The Earth’s crust is divided into large, rigid tectonic plates that constantly move across the planet’s surface. While the vast majority of the United States rests firmly on the North American Plate, certain areas along the western margin and in the far north are exceptions. These regions are either situated on a different plate entirely or are defined by complex boundaries where the North American Plate interacts with other major and minor plates.
The Main Boundary: California’s Pacific Plate Territory
The most recognized area of the contiguous United States not on the North American Plate is a sliver of California west of the San Andreas Fault system. This narrow strip of land is attached to the Pacific Plate, the largest of the Earth’s tectonic plates. Lands west of the fault, including portions of major metropolitan areas like San Diego and Los Angeles, are slowly moving northwestward relative to the rest of the continent.
This boundary is characterized as a transform plate boundary, meaning the plates are not colliding or pulling apart, but rather sliding horizontally past one another. The Pacific Plate is grinding past the North American Plate at an average rate of approximately 46 millimeters, or about two inches, per year. This sideways motion, known as right-lateral strike-slip faulting, is the fundamental cause of California’s frequent earthquakes.
The San Andreas Fault does not run directly along the coastline, which is why continental crust is included on the Pacific Plate. This inclusion is a geological consequence of a former subduction zone that existed before plate motions changed around 30 million years ago. Material scraped off the subducting plate was accreted to the western edge of the North American Plate, only to be later captured by the Pacific Plate’s movement.
Alaska’s Complex Tectonic Landscape
Alaska represents the most tectonically complex and seismically active region of the country, with its southern margin defined by a major convergent plate boundary. Here, the dense oceanic crust of the Pacific Plate is actively plunging beneath the lighter continental crust of the North American Plate, a process known as subduction. This boundary, which extends for 2,500 miles, forms the dramatic feature known as the Aleutian Trench.
The Pacific Plate subducts beneath Alaska at varying rates, creating immense stress along the plate interface. This descent is responsible for generating the largest earthquakes in the United States, including the magnitude 9.2 Great Alaska Earthquake of 1964. Subduction also drives the formation of the Aleutian Volcanic Arc, a chain of active volcanoes that parallels the trench.
In southeast Alaska, the tectonic setting transitions from subduction to a transform boundary, similar to California’s, along the Queen Charlotte Fault. The Pacific Plate here is primarily sliding past the North American Plate, causing shallow, strike-slip earthquakes. Alaska’s unique position at the corner of the two major plates means it experiences all three types of plate boundaries—convergent, transform, and divergent—making its geology exceptionally dynamic.
The Hidden Juan de Fuca Plate
Further south, off the coasts of Washington and Oregon, a smaller tectonic entity known as the Juan de Fuca Plate defines the region’s geological risk. This microplate is a remnant of the much larger ancient Farallon Plate, which has mostly been subducted beneath North America over millions of years. The Juan de Fuca Plate is bordered on its west by the Pacific Plate and on its east by the North American Plate.
This oceanic plate is actively subducting beneath the North American Plate at the Cascadia Subduction Zone. While the land of the Pacific Northwest states sits atop the North American Plate, the offshore Juan de Fuca Plate creates a massive seismic hazard. The subduction zone extends from northern California up to Vancouver Island in British Columbia.
The Juan de Fuca Plate is fractured into several smaller pieces, including the Gorda Plate to the south. The constant pressure exerted by these subducting remnants is the reason for the volcanic Cascade Range that runs through Washington, Oregon, and northern California. As the plate descends, it releases water that causes the overlying mantle to melt, fueling the chain of volcanoes like Mount St. Helens.
Why Plate Boundaries Matter
The locations where the United States is not on the North American Plate represent the country’s most geologically active zones. The type of plate interaction directly dictates the natural hazards faced by the population. The transform boundary along the San Andreas Fault primarily generates frequent, shallow, strike-slip earthquakes.
In contrast, the subduction zones in Alaska and the Cascadia region pose the threat of megathrust earthquakes, which are the most powerful seismic events globally, capable of reaching magnitudes over 9. The Cascadia Subduction Zone, in particular, is capable of producing a massive earthquake and tsunami when the locked plates eventually slip.
These boundaries also explain the distribution of volcanoes across the American West. The subduction of the Pacific Plate in Alaska feeds the Aleutian Arc, while the Juan de Fuca Plate’s descent powers the Cascade Volcanic Arc. Understanding the location and mechanism of these plate boundaries is fundamental to assessing seismic risk and preparing for the geological events that shape the American landscape.