Tectonics and Landforms

Plate Boundaries and Volcanoes

Looking at a map of lithospheric plate boundaries shows volcanic activity along the boundaries of some lithospheric plates. Convergent plate boundaries where subduction occurs commonly experience volcanic activity . The "Ring of Fire" that extends from the west coast of the United States toward the Aleutian Islands and over to Japan is a good example of volcano activity associated with the margins of plates. Mt. St. Helens (Washington), Mt. Shishaldin (Alaska), and Mt. Fujiyama (Japan) were born from the collision and associated melting of plates beneath the surface of the Earth. These "stratovolcanoes" are built by enormous eruptions of viscous lava and ash created from magma rich in silica.

Figure 15.22 Shishaldin Volcano, Alaska is a volcano found on the "Ring of Fire" (Source: Volcano Hazards Program, USGS Used with permission)

Volcanoes are also found along spreading plate boundaries.  Here, basaltic magma is extruded onto the floor of the ocean. In some places, the magma solidifies into great volcanic cones rising from the ocean floor. Iceland's Heimaey volcano is a good example of volcanic activity along the midocean ridge.

One of the most well-known locations for volcanic activity, the Hawaiian Islands, is not a product of plate margin processes. The Hawaiian Islands have formed over a "hot spot" or mantle plume where magma rises to the surface and flows out onto the ocean floor. The chain of islands were created as the Pacific Plate moves over the mantle plume creating a succession of volcanoes oriented in a northwest (oldest) to southeast direction (youngest).

Assess your basic understanding of the preceding material by "Looking Back at Plate Tectonics and Boundaries" or continue reading.

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