Plate Tectonics

All tectonic plates move relative to other plates, and the Pacific plate is moving at a rate of roughly 50 millimeters/year (2 inches/year) relative to the North American plate. Most, but not all, of this motion occurs along the San Andreas Fault. The Sierra Nevada, whose granite plutons were created by the subduction of an ancient plate known as the Farallon plate, are located on the North American plate, but they are being pulled along by the Pacific plate. 288 The Juan de Fuca plate, which is all that remains of the Farallon, is moving towards North America at an average rate of 36 millimeters/year (1.4 inches/ year), creating the Cascade Range’s volcanoes. As the Pacific plate continues to move northwest, it is causing the Basin and Range to pull apart. As a result, Nevada is full of sinking valleys and tilted mountains, and is growing wider in the process (Figure 10.1). Although this diagram does not show Alaska, the Pacific plate is subducting underneath the North American plate, creating the volcanic Aleutian Island Arc.

Figure 10.1: The general tectonic setting of the West Coast.

Figure 10.1: The general tectonic setting of the West Coast.

Convergent Boundaries

Where two plates collide, the denser plate (usually oceanic) subducts under the lighter one (either a younger oceanic plate or a continental plate) and creates a subduction zone. Subduction zones produce the largest and deepest earthquakes in the world. The water in the subducting plate is carried deep into the mantle and causes the melting of the overlying mantle rock. The resulting magma is hot and buoyant, so it rises to the surface and creates volcanoes. In the Western US, the Aleutian Island Arc and the Cascade volcanoes provide surface evidence of convergent boundaries.

Divergent Boundaries

Divergent plate boundaries exist where tectonic plates are under tension— essentially being pulled apart. This tension leads to the upwelling of magma, which forces its way through the separating crust onto the surface. In oceanic settings, such as the Juan de Fuca Ridge, new oceanic crust is created, and shallow, minor earthquakes occur. In continental settings, such as the Basin and Range of Nevada, the result of the tension is faulting and shallow earthquakes. As the faults pull apart and thin the crust, the decrease in overlying pressure can result in the formation of basaltic magma, which, in turn, can lead to the formation of small volcanoes called cinder cones. The faulting also creates the down-dropped basins and tilted mountain ranges of the Basin and Range.

Transform Boundaries

Transform boundaries exist where adjacent tectonic plates are moving alongside each other, causing a transform fault to develop. Most transform fault boundaries can be found in the ocean, where they connect divergent boundaries. One exception is the San Andreas Fault, which is mainly located on land. It separates the small divergent boundaries in Mexico (between the mainland and the Baja Peninsula) from the Juan de Fuca Ridge, and runs through continental crust for the majority of its length. The San Andreas Fault produces earthquakes when the plates slip, but it can also create ridges parallel to the fault, as well as mountains and valleys when the fault changes direction.