Volcano

Volcanoes are landforms whose shapes may remain unchanged for centuries or may change drastically in minutes. Some exist singly, looming over a flat landscape. Others exist in groups, forming mountain ranges. Scores of volcanoes remain unseen, hidden beneath the surface of the planet's oceans. These submarine volcanoes are known as seamounts. (For further information on oceanic landforms, see the Ocean basin chapter.)

The material and processes deep within Earth that form volcanoes have shaped the planet's surface since its beginning more than four billion years ago. Volcanologists, scientists who study volcanoes and volcanic phenomena, have identified the existence of more than forty thousand volcanoes on Earth. Currently, there are about six hundred active volcanoes scattered around the world. Volcanologists classify an active volcano as one that has erupted in the last few hundred years or shows signs of erupting in the near future. A dormant volcano is one that has not erupted for a few hundred years, but has erupted in the last few thousand years. An extinct volcano is one that has not erupted in the last few thousand years and will not, volcanologists believe, ever erupt again.

The word volcano comes from the name of the Roman god of fire and a small island that is part of a group of volcanic islands located just north of Sicily in the Tyrrhenian Sea. Ancient Romans believed that Vulcan, the mythological god who made tools and weapons for other Roman gods, operated his forge beneath the island of Vulcano. One of the present-day Aeolian (pronounced ee-OH-lee-an) Islands, Vulcano has been volcanically active for thousands of years. During the Middle Ages in Western Europe (roughly from 500 to 1500 c.e.), many people considered the smoking crater on Vulcano to be the entrance to hell. After this period, the word volcano was applied to all such eruptive landforms.

The shape of the land

Technically, a volcano is a vent or hole in Earth's surface through which heated material escapes from underground. That material could be any combination of magma (called lava once it reaches Earth's surface), rock fragments, ash, and gas. Ejected through the vent, volcanic material accumulates to form a hill or, if over 1,000 feet (305 meters), a mountain around the opening. It is this accumulating landform that is more commonly referred to as a volcano.

Volcanologists recognize various volcanic landforms, such as shield volcanoes, stratovolcanoes, cinder cones, lava domes, calderas, and lava plateaus. The differing shapes of these landforms are determined by the composition of the magma flowing into the specific volcano from underground.

Magma is molten (melted) rock that contains particles of mineral grains and dissolved gas (primarily water vapor and carbon dioxide). The most abundant element found in magma is silicon, in the form of the oxide silica. (An oxide is a compound of an element and oxygen. As magma cools, the silica crystallizes to become the mineral quartz.) The

Volcano: Words to Know

Asthenosphere:

The section of the mantle immediately beneath the lithosphere that is composed of partially melted rock.

Basalt:

A dark, dense volcanic rock, about 50 percent of which is silica.

Convection current:

The circular movement of a gas or liquid between hot and cold areas.

Crust:

The thin, solid outermost layer of Earth.

Hot spot:

An area beneath Earth's crust where magma currents rise.

Lava:

Magma that has reached Earth's surface.

Lithosphere:

The rigid uppermost section of the mantle combined with the crust.

Magma:

Molten rock containing particles of mineral grains and dissolved gas that forms deep within Earth.

Magma chamber:

A reservoir or cavity beneath Earth's surface containing magma that feeds a volcano.

Mantle:

The thick, dense layer of rock that lies beneath Earth's crust.

Mountain:

A landmass that rises 1,000 feet (305 meters) or more above its surroundings and has steep sides meeting in a summit that is much narrower in width than the base of the land-mass.

Plates:

Large sections of Earth's lithosphere that are separated by deep fault zones.

Plate tectonics:

The geologic theory that Earth's crust is composed of rigid plates that "float" toward or away from each other, either directly or indirectly, shifting continents, forming mountains and new ocean crust, and stimulating volcanic eruptions.

Pyroclastic material:

Rock fragments, crystals, ash, pumice, and glass shards formed by a volcanic explosion or ejection from a volcanic vent.

Ring of Fire:

The name given to the geographically active belt around the Pacific Ocean that is home to more than 75 percent of the planet's volcanoes.

Silica:

An oxide (a compound of an element and oxygen) found in magma that, when cooled, crystallizes to become the mineral quartz, which is one of the most common compounds found in Earth's crust.

Subduction zone:

A region where two plates come together and the edge of one plate slides beneath the other.

Viscosity:

The measure of a fluid's resistance to flow.

amount of silica in magma determines how easily the magma flows. When discussing the flow rate of magma, volcanologists refer to its viscosity (pronounced vis-KOS-eh-tee), which is the measure of a fluid's resistance to flow. If a fluid is thin and runny, like water, it has a low viscosity or is less viscous (pronounced VIS-kus). If a fluid is thick and slow-moving, like tar, then it has a high viscosity or is more viscous. Magma's viscosity is directly related to its silica content: The higher the silica content, the higher the viscosity and the slower it flows. Temperature and the amount of gas contained in magma also affect its viscosity, but opposite that of silica. When its temperature is high and it contains a vast amount of dissolved gas, magma has a low viscosity and flows quite readily.

Lava (magma that has erupted from a volcano) can create interesting rock formations once it has run down the side of a volcano and cooled. Two such formations generated by lava flows include aa (pronounced AH-ah) and pahoehoe (pronounced pa-HOY-hoy), which are quite common in the Hawaiian Islands. In fact, about 99 percent of the island of Hawaii is composed of aa and pahoehoe. The different textures of these volcanic rocks are caused by a difference in the viscosity of the lava flows that created them. Slow-moving, cooler, and more viscous lava hardens to form aa, rough blocks that have sharp, jagged edges. Lava flows that are hotter, have a high concentration of gas, and are less viscous harden to form pahoehoe, which has a smooth, coiled surface that resembles rope. While a hard skin develops on the surface of pahoehoe as it cools, hot lava continues to flow beneath the skin, causing the coiled-rope wrinkles that mark its surface. Lava may continue to flow inside hardened pahoehoe for miles.

Shield volcano

Shield volcanoes are broad landforms with gently sloping sides, resembling a warrior's shield lying on the ground with the curved face up. These types of volcanoes are typically created by successive nonexplosive eruptions of lava that have low silica content and, consequently, relatively low viscosity. The runny lava flows great distances over the wide surface of the volcano, forming thin sheets of nearly uniform thickness. The slope of a shield volcano is seldom more than 10 degrees at its summit and 2 degrees at its base. Hawaii, Tahiti, Samoa, the Galápagos, and many other oceanic islands are actually the upper portions of large shield volcanoes.

Stratovolcano

Stratovolcanoes, also called composite volcanoes, have the most symmetrical cone shape of any volcano types. They are among the most picturesque landforms on Earth. These steep mountains have slopes of up to 30 degrees at the summit, tapering off to 5 degrees at the base. They are built up of alternating layers of lava and layers of pyroclastic (pronounced pie-row-KLAS-tic; fragmented rock, crystals, ash, pumice, and glass shards) material, which have flowed down on different sides of the volcano at different times. The steep slope near the summit is due partly to thick, viscous lava flows that did not travel far downward from the vent. The gentler slope near the base is due to the accumulation of pyroclastic material that the volcano erupted violently and material eroded from the sides of the volcano. Examples of stratovolcanoes include Mount Fuji in Japan, Mount Mayon in the Philippines, and Mount Vesuvius in Italy. In the United States, Mount Rainier and Mount Baker in Washington and Mount Hood in Oregon are classic examples of stratovolcanoes.

Cinder cone

Cinder cones are the steepest of volcanoes, with slopes of 30 to 40 degrees. They are seldom taller than 1,640 feet (500 meters); many are not more than a few hundred feet high. Cinder cones are built entirely or almost entirely of blobs of lava that have broken up into small fragments

or cinders after being ejected in mildly explosive eruptions. The cinders rain back down to form a cone around a bowl-shaped depression or crater at the summit. These volcanoes can grow very rapidly, but they are usually not active for long. Because the material that forms them is fragmented and easily eroded, cinder cones usually do not remain as landforms for an extended period of time. All cinder cones currently on Earth originated within the last 1 to 2 million years; most are less than 150,000 years old. Sunset Crater in Arizona, Stromboli in the Mediterranean Sea, Parícutin in Mexico, and Cerro Negro in Nicaragua are all examples of cinder cones.

Lava dome

Lava domes, also called volcanic domes, form in the craters of volcanoes (mostly stratovolcanoes) after a major eruption. Highly viscous lava with little gas content oozes out of the volcano's vent like toothpaste out of a tube. The pasty lava is too thick to flow, and it solidifies on top of the vent, forming a rounded, steep-sided mound. The building of lava domes can be a forceful process, with small but violent explosions blasting out pieces of the dome. Fresh viscous lava flows then replace and build on the old material, causing the dome to take on a variety of strange shapes. Examples of lava domes include Mono Dome in California and Santiaguito Dome in Guatemala. After Mount St. Helens in the state of Washington erupted in May 1980, a lava dome began to form in its crater. Currently, the dome rises more than 900 feet (275 meters) above the crater floor.

Caldera

Below the vent in a volcano is a passageway called a pipe. The pipe leads down to the magma chamber, a large area where magma collects below Earth's surface. In some highly explosive eruptions, a great percentage or all of a volcano's magma chamber may be emptied. When this occurs, the roof of the magma chamber may be left unsupported. It may then collapse under its own weight, forming a large, usually circular, steep-walled basin known as a caldera (Spanish for cauldron or kettle) across the top of the volcano. Calderas have a diameter ranging from 1 to 15 miles (1.6 to 25 kilometers). Over time, rain and snowmelt may collect in a caldera, forming a lake. Lake Toba, in Sumatra, Indonesia, fills the world's largest caldera. It measures 18.6 miles (30 kilometers) wide and 62 miles (100 kilometers) long. The central part of Yellowstone National Park in Wyoming is a caldera measuring 28 miles (45 kilometers) by 47 miles (75 kilometers). A huge eruption 600,000 years ago created the depression at the heart of the park.

Lava plateau

In some eruptions, lava pours forth through fissures or long, narrow cracks in the ground instead of through a central vent in a volcano. This thin lava tends to spread out rapidly and widely, flooding the surrounding landscape. Repeated outpourings of lava eventually build up to create flat lava plains called lava plateaus. Lava plateaus are also known as flood basalts, after the dark, dense volcanic rock called basalt that floods across the surface of the land. Although erupted in thin sheets, the lava flows accumulate to form deposits thousands of feet thick.

The most famous example of a lava plateau in the United States is the Columbia Plateau, covering most of southern Washington from its border with Idaho west to the Pacific Ocean and extending south into Oregon. In places, it measures 5,000 feet (1,524 meters) thick. The Deccan Plateau of west-central India is much larger, covering some 300,000 square miles (770,000 square kilometers) and accumulating in spots to a thickness of 6,000 feet (1,829 meters). Larger still is the Siberian Traps in central Russia, which formed about 250 million years ago. It covers about 750,000 square miles (1,942,500 square kilometers). If the lava that poured forth to create the Siberian Traps were spread out evenly across the entire planet, it would create a layer 10 feet (3 meters) thick.

A Venutian Volcano on Earth?

The only active volcano in the East African Rift Valley in northern Tanzania, Ol Doinyo Lengai (pronounced ol DOYN-yo LEN-guy) rises 9,479 feet (2,890 meters). Its name means "Mountain of God" in the language of the Masai people who inhabit this region. What makes the strato-volcano so strange and interesting to volcanologists and other scientists is the lava it erupts.

While most of the world's volcanoes erupt silicate lavas, which are made up primarily of silicon and oxygen, Ol Doinyo Lengai erupts a lava rich in calcium and sodium. Volcanologists call this type of lava natrocarbonatite, and it is erupted from no other active volcano on Earth. At about 950°F (510°C), natrocarbonatite lava is approximately one-half as hot as normal lava. It is also the most fluid lava in the world. With a very low gas content, it has a very low viscosity and flows like water.

Since it is not as hot as normal lava, natrocarbonatite lava does not glow as brightly. In sunlight, lava flows from Ol Doinyo Lengai may be mistaken for dark brown or black mudflows. The dark lava quickly solidifies, then changes color to gray. When it comes in contact with moisture, the lava undergoes a chemical reaction that turns it white. This change may occur immediately if it is raining or may take several months in dry conditions. The reaction to moisture also softens the lava, so a person walking on it may sink in slightly.

Ol Doinyo Lengai is unique on Earth, but not in the solar system. Scientists have discovered through satellite photos that similar volcanic structures and lava flows exist on the planet Venus.

Forces and changes: Construction and destruction

A volcanic eruption is among the most powerful forces on Earth. When Mount St. Helens in the state of Washington erupted in May 1980, the energy it released was equivalent to the largest hydrogen bomb ever exploded (a hydrogen bomb equals the power of one thousand atomic bombs). The eruption of the ancient volcano that produced Crater Lake in Oregon, one of the best-known calderas in the world, was forty-two times as powerful as that of Mount St. Helens. One hundred times more powerful was the 1815 eruption of the stratovolcano Tambora on the Indonesian island of Sumbawa. The ash cloud released by the volcano lowered global temperatures by as much as 5°F (3°C) from late spring to early autumn the following year. Subsequent crop failures produced widespread famine.

Earth's interior

The power to change the shape both of a volcano and the landscape around it comes from miles beneath Earth's surface. The interior of the planet is divided into different layers of varying composition. The rocky outer layer that forms Earth's surface is the crust, which varies in thickness from 3 to 31 miles (5 to 50 kilometers). The crust is thickest below land and thinnest below the oceans. Underlying the crust is a thick layer of rocks (different from those composing the crust) known as the mantle. The mantle extends down roughly 1,800 miles (2,900 kilometers) beneath the planet's surface.

The uppermost section of the mantle is a rigid or firm layer. Combined with the overlying solid crust, it makes up the lithosphere (pronounced LITH-uh-sfeer; from the Greek word lithos, meaning "stone"). On average, the lithosphere measures about 60 miles (100 kilometers) thick. The part of the mantle immediately beneath the rigid, cold lithosphere is composed of partially melted rock that is pliable, like putty. This section, called the asthenosphere (pronounced as-THEN-uh-sfeer; from the Greek word asthenes, meaning "weak"), extends to a depth of about 155 miles (250 kilometers).

Cultural Landforms

According to Navajo legend, on a towering landform in what is now northwestern New Mexico, a battle took place between a warrior and monster birds that had been terrorizing the ancestors of the Navajo people. Because the warrior was victorious, the Navajo were able to settle in this area around the landform they came to call Tsé Bit'a'i (pronounced say bid-ahih; meaning "rock with wings"). When European settlers arrived centuries later, they thought the landform the Navajo held sacred looked like a giant sailing ship, and so they called it Shiprock.

Shiprock is located in the four corners area of America's Southwest, the only spot where four states (Utah, Colorado, Arizona, and New Mexico) meet. An area of sandstone cliffs and rugged mountains, it is the traditional homeland of several Native American tribes. Shiprock sits in the middle of what volcanologists call the Navajo volcanic field. It is littered with the remains of dozens of extinct volcanoes that last erupted twenty-five to thirty million years ago.

Shiprock stands 1,969 feet (600 meters) above the surrounding plain and measures 1,640 feet (500 meters) in diameter at its greatest width. It is a spectacular example of a volcanic neck or plug. When a volcano stops erupting and becomes extinct, remaining magma and other volcanic material may harden in the volcano's pipe, the passageway between the magma chamber and the vent. Typically, this material tends to be more resistant to erosion than the enclosing volcanic landform. Thus, long after the volcano has eroded away, the volcanic neck stands out against the landscape as the fossil remains of a once great volcano.

Volcanologists estimate that the summit of the original volcano stood almost 1,000 feet (305 meters) above Shiprock's current height. Radiating out from the volcanic neck into the flat, eroded plain are six long, thin, wall-like structures, the ancient volcano's only other visible remains. These vertical structures, called dikes, were created when magma pushed up through cracks that formed in the rock layers beneath the volcano. The rock layers have long since eroded away, leaving the dikes above the surface. The dikes range in length from 0.6 to 5.5 miles (1 to 9 kilometers).

Beneath the mantle is Earth's core, which is composed of a solid inner portion and a liquid outer portion. Both layers of the core are made of the metal elements iron and nickel. Scientists believe temperatures in the core exceed 9,900°F (5,482°C). If the heat energy created by such high temperatures were not released, Earth would become so hot its entire interior would melt. This energy is carried to the surface of the planet by convection currents, the circular movement of molten material deep within Earth.

When a gas or liquid is heated, it expands and becomes less dense (or lighter). It then rises above cooler, denser gas or liquid that surrounds it. This action takes place in Earth's mantle. Under tremendous pressure and

Volcano Facts

• There are 224 volcanoes located in the continental United States and Alaska. About half of them are located in Alaska alone.
• The ash cloud from a volcanic eruption can produce lightning. Collisions between the rapidly moving ash particles produce electric charges, which build up until they discharge to form a lightning bolt.
• The most active volcano in the United States is Kilauea (pronounced kee-low-AY-ah) on the island of Hawaii. Kilauea has been erupting nearly continuously since 1983.
• Stromboli, one of the Aeolian Islands of Italy, has been erupting almost continuously with relatively small explosions and occasional bigger explosions and lava flows for the last 2,000 years.
• The first volcanic eruption ever to be described in detail was that of Mount Vesuvius, which erupted in 79 c.e. and buried the Roman cities of Pompeii and Herculaneum. Pliny the Younger, who witnessed the eruption from a distance of 18 miles (29 kilometers), wrote down his observations afterward.
• Volcanologists estimate that fifteen to twenty volcanoes are erupting around the world at any given moment.
• The 1883 eruption of Krakatau in Indonesia created a tsunami (pronounced tsoo-NAH-mee; a series of great ocean waves caused by a large displacement of water) greater than 115 feet (35 meters) in height that drowned an estimated 36,000 people. The volcanic explosion was heard almost 3,000 miles (4,827 kilometers) away.
• The most common elements in materials erupted by volcanoes are silicon, oxygen, magnesium, iron, aluminum, calcium, sodium, potassium, titanium, phosphorous, carbon, hydrogen, and sulfur.
• The fastest recorded speed of a lava flow was about 37 miles (60 kilometers) per hour at Nyiragongo volcano in Zaire.
• The temperature of basalt, the hottest type of lava, can reach almost 2,150°F (1,200°C).

high temperature, mantle rock near the core heats up and expands. This makes it less dense and more buoyant, and it slowly rises to the surface. Near the surface, the hot rock moves sideways along the underside of the lithosphere, losing its heat. As it cools, the rock becomes denser, or heavier, and sinks back toward the core, only to be heated once again. This continuous motion of heated material rising, cooling, and sinking within Earth's mantle forms circular currents: convection currents. The time involved for heated rocks to rise from the lower mantle to the surface, cool, and return to the interior is estimated to be around 200 million years.

Plate tectonics

The slowly moving convection currents are able to release their heat energy near the surface of the planet because both Earth's interior and its surface are in motion. Earth's lithosphere is not solid, but is broken into many large slabs or plates that "float" on the hot, soft asthenosphere. There are seven large plates, eight medium-sized plates, and a number of smaller ones. These plates are in constant contact with each other, fitting together like a jigsaw puzzle. When one plate moves, it causes other plates to move. The movement of the plates toward or away from each other is in response to the pressure exerted by the convection currents. Scientists used the word tectonics (from the Greek word tekton, meaning "builder") to describe the movements. The scientific theory explaining the plates and their movements and interactions is known as plate tectonics.

The major geologic features of Earth, from volcanoes to mountains to basins to oceanic trenches, are all the result of plate movement. Plates move at rates from about 1 to 6 inches (2.5 to 15 centimeters) per year. The plates interact with each other in one of three ways: they move toward each other (converge), they move away from each other (diverge), or they slide past each other (transform). The boundaries where plates meet and interact are known as plate margins.

Walking in the Footsteps of Giants

It is called the Giant's Causeway: a mass of closely packed lava columns whose tops seem to form stepping stones that lead from a cliff along the coast of Northern Ireland and disappear under the North Channel. There are approximately forty thousand of these columns, most of which are roughly hexagonal (six-sided) in shape. The rest have between four and eight sides. The tallest columns stand 40 feet (12 meters) high, and the hardened lava in the cliff is 90 feet (27 meters) thick in places.

A causeway is a raised path or road over water or across land that is sometimes covered by water. The ancient Irish believed that a giant created this landform for such a purpose. According to legend, Finn MacCool (Fionn mac Cumhail is the Gaelic spelling of his name) was a warrior and commander of the armies of the kingdom of Ireland. One day, his fighting abilities were questioned by Benendoner, a Scottish giant. MacCool then challenged the giant, but Benendoner could not swim and so was not able to come to Ireland to answer the challenge. Enraged, MacCool used his sword to cut sections from the cliff, which he then threw into the channel between Ireland and Scotland to create the causeway so the two giants could meet to settle their dispute.

Scientists have known since the late eighteenth century that lava flows formed the strange columns, but the cause behind their regular geometric shape remained a mystery until the twentieth century. Scientists now know that the volcanic rock in the area is made of basalt, which flowed smoothly out of fissures or cracks in Earth's surface about sixty million years ago. As basalt cools, it shrinks. The basalt that formed the causeway, however, did not cool slowly and shrink evenly. Scientists believe water most likely may have washed over it, accelerating its cooling. As the basalt cooled rapidly, randomly scattered areas across its surface cooled before other areas. The stress of the rapid cooling and uneven shrinking would have caused cracks to form along the surface and continue downward, much in the same way that mud dries in a puddle of water that has evaporated.

There are three types of convergent plate margins or areas where two plates move toward each other: continental-continental, continental-oceanic, and oceanic-oceanic. When two continental (land) plates converge, they crumple up and compress, forming complex mountain ranges of great height. The rocks making up oceanic crust are denser (heavier) than those making up continental crust. So when an oceanic plate converges with another plate, either continental or oceanic, it slides beneath the other. The region where this occurs is known as the subduction zone, and this is where most of the world's explosive volcanoes form.

When an oceanic plate subducts, or sinks, beneath a continental plate, the leading edge of the oceanic plate is pushed farther and farther beneath the continent's surface. When it reaches about 70 miles (112 kilometers) into the mantle, high temperature and pressure melt the rock at the edge of the plate, forming thick, flowing magma. Since magma is less dense than the rock that typically surrounds it deep underground, the magma tends to rise toward Earth's surface. Driven by pressure created by gas bubbles within it, the magma forces its way through weakened layers of rock to collect in underground reservoirs called magma chambers.

As magma rises through the mantle to Earth's surface, the surrounding pressure on it decreases, allowing the magma to expand. As more and more magma collects in a magma chamber, pressure from the expanding magma increases until it exceeds that of the overlying rock. An explosion then occurs, and the magma is forced out of the chamber upward through cracks or vents in the planet's surface. The severity of the explosion is dependent on the composition of the magma. If it has a low viscosity, its gases can escape rapidly, and it flows rather than exploding. If it has a high viscosity, its gases cannot escape as quickly. Pressure builds until the gases escape violently in an explosion.

Ring of Fire

Volcanoes are not scattered widely across the surface of Earth. Most are concentrated on the edges of continents, along island chains, or beneath the sea forming long mountain ranges. A majority of the world's active volcanoes above sea level are located in a geographic belt called the Ring of Fire. This belt encompasses the lands on the edges of the Pacific Ocean. It also marks the boundary where the Pacific Plate subducts beneath the plates surrounding it. The Ring of Fire follows the west coast of the Americas from Chile to Alaska. It runs through the Andes Mountains, Central America, Mexico, California, the Cascade Mountains, and the Aleutian Islands. It continues down the east coast of Asia from Siberia to New Zealand, through Kamchatka, the Kurile Islands, Japan, the Philippines, Celebes, New Guinea, the Solomon Islands, and New Caledonia.

Since a vast percentage of all magma reaching Earth's surface does so under the oceans, most of the planet's volcanoes are located there. Some protrude above the water as islands; others lie entirely beneath the surface. Most form along mid-ocean ridges, which are long, narrow structures where oceanic plates are diverging or moving apart and magma rises to fill the gap. The Mid-Atlantic Ridge, where the North and South American plates are diverging from the Eurasian and African plates, is a submerged mountain range with many volcanic features. The volcanic-rich island of Iceland lies directly over the Mid-Atlantic Ridge. (For further information on oceanic landforms, see the Ocean basin chapter.)

Hot spots are special areas where volcanoes form apart from plates converging or diverging, such as in the middle of the Pacific Ocean. At these places, magma rises from the mantle and erupts through Earth's crust

to form a volcano. Volcanologists believe these plumes of magma can exist for millions of years. A famous example of hot spot volcanoes is the string of Hawaiian islands. These islands form a chain of volcanoes because the magma plume that created them has remained stationary while the Pacific plate has shifted. As the plate moved slowly over millions of years, each volcano was cut off from its magma source and a new one formed in its place over the hot spot. The northwesternmost island in the Hawaiian chain contains the oldest rocks and is volcanically extinct; the island of Hawaii, the southeasternmost in the chain, contains the youngest rocks and is the most volcanically active of all the islands. Volcanologists believe it sits directly over the hot spot. More than one hundred hot spots have been active around the planet during the last ten million years, creating some of the world's largest volcanoes, including the present-day Hawaiian volcanoes Mauna Loa and Kilauea.

Spotlight on famous forms

Mauna Loa, Hawaii

Mauna Loa, a very broad, flat shield volcano on the island of Hawaii, is the world's largest volcano and one of the most active. It rises 13,680 feet (4,170 meters) above sea level, and it extends more than 18,000 feet (5,544 meters) to the floor of the Pacific Ocean. From its base under the ocean to its summit, Mauna Loa measures about 32,000 feet (9,754 meters), which makes it taller than Mount Everest in the Himalayas, the

Largest Volcano in the Solar System

Olympus Mons on Mars is the largest of all known volcanoes in the solar system. It is a shield volcano that stands 16 miles (26 kilometers) high and measures 388 miles (624 kilometers) in diameter. A 4-mile-high (6-kilometer-high) scarp, or steep cliff, rims its outer edge. A caldera 50 miles (80 kilometers) wide is located at its summit. If Olympus Mons were placed on the face of Earth, it would cover an area the size of the state of Arizona.

Scientists believe Olympus Mons and other volcanoes on Mars were able to become extremely large because of two main reasons: First, there is less gravity on Mars, which allowed the volcanoes to grow without collapsing under their own weight. Second, the crust on Mars does not move as it does on Earth. This allowed lava to pile up in one large volcano instead of being distributed in an arc of volcanoes, such as the Hawaiian Islands on Earth.

Olympus Mons and all other volcanoes on Mars are extinct. Scientists estimate the youngest lava flows on the gigantic volcano are 20 to 200 million years old.

world's tallest mountain on land. The volcano, whose name means "Long Mountain" in Hawaiian, makes up just more than 50 percent of the island of Hawaii. Mauna Loa encompasses an area of about 2,035 square miles (5,271 square kilometers).

Thousands of thin flows of lava have built the volcano over hundreds of thousands of years. It currently has a volume of approximately 9,600 cubic miles (40,000 cubic kilometers). Since its first documented historical eruption in 1834, Mauna Loa has erupted thirty-three times. Its most recent eruption occurred in the spring of 1984. The lava flows it has produced in the past two centuries have covered 310 square miles (803 square kilometer).

At Mauna Loa's summit is a caldera named Mokuaweoweo (pronounced MO-koo-AH-WAI-o-WAI-o; translated literally as "fish section"). It measures 3 miles (5 kilometers) long, 1.5 miles (2.4 kilometers) wide, and 600 feet (183 meters) deep. Volcanologists believe the caldera formed 600 to 700 years ago when the volcano's magma chamber collapsed after an eruption.

Mount Cotopaxi, Ecuador

The world's highest active volcano, Mount Cotopaxi, rises 19,388 feet (5,909 meters) above the surrounding highland plain in central Ecuador. A stratovolcano with an almost perfectly symmetrical cone, Mount Cotopaxi is covered in glaciers above 14,930 feet (4,550 meters). Deep valleys are cut into its steep sides, radiating downward from its summit. The crater at its summit measures 2,625 feet (800 meters) by 2,133 feet (650 meters).

Cotopaxi, which in the local Quechua (pronounced KECH-wa) language means "neck of the Moon," has a well-recorded history of explosive eruptions. In 1534, when an invading Spanish army began an attack against the native Incas, the battle was cut short by a massive eruption. Since then, Mount Cotopaxi has erupted more than fifty times, most recently in 1904. An eruption in 1877 melted snow and ice from the summit, producing mudflows (thick mixtures of mud, water, and other surface fragments) that traveled more than 60 miles (100 kilometers) from the volcano.

Mount St. Helens, Washington

Located in southwestern Washington, Mount St. Helens is a relatively young stratovolcano, approximately forty thousand years old. Before its spectacular eruption in May of 1980, it had lain dormant since 1857. Native Americans in the area surrounding the volcano called it Louwala-Clough, meaning "smoking mountain." In the past five centuries, the volcano has produced four major eruptions and dozens of lesser ones. It is the most active volcano in the Cascade Range, a mountain chain that extends about 700 miles (1,125 kilometers) from British Columbia, Canada, to northern California.

Prior to May 18, 1980, Mount St. Helens rose to a height of 9,677 feet (2,950 meters). The fifth-tallest mountain in Washington, it had a nearly perfect cone shape, and its summit glistened in a perpetual cap of snow and ice. When the volcano exploded on that May morning in a continuous nine-hour eruption, it lost its uppermost 1,300 feet (396 meters). A massive earthquake under Mount St. Helens caused its north flank to slide away in the largest landslide in recorded history. This landslide triggered a destructive, lethal sideways blast of hot gas, steam, and rock debris that swept across the landscape as fast as 680 miles (1,095 kilometers) per hour. Within minutes of the blast, an ash plume from the destroyed crater rose 15 miles (24 kilometers) into the sky.

After the eruption, Mount St. Helens's summit stood only 8,364 feet (2,549 meters) above the destroyed landscape surrounding it—about 0.67 cubic miles (2.75 cubic kilometers) of material had been removed from the volcano. Its new horseshoe-shaped crater measured 1.2 miles (1.9 kilometers) by 1.8 miles (2.9 kilometers), and the crater floor was 2,084 feet (635 meters) deep. Within months after the main eruption, small eruptions produced thick lava that formed a lava dome in the crater. Over the next six years, subsequent eruptions generated viscous lava that added layer upon layer to the dome. At the beginning of the twenty-first century, the lava dome towered more than 900 feet (275 meters) above the crater floor.

Crater Lake, Oregon

Measuring 5 miles (8 kilometers) in diameter, Crater Lake has a surface area of 20.6 square miles (53.4 square kilometers). It is enclosed by steep rock walls that rise up almost 2,000 feet (610 meters) above its surface. With a depth of 1,943 feet (592 meters), Crater Lake is the deepest lake in the United States and the seventh-deepest in the world. No springs or other inlets feed the lake. The water that evaporates or seeps out of the lake is replaced mostly by winter snows, which average 44 feet (13.5 meters) a year.

Crater Lake occupies the remains of Mount Mazama, the name given by volcanologists to this ancient cluster of overlapping stratovolcanoes that originally stood at about 12,000 feet (3,658 meters). The caldera formed when Mazama exploded violently 7,700 years ago. The eruption spewed about 18 cubic miles (75 cubic kilometers) of pyroclastic material onto thousands of square miles of surrounding landscape and caused the volcano's magma chamber to collapse. Minor eruptions afterward built a cinder cone on the floor of the caldera. Now known as Wizard Island, the cone rises 764 feet (233 meters) above the surface of the lake on its west side.

After forming, the caldera gradually filled with more than 5 trillion gallons (19 trillion liters) of water from rainfall and melting snow. The deep blue color of the lake is due to its great depth, the clarity of its water, and the way light interacts with water. Sunlight (white light) is made up of all the colors in the spectrum: red, orange, yellow, green, blue, indigo, and violet. Red light has the longest wavelength, violet the shortest. Just as a prism splits sunlight into different bands of color, so does water. As sunlight enters water, the water molecules easily absorb the longer wavelengths of light (reds, oranges, and yellows). The shorter wavelengths are more easily scattered than absorbed. Because of the depth of Crater Lake, all of the longer wavelengths of light are absorbed. Since the lake's water is so clear, the shorter wavelengths of blue and violet are able to penetrate much farther down before being scattered and redirected toward the surface. This creates what the human eye sees as an intense blue color.

For More Information

Books

Decker, Robert, and Barbara Decker. Volcanoes. New York: W. H. Freeman, 1997.

Morris, Neil. Volcanoes. New York: Crabtree Publishing, 1995.

Rosi, Mauro, Papale, Paolo, Lupi, Luca, and Marco Stoppato. Volcanoes. Toronto: Firefly Books, 2003.

Thompson, Luke. Volcanoes. New York: Children's Press, 2000.

Tilling, Robert I. Born of Fire: Volcanoes and Igneous Rocks. Berkeley Heights, NJ: Enslow, 1991.

Trueit, Trudy Strain. Volcanoes. New York: Franklin Watts, 2003.

Van Rose, Susanna. Volcano and Earthquake. New York: DK Publishing, 2000.

Web Sites

"Cascades Volcano Observatory: Learn About Volcanoes." USGS. http://vulcan.wr.usgs.gov/Outreach/AboutVolcanoes/framework.html (accessed on September 2, 2003).

The Electronic Volcano. http://www.dartmouth.edu/~volcano/ (accessed on September 2, 2003).

"Global Volcanism Program." Smithsonian Institution. http://www.volcano.si.edu/gvp/ (accessed on September 2, 2003).

Plate tectonics. http://www.platetectonics.com/ (accessed on September 2, 2003).

Tilling, Robert I. "Volcanoes." USGS. http://pubs.usgs.gov/gip/volc/ (accessed on September 2, 2003).

Volcanic Landforms. http://volcano.und.nodak.edu/vwdocs/vwlessons/landforms.html (accessed on September 2, 2003).

Volcanic Landforms of Hawaii Volcanoes National Park. http://volcano.und.nodak.edu/vwdocs/vwlessons/havo.html (accessed on September 2, 2003).

Volcano World. http://volcano.und.nodak.edu/ (accessed on September 2, 2003).

"Volcanoes of the United States." USGS. http://pubs.usgs.gov/gip/volcus/ (accessed on September 2, 2003).

Volcano

A volcano is a hole in Earth's surface through which magma (called lava when it reaches Earth's surface), hot gases, ash, and rock fragments escape from deep inside the planet. The word volcano also is used to describe the cone of erupted material (lava and ash) that builds up around the opening.

Volcanic activity is the main process by which material from Earth's interior reaches its surface. Volcanoes played a large part in the formation of Earth's atmosphere, oceans, and continents. When Earth was new, the superheated gases within it (including carbon dioxide) streamed out through countless volcanoes to form the original atmosphere and oceans.

Volcanoes are found both on land and under the oceans (where they are called seamounts). Geologists label volcanoes by their periods of activity. If a volcano is erupting, it is called active. If a volcano is not presently erupting but might at some future date, it is called dormant. If a volcano has stopped erupting forever, it is called extinct. Generally, volcanoes are labeled extinct when no eruption has been noted in recorded history.

Words to Know

Caldera: Large circular depression formed when an empty magma chamber causes the collapse of the volcano above it.

Chemosynthesis: Process by which the energy from certain chemical reactions, rather than light (as in photosynthesis), is used by some organisms to manufacture food.

Hot spot: An upwelling of heat from beneath Earth's crust.

Ignimbrite: Rock formation that results from a large pyroclastic flow.

Lava: Magma at Earth's surface.

Magma: Molten rock deep within Earth that consists of liquids, gases, and particles of rocks and crystals.

Photosynthesis: Process by which light energy is captured from the Sun by pigment molecules in plants and algae and converted to food.

Pyroclastic flow: A dense wave of superheated air and rock that moves as a fluid from an erupting volcano, sometimes crossing thousands of square miles of landscape.

Seafloor spreading: Spreading of the seafloor outward at ridges where two oceanic plates are diverging.

Seamount: Large, submarine volcano.

Tuff: Fused hard rock formed from a large pyroclastic flow.

How volcanoes form

According to the geologic theory called plate tectonics, Earth's crust is broken into various rigid plates that "float" on the surface of the planet. The plates move in response to intense pressure created underneath by the movement of currents carrying heat energy from the center of the planet to the surface. This pressure causes plates to move toward or away from each other (and also past each other in a horizontal motion).

Volcanoes form on land near coastal areas when a continental (land) plate and an oceanic plate converge or move toward each other. Since the oceanic plate is denser, it subducts or sinks beneath the continental plate. As the rock of this subducted oceanic plate is pushed farther and farther beneath the continent's surface, extremely high temperatures and pressure melt the rock. This creates hot, buoyant magma that then rises toward the surface. When the magma reaches the crust, it collects in a magma reservoir or chamber. When pressure inside the reservoir exceeds that of the overlying rock, magma is forced upward through cracks in Earth's crust.

Hydrothermal Vents

Hydrothermal vents are cracks in the ocean floor or chimney-like structures extending from the ocean floor up to 150 feet (45 meters) high. Due to nearby volcanic activity, these vents release hot mineral-laden water into the surrounding ocean. Temperature of this fluid is typically around 660°F (350°C).

Often, the fluid released is black due to the presence of very fine sulfide mineral particles (iron, copper, zinc, and other metals). As a result, these deep-ocean hot springs are called black smokers. Hydrothermal vents usually occur at midocean ridges where new seafloor is created.

Hydrothermal vents are surrounded by unusual forms of sea life, including giant clams, tube worms, and unique types of fish. These organisms live off bacteria that thrive on the energy-rich chemical compounds transported by hydrothermal fluids. This is the only environment on Earth supported by a food chain that does not depend on the energy of the Sun or photosynthesis. The energy source is chemical, not solar, and is called chemosynthesis.

Seamounts (underwater volcanoes) form when oceanic plates both converge (move toward each other) and diverge (move away from each other). When oceanic plates converge, one sinks beneath the other, creating a deep-sea trench. Rising magma from the subducted plate then rises to form volcanoes along the trench. When oceanic plates diverge, magma seeps upward at the ridge between the plates to create new seafloor (a process called seafloor spreading). Volcanoes form on either side of the ridge.

Hot spots are special areas where volcanoes form apart from plates converging or diverging. Hot spots are a common term for thermal plumes of magma welling up through the crust far from the edges of plates. As a plate drifts over a hot spot, magma from Earth's interior rises and volcanic activity takes place. Some famous hot spots are Hawaii, Yellowstone National Park (United States), Iceland, Samoa, and Bermuda.

Volcanic eruptions

Volcanoes erupt different material, and they each have their own style of erupting. These varied eruptions result from the differences in magma that each volcano contains. Magma that is low in gas and silica (silicon dioxide, a compound found widely in rocks and minerals) yields a gentle flow of thin, quickly spreading lava. In contrast, magma that is rich in gas and silica gives rise to violent explosions: the thick, tarlike magma may plug up the volcanic vent, blocking the upward movement of the magma until built-up pressure blows away the overlying rock. Geologists classify volcanic eruptions according to four chief forms or phases: Hawaiian, Strombolian, Vulcanian, and Peleean.

In a Hawaiian phase, runny lava gushes out in a fountain without any explosive eruptions. In a Strombolian phase (named after the Stomboli volcano on an island north of Sicily), thick lava is emitted in continuous

but mild explosions. Lava arcs and steam-driven clouds of ash shower the dome with molten drizzle. A Vulcanian phase occurs when a magma plug has blocked the volcanic vent. The resulting explosive eruption hurls tons of almost solid magma into the sky, and a vapor cloud forms over the crater. The most violent eruption is the Peleean, named after Mount Pelee on the Caribbean island of Martinique. Fine ash, thick lava, and glowing, gas-charged clouds are emitted, traveling downhill at a tremendous speed.

Fierce rains often accompany eruptions because of the release of steam from the volcano, which then condenses in the atmosphere to form clouds. Volatile gases in the magma also fly into the atmosphere upon eruption. These include hydrogen sulfide, fluorine, carbon dioxide, and radon. A dense wave of ash, superheated gases, and rock that moves as a fluid from an erupting volcano is known as a pyroclastic flow. Flows travel downhill at speeds more than 60 miles (100 kilometers) per hour, filling existing valleys with the fluid mixture. This material deflates as it cools. The rock formation that results is called an ignimbrite (pronounced IG-nim-bright), and the fused rock is called tuff. Ignimbrites can cover hundreds of square miles of landscape, such as the Mitchell Mesa Tuff of West Texas.

When a volcano erupts such a large volume of material, often emptying its magma chamber, the central part of the cone is left unsupported. As a result, the crater and walls of the vent collapse into the hollow chamber, creating a large circular depression known as a caldera across the summit. The famous Crater Lake in southern Oregon formed in this way.

Volcanic structures

The size and shape of a volcano is dependent on the history and type of its eruptions. Based on this, geologists classify volcanoes into four shapes: cinder cones, composite cones, shield volcanoes, and lava domes.

Cinder cones are built of lava fragments. They have slopes of 30 to 40 degrees and seldom exceed 1,640 feet (500 meters) in height. Sunset Crater in Arizona and Parícutin in Mexico are examples of cinder cones.

Composite cones (or stratovolcanoes) are made up of alternating layers of lava, ash, and solid rock. They are characterized by slopes of up to 30 degrees at the summit, tapering off to 5 degrees at the base. Mount Fuji in Japan and Mount St. Helens in Washington are composite cone volcanoes.

Shield volcanoes are built primarily by a series of lava flows that pile one on top of another. Their slopes are seldom more than 10 degrees at the summit and 2 degrees at the base. The Hawaiian Islands are clusters of shield volcanoes. Mauna Loa (on the island of Hawaii) is the world's largest active volcano, rising 13,680 feet (4,170 meters) above sea level. Kenya's Mount Kilamanjaro, the tallest mountain in Africa, is a shield volcano.

Lava domes are made of thick, pasty lava squeezed like toothpaste from a tube. Examples of lava domes are Lassen Peak and Mono Dome in California.

Volcanic catastrophes

Numerous volcanoes erupt around the world every century, usually in sparsely populated areas. Even so, volcanoes have threatened human civilization throughout history and will do so as long as people live on Earth's often violent surface.

An ash fall from Mount Vesuvius buried the Roman city of Pompeii in a.d. 79. The volcano, which sent a column of hot ash 12 miles (19 kilometers) into the sky, struck down the people where they lived, preserving the shapes of their bodies where they fell in the ash. The nearby city of Herculaneum was covered by a pyroclastic flow that destroyed it in seconds. Pompeii remained buried until 1748, when construction workers first unearthed parts of the ancient city—much of it appearing as it did on the morning Vesuvius erupted.

On August 27, 1883, the volcanic island of Krakatoa in Indonesia erupted, blowing an ash cloud 50 miles (80 kilometers) high then collapsing into a caldera. The collapse was heard almost 2,500 miles (4,020 kilometers) away. Resulting tidal waves reaching 130 feet (40 meters) killed 36,000 people in coastal Java and Sumatra. Spectacularly weird sky phenomena from this eruption included brilliant green sunrises and moon-rises in the equatorial latitudes, followed by day-long blue sunlight and bright green sunsets.

On the morning of May 18, 1980, Mount St. Helens in Washington erupted with the force of more than 500 atomic bombs—one of the largest volcanic explosions in North American history. The blast, which sent a mushroom-shaped ash plume 12 miles (20 kilometers) high, reduced the summit (peak) by more than 1,300 feet (400 meters). Sixty people and countless animals were killed, and every tree within 15 miles (24 kilometers) was flattened. Ensuing landslides carried debris for nearly 20 miles (32 kilometers).

Volcanic benefits

The eruption of volcanoes through geologic time built the continents. The soil of some of the world's richest farmland draws its fertility from minerals provided by nearby volcanoes. The heat of magma boils water into steam that spins the turbines of geothermal power stations. Geothermal stations now light electric power grids in Iceland, Italy, New Zealand, and a other places. Enough heat flows from the world's volcanic regions and midoceanic ridges to power industrial civilization for several hundred million years. This power source awaits only the development of feasible geothermal technology.

[See also Island; Ocean; Plate tectonics; Rocks ]

volcano vents or fissures in the earth's crust through which gases, molten rock, or lava , and solid fragments are discharged. Their study is called volcanology. The term volcano is commonly applied both to the vent and to the conical mountain (cone) built up around the vent by the erupted rock materials. Volcanoes are described as active, dormant, or extinct. The soil resulting from decomposition of volcanic materials is extremely fertile, and the ash itself is a good polishing and cleansing agent.

Occurrence

Volcanoes are found in association with midocean ridge systems (see seafloor spreading ) and along convergent plate boundaries, such as around the Pacific Ocean's "Ring of Fire" (see plate tectonics ), the ring of plate boundaries associated with volcanic island arcs and ocean trenches surrounding the Pacific Ocean. Continental volcanoes are also associated with converging plate boundaries, such as the volcanoes of the Cascade Range along the W coast of the United States. Isolated volcanoes also form in the midocean area of the Pacific apparently unrelated to crustal plate boundaries. These sea mounts and volcanic island chains, such as the Hawaiian chain, may form from rising magma regions called hot spots.

Volcanic Cones and Craters

Shapes of volcanoes include composite cones, or stratovolcanoes, with steep concave sides such as Mt. St. Helens in the W United States; shield cones have gentle slopes and can be relatively large such as the Hawaiian Islands; and cinder cones as Parícutin in Mexico, with steep slopes made of cinderlike materials. Explosive eruptions build up steep-sided cones, while the nonexplosive ones usually form broad, low lava cones. Cones range in height from a few feet to nearly 30,000 ft (9 km) above their base. Usually the cone has as its apex a cavity, or crater, which contains the mouth of the vent. Such craters are typically less than 1 mi (1.6 km) across, but larger craters, called calderas, ranging in diameter from 3 mi to—in a few instances—50 mi (5-80 km), are formed by particularly large eruptions (see crater ).

Volcanic Eruptions

More than 500 volcanoes are known to have erupted on the earth's surface since historic times, and many more have erupted on the ocean floor unobserved by humans. Fifty volcanoes have erupted in the United States, which ranks third, behind Indonesia and Japan, in the number of historically active volcanoes. Of the world's active volcanoes, more than half are found around the perimeter of the Pacific, about a third on midoceanic islands and in an arc along the south of the Indonesian islands, and about a tenth in the Mediterranean area, Africa, and Asia Minor.

Evidence of extraterrestrial volcanic activity also has been found. Space probes have detected the remnants of ancient eruptions on earth's moon, Mars (which has the largest volcano in the solar system, Olympus Mons, 340 mi/550 km across and 15 mi/24 km high), and Mercury; these probably originated billions of years ago, since these bodies are no longer capable of volcanic activity. Triton (a satellite of Neptune), Io (a satellite of Jupiter), and Venus are the only bodies in the solar system besides earth that are known to be volcanically active. The volcanic processes that occur in the outer portion of the solar system are very different from those in the inner part. Eruptions on earth, Venus, Mercury, and Mars are of rocky material and are driven by internal heat. Io's eruptions are probably sulfur or sulfur compounds driven by tidal interactions with Jupiter. Triton's eruptions are of very volatile compounds, such as methane or nitrogen, driven by seasonal heating from the sun.

Terrestrial volcanic eruptions may take one or more of four chief forms, or phases, known as Hawaiian, Strombolian, Vulcanian, and Peleean. In the Hawaiian phase there is a relatively quiet effusion of basaltic lava unaccompanied by explosions or the ejection of fragments; the eruptions of Mauna Loa on the island of Hawaii are typical. The Strombolian phase derives its name from the volcano Stromboli in the Lipari, or Aeolian, Islands, N of Sicily. It applies to continuous but mild discharges in which viscous lava is emitted in recurring explosions; the ejection of incandescent material produces luminous clouds. A more explosive volcanic eruption is the Vulcanian, where the magma (lava before emission) accumulates in the upper level of the vent but is blocked by a hardened plug of lava that forms between consecutive explosions. When the explosive gases have reached a critical pressure within the volcano, masses of solid and liquid rock erupt into the air and clouds of vapor form over the crater. The Peleean, derived from Mt. Pelée, is the most violent, emitting fine ash; hot, gas-charged fragments of lava; and superheated steam in an incandescent "cloud" that travels downhill at great speed. Eruptions are often accompanied by torrential rains caused by the condensation of steam. The erupted fragments vary in size, including minute particles of volcanic dust and ash, lapilli (cinders or pellets), bombs (rounded or ellipsoidal masses of hardened magma), and huge masses called blocks.

Historical Volcanoes

Notable eruptions within historic times have been those of Vesuvius, in Italy (AD 79, 1906, and other times); Tambora, in Indonesia, where between 30 and 50 cu mi (125-210 cu km) of molten and shattered rock were blown into the air (1815); Krakatoa, near Java, material from which was sent 17 mi (27 km) into the atmosphere (1883); Parícutin, in Mexico, the volcano that began in a cornfield (1943); Hibok Hibok, on Camiguin island in the Philippines, which killed 84 people (1948); Besymianny, in Kamchatka, where 2 cu mi (8 cu km) of material were hurled into the air (1956); the peak of Tristan da Cunha, whose eruption caused the entire settlement to be evacuated (1961); Agung, in Bali, which killed 1,100 people (1963); Mt. St. Helens in Washington, which exploded with an energy equivalent to 10 million tons of TNT, killing 35, with 25 missing (1980); El Chichon in Mexico, which expelled about 500 million tons of ash and gas (1982); and Mt. Pinatubo in the Philippines, which killed over 500 people and ejected over 2 cu mi (8 cu km) of material (1991). Other notable volcanoes are Cotopaxi and Chimborazo (Ecuador), Iztaccihuatl and Popocatépetl (Mexico), Lassen Peak and Katmai (United States), and Etna (Sicily). Mauna Loa (Hawaii) is the world's largest active volcano, projecting 13,677 ft (4,170 m) above sea level and over 29,000 ft (8,850 m) above the ocean floor; from its base below sea level to its summit, Mauna Loa is taller than Mt. Everest. In 1963 the birth of the volcanic island Surtsey near Iceland was observed. In November of that year events began with a submarine eruption along the Mid-Atlantic Ridge. Eruption followed eruption until June, 1967, by which time the island stood 492 ft (150 m) above sea level and covered an area of almost 2 sq mi (3 sq km). The island has not grown since the last eruption, and it is presently volcanically quiet.

Bibliography

See S. Van Rose and I. Mercer, Volcanoes (2d ed., 1991); F. Martin, Volcano (1996); H. Sigurdsson, Melting the Earth: The History of Ideas on Volcanic Eruptions (1999); H. Sigurdsson et al., ed., Encyclopedia of Volcanoes (1999).

Volcano

Volcanoes are vents or fissures in Earth's crust through which lava , gases, and pyroclastic debris are released. More commonly, the term volcano refers to the landform built up from the accumulation of lava and/or pyroclastic debris. Based on the timing of their last eruption, volcanoes are classified as active (having erupted during historic time), dormant (having no recent eruptions, but with the potential to erupt again), or extinct (having no historic eruptions and showing no evidence of future eruptions). There are currently over 500 active volcanoes on Earth's surface, including famous examples such as Mt. Fuji, Mt. St. Helens, and Mauna Loa. Mt. Vesuvius, which last erupted in a.d.79, is an example of a dormant volcano; Mt. Kilimanjaro is an extinct volcano.

Fueled by Earth's internal processes, volcanoes occur primarily along plate boundaries but also form above hot spots. Eruptive activity may include lava flows, lateral blasts, ash flows, lahars, the release of volcanic gases, or any combination of these. Different types of volcanoes, each with a unique set of characteristics and eruptive styles, include shield volcanoes, composite volcanoes, lava domes, calderas, and cinder cones. Different types of magma form under different plate tectonic settings, and the type of magma present determines the type of volcano that will form in a given area .

Shield volcanoes, with their gentle slopes and curved profile, are the largest of all volcanoes. They are built up from repeated basaltic flows, often beginning at the ocean floor. Basaltic magma has a relatively low silica content, allowing it to flow readily. As a result, shield volcanoes are characterized by lava flows rather than explosive pyroclastic activity. Shield volcanoes are most commonly formed above hot spots under basaltic oceanic crust. They are also formed in areas where the mid-ocean ridge intersects with land, as in Iceland, or in areas of active rifting , like east Africa . In these areas, as the magma is rising to the surface, it mixes with only basaltic rocks, allowing it to preserve its mafic composition and flow readily. Probably the most famous shield volcanoes, Mauna Loa and Mauna Kea, currently rest above the Hawaiian hotspot. Measured from its base on the ocean floor to its summit, Mauna Kea is 5.6 mi (9 km) tall—slightly taller than Mt. Everest.

Composite volcanoes, also known as stratovolcanoes, have steep sides and a characteristic cone shape. They are built up from alternating layers of lava and pyroclastic debris. Lava associated with composite volcanoes generally has an intermediate composition, and is more resistant to flow than basaltic lava. This results in the mixture of flows and explosions. Composite volcanoes occur above subduction zones, where rising magma mixes with both oceanic and continental crust raising the overall silica content. They are ubiquitous along the subduction zones of the Pacific Rim, and some famous examples include Mt. Fuji in Japan and Mt. Rainier in Washington. Their ability to erupt explosively, as demonstrated by Mt. St. Helens in 1980, makes these some of the most dangerous volcanoes on Earth.

Lava domes are steep-sided, rounded domes, formed because of pressure exerted by rising viscous magma. Rhyolite , a felsic magma, is usually associated with lava domes. Its felsic composition makes it highly viscous, forcing it to move slowly, building up pressure and deforming the ground surface above. Lava domes are generally associated with composite volcanoes, although they can occur on their own. They are capable of causing deadly eruptions as tremendous amounts of built-up pressure are suddenly released in giant explosions. Eruption of a lava dome was responsible for the death and destruction caused by the 1902 eruption of Mt. Pelée on Martinique.

Calderas are massive depressions created by rare, violent explosions. Also associated with rhyolitic magma, caldera eruptions are capable of expelling enormous amounts of ash and debris in a single explosion. Calderas form where hotspots occur under continental crust. As magma rises, it mixes with the felsic continental crust, resulting in a high silica content. As is the case with lava domes, the resultant viscous magma cannot flow, and explodes when sufficient pressure has built up. Although there have been none in recent geologic history, about 600,000 years ago a large caldera eruption occurred at what is presently the site of Yellowstone National Park in Wyoming and Montana. The famous hot springs and geysers of the area are the legacy of that eruption, and it is believed that the site has the potential to produce another eruption in the future.

Cinder cones are steep-sided, cone-shaped, relatively small volcanoes that are formed by the accumulation of pyroclastic debris. They are not associated with any one particular lava type, and occur in a number of settings. They are commonly found on the flanks or inside the summit craters of larger volcanoes, and form when pyroclastic debris ejected by the main volcano accumulates to form the smaller cone. Perhaps the most famous cinder cone, Parícutin volcano in Mexico, grew suddenly out of a farmer's cornfield and within one month had risen to a height of almost 1,000 ft (305 m). Cinder cones tend to have short life spans; lava flows released by Parícutin eventually covered an extensive area, but within 10 years the volcano became dormant.

See also Convergent plate boundary; Nuee ardent

Volcanic eruptions

A volcanic eruption is the release of molten rock and volcanic gases through Earth's crust to the surface. Molten rock within the earth, or magma , is driven to erupt by buoyancy because it is lighter than the surrounding rock. Dissolved gases within the magma are under great pressure and force magma upwards. The upward migrating magma takes advantage of preexisting zones of weaknesses such as fractures or established volcanic necks until it eventually breaks through the surface.

An eruption may last for a few minutes or many hours and days. An eruption may be only a discharge of steam and gases through a small vent, a relatively mild oozing of lava from a fissure in a shield volcano , or a spectacular explosion that shoots huge columns of gases and debris into the sky. The explosiveness of an eruption depends to a great extent on the composition of the molten rock. Magma high in silica will be more viscous than one low in silica. A high-viscosity magma (such as a rhyolite ) will tend to trap dissolved gases. The pressure of the gases can build up to the point where they are released in a spontaneous explosive eruption. A less viscous magma (such as a basalt ) allows volcanic gases to bubble through more easily, preventing great build-ups of pressure, and resulting in calmer outpourings of lava.

The length an eruption is described as an eruptive pulse, eruptive phase, or eruptive episode. An eruptive pulse is a very short event lasting a few seconds to minutes. An eruption that lasts a few hours to days and consists of numerous eruptive pulses is called an eruptive phase. Eruptions that involve repeated pulses and phases over days, months, or years is an eruptive episode.

Volcanic eruptions are described according to explosivity, lava type, and other constituents such as ash, gas, and steam content or the nature of rock fragments produced. Some common eruption types are named for classic types of volcanoes that characterize the eruption. These include Hawaiian, Plinian (Vesuvian), Strombolian, and Vulcanian. Some types of eruptions have more descriptive names, such as effusive and phreatic.

A Hawaiian-type eruption consists of a highly fluid basaltic lava that tends to flow effusively from linear fissures or from a central vent in the production of shield volcanoes. The release is not generally explosive as lava gently flows in streams or through lava tubes. Sometimes the lava accumulates in lava lakes . Occasionally, however, more spectacular fountains of lava spurting out from a vent do occur.

A Plinian, or Vesuvian, eruption is a more explosive and potentially destructive event where large amounts of ash, dust, and gas are blown out of a central source at a high velocity. The eruptive cloud often forms a large column extending high into the air above the volcano. Avalanches of hot ash, rock, and gas, called nuee ardentes, can travel down the side of the volcano at up to 100 mph (160 kph) are possible, such as the one that covered the Italian city of Pompeii. Rhyolitic to dacitic compositions are common. The name is derived from the historian Pliny, who recorded the eruption of Vesuvius in a.d.79.

Strombolian eruptions are characterized by discrete episodic explosions or fountains of basaltic lava from a single vent or crater. The eruptive pulses are caused by the release of volcanic gases, and are separated by periods of a few seconds to hours. Lava fragments consisting of partially molten volcanic bombs that become rounded as they fly through the air are commonly produced.

Vulcanian, or hydrovolcanic eruptions are explosive events that release a combination of ash and steam into the air, producing an eruptive column. Fragments of lava are ejected, but owing to a high viscosity or previous cooling, the fragments do not form aerodynamic bombs. The composition of the lava is generally andesitic to dacitic.

An effusive eruption is a general term for any non-explosive release of lava. The lava gently wells up from the ground and overflows, cooling on its way down the slope. Effusive eruptions are common in a Hawaiian type event. When a basaltic effusive eruption occurs on the ocean floor, pillow lavas often form. As the name suggests, pillow basalts are rounded elongate shapes the lava takes due to extrusion under the pressure of the ocean. As pillow lavas continually erupt, they form stacked mounds of pillows. Effusive eruptions may occur with a range of compositions, although they are most common in low viscosity lavas such as basalt.

If cool ground water or surface water comes in contact with magma below the surface, a phreatic eruption may occur. This is caused by water that is heated into pressurized steam, creating an explosive eruption driven solely by the steam. Because the eruption is driven by steam, no new rock is formed.

See also Extrusive cooling; Fumerole; Hawaiian island formation; Hotspots; Lahar; Nuee ardent; Pipe, volcanic; Tuff; Volcanic vent

Crater, volcanic

A crater is a steep-sided roughly circular to elliptical depression in the earth caused either by volcanic activity or by the impact of an extraterrestrial body. Volcanic craters are formed by explosive events, and/or by the collapse of part of a volcano following withdrawal of magma . Impact craters are the result of collisions between Earth and extraterrestrial bodies such as meteors or comets .

Large volcanic craters are known as calderas among vulcanologists. There are two often-complementary processes involved in their formation; violent eruptions of ash and magma, and/or the collapse of a volcanic surface following withdrawal of a large body of magma from the subsurface. An example of the first type may be Crater Lake in Oregon, thought to have been produced by a violent explosion that destroyed a volcano the size of Mount St. Helens. The caldera at Kilauea, in contrast, is thought to be the result of magma drainage from beneath the summit. There is still significant discussion about whether volcanic calderas are formed directly by explosion, indirectly by collapse of the surface following magma ejection or withdrawal, or by both.

Impact craters are the result of collisions of extraterrestrial bodies with the earth. Only recently have scientists begun to understand the importance of impact processes in shaping the planet and life on it. Exploration of our solar system has revealed that essentially all planetary bodies are cratered. The density of craters on the older surfaces of the Moon indicates an intense bombardment from approximately 4.6 to 3.9 billion years ago. The Moon itself is likely the result of a collision of a Mars size object with a young Earth. The earth experienced the same bombardment as the other planetary bodies. In fact, Earth is subject to about twice as many impacts as the moon because of the difference in gravity . This is not obvious because tectonic and erosion activity on the earth have removed evidence of most of the impacts that have occurred. Nevertheless, approximately 150 craters have been identified, with more recognized every year.

Perhaps the most well-known impact crater on Earth is Chicxulub, a buried crater in the Yucatan, Mexico, that is 110 miles (180 kilometers) in diameter. Most geoscientists now believe that this impact event was responsible for the great mass extinction of the dinosaurs and many other species at the Cretaceous/Tertiary (K-T) boundary, 65 million years ago. Impacts this size occur infrequently, on the order of one every 100 million years. However, impacts that could cause damage similar to a nuclear winter , occur at time scales estimated as two or three every million years. This estimate is significant because the most recent known event, Zhamanshin in Kazaksthan, occurred about a million years ago.

See also Meteoroids and meteorites; Volcanic eruptions

Volcanic vent

Volcanic vents are openings in Earth's crust where molten lava and volcanic gases escape onto the land surface or into the atmosphere. Most volcanoes have a circular central vent near their summit crater that serves as a conduit for ongoing volcanic construction. Basaltic lavas that cool to form oceanic crust, oceanic plateaus, and continental flood basalts erupt from large, elongate, planar vents called fissures. New oceanic crust is created at axial fissures along the globe-encircling ocean ridge system. Small cracks and ducts in volcanic and hydrothermal provinces serve as vents for escaping lava, gas, and water that create smaller-scale volcanic features like gaseous fumaroles, hot springs , geysers, and rootless splatter cones called hornitos.

Each of the three main types of volcanoes—cinder cones, shields, and composite volcanoes—forms by eruption of lava, volcanic ash and gases from a central vent. A cinder cone, like Volcan Parícutin in Mexico, begins with an eruption from a vent in the land surface and grows into a steep-sloped, circular mountain as cinders from successive eruptions form a cone around the vent. Shield volcanoes, like the Hawaiian Islands, are composed of low-viscosity basaltic lava that flows easily and rapidly from a central vent. Though sometimes very large, shield volcanoes have a simple structure of stacked, low-angle lava flows around the central vent.

Composite volcanoes, or stratovolcanoes, are very large volcanic edifices composed of alternating layers of volcanic ash, volcanic ejecta and lava flows. Mt. Rainier in Washington, Cotopaxi in Ecuador, Mt. Etna in Sicily, and Mt. Fuji in Japan are stratovolcanos. Extremely large, pyroclastic eruptions of gas-charged, viscous lava issue from a central vent, or group of vents, in the summit crater of a composite volcano . However, because the andesitic and rhyolitic lava that composes a stratovolcano is so viscous, the central vent system is often plugged between large eruptions. Lava fills

fissures on the flanks of the mountain creating radial dikes. Gases and fluids also escape from secondary vents, creating fumaroles and hot springs on the slopes of a stratovolcano. When a composite volcano becomes dormant, erosion wears away the volcano, leaving the vertical column that cooled in the feeder duct beneath the volcanic vent. Shiprock in New Mexico and Devil's Tower in Wyoming are examples of volcanic necks that formed this way.

See also Mid-ocean ridges and rifts; Volcanic eruptions

VOLCANOES

VOLCANOES are mountains with a vent from which molten material from deep within the earth can spew under the appropriate conditions. Volcanoes have existed for geologic eons, but many are no longer active. The number of volcanoes worldwide that earth scientists consider active—those that can erupt—was about five hundred in the

mid-1990s. Volcanoes are usually located at the junction of the earth's lithospheric plates. In the United States most active volcanoes are located in Alaska or in Hawaii, which consists of a group of islands formed by earlier volcanic eruptions. The West Coast of the continental United States also has a relatively inactive volcanic zone.

The two principal volcanoes in the United States are Mauna Loa and Kilauea, both in the Hawaiian island chain. Mauna Loa, the world's largest volcano, erupted most recently in 1975 and 1984. Kilauea is in almost continual eruption. Alaskan eruptions occurred in 1989, when Mount Redoubt, along Cook Inlet, southwest of Anchorage, erupted; in 1992, when Mount Spurr erupted; and in 1996, when an unnamed volcano on Augustine Island (also in Cook Inlet) erupted. Although not in the United States, Mount Pinatubo in the Philippines projected enough ash into the stratosphere during its eruption in 1991 to have a significant cooling effect on the U.S. climate for several years. Eruptions in the lower forty-eight states are rare but certainly not unknown: for example, the widely publicized eruption of Mount St. Helens in Washington State in 1980. Despite dire predictions and a minor eruption in 1990, the area surrounding Mount St. Helens had largely recovered from the effects of the 1980 eruption by 2000.

There are two volcanic observatories in the United States. One, established on Kilauea in 1912, is the second oldest in the world, ranking behind only one in Italy, on Mount Vesuvius. Following the eruption of Mount St. Helens in 1980, an observatory was established there.

BIBLIOGRAPHY

Decker, Robert W., and Barbara B. Decker. Mountains of Fire: The Nature of Volcanoes. New York: Cambridge University Press, 1991.

Scarth, Alwyn. Volcanoes: An Introduction. College Station: Texas A&M University Press, 1994.

Nancy M.Gordon/c. w.

See alsoAlaska ; Geology ; Hawaii ; Paleontology .

volcanogenic earthquakes On a global scale the association of earthquakes, particularly those with deep foci, and volcanicity is very close, as is indicated by the fact that both occur at destructive and constructive plate boundaries. In contrast, the earth tremors that are the direct consequence of volcanic activity are small, minor shocks that are spread over a relatively large area.

Before the commencement of a volcanic eruption quite local shocks are commonly registered which are caused by the opening of fissures in the underlying rocks or in the volcanic structure itself. These are due to the sluggish movement of very hot viscous magma several kilometres below the volcanic vent from one storage chamber to another. This motion takes place under great steam pressure through a network of tubes and pipes. In the process, various parts of the surrounding rock become hotter and more strained as the magma pushes through them. This results in fractures in the neighbouring rocks, the strain being relieved by elastic rebound, which is felt as an earthquake.

After a major eruption has ceased, more-or-less circular volcano-tectonic basins appear, termed calderas. These form by collapse of the surface rocks owing to partial evacuation of the underlying magma chamber. This caldera collapse induces very complex ground movements that are accompanied by earthquakes.

Harold G. Reading

volcanic cone A conical mound of volcanic ejecta accumulated around an eruptive vent. Cones have outer slope angles of about 30° and are topped by a depression or crater over the site of the vent. The type of material which accumulates to form the cone can be used to name the type of cone. For example, alternate layers of lava with beds of ash and other pyroclastic material characterize a strato-volcano (composite volcano), and spatter ejected from a vent during a Hawaiian-type eruption would accumulate to form a spatter cone around the vent. Scoria ejected from a vent during a Strombolian-type eruption would accumulate to form a scoria cone around the vent. See VOLCANO.

volcano A naturally occurring vent or fissure at the Earth's surface through which erupt molten, solid, and gaseous materials. The viscosity, gas content, and rate of extrusion of the magma probably determine the shape of the mountain built by the eruptions. The magma may reach the surface either through a single channel (see CENTRAL VENT VOLCANO), or through a series of vertical fractures (see FISSURE VOLCANO). Types of eruptions are named after volcanoes associated with them. See HAWAIIAN ERUPTION; PELÉEAN ERUPTION; PLINIAN ERUPTION; STROMBOLIAN ERUPTION; SURTSEYAN ERUPTION; VESUVIAN ERUPTION; and VULCANIAN ERUPTION.

vol·ca·no / välˈkānō; vôl-/ • n. (pl. -noes or -nos) a mountain or hill, typically conical, having a crater or vent through which lava, rock fragments, hot vapor, and gas are or have been erupted from the earth's crust. ∎ fig. an intense suppressed emotion or situation liable to burst out suddenly: what volcano of emotion must have been boiling inside that youngster.

volcano a mountain or hill, typically conical, having a crater or vent through which lava, rock fragments, hot vapour, and gas are or have been erupted from the earth's crust; in figurative usage, an intense suppressed emotion; a situation liable to burst out suddenly. Recorded from the early 17th century, the word comes from Italian, from Latin Volcanus Vulcan.

411. Volcanoes

See also 179. GEOLOGY ; 283. MOUNTAINS .

volcanism

the phenomena connected with volcanoes and volcanic activity. Also vulcanism. —volcanist, n.

volcanology

Geology. the scientific study of volcanoes and volcanic phenomena. Also vulcanology. —volcanologist, n. —volcanologic, volcanological, adj.

volcano Vent from which molten rock or lava, solid rock debris, and gases issue. Volcanoes may be of the central vent type, where the material erupts from a single pipe, or of the fissure type, where material is extruded along an extensive fracture. Volcanoes are usually classed as active, dormant or extinct. See also volcanism

volcanic plug (volcanic neck) The cylindrical filling of an ancient volcano which, due to its greater resistance, may be preserved after the volcanic edifice has been eroded away. See also PUY.

volcano XVII. — It. — L. Volcānus, Vulcānus Rom. god of fire.
So volcanic XVIII. — F. volcanique.

earthquakes, volcanogenic see volcanogenic earthquakes

volcano •Mano, piano •Arno, boliviano, Bolzano, Carnot, chicano, guano, Kano, llano, Locarno, Lugano, Marciano, Marrano, meccano, oregano, Pisano, poblano, Romano, siciliano, soprano, Sukarno •Renault, steno, tenno •techno • Fresno • Pernod •ripieno, volcano •albino, bambino, beano, Borodino, Borsalino, cappuccino, casino, chino, Comino, concertino, Filipino, fino, Gino, keno, Ladino, Latino, Leno, maraschino, merino, Monte Cassino, Navarino, neutrino, Pacino, palomino, pecorino, Reno, San Marino, Sansovino, Torino, Trevino, Valentino, vino, Zeno •minnow, winnow •Llandudno • Gobineau • domino •Martineau •lino, rhino, wino •tonneau • Grodno •Livorno, porno •Mezzogiorno •cui bono?, kimono, Mono, no-no, phono •Bruno, Gounod, Juneau, Juno, Uno •Huguenot • pompano •Brno, inferno, journo, Salerno, Sterno