting of pre-existing rock. Generally, this occurs in one of two ways: (1) by convection of rock upwards through the mantle until it melts, or (2) by melting rock at a subduction zone. Mantle convection occurs because deep within the earth, radioactive decay raises the temperature of rock, making it expand. This expansion lowers the rock's density , causing it to rise, or convect. As the rock rises through the mantle, the surrounding pressure decreases and eventually the convecting rock melts as a result. Geologists call this pressure-relief melting. The magma moves upward and erupts to form either an oceanic ridge or a hot spot volcano. At subduction zones, volatile compounds (especially water ) escape from the subducting plate and lower the melting temperature of the overlying mantle rocks. This triggers melting and magma forms as a result.

Magma comes from a variety of sources and may have a complicated history. For example, as magma rises in the mantle and crust, it undergoes a process known as fractional crystallization. Each mineral in a rock has its own crystallization (or melting) temperature. Because different minerals crystallize at different temperatures, certain minerals form from magma earlier than others. This produces a magma with a composition different
from that when the minerals first began to crystallize. Therefore, the minerals that crystallize later, and the rocks that they form, will be of a different composition than those that form earlier. Fractional crystallization is thought to be one way of producing rocks of different compositions from the same magma. Partial melting and magma contamination are also important.

If a rock is not exposed to a high enough temperature to melt all of its minerals, only some minerals will melt. This is known as partial melting. If a rock melts only partially, the magma produced will have a different chemical composition than the rock from which the magma originated. As magma rises toward the earth's surface it may also cause rocks in the overlying crust to partially melt, contaminating the magma with molten rock of a different composition. The composition of magma therefore depends on many factors, including original magma composition resulting from partial melting, fractional crystallization, and magma contamination.

Volcanic rocks produced from partially melted continental crust usually appear red, brown, or gray in color and are known as felsic rocks. Felsic rocks such as rhyolite are rich in the minerals feldspar and quartz, both of which contain abundant silica. Lava formed by melting of mantle rocks contains abundant iron- and magnesium-rich minerals, which are poorer in silica than quartz and feldspar, and produces mafic volcanic rocks such as basalt. Lava with a chemical composition that falls between these two extremes is said to be of intermediate composition. Andesite is an example of a volcanic rock of this type.

Types of volcanic eruptions

Lava flows are streams of molten rock that flow onto Earth's surface from a vent or fissure, and most commonly have a basaltic composition. Two common types of basaltic lava have been distinguished in the Hawaiian language. The Hawaiian words adopted into English to describe these different lava rocks are aa (pronounced AH-ah), also called blocky lava, and pahoehoe (pronounced pa-HOY-hoy) or ropey lava. Aa forms from viscous and slow-moving, and aa flows are characterized by an irregular, jagged appearance. Pahoehoe flows, in contrast, are characterized by smooth, wavy surfaces. Submarine eruptions of basalt form large lobes known as pillows, and are commonly referred to as pillow basalts.

Pyroclastic ("fiery fragment") deposits are the result of explosive eruptions. Explosive eruptions occur when magma containing water or gases (or magma that has been in contact with ground water) rises near enough to the surface that the pressure exerted by the rock above it can no longer keep the magma from boiling. The result is an explosive eruption of pyroclastic debris. Volcanic dust, ash, cinders, and blocks are collectively known as tephra. Ash from pyroclastic eruptions can cover large areas, thinning with distance from the volcano. The rock produced by a volcanic ash fall is known as tuff.

An ash flow, or pyroclastic flow, is a dense body of ash, superheated gases, and rock that moves as a fluid from an erupting volcano, crossing the landscape and filling valleys with the fluid mixture. This material deflates as it cools and produces a rock known as ignimbrite, or welded tuff. Ignimbrites can cover hundreds of square kilometers of landscape, such as the Mitchell Mesa Tuff of West Texas. Pyroclastic flows from a prehistoric eruption of Taupo, a volcano in New Zealand, produced ignimbrite deposits that covered the tops of hills hundreds of meters tall.

A pyroclastic surge is a kind of pyroclastic flow that occurs when magma encounters groundwater close to Earth's surface. Also called a nuee ardente, a French phrase meaning "glowing cloud," this was the kind of eruption that destroyed the city of St. Pierre, on the Caribbean island of Martinique, in 1902. The volcano that is formed by this kind of eruption is called a maar or tuff ring.

Controls on the explosivity of volcanic eruptions

Generally, the viscosity of a magma controls the type and violence of eruptions. Viscosity is the resistance of a fluid to flow. The more viscous the magma, the more explosive its eruption is likely to be. Very viscous magmas tend to resist eruption, and so gas pressure builds within the magma pipe leading to the volcanic vent. By the time sufficient pressure builds to displace a viscous magma, the force released by the eruption will be much greater than for a fluid magma. This leads to explosive eruptions. The most important controls on viscosity are the silica content of the magma and its temperature. Basaltic (mafic) lavas are very fluid due to their low silica content. Conversely, rhyolitic lavas are very viscous due to their high silica content. Magma and lava viscosity is also a function of temperature; therefore, a the viscosity of a lava flow will increase as its temperature decreases.

Volatile substances are elements or compounds—hydrogen sulfide, water, carbon dioxide , radon , and other gasses—that escape during eruptions. The Latin root for volatile means "winged." Volatile compounds in magma can cause violent explosive eruptions.

Different kinds of volcanic structures

As new rock forms around a volcanic vent, the resulting volcano takes shape according to the kind of erupted material, which is in turn related to lava composition. The most common types of volcanoes, from largest to smallest, are the shield volcano, composite volcano, and cinder cone.

A shield volcano is a very large, broad, and low profile volcano consisting of layers of basaltic rocks. Shield volcanoes most commonly form in the middle of oceanic plates or in continental rifts, which are areas in which the continental crust is being pulled apart. The shape of shield volcanoes resembles the round shields used by warriors of ancient times to protect themselves in battle. The tallest individual mountain on Earth—the island of Hawaii—is a shield volcano. This volcanic island slopes gently down from the 13,796 ft (4205 m) summit of Mauna Kea to the ocean abyss more than 30,000 ft (9 km) below. Kenya's Mt. Kilimanjaro, the tallest mountain in Africa , is a shield volcano, as is Olympus Mons, the tallest mountain on Mars . Mountains very much like shield volcanoes have been mapped on Venus by the Magellan spacecraft radar mapping expedition. Shield volcanoes can produce large amounts of lava and bury large areas, but are not known for for violent explosive eruptions.

A composite volcano, or stratovolcano, is a large, steep-sided andesitic volcano made of alternating sequences of lava and pyroclastic debris. Composite volcanoes are most commonly located along plate boundaries. Japan's Mt. Fuji is a composite volcano, as are Mount St. Helens in Washington, Mt. Ararat in the Caucasus, and Popocatepetl, near Mexico City. Composite volcanoes can grow over millions of years and then collapse in a cataclysmic event, forming a large volcanic crater known as a caldera.

A cinder cone is a small, steep-sided volcano made of pyroclastic material, with lava composition ranging from basaltic to rhyolitic. Cinder cone eruptions occur as an incandescent liquid solidifies in midair and falls into a heap. Landslides sculpt the sides of the still-hot rock
pile, forming such cinder cones as Mexico's Paricutin, New Mexico's Mt. Capulin, and Arizona's Sunset Crater.

Some mafic eruptions occur through cracks known as fissures. Instead of building a mountain in one place, fissure eruptions cover broad areas with basaltic lava flows known as flood basalts. A fissure eruption occurred in Iceland in 1783, and the resulting environmental catastrophe wiped out one-fifth of its population. Fissure eruptions have:

Filled the Rio Grande Rift with hundreds of feet (100 m) of volcanic rock at Taos, New Mexico.
Constructed the Columbia River Plateau in Washington and Oregon.
Covered southern India with approximately 240,000 cubic mi (1 million cubic km) of basalt 65 million years ago, forming the Deccan Traps.
Formed the Siberian Flood Basalt plateau in northern Russia 250 million years ago, the largest flood basalt in the world.

Some of these basaltic eruptions happened at the same time great extinctions occurred. No conclusive evidence has been found for a connection between fissure eruptions and global extinction , but there is no doubt that the gas and heat released could affect environmental conditions for the worse all over the planet's surface.

Calderas

The most violent large volcanic eruption is the collapse of a composite volcano. This normally happens on the active margins of tectonic plates, that is, at subduction zones or along a continental rift valley (where a continent is breaking apart). The process is part of the evolution of a composite volcano, which starts with a reservoir of molten rock, several miles wide and under high pressure. This magma rises in the earth's crust and forces its way to the surface. A composite volcano is born in clouds of ash, supersonic steam explosions filling the air with hot rock, ash, and various gases.

After a series of eruptions, perhaps over millions of years, the volcano forms a mountain of lava and pyroclastic material as much as 2-3 mi (3-4 km) high. Eventually, there is one last eruption of ash and pyroclastic flows. The magma begins to boil, gas bubbles expand the magma to many times its original volume , and it explodes upward. The magma chamber rapidly empties its contents onto the landscape above and the volcano collapses into the void, forming a depression known as a caldera.

Volcanic catastrophes

According to the United States Geological Survey, between 50 and 60 volcanoes erupt each year, usually in sparsely populated areas. In the past 500 years more than 200,000 people have died as a result of volcanic activity. Between 1900 and 1986, volcanoes directly or indirectly killed an average of 845 people each year. The 1980 eruption of Mount St. Helens, the last volcanic eruption within the contiguous United States, killed 57 people.

During the 1980s and 1990s the science of volcanology greatly improved our knowledge of volcano behavior. Consequently, volcanologists can now warn civil authorities when eruptions are likely to occur. Mt. Pinatubo, a composite volcano in the Philippine Islands, erupted explosively in 1991. In 1997, a pyroclastic flow burst from the Sourfriere Hills volcano on the island of Montserrat in the Caribbean. Lives were lost in both of these disasters; however, many more lives were saved by timely evacuation of the populations flanking the volcanoes.

Mt. Mazama, which existed in what is now southern Oregon, erupted and collapsed nearly 7000 years ago, creating Crater Lake. The story of the eruption evolved into a Native American myth, the Battle of Llao and Skell, that was eventually translated to English. Twentieth century geologists found the myth to be an accurate description of the development of a caldera nearly 250 generations ago.

An ash fall from Mt. Vesuvius buried the Roman city of Pompeii in a.d. 79. The volcano struck down the people where they lived, and preserved the shapes of their bodies where they fell in the ash. Pompeii is a remarkable volcanic disaster because much of the city was preserved along with the names, portraits, writings, and even graffiti of those who lived there. Plaster casts were made of them as the city was excavated in the late 1700s. The nearby city of Herculaneum was covered by a pyroclastic flow that destroyed it in seconds.

Kuwae, in Melanesia, erupted in 1453. A legendary chieftain hastily assembled the population, moving them to safety in the last-minute. The eruption destroyed all remaining life on the island, and split it into several sections. Decades after the caldera eruption, the people returned to the new archipelago of which Epi and Tongoa are the principal islands. Eruptions of this size affect weather and climate worldwide, and cause peculiar optical effects in the atmosphere. Ash in the stratosphere causes the sky to appear strangely colored and dims the sunlight. Some geologists speculate that the eruption's optical effects in 1453 may have filled the defenders of Constantinople with superstitious dread, hastening the city's demise.

The Laki fissure eruption of 1783 produced huge volumes of fluorine gas, which poisoned the grass that fed Icelanders' flocks. Approximately 229,000 animals died as a result, and 10,000 Icelanders subsequently starved to death, reducing the population by one-fifth. Benjamin Franklin observed the blue haze that covered Europe , and deduced that the pollution from the eruption must have caused the abnormally cold winter that year.

Tambora, an Indonesian composite volcano, erupted and collapsed in 1815, chilling the world for the next year. The explosion destroyed the volcano's summit and filled the stratosphere with volcanic dust, significantly decreasing the amount of sunlight that reached Earth's surface. Tambora's collapse killed 10,000 people, and another 80,000 starved to death as a result of crop losses.

Krakatau, another Indonesian composite volcano, erupted and collapsed in 1883, causing worldwide cooling similar to Tambora. The collapse was heard by people 2,500 mi (4,000 km) away. Tsunamis killed 36,000 people in coastal Java and Sumatra. The atmospheric effects of this eruption in the equatorial latitudes included brilliant green sunrises, sunsets, and moonrises, followed by blue sunlight throughout the day.

Volcanologists know of even larger caldera eruptions in the geologic past than those in this list. Likewise, there are many volcanoes that have the potential to erupt catastrophically in the near future. Some of them are located in the world's most populous areas: Seattle, Washington; Guadalajara, Mexico; the Bay of Naples, Italy; and Rabaul, Papua New Guinea.

Volcanic benefits

Through geologic time, volcanic eruptions have shaped Earth's environment in many ways. The eruption of volcanoes built the continents on which we live. We owe much of the composition of our atmosphere to volcanic eruptions on the early Earth. Our oceans formed from water expelled during these same eruptions. Some of the world's richest farmland draws its fertility from minerals provided by nearby volcanoes. Volcanoes, particularly collapsed calderas, can develop geothermal systems when groundwater or rainwater seep into the volcano. Geothermally powered electric generating stations provide electricity in Iceland, Italy, and New Zealand. Volcanic processes are also responsible for precious and non-precious mineral deposits, which are formed when minerals precipitate from geothermal waters circulating in the rocks beneath and around volcanoes.

Resources

books

De Boer, Jelle Zeilinga, and Donald T. Sanders. Volcanoes in Human History: The Far-Reaching Effects of Major Eruptions. Princeton, NJ: Princeton University Press, 2001.

Harris, Stephen. Fire Mountains of the West. Missoula: Mountain Press Publishing Co., 1988.

periodicals

Gore, Rick. "Cascadia: Living On Fire." National Geographic (May 1998): 6-37.

Heiken, Grant. "Will Vesuvius Erupt? Three Million People Need to Know." Science (November 26, 1999): 1685

McClintock, Jack. "Under the Volcano." Discover (November 1999): 82.

Mothes, Patricia. "Waiting for the Eruption: Tungurahua Volcano, Ecuador." Geotimes (March 2000): 26-27.

Williams, A.R. "Under the Volcano: Montserrat." National Geographic (July 1997): 58-73.

other

U.S. Forest Service. "Mount St. Helens National Volcanic Monument." January 13, 2003 [cited February 9, 2003]. <http://www.fs.fed.us/gpnf/mshnvm/>.

U.S. Geological Survey. "U.S. Geological Survey Volcano Hazards Program." January 17, 2003 [cited February 9, 2003]. <http://volcanoes.usgs.gov/>.

Clay Harris

KEY TERMS

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Ash fall: —A layer of volcanic ash that falls from an erupted ash cloud.
Cinder cone: —A small, steep-sided volcano made of pyroclastic material. A cinder cone is an accumulation of loose volcanic material that erupts as a liquid, and cools into cinders in the air, falling to the ground in a heap.
Composite volcano: —A large, steep-sided volcano made of alternating sequences of lava and pyroclastic debris. Sometimes called a stratovolcano.
Convection current: —The motion of a fluid that rises as it is heated and sinks as it cools, moving in a circular path.
Felsic: —A term applied to light-colored igneous rocks, such as rhyolite, that are rich in silica. Felsic rocks are rich in the minerals feldspar and quartz.
Fissure: —A crack through which lava erupts onto Earth's surface
Hot spot: —An upwelling of magma from beneath the earth's crust, caused by a disturbance at the boundary between the solid mantle and the liquid outer core. This upwelling is not related to the convection currents associated with oceanic ridges, although some hot spots do occur there.
Lava: —Molten rock erupted onto Earth's surface.
Mafic: —A term applied to dark-colored igneous rocks, such as basalt, that are poor in silica and contain large amounts of the iron and magnesium.
Magma: —Molten rock beneath Earth's surface.
Oceanic ridge system: —A long (40,000 mi; 64,000 km) crack in the earth's crust where new ocean crust is continuously forming, causing ocean basins to grow wider.
Pyroclastic flow: —A fast moving body of pyroclastic material from an erupting volcano. It moves as a fluid, in some cases covering thousands of square kilometers.
Pyroclastic material: —Volcanic debris formed by solidification of erupted lava in air; includes dust, ash, cinders, and blocks of rock.
Shield volcano: —A broad, low profile volcano consisting of layers of basaltic rock, typically formed in the middle of oceanic plates or on continental rifts.
Silica: —Any of the mineral forms of silicon dioxide.
Subduction zone: —A boundary between tectonic plates in which a dense oceanic plate is forced beneath a less dense continental plate.
Viscosity: —The internal friction within a fluid that makes it resist flow.
Volatile: —Readily able to form a vapor at a relatively low temperature.