"Learn About Volcanoes"
What Types of Volcanoes Are There? |
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| Calderas | The largest and most explosive volcanic eruptions eject tens to hundreds of cubic kilometers of magma onto the Earth's surface. When such a large volume of magma is removed from beneath a volcano, the ground subsides or collapses into the emptied space, to form a huge depression called a caldera. Some calderas are more than 25 kilometers in diameter and several kilometers deep. -- Excerpt from: Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication | ||
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Cinder Cones and
Scoria 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. -- Excerpt from: Tilling, 1985, Volcanoes: USGS General Interest Publication | ||
| Composite Volcanoes and Stratovolcanoes |
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,
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.
-- Excerpt from: Tilling, 1985, Volcanoes: USGS General Interest Publication
Composite volcanoes tend to erupt explosively and pose considerable danger to nearby life and property. -- Excerpt from: Tilling, Topinka, and Swanson, 1990, Eruptions of Mount St. Helens: Past, Present, and Future: USGS General Interest Publication |
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| "Continental" Volcanoes | In the typical "continental" environment, volcanoes are located in unstable, mountainous belts that have thick roots of granite or granitelike rock. Magmas, generated near the base of the mountain root, rise slowly or intermittently along fractures in the crust. During passage through the granite layer, magmas are commonly modified or changed in composition and erupt on the surface to form volcanoes constructed of nonbasaltic rocks. -- Excerpt from: Tilling, 1985, Volcanoes: USGS General Interest Publication | ||
| "Island-Arc" Volcanoes | In a typical "island-arc" environment, volcanoes lie along the crest of an arcuate, crustal ridge bounded on its convex side by a deep oceanic trench. The granite or granitelike layer of the continental crust extends beneath the ridge to the vicinity of the trench. Basaltic magmas, generated in the mantle beneath the ridge, rise along fractures through the granitic layer. These magmas commonly will be modified or changed in composition during passage through the granitic layer and erupt on the surface to form volcanoes built largely of nonbasaltic rocks. -- Excerpt from: Tilling, 1985, Volcanoes: USGS General Interest Publication | ||
| Lava Plateaus | In some shield-volcano 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. -- Excerpt from: Tilling, 1985, Volcanoes: USGS General Interest Publication | ||
| Lava Domes | Lava (usually dacite or rhyolite) that is too sticky to flow far from its vent forms steep-sided mounds called lava domes. -- Excerpt from: Wright and Pierson, 1992, USGS Circular 1973 | ||
| Maars and Tuff Cones | Also called "tuff cones", maars are shallow, flat-floored craters that scientists interpret have formed above diatremes as a result of a violent expansion of magmatic gas or steam; deep erosion of a maar presumably would expose a diatreme. Maars range in size from 200 to 6,500 feet across and from 30 to 650 feet deep, and most are commonly filled with water to form natural lakes. Most maars have low rims composed of a mixture of loose fragments of volcanic rocks and rocks torn from the walls of the diatreme. -- Excerpt from: Tilling, 1985, Volcanoes: USGS General Interest Publication | ||
| "Oceanic" Volcanoes | In a typical "oceanic" environment, volcanoes are alined along the crest of a broad ridge that marks an active fracture system in the oceanic crust. Basaltic magmas, generated in the upper mantle beneath the ridge, rise along fractures through the basaltic layer. Because the granitic crustal layer is absent, the magmas are not appreciably modified or changed in composition and they erupt on the surface to form basaltic volcanoes. -- Excerpt from: Tilling, 1985, Volcanoes: USGS General Interest Publication | ||
| Shield Volcanoes | Shield volcanoes 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 a warrior's shield. They are built up slowly by the accretion of thousands of flows of highly fluid basaltic (from basalt, a hard, dense dark volcanic rock) 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. -- Excerpt from: Tilling, 1985, Volcanoes: USGS General Interest Publication | ||
| Submarine Volcanoes, Ridges, and Vents | Submarine volcanoes and volcanic vents are common features on certain zones of the ocean floor. Some are active at the present time and, in shallow water, disclose their presence by blasting steam and rock-debris high above the surface of the sea. Many others lie at such great depths that the tremendous weight of the water above them results in high, confining pressure and prevents the formation and explosive release of steam and gases. Even very large, deepwater eruptions may not disturb the ocean surface. -- Excerpt from: Tilling, 1985, Volcanoes: USGS General Interest Publication | ||
| Tuyas | A volcano that erupted under a glacier. -- Excerpt from: British Columbia Ministry of Environment, Lands & Parks, Website, 2000 | ||
| Volcanic Fields; Lava Fields; Mafic Volcanoes; Monogenetic Volcanoes |
Mafic volcanoes typically erupt for brief time intervals (weeks to perhaps centuries), but some can grow almost as
large as composite volcanoes. Subsequent eruptions in the region typically issue from new vents and, over tens to
hundreds of thousands of years, build broad fields of many volcanoes. Prominent mafic volcanoes in the Three
Sisters region include North Sister, Mount Bachelor, Belknap Crater, Black Butte, and Mount Washington.
Hundreds more mafic volcanoes form the High Cascades of central Oregon between the neighboring composite
volcanoes of Mount Jefferson, 60 kilometers (40 miles) north of Three Sisters, Newberry volcano, a similar
distance southeast, and Crater Lake, 120 kilometers (75 miles) south.
-- Excerpt from: Scott, et.al., 2001, Volcano Hazards in the Three Sisters Region, Oregon: USGS Open-File Report 99-437
Composite volcanoes erupt episodically over tens to hundreds of thousand of years and can display a wide range of eruption styles. Monogenetic volcanoes typically erupt for only brief time intervals -- weeks to perhaps centuries -- and generally display a narrower range in eruptive behavior. Most monogenetic volcanoes are basaltic in composition but... a few are of andesite and dacite composition -- that is, with a relatively higher silica content. -- Excerpt from: Walder, et.al., 1999, Volcano Hazards in the Mount Jefferson Region, Oregon: USGS Open-File Report 99-24 Monogenetic volcanic fields are collections of cinder cones, and/or Maar vents and associated lava flows and pyroclastic deposits. Sometimes a stratovolcano is at the center of the field. -- Excerpt from: Wood and Kienle, 1990, Volcanoes of North America: United States and Canada: Cambridge University Press, 354p. |
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