Earth’s surface is not a single, solid shell but is instead broken into several large pieces known as tectonic plates. These segments of the Earth’s lithosphere, which includes the crust and upper mantle, are constantly moving, albeit very slowly. The interactions at the boundaries of these plates are responsible for many of the planet’s geological features and events, such as earthquakes, volcanoes, and the formation of mountain ranges.
Understanding the North American Plate
The North American Plate encompasses most of the North American continent, Greenland, Cuba, the Bahamas, and significant portions of the Atlantic Ocean, making it the second-largest tectonic plate globally. The majority of the United States rests directly on this plate. The plate moves in a southwest direction at a rate of about 2.3 centimeters (approximately 1 inch) per year.
The US Region Off the Plate
While most of the United States is situated on the North American Plate, a section lies on a different tectonic plate. This region is primarily located in parts of California, particularly the land situated west of the San Andreas Fault. This narrow strip of western California, including major cities like Los Angeles and parts of San Francisco, is a segment of the Pacific Plate. Portions of the Pacific Northwest, specifically offshore areas of northern California, Oregon, and Washington, are influenced by the smaller Juan de Fuca Plate.
The Underlying Plate Dynamics
This part of the United States is not on the North American Plate due to complex interactions of several tectonic plates. The San Andreas Fault marks the primary boundary between the Pacific Plate and the North American Plate. This fault is a transform boundary, meaning the two plates are sliding horizontally past each other. The Pacific Plate moves northwestward at a rate of 7 to 11 centimeters (approximately 3 to 4 inches) per year relative to the North American Plate.
Further north, off the coast of the Pacific Northwest, the Juan de Fuca Plate interacts with the North American Plate through a different type of boundary. This small oceanic plate is actively subducting beneath the North American Plate. This process occurs along the Cascadia Subduction Zone, a convergent plate boundary. The Juan de Fuca Plate moves eastward to northeastward, subducting at an average rate of 2.6 to 4 centimeters (about 1 to 1.6 inches) per year.
Geological Consequences
The dynamic plate interactions in these regions lead to geological phenomena. Along the San Andreas Fault, the grinding motion between the Pacific and North American Plates accumulates stress, which is periodically released as earthquakes. California experiences frequent seismic activity, with thousands of annual earthquakes, most too small to be felt. The U.S. Geological Survey estimates a 72% chance of a magnitude 6.7 or greater earthquake in the San Francisco Bay Area within 30 years, and a 60% chance in the Los Angeles area.
In the Pacific Northwest, the subduction of the Juan de Fuca Plate beneath the North American Plate fuels the volcanic activity of the Cascade Volcanic Arc. As the oceanic plate descends, it partially melts, and the resulting magma rises to form volcanoes such as Mount St. Helens, Mount Rainier, Mount Hood, Mount Shasta, and Lassen Peak. This subduction zone is also capable of producing megathrust earthquakes, with the last major event, the 1700 Cascadia earthquake, estimated to be between magnitude 8.7 and 9.2. The forces at these plate boundaries also contribute to the formation of distinct landforms, including coastal mountain ranges and valleys.