Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 36

FIGURE 2.11 Schematic sections at various locations showing relationships among lithospheric
plates, continents, oceans, spreading centers, island arcs, deep-sea trenches (subduction zones),
and mountain building. (Adapted from various sources.)
Island-arc volcanoes grow larger over time as material is brought into the subduction
zone, consumed by melting, and reinjected upward as magmas. Island arcs contain
many of the world's most destructive volcanoes. These are primarily the large compos-
ite cones built from silica-rich andesite and rhyolite lavas, which are cooler and more
viscous than the low-silica basalt types, with the result that the silica-rich types choke
their vents and cause huge explosions of pyroclastics (blocks, bombs, cinders, and ash).
Island arcs, and the volcanoes they contain, are especially common in the Pacific Ocean
(Fig. 2.11), where they occur around the periphery in a zone known as the “ring of fire.”
A few island arcs also occur along the American coasts, including the Aleutians of the
Alaskan coast, the Lesser Antilles of the Caribbean, and the South Sandwich Islands in
the Scotia Arc at the tip of South America.
At different times in the geologic past, island arcs have occurred off the east and
west coasts of North America and the west coast of South America, but these have been
incorporated into the main continental blocks by plate-tectonic collisions. Ophiolite se-
quences commonly mark the suture of their collisions. On the west coast of North Amer-
ica, the Cascades and the Sierra Nevada mountains are made in part of ancient island
arcs, and in the east, the low Taconic and Berkshire Mountains along the borders of
Vermont, Massachusetts, and New York State are the remains of a very old island arc.
Numerous other island arc fragments are also known to be incorporated into the Ap-
palachian Mountains (Fig. 2.12).