The North American Plate is a massive, rigid segment of the Earth’s lithosphere, which includes the crust and uppermost mantle. This plate serves as the foundation for nearly all of North America, Greenland, and a significant portion of the western Atlantic Ocean floor. It ranks as the second-largest tectonic plate globally. Its slow, continuous movement drives the geological processes that define the continent’s features, such as the formation of mountain ranges and the occurrence of earthquakes.
The Rate and Direction of Movement
The North American Plate is currently moving in a west-southwesterly direction. This motion is a slow, steady drift away from its eastern boundary in the Atlantic Ocean. The rate of movement averages between 1 and 2.5 centimeters per year. The plate also moves in a rotational, counter-clockwise manner, causing the speed to vary from about 1 cm/year in the south to almost 4 cm/year in the northern regions. Scientists track this displacement using geodesy, a field that employs satellite-based systems like the Global Positioning System (GPS) to monitor crustal changes over time.
Tectonic Forces Driving Plate Motion
The movement of all tectonic plates is driven by the dissipation of heat from the Earth’s interior through mantle convection. This involves the slow motion of solid mantle material beneath the lithosphere, which translates movement to the plates. Two primary forces influence plate motion. The first is “ridge push,” a gravitational force where new, hot crust forms at divergent boundaries, like the Mid-Atlantic Ridge, and slides away, pushing the plate forward. The second is “slab pull,” the weight of cold, dense oceanic crust sinking into the mantle at subduction zones.
Slab pull is generally considered the strongest global driver, but the North American Plate is unique because it lacks active subduction along most of its western edge. Instead, its motion is heavily influenced by forces exerted at its base by the underlying mantle flow. Studies suggest that the deeper mantle moves faster than the plate itself, indicating that convective currents are propelling the plate from below. This mechanism, involving mechanical interaction between the plate and the mantle, explains its continued movement.
Geological Consequences at Plate Boundaries
The west-southwesterly motion of the North American Plate results in three distinct types of interactions along its boundaries.
Eastern Boundary: Divergent
The eastern edge is a divergent boundary, where the North American Plate pulls away from the Eurasian and African Plates at the Mid-Atlantic Ridge. Magma rises here to fill the gap, continuously creating new oceanic crust through seafloor spreading, which causes the Atlantic Ocean to widen.
Western Boundary: Transform
Along the western side, the interaction with the Pacific Plate is a transform boundary, where the two plates slide horizontally past one another. This boundary is defined by the San Andreas Fault system in California, an active zone of shearing and friction. Frequent earthquakes occur as stress builds up and is suddenly released along the fault lines.
Northern Boundary: Convergent
Further north, off the coast of the Pacific Northwest, the North American Plate forms a convergent boundary with the small Juan de Fuca Plate. The denser Juan de Fuca Plate is forced beneath the North American Plate in a process called subduction, forming the Cascadia Subduction Zone. This subduction is responsible for the formation of the Cascade Range volcanoes and carries a risk of generating large earthquakes and tsunamis. Similar subduction occurs further north along the Aleutian Trench, where the Pacific Plate slides beneath the North American Plate, creating a seismically active volcanic arc.