Volcanes
Chapter 2 Volcanoes
from Perilous Earth: Understanding Processes Behind Natural Disasters, ver. 1.0, June, 2009 by G.H. Girty, Department of Geological Sciences, San Diego State University
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Introduction
When plates are bounded on one side by a mid‐ocean ridge and on the other side by a convergent boundary, then the leading edge occurs on the convergent side while the trailing edge lies on the divergent side (Figure 1). Volcanic arcs are long linear chains of volcanoes that lie on the leading edge of a lithospheric plate. They develop on average about 100 km from a trench, but this distance will vary as a function of the dip on the subducting plate. Regardless of the dip, all island arcs tend to lie vertically about 100‐125 km above a subducting plate.
Figure 1. Digital Elevation Map (DEM) of South America showing the position of the leading and trailing edges. Note that the South America plate extends from the Mid‐Atlantic ridge to the trench along its western border. The Andes (brown strip along leading edge) represent a continental‐margin island arc. If a volcanic arc develops on the leading edge of continental lithosphere, then the volcanic arc is described as a continental‐margin island arc. In contrast, if the volcanic arc develops along the leading edge of oceanic lithosphere, then it forms an intra‐oceanic island arc. The Andes along the western edge of the South American plate represent an excellent example of a continental‐margin island arc (Figure 1), while the western Aleutians extending westward from the Alaskan Peninsula are an example of an intra‐oceanic volcanic arc (Figure 2).
Figure 2. Earth Explorer 5.0 DEM of Aleutian Islands and adjacent regions.
from Perilous Earth: Understanding Processes Behind Natural Disasters, ver. 1.0, June, 2009 by G.H. Girty, Department of Geological Sciences, San Diego State University
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In the continental United States, the chain of volcanoes extending from northern California, through Oregon and Washington occur on the leading edge of the North American plate and are the result of the subduction of the Juan de Fuca microplate. They therefore represent a continental‐margin island arc. Types of Volcanoes
There are three major types of volcanoes: strato or composite, shield, and cinder cones. Composite volcanoes are most commonly found within island arcs but also occur in continental rift settings. For example, they are common along the leading edge of the North American plate in northern California, Oregon, and Washington, and they rim the western margin of South America. In addition, they are ubiquitous features in the Aleutian Islands and the western Pacific island arc chain that extends from the Japanese Islands southward to New Zealand. This circum‐Pacific belt of mostly stratovolcanoes is commonly referred to as the "Ring of Fire".
Figure 3. A cut away view of a stratovolcano. A composite volcano (or stratovolcano), as its name implies, is made up of alternating layers of pyroclastic material and solidified lava (Figure 3). The layers of pyroclastic material tend to occur in greater abundance than do layers derived from lava flows. Compositionally, most of the material found on composite volcanoes, whether pyroclastic or derived from lava flows, is intermediate (andesitic) in composition though both mafic (basaltic) and silicic (rhyolitic) are additional volumetrically less significant components. Eruptions of composite volcanoes occur intermittently, but such systems tend to be long lived. As a result of their longevity, composite volcanoes can grow to heights in excess of 6,900 meters. Because much material is built up around the central vent areas slopes leading to ...
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