The San Andreas Fault, pictured here near Sleepy Valley, California, was the source of the great San Francisco earthquake in 1906.
The devastating earthquakes and tsunami experienced in Japan in 2011 served as a stark reminder of just how violent nature can be. For residents of California, it was an unwelcome reminder of the dangers they live with on a daily basis. The two locations, though an ocean apart, share the same fundamental reasons for their geological threats, namely the convergence of tectonic plates. Although the movement of these colossal plates is measured in terms of millions of years, when they do shift, they unleash untold devastation.
Plate Tectonics
Born in the 1960s, the theory of plate tectonics and continental drift holds that the Earth's crust is comprised of about a dozen rigid plates, like a cracked eggshell. These plates move, or drift, on top of the Earth's fluid mantle. These plates interact with each other, creating divergent, convergent and transformative plate boundary zones. New crust is created in divergent zones, generally found in the oceans, while old crust is recycled in convergent zones.
Convergent Zones
Convergent zones are created wherever two plates are colliding against each other. If one of the plates is forced under the other, it is called a subduction zone. For instance, when a heavier oceanic plate collides with a lighter, more buoyant continental plate, it is forced underneath. As this downward path reaches a depth of 100 kilometers, trapped water and gases are released. As these rise to the surface, they create a chain of chemical reactions that melt the mantle. This creates magma chambers, leading to the rise of volcanoes.
California Boundary Zone
California is situated on top of one of these collision zones, resulting in a convergent boundary. Furthermore, it is an example of a subduction zone, as one plate is being forced under the other. Here, the Juan de Fuca plate is colliding with the North American plate. The Juan de Fuca plate, a heavier oceanic plate, is forcing its way under the lighter, more buoyant North American plate. This tectonic boundary is part of the Pacific Ring of Fire, stretching from the Kermadec trench of New Zealand to the Peru-Chili trench in South America, including Japan and the western coast of the United States.
Effects of the California Subduction Zone
As is to be expected with a subduction zone, one effect is the volcanic activity that is prevalent along the Northwest coast of the United States. On May 18, 1980, the violent eruption of Mount St. Helens provided vivid evidence of this volcanic product of subduction zone activity. Furthermore, the San Andreas Fault is another product of this ongoing collision. This infamous fault line runs along the collision zone and extends southward through California. Subduction zones are known for producing extreme earthquakes. As the pressure builds between the two plates, massive amounts of energy are stored in the plates. When friction is no longer able to hold them in place, the plates shift, releasing their potential energy in the form of earthquakes. This is what occurred along the San Andreas Fault during the great San Francisco earthquake on May 18, 1906.
Tags: subduction zone, Andreas Fault, American plate, California Subduction, coast United, coast United States
