Pahoehoe near the coast of Kilauea. Photo by Steve Mattox, 1989.

Photo of the "fire hose" flow, a small lava tube pouring into the ocean. Photograph by J.D. Griggs, U.S. Geological Survey, November 27, 1989.

Igneous rocks begin as magma. Intrusive igneous rocks, like granite, form when magma cools inside the Earth. Extrusive igneous rocks, like the basalt lava flow in this photo, form at the Earth's surface. Volcanic rocks are extrusive igneous rocks. Photograph by Steve Mattox, July 1990.

Measuring the temperature of lava is one method used to monitor volcanic eruptions. Photograph by R.L. Christiansen, U.S. Geological Survey, January 9, 1973.

The lava at Mount St. Helens is not like the type of lava that erupts from Hawaii's Kilauea Volcano. Kilauea's lava easily flows across the ground while Mount St. Helen's lava oozes onto the dome, like taffy candy being squeezed from a giant tube. Photo courtesy of the U.S. Geological Survey.

The Kilauea Iki eruption began on the morning of November 14, 1959, three months after a rapid increase in inflation of the ground surface and the number and size of earthquakes. Photo courtesy of the U.S. Geological Survey.

During the second phase of the Kilauea eruption, lava spilled from the lava lake in the deeper east crater and began to flow into the west crater. Photo courtesy of the U.S. Geological Survey.

After many eruption episodes the unstable walls of the Kilauea growing cinder cone collapsed into the lava lake. Photo courtesy of the U.S. Geological Survey.

The Kilauea eruption continued for 4 more weeks and included 16 additional eruptive phases. Each eruptive phase produced a lava fountain. The views of the fountain and lava lake at night were spectacular. Photograph by Jerry Eaton, U.S. Geological Survey, 6:30 pm, December 5, 1959.

The April 1982 eruption at the summit of the Kilauea lasted only 19 hours. It was preceded by rapid inflation and a swarm of small earthquakes that lasted 3 hours. Photography by J. P. Lockwood, U.S. Geological Survey, April 30, 1982.

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1.     Mount Mazama/Crater Lake, Oregon-Over 6,000 years ago Mount Mazama (posthumously named) erupted. Before the explosion the mountain was 12,000 feet high; when it was over it had been replaced by a 1,900-foot deep crater. Crater Lake, famed for its intense blue waters, was made a National Park in 1902. Volcanic activity occurred some time after the Mount Mazama explosion, creating Wizard Island in the middle of the lake. See Crater Lake National Park Homepage for more information. Picture: David Muench/Corbis |

2. Mount Etna, Sicily-Although Mount Etna (or Aetna) is the highest active volcano in Europe, its renown comes from its role in Greek legends and in ancient works by writers such as Hesiod, Pindar and Aeschylus. According to Greco-Roman mythology, the giants -- the enemies of the gods -- were buried beneath Mount Etna. In their efforts to break free, the Giants caused frequent earthquakes around the mountain. The most recent eruption, in the Bove Valley section of Etna Volcano Park, occurred in December 1991. © 1998 Parco dell'Etna

3. Mount Vesuvius, Italy-Vesuvius' eruption in 79 A.D. covered the cities of Pompeii and Herculaneum, preserving them for generations to come. But this eruption also holds a place in history because of its documentation. Pliny the Younger left a detailed description of the event in two letters to Tacitus. A type of eruption -- the Plinian type -- is named for Pliny the Elder who died in the catastrophe. The volcano is still active and has had several eruptions -- the most deadly being in 1631. Check out this Mount Vesuvius page. Picture: UPI/Corbis-Bettmann |

4. Mount Tambora, Indonesia-The largest eruption during the last two  centuries, as well as the deadliest volcano in recorded history, Mount Tambora exploded April 10-11, 1815. It killed an estimated 92,000 people. Almost 80,000 of the victims died of starvation brought on by the agricultural devastation in the volcano's wake. The eruption and the resulting massive clouds of dust and ash affected most of the Northern Hemisphere, causing unusually cool temperatures and failed crops in 1816 -- sometimes referred to as "the year without a summer." See Mount Tambora. Picture: NASA |

5. Mount Krakatau, Indonesia-On August 27, 1883, Mount Krakatau  exploded with such force that it was heard in Australia, over 2,000 miles away. The force of the eruption triggered a series of tsunamis that reached the Hawaiian islands and the coast of South America, killing more than 36,000 people. The five cubic miles of ejecta covered the surrounding areas in darkness for over two days and caused a series of dramatic sunsets around the world throughout the following year. The explosion and subsequent collapse of the volcano left only a remnant of the island above sea level. By 1928, another small island had emerged from a rising volcanic cone. Visit Krakatau, Indonesia.  Picture: Corbis-Bettmann |

6. Mount Pelee, Martinique-The eruption on May 8, 1902, killed 29,000,  destroying the port town of Saint-Pierre four miles away. Almost all the deaths were caused by the resulting pyroclastic flow -- a deadly, fast-moving cloud of hot gas and dense liquidized volcanic particles.Only two residents of the town survived the flow. Volcanology (also called Volcanism) was at best a primitive science in 1902, and the existence of pyroclastic flows was unknown. After this disaster a "new" type of eruption was named after Mount Pelee - the Pelean-type eruption. See Mount Pelee eruption. Photograph of the remains of St. Pierre by Heilprin, 1902.

7. Parícutin, Mexico-In February 1943, a pile of ash began to rise from a  corn field near the town of Parícutin, Mexico. A mountain began to emerge from the earth, reaching a height of 1,200 feet in one year. Although the ensuing nine-year eruption resulted in the destruction of the town of Parícutin, it presented the modern world with a remarkable opportunity to see the birth of a volcano. Only three people died, all by lightning associated with the eruption. Visit Parícutin. Picture: Daniel Laine/Corbis |

8. Mount St. Helens, Washington-One of the more highly publicized and studied volcanic explosions, Mount St. Helens erupted on May 18, 1980. The volcano, which had been dormant since 1857, began erupting steam after a series of earthquakes in March 1980. The 1978 prediction of the U.S. Geological Survey that violent and intermittent volcanic activity would begin, "within the next 100 years, and perhaps even before the end of this century," had come true. Luckily, close study of St. Helens prevented a major loss of life. Even so, 60 deaths resulted from the May 18 eruption. Learn more about Mount St. Helens. Picture: Cascades Volcano Observatory/USGS/Photodisc |

10. Mount Pinatubo, Philippines-Killing almost 800 and leaving an estimated 100,000 homeless, Mount Pinatubo's eruption in June 1991 was 10 times larger than the Mount St. Helens' eruption and one of the biggest of the 20th century. It emitted a cloud of smoke and ash over 19 miles high. The evacuation of more than 70,000 people and the volcanic event were broadcast worldwide, making Pinatubo (in)famous throughout the world. Learn more about Mount Pinatubo. Picture: US Geological Survey

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Geologists generally group volcanoes into four main kinds—cinder cones, composite volcanoes, shield volcanoes, and lava domes.

Cinder cones

Cinder cones are the simplest type of volcano. They are built from particles and blobs of congealed lava ejected from a single vent. As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall as cinders around the vent to form a circular or oval cone. Most cinder cones have a bowl-shaped crater at the summit and rarely rise more than a thousand feet or so above their surroundings. Cinder cones are numerous in western North America as well as throughout other volcanic terrains of the world.

In 1943 a cinder cone started growing on a farm near the village of Parícutin in Mexico. Explosive eruptions caused by gas rapidly expanding and escaping from molten lava formed cinders that fell back around the vent, building up the cone to a height of 1,200 feet. The last explosive eruption left a funnel-shaped crater at the top of the cone. After the excess gases had largely dissipated, the molten rock quietly poured out on the surrounding surface of the cone and moved downslope as lava flows. This order of events—eruption, formation of cone and crater, lava flow—is a common sequence in the formation of cinder cones.

During 9 years of activity, Parícutin built a prominent cone, covered about 100 square miles with ashes, and destroyed the town of San Juan. Geologists from many parts of the world studied Parícutin during its lifetime and learned a great deal about volcanism, its products, and the modification of a volcanic landform by erosion.

Composite volcanoes

Some of the Earth's grandest mountains are composite volcanoes—sometimes called stratovolcanoes. They are typically steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs and may rise as much as 8,000 feet above their bases. Some of the most conspicuous and beautiful mountains in the world are composite volcanoes, including Mount Fuji in Japan, Mount Cotopaxi in Ecuador, Mount Shasta in California, Mount Hood in Oregon, and Mount St. Helens and Mount Rainier in Washington.

Most composite volcanoes have a crater at the summit which contains a central vent or a clustered group of vents. Lavas either flow through breaks in the crater wall or issue from fissures on the flanks of the cone. Lava, solidified within the fissures, forms dikes that act as ribs which greatly strengthen the cone.

The essential feature of a composite volcano is a conduit system through which magma from a reservoir deep in the Earth's crust rises to the surface. The volcano is built up by the accumulation of material erupted through the conduit and increases in size as lava, cinders, ash, etc., are added to its slopes.

When a composite volcano becomes dormant, erosion begins to destroy the cone. As the cone is stripped away, the hardened magma filling the conduit (the volcanic plug) and fissures (the dikes) becomes exposed, and it too is slowly reduced by erosion. Finally, all that remains is the plug and dike complex projecting above the land surface—a telltale remnant of the vanished volcano.

An interesting variation of a composite volcano can be seen at Crater Lake in Oregon. From what geologists can interpret of its past, a high volcano—called Mount Mazama- probably similar in appearance to present-day Mount Rainier was once located at this spot. Following a series of tremendous explosions about 6,800 years ago, the volcano lost its top. Enormous volumes of volcanic ash and dust were expelled and swept down the slopes as ash flows and avalanches. These large-volume explosions rapidly drained the lava beneath the mountain and weakened the upper part. The top then collapsed to form a large depression, which later filled with water and is now completely occupied by beautiful Crater Lake. A last gasp of eruptions produced a small cinder cone, which rises above the water surface as Wizard Island near the rim of the lake. Depressions such as Crater Lake, formed by collapse of volcanoes, are known as calderas. They are usually large, steep-walled, basin-shaped depressions formed by the collapse of a large area over, and around, a volcanic vent or vents. Calderas range in form and size from roughly circular depressions 1 to 15 miles in diameter to huge elongated depressions as much as 60 miles long.

Shield volcanoes

Shield volcanoes, the third type of volcano, are built almost entirely of fluid lava flows. Flow after flow pours out in all directions from a central summit vent, or group of vents, building a broad, gently sloping cone of flat, domical shape, with a profile much like that of a warrior's shield. They are built up slowly by the accretion of thousands of highly fluid lava flows called basalt lava that spread widely over great distances, and then cool as thin, gently dipping sheets. Lavas also commonly erupt from vents along fractures (rift zones) that develop on the flanks of the cone. Some of the largest volcanoes in the world are shield volcanoes. In northern California and Oregon, many shield volcanoes have diameters of 3 or 4 miles and heights of 1,500 to 2,000 feet. The Hawaiian Islands are composed of linear chains of these volcanoes including Kilauea and Mauna Loa on the island of Hawaii—two of the world's most active volcanoes. The floor of the ocean is more than 15,000 feet deep at the bases of the islands. As Mauna Loa, the largest of the shield volcanoes (and also the world's largest active volcano), projects 13,677 feet above sea level, its top is over 28,000 feet above the deep ocean floor.

In some eruptions, basaltic lava pours out quietly from long fissures instead of central vents and floods the surrounding countryside with lava flow upon lava flow, forming broad plateaus. Lava plateaus of this type can be seen in Iceland, southeastern Washington, eastern Oregon, and southern Idaho. Along the Snake River in Idaho, and the Columbia River in Washington and Oregon, these lava flows are beautifully exposed and measure more than a mile in total thickness.

Lava domes

Volcanic or lava domes are formed by relatively small, bulbous masses of lava too viscous to flow any great distance; consequently, on extrusion, the lava piles over and around its vent. A dome grows largely by expansion from within. As it grows its outer surface cools and hardens, then shatters, spilling loose fragments down its sides. Some domes form craggy knobs or spines over the volcanic vent, whereas others form short, steep-sided lava flows known as “coulees.”

Volcanic domes commonly occur within the craters or on the flanks of large composite volcanoes. The nearly circular Novarupta Dome that formed during the 1912 eruption of Katmai Volcano, Alaska, measures 800 feet across and 200 feet high. The internal structure of this dome—defined by layering of lava fanning upward and outward from the center—indicates that it grew largely by expansion from within. Mont Pelée in Martinique, Lesser Antilles, and Lassen Peak and Mono domes in California are examples of lava domes. An extremely destructive eruption accompanied the growth of a dome at Mont Pelée in 1902. The coastal town of St. Pierre, about 4 miles downslope to the south, was demolished and nearly 30,000 inhabitants were killed by an incandescent, high-velocity ash flow and associated hot gases and volcanic dust.

Only two men survived; one because he was in a poorly ventilated, dungeon-like jail cell and the other who somehow made his way safely through the burning city.

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