Describe three ways volcanoes form?

Question

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Answer 1

A volcano is a mound, hill or mountain constructed by solid fragments, lava flows, and or dome-like extrusions deposited around a vent from which the material is extruded. The vent is a conduit that extends from the earth's upper mantle or lithosphere to the surface. Most of the material is deposited close to the vent, but some is carried high into the atmosphere to be spread by winds hundreds or thousands of kilometers from the source.


Types of Volcanoes
The form, or shape, of a volcano is governed by the composition of erupting magma and type of erupted products (volcaniclastic products of various kinds such as pyroclastic and autoclastic fragments; or effusive lava). Their shapes are determined in large part by the explosivity of eruptions, and volume of water that interacts with magma.

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Shield Volcanoes

View northward toward Mauna Loa volcano from Pohue Bay, south coast of Hawaii. The broad curving horizon line is the summit of Mauna Loa that stands over 14,000 feet above sea level and 30,000 feet above the sea floor. It is the highest mountain on earth.
Shield volcanoes are large volcanoes with broad summit areas and low-sloping sides (shield shape) because the extruded products are mainly low viscosity basaltic lava flows. A good example of a shield volcano is the Island of Hawaii (the "Big Island"). The Big Island is formed of five coalesced volcanoes of successively younger ages, the older ones apparently extinct. Mauna Loa, one of the main volcanoes, has a higher elevation than any mountain on earth -- 9090 meters (30,000 feet) from the floor of the ocean to its highest peak.

Shield volcanoes have summit calderas formed by piston-like subsidence. Subsidence occurs when large volumes of lava are emptied from underground conduits; withdrawal of support leads to collapse. Many smaller pit craters also occur along fissure zones on the flanks of the volcanoes. These form by collapse due to withdrawal of magma along conduits.



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Cinder cones (scoria cones)

Cinder cone at Little Lake, California.
Cinder cones are mounds of basaltic fragments. Streaming gases carry liquid lava blobs into the atmosphere that rain back to earth around the vent to form a cone. The lava blobs commonly solidify, or partially solidify, during flight through the air before landing on the ground. They are called "bombs." If gas pressure drops, the final stage cinder cone construction may be a lava flow that breaks through the base of the cone. If abundant water in the environment has access to the molten magma, their interaction may result in a maar volcano rather than a cinder cone.

The longer the eruption the higher the cone. Some are no higher than a few meters and others rise to as high as 610 meters or more, such as Paricutin volcano, Mexico, that was in nearly continuous eruption from 1943 to 1952. Along with pyroclastic activity were lava flows that flowed from its base to destroy the village of Paricutin. Cinder cones can occur alone or in small to large in groups,, or fields.



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Composite Volcanoes ("Stratovolcanoes")

St. Augustine volcano, Alaska. Composite cone. Photograph by Harry Glicken.
Composite volcanoes are built by multiple eruptions, sometimes recurring over hundreds of thousands of years, sometimes over a few hundred. Andesite magma, the most common but not the only magma type, tends to form composite cones. Although andesitic composite cones are built mostly of fragmental debris, some of the magma intrudes fractures within the cones to form dike or sills. In this way, multiple intrusive events build a structural framework of dikes and sills that knits together the voluminous accumulation of volcanic rubble. Such a structure can stand higher than cones composed only of fragmental material. Composite cones can grow to such heights that their slopes become unstable and susceptible to collapse from the pull of gravity.

Famous examples of composite cones are Mayon Volcano, Philippines, Mount Fuji in Japan, and Mount Rainier, Washington, U.S.A. Some composite volcanoes attain two to three thousand meters in height above their bases. Most composite volcanoes occur in chains and are separated by several tens of kilometers. There are numerous composite volcano chains on earth, notably around the Pacific rim, known as the "Rim of Fire".



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Domes

Mount Unzen and Shimabara City, Kyushu, Japan. Photograph shows Unzen dome and pyroclastic flow and pyroclastic surge pathways down the mountain to the sea. Photograph 20 January 1992 by Asia Air Survey Co., Ltd.
Lava domes form by the slow extrusion of highly viscous silica-rich magma. Most domes are rather small, but can exceed 25 cubic km in volume. Domal extrusions may end up as rather slow-moving lavas but many begin explosively, forming reamed-out explosion pits blanketed by pyroclastic debris. The explosive activity wanes as the gas content decreases. With lowered gas pressures, the magma extrudes slowly as viscous lava that forms thick stubby flows, or domes that are spinal or dome-shaped. As a dome enlarges, its margins slowly creep outward as a lava flow with steep cliff-like margins and a rubbly surface. If protrusion occurs on a steep slope, dome margins can collapse in a dangerous mass of hot rubble that can form pyroclastic flows. Domes can be solitary volcanoes, form in clusters, grow in craters or along the flanks of composite cones. A dome has been growing slowly within the crater of Mount St. Helens since the eruption of 1980. Domes have also filled the crater of Mt. Pelée, Martinique, and many other volcanoes.

The eruption of Mount Unzen, Japan taught volcanologists a valuable lesson -- that the collapse of an active dome can cause pyroclastic flows to develop. From 1991 to 1995, the continued growth and partial collapse of the Mount Unzen dome initiated hundreds of small but highly destructive pyroclastic flows and surges.



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Calderas

Photograph encompassing part of Crater Lake caldera, Oregon, U.S.A. Diameter about 8 kilometers. View is toward the east and includes a late-stage volcano in crater named Wizard Island.

Types and Origins of Calderas
Calderas are circular to oblong depressions formed by collapse along arcuate fractures associated with extrusion of pyroclastic materials. Their diameters are many times larger than those of associated vents. They may attain diameters up to 60 km across. The largest estimated volume of erupted products is over 3500 cubic kilometers, and deposits are known to have covered 25,000 square km. The frequency of such voluminous eruptions is very low. Those with volumes of 500 cubic km have a frequency of about 100,000 years. Such eruptions have occurred at Long Valley, California (Mammoth area) with a caldera of 20 km diameter; several have occurred in the Colorado Rocky Mountains (San Juan Mountains), southern New Mexico, Los Alamos area, New Mexico (Valles Caldera in the Jemez Mountains), and many other places. Most calderas in western North America have developed on thick continental crust. Intraoceanic calderas are commonly smaller in size and eruptive volume and less silicic. Another source of information about calderas is the U.S. Geological Survey.
The area of caldera collapse is about proportional to the volume of erupted material. Depths of subsidence as indicated by thickness of caldera fill is 1 to 3 km or more. Structural boundaries of calderas are commonly single or composite ring fault zones along which initial collapse took place. In deeply eroded calderas these structural boundaries may be expressed by a ring dike emplaced along arcuate faults during or after collapse.