The North American Plate is one of Earth’s largest tectonic plates, spanning roughly 75 million square kilometers from the Mid-Atlantic Ridge in the east to the Pacific coast in the west, and from the Arctic Ocean down through Central America and the Caribbean. It carries an entire continent along with a substantial area of ocean floor, and its edges host some of the planet’s most studied geological features: the San Andreas Fault, the Cascadia subduction zone, the Aleutian Trench, and the volcanic spine of the Caribbean. But the plate’s interior is far from geologically boring. Ancient failed rifts, intraplate earthquakes, hotspot tracks, and the slow rebound of land once buried under ice sheets all make the story of the North American Plate richer than its boundaries alone suggest.
What Surrounds the Plate
The North American Plate meets different neighbors along each of its edges, and the style of interaction at each boundary determines the kinds of geological hazards and landforms that develop there. Along the west coast of the United States, the plate slides laterally past the Pacific Plate along the San Andreas Fault system, a transform boundary. In coastal California, more than 310 kilometers of lateral displacement has accumulated on the San Andreas Fault system over tens of millions of years, though the slip is not confined to a single strand. A detailed model of slip distribution in the San Francisco Bay area found that more than 250 kilometers of that motion has been accommodated by faults east of the San Francisco Peninsula, with the patterns of active faulting shifting irregularly over time rather than migrating steadily eastward as older models proposed.1Tectonics. Distribution of displacement on and evolution of a young transform fault system: The northern San Andreas fault system, California
Farther north, the boundary switches from transform to convergent. Off the coasts of Washington, Oregon, and British Columbia, the small Juan de Fuca Plate dives beneath North America at the Cascadia subduction zone. This boundary is accumulating elastic strain that could be released in a very large earthquake. The locked portion of the fault is relatively narrow, confined to a roughly 60-kilometer-wide band beneath the continental shelf and slope, and the seismogenic zone extends to a depth of less than about 15 kilometers. That shallow, narrow locked zone is a consequence of the unusually young and hot oceanic crust being subducted, combined with thick insulating sediments on the seafloor.2Journal of Geophysical Research: Solid Earth. Current deformation and the width of the seismogenic zone of the northern Cascadia subduction thrust Recent deep seismic imaging spanning about 900 kilometers of the margin has revealed that the Juan de Fuca Plate is fragmenting within the seismogenic zone as it bends beneath the upper plate, creating structural domains that appear to align with past earthquake rupture segments.3PubMed Central. Subducting plate structure and megathrust morphology from deep seismic imaging linked to earthquake rupture segmentation at Cascadia
Along the Aleutian Islands, the Pacific Plate subducts beneath North America along a trench that stretches thousands of kilometers across the North Pacific. Seismic imaging beneath the Aleutian arc shows that partial melt accumulates in the upper mantle below the arc’s volcanoes, and the structural variation of both the sinking Pacific slab and the overriding plate shapes which parts of the arc are volcanically active and which are relatively quiet.4Journal of Geophysical Research: Solid Earth. Segmentation of the Aleutian‐Alaska Subduction Zone Revealed by Full‐Wave Ambient Noise Tomography: Implications for the Along‐Strike Variation of Volcanism Intermediate-depth earthquakes along this boundary tend to occur near the subducted oceanic crust itself, suggesting that mineral phase changes within the sinking slab rather than temperature alone trigger them.5Geophysical Research Letters. Relationship between shallow‐ and intermediate‐depth seismicity in the Eastern Aleutian Subduction Zone
To the south and southeast, the plate boundary with the Caribbean Plate is complex. GPS measurements show the Caribbean moves relative to North America at roughly 18 to 20 millimeters per year, significantly faster than older geological models predicted. That motion is obliquely convergent along much of the boundary east of Cuba, meaning the plates are not simply sliding past one another; they are also pushing together.6Geophysical Research Letters. GPS geodetic constraints on Caribbean‐North America Plate Motion
The Ancient Core Beneath Canada
The oldest and most stable part of the North American Plate is its cratonic core, a vast block of ancient continental crust centered beneath the Canadian Shield. These cratons have survived billions of years of tectonic collisions and break-ups because they sit atop thick, cold, chemically depleted roots of mantle rock called lithospheric keels. These keels are buoyant enough to resist being dragged down or eroded by the convecting mantle beneath them, and their thickness can reach around 250 kilometers.7Journal of Geophysical Research: Solid Earth. Subduction beneath Laurentia modified the eastern North American cratonic edge: Evidence from P wave and S wave tomography
Beneath Hudson Bay, seismic imaging has revealed that the keel is not a uniform slab of cold rock. It has significant internal architecture, varying from about 180 kilometers thick beneath a Paleozoic rift zone in the northeast to more than 260 kilometers thick beneath Southampton Island and central-southern Hudson Bay. The keel was assembled episodically, growing in stages as separate blocks of ancient crust were welded together during past continent-building events.8Earth and Planetary Science Letters. Seismic imaging of the lithosphere beneath Hudson Bay: Episodic growth of the Laurentian mantle keel This deep root is what gives the continental interior its long-term geological stability, shielding places like the Great Plains from the tectonic drama unfolding at the plate’s edges.
Mountain Building on the Plate
Two of the continent’s most prominent mountain systems formed through very different mechanisms, and they bookend the plate east and west.
The Appalachian Mountains formed along what is now the eastern margin, built during a series of collisions as ancient continents converged to assemble the supercontinent Pangea. During the final stage of Appalachian mountain building, known as the Alleghanian orogeny, the crust thickened dramatically. Geological and geochemical data show crustal thickness locally reached about 39 kilometers, based on reconstruction of the thrust sheets that stacked up during the collision. This thickening generated the heat and pressure required to form granitic magmas at depth, and coincided with sediment deposition in a foreland basin between roughly 350 and 320 million years ago.9Lithosphere. Linking metamorphism, magma generation, and synorogenic sedimentation to crustal thickening during Southern Appalachian mountain building, USA
The Rocky Mountains and their associated foreland uplifts formed much later, during the Late Cretaceous to early Paleogene, through a process that has puzzled geologists for over a century. The Laramide orogeny produced basement-cored arches and deep basins far inland from the plate’s western margin, hundreds of kilometers east of where mountain building would normally be expected. The leading explanation is flat-slab subduction: instead of diving steeply into the mantle, the subducting oceanic plate flattened out beneath North America, coupling mechanically to the continental plate and transmitting compressive stress deep into the interior.10Earth and Planetary Science Letters. Thermal modelling of the Laramide orogeny: testing the flat-slab subduction hypothesis The slab’s flattening was accompanied by a gap in volcanic activity in the Sierra Nevada, an eastward jump of igneous activity, and a wave of subsidence and uplift sweeping across the foreland.11Geological Society of America Memoirs. The Laramide orogeny: Current understanding of the structural style, timing, and spatial distribution of the classic foreland thick-skinned tectonic system
There is debate about exactly how the flat slab transmitted stress. One view holds that it applied strong basal shear traction to the overriding plate; another argues, partly by analogy with modern flat-slab segments in South America, that the main effect was increased horizontal end-load stress at the fore-arc, which was then transmitted laterally across the rigid Colorado Plateau into the foreland.12Geology. Isotopic evidence for preservation of Cordilleran lithospheric mantle during the Sevier-Laramide orogeny, western United States
A Rift That Almost Broke the Continent
About 1.1 billion years ago, the interior of what is now North America nearly split in two. The Midcontinent Rift System stretches from Lake Superior southwest into Kansas and southeast into lower Michigan, a massive scar filled with flood basalts and sedimentary rock. Surface exposures, seismic, and gravity data reveal a basin filled with inward-dipping basalt layers above thinned and underplated crust, features so similar to a rifted volcanic margin that the rift appears to have come close to continental breakup before it failed.13Tectonophysics. Insights from North America’s failed Midcontinent Rift into the evolution of continental rifts and passive continental margins
The rift developed while Laurentia, the ancient core of North America, was moving rapidly southward during the assembly of the supercontinent Rodinia. Massive magmatic activity accompanied the rifting, likely driven by rising mantle material displaced by subducting oceanic crust along the continent’s southern margin. The hot, ductile crust was stretched apart by extensional stresses until, for reasons still debated, the rifting stopped.14Precambrian Research. Reviewing the configuration and extent of the Midcontinent rift system The Midcontinent Rift was not unique in North American history; two other major failed rifts followed similar evolutionary paths of extension, magmatism, subsidence, and later compression, each recording different phases of supercontinent assembly and breakup.15GSA Today. Three Major Failed Rifts in Central North America: Similarities and Differences
Hotspot Tracks Across the Plate
As the North American Plate drifts, it passes over relatively fixed sources of hot rock rising from deep in the mantle. These hotspots burn trails of volcanic activity across the plate, like a blowtorch held under a moving sheet of metal. The most famous example is the Yellowstone hotspot. The Snake River Plain in southern Idaho represents about 17 million years of volcanic activity produced as the continent migrated over this magma source. Seismic imaging has identified a clear plume beneath Yellowstone, and the trail of volcanism stretching from the Columbia Plateau through the Snake River Plain to the Yellowstone Plateau is now considered a single magmatic system related to that plume.16Geology. Yellowstone plume–continental lithosphere interaction beneath the Snake River Plain
The plume does not rise vertically. Imaging shows the thermal anomaly beneath Yellowstone plunges roughly 65 degrees to the west-northwest, a tilt that reflects the plume tail being forced beneath the edge of thinned cratonic lithosphere as the continent overrode it. A revised model proposes that much of the early volcanism in southeastern Oregon and southwestern Idaho was produced by the plate overriding the broader plume head, and that a distinct, narrower plume-tail track was not established until about 12 million years ago.17Tectonics. Lithospheric topography, tilted plumes, and the track of the Snake River–Yellowstone hot spot
The eastern side of the plate has its own hotspot story. The Monteregian Hills of Quebec, the younger White Mountain igneous intrusions, and the New England Seamounts extending into the Atlantic form a coherent hotspot track.18Journal of Geophysical Research: Solid Earth. Monteregian hotspot track: A long‐lived mantle plume Radiometric dating of the seamounts shows construction ages increasing from southeast to northwest, consistent with the plate moving over a fixed magma source between about 103 and 82 million years ago at a rate of roughly 4.7 centimeters per year.19Journal of Geophysical Research: Solid Earth. Age progressive volcanism in the New England Seamounts and the opening of the central Atlantic Ocean
Earthquakes Far from the Plate’s Edges
The New Madrid Seismic Zone in the central Mississippi Valley sits about as far from a plate boundary as you can get in North America, yet it produced some of the strongest earthquakes in the continent’s recorded history during the winter of 1811–1812. The zone lies along ancient fault structures that have been reactivated repeatedly throughout geologic time. Seismic reflection profiling has revealed major faults coincident with the modern earthquake trends, along with deformation caused by repeated episodes of igneous intrusion.20PubMed. Recurrent intraplate tectonism in the new madrid seismic zone The Reelfoot fault and related structures are interpreted as ancient normal faults that were later inverted into reverse faults by compressive stress.21Geosphere. New Madrid seismic zone fault geometry
A persistent puzzle is why these faults remain active at all. Modeling work suggests that reactivating them under the current stress field requires either unrealistically weak fault surfaces or elevated pore fluid pressure at depth. The preferred explanation among some researchers is that deep fluids, possibly rising from the upper mantle where a low-velocity seismic anomaly hints at their presence, are reducing the effective strength of the faults enough for them to slip.22Journal of Geophysical Research: Solid Earth. A Parametric Analysis of Fault Reactivation in the New Madrid Seismic Zone: The Role of Pore Fluid Overpressure
Charleston, South Carolina, presents a similar riddle. The area experienced a devastating earthquake in 1886, but analysis of stress patterns in the southeastern United States shows that regional and local stresses can explain the orientation of earthquake mechanisms without concentrating stress specifically at Charleston. Other factors, still not fully understood, must contribute to making that particular area earthquake-prone.23Tectonophysics. Intraplate seismicity and stress in the southeastern United States
What Drives the Plate and How Fast It Moves
The question of what actually pushes the North American Plate has a more interesting answer than the simple textbook cartoon of slab pull and ridge push. Modeling of the forces acting on the plate suggests that boundary loads, such as ridge push from the Mid-Atlantic Ridge and resistance at subduction zones, are the most important, followed by internal forces like gravitational potential energy differences across the continent and basal drag from the mantle beneath.24Reviews of Geophysics. North American dynamics and western U.S. tectonics But the deeper mantle is not merely a passive layer the plate slides over. Analysis indicates the mantle beneath North America moves faster than the plate itself, meaning mantle convection actively drives the plate rather than simply resisting it.25Geology. Which forces drive North America?
The plate’s speed and direction have not been constant. Modeling of absolute plate motions over the past 20 million years shows at least one significant shift in the plate’s angular velocity around 9 million years ago.26Tectonics. Episodic North America and Pacific Plate motions In very rough terms, most of the continent moves westward to southwestward at a couple of centimeters per year, though the speed varies depending on where you measure and which reference frame you use.
The Land Is Still Bouncing Back from the Ice Age
During the last glacial maximum, roughly 20,000 years ago, an ice sheet several kilometers thick covered much of Canada and the northern United States. The weight of that ice pushed the crust downward into the mantle, and even though the ice melted between about 18,000 and 7,000 years ago, the land is still rising.27Journal of Geophysical Research: Solid Earth. Deglaciation‐induced vertical motion of the North American continent and transient lower mantle rheology GPS measurements from 360 sites across the stable interior of the plate show present-day uplift of about 10 millimeters per year near Hudson Bay, the area where the ice was thickest. Uplift rates decrease with distance from that center and shift to subsidence of 1 to 2 millimeters per year south of the Great Lakes. The hinge line separating uplift from subsidence is visible in water-level gauge records along the Great Lakes: northern shores are rising while southern shores are sinking.28Geophysical Research Letters. Observation of glacial isostatic adjustment in “stable” North America with GPS
This glacial isostatic adjustment is the single strongest deformation signal in the plate’s interior today, dwarfing any tectonic motion within the continental core. It has practical consequences: shoreline positions along the Great Lakes and Hudson Bay are shifting, relative sea-level trends differ from place to place along the Atlantic coast, and the long-term stability assumptions used for infrastructure planning need to account for ground that is still in slow motion.
Fossil Slabs from a Vanished Ocean
Before the Pacific Plate bordered western North America, a much larger plate called the Farallon Plate dominated the eastern Pacific. As the spreading ridge that separated the Farallon and Pacific plates approached North America, the Farallon broke into smaller fragments. Most of those fragments were swallowed by subduction, but not all of them were consumed completely. Seismic tomography has identified fossil slabs, high-velocity anomalies in the mantle, still attached to microplate remnants that stopped subducting before the spreading center reached the trench. One such slab lies beneath Baja California, attached to the Guadalupe and Magdalena microplates, and another extends to depths of 200 kilometers or more beneath the southern Central Valley of California, attached to the former Monterey microplate.29PubMed Central. Fossil slabs attached to unsubducted fragments of the Farallon plate The Baja slab’s location correlates with surface eruptions of high-magnesium andesites, a type of lava thought to form by partially melting subducted oceanic crust. These fossil slabs are, in a sense, the last physical traces of a plate that once spanned much of the Pacific.
How the Plate Rearranged Global Climate
Tectonic movements of the North American Plate have had consequences far beyond the continent. One of the most significant was the closure of the Central American Seaway, the ocean gap that once separated North and South America. Evidence from Caribbean plate interactions suggests that incipient subduction caused progressive uplift of the overriding plate, cutting off deep-water flow around 7.5 million years ago and shallow-water flow around 5 million years ago, with a full land bridge forming by about 2.7 million years ago.30Geochemistry, Geophysics, Geosystems. Evidence for Caribbean Plate Subduction Beneath the Isthmus of Panama and Implications for Subduction Initiation and the Closure of the Central American Isthmus During the Miocene
The consequences rippled through the oceans and atmosphere. As the seaway shoaled, surface-water exchange between the tropical Atlantic and Pacific was restricted. Atlantic surface salinity increased, and ocean carbon isotope records from the Caribbean suggest an intensification of the deep overturning circulation in the North Atlantic. These shifts created new feedback loops between the ocean and atmosphere that have influenced global climate from the early Pliocene onward.31Geology. Role of Panama uplift on oceanic freshwater balance The strengthened Atlantic circulation is thought to be one of the factors that made the Northern Hemisphere more susceptible to the glacial-interglacial cycles that dominated the past few million years.
A Supercontinent on the Horizon
Plate tectonics is a continuous process, and the North American Plate’s current trajectory points toward a distant reunion with other continents. Numerical modeling of plate-motion-driven flow coupled with deep mantle circulation projects that Australia, Eurasia, North America, and Africa will merge in the Northern Hemisphere to form a new supercontinent within roughly 250 million years.32Geology. Formation of a future supercontinent through plate motion–driven flow coupled with mantle downwelling flow The specifics depend heavily on modeling assumptions, but the broad pattern of convergence in the Northern Hemisphere is a consistent feature of several independent simulations. It would be the latest in a cycle of supercontinent assembly and breakup that has repeated at least four or five times in Earth’s history, with Pangea being only the most recent example.