| Some volcanoes crown island areas lying near the continents, and others form chains of islands in the deep ocean basins. Volcanoes tend to cluster along narrow mountainous belts where folding and fracturing of the rocks provide channelways to the surface for the escape of the magma. Significantly, major earthquakes also occur along these belts, indicating that volcanism and seismic activity are often closely related, responding to the same dynamic Earth forces. -- 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 |
Aleutian Volcanic Arc:
Most Alaskan volcanoes are in the Aleutian arc which extends approximately 2,500 kilometers along the southern edge of the Bering Sea and Alaskan mainland. This classic volcanic arc contains some 80 Quaternary stratovolcanoes and calderas. Aleutian arc volcanism is the result of subduction of the Pacific Plate beneath the North American Plate. The 3,400-kilometer-long Aleutian trench that extends from the northern end of the Kamchatka trench to the Gulf of Alaska marks the boundary between the two plates. -- Excerpt from: Wood and Kienle, 1990, Volcanoes of North America: Cambridge University Press, contribution by J. Kienle and C.J. Nye
Augustine volcano, a postglacial island volcano in lower Cook Inlet, is part of the eastern Aleutian arc. -- Excerpt from: Wood and Kienle, 1990, Volcanoes of North America: Cambridge University Press, contribution by Jürgen Kienle |
| 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 |
Hawaiian Islands:
The Hawaiian islands are the tops of gigantic volcanic mountains formed by countless eruptions of fluid lava over several million years; some tower more than 30,000 feet above the sea floor. These volcanic peaks rising above the ocean surface represent only the tiny, visible part of an immense submarine ridge, the Hawaiian Ridge-Emperor Seamount Chain, composed of more than 80 large volcanoes. This range stretches across the Pacific sea floor from the Hawaiian Islands to the Aleutian Trench. The length of the Hawaiian Ridge segment alone, between the Big Island and Midway Island to the northwest, is about 1,600 miles, roughly the distance from Washington D.C., to Denver, Colorado. The amount of lava erupted to form this huge ridge, about 186,000 cubic miles, is more than enough to cover the State of California with a mile-thick layer. -- Excerpt from: Tilling, Heliker, and Wright, 1987, Eruptions of Hawaiian Volcanoes: Past, Present, and Future: Department of the Interior/U.S. Geological Survey Publication
Mauna Loa is considered the world's largest active volcano, with an estimated volume of around 40,000 cubic kilometers, based on the assumption that the volcano extends from the surface to the top of the isostatically depressed oceanic crust. -- Excerpt from: Wood and Kienle, 1990, Volcanoes of North America: United States and Canada: Cambridge University Press, 354p., p.342-344, Contribution by Jack Lockwood |
| 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 |
Cascade Range:
In the Pacific Northwest, the Juan de Fuca Plate plunges beneath the North American Plate. As the denser plate of oceanic crust is forced deep into the Earth's interior beneath the continental plate, a process known as subduction, it encounters high temperatures and pressures that partially melt solid rock. Some of this newly formed magma rises toward the Earth's surface to erupt, forming a chain of volcanoes (the Cascade Range) above the subduction zone. -- Excerpt from: Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication
Mount Rainier at 4,393 meters (14,410 feet) the highest peak in the Cascade Range is a dormant volcano whose load of glacier ice exceeds that of any other mountain in the conterminous United States. -- Excerpt from: Hoblitt, et.al., 1995, Volcano Hazards from Mount Rainier, Washington USGS Open-File Report 95-273 |
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