Tectonic plates are large, relatively rigid pieces of Earth's lithosphere that move slowly over geological time. The lithosphere consists of Earth's crust together with the rigid uppermost part of the mantle. Plates vary greatly in size and can contain continental crust, oceanic crust or both.
Major plates include the Pacific, North American, South American, African, Eurasian, Antarctic, Indo Australian and South American plates, although the exact subdivision of the Indo Australian region and the recognition of numerous smaller plates can vary between geological models. Plates are not simply continents. A single plate may contain a continent and a large surrounding area of oceanic lithosphere, while a continent can extend across or near plate boundaries in some complex regions.
The crust and lithospheric mantle differ from the underlying asthenosphere in mechanical behaviour rather than being separated by one universal chemical boundary. Oceanic crust is predominantly basaltic and is generally only several kilometres thick, while continental crust is compositionally more diverse and typically thicker. Beneath the crust lies lithospheric mantle composed mainly of peridotitic rocks. Together these materials form the mechanically coherent plate. The asthenosphere is hotter and weaker and can deform over geological timescales, although it is still solid rock under normal conditions. The distinction between lithosphere and asthenosphere is therefore primarily rheological, meaning it concerns how materials respond to stress and deform.
Plates move at rates that are generally measured in centimetres per year. This may seem negligible on a human timescale, but over millions of years it can move continents thousands of kilometres and create or destroy entire ocean basins. Plate motion can be measured today using satellite geodesy, including networks based on precise positioning systems. Geological evidence independently records ancient movements through magnetic anomalies, radiometric ages, fossil distributions, geological correlations and palaeoclimatic indicators. Present day measurements therefore connect directly with the much longer geological record. The speed and direction of individual plates vary, and plate motions can change as boundaries form, disappear or reorganise.
Tectonic plates interact mainly at divergent, convergent and transform boundaries. Divergent boundaries separate plates and commonly create new oceanic lithosphere. Convergent boundaries bring plates together and can produce subduction or continental collision. Transform boundaries involve lateral relative motion. Some plate boundaries are broad deformation zones rather than perfectly sharp lines, particularly within continental regions. Plates can also contain hotspots or other regions of significant volcanic activity that are not necessarily located at boundaries. The movement of plates therefore reflects a combination of rigid body motion and deformation concentrated near boundaries or within weaker regions.
Oceanic and continental lithosphere behave differently because they have different thicknesses, compositions, ages and densities. Newly formed oceanic lithosphere is relatively hot and buoyant but cools, thickens and becomes denser as it ages. Old oceanic lithosphere can become dense enough to sink into the mantle at subduction zones. Continental lithosphere is generally more buoyant and resistant to subduction because much continental crust has relatively low density. This difference helps explain why continents can persist for billions of years while oceanic crust is commonly recycled on timescales of roughly hundreds of millions of years. Plate composition is therefore an important factor in determining tectonic behaviour.
Tectonic plates are part of a global system in which material is continually created, modified and recycled. At mid ocean ridges, mantle derived magma produces new oceanic crust. At subduction zones, oceanic lithosphere descends into the mantle and can contribute water and other materials to mantle processes. Continental collision can thicken crust and produce mountain belts. Transform motion can offset geological structures and redistribute stress. Over long periods, these processes repeatedly assemble and break apart supercontinents, open and close ocean basins and rearrange the positions of continents. The Wilson cycle is a conceptual framework describing repeated opening and closing of ocean basins through tectonic processes.
The boundaries of tectonic plates are identified using combinations of geological and geophysical evidence. Concentrations of earthquakes often outline active boundaries, while volcanoes and mountain belts reveal regions of deformation and magma generation. Seafloor magnetic anomalies reveal spreading centres and past plate movement. GPS measurements provide present day velocities. Ocean trenches identify many subduction zones, while major fault systems mark transform or continental deformation zones. Plate maps are therefore scientific models based on multiple observations rather than arbitrary divisions drawn from the appearance of continents. Some small plates and microplates remain subjects of detailed geological research because their boundaries can be diffuse or change through time.
A common misconception is that tectonic plates float on a global ocean of molten rock. Most of the asthenosphere is solid, although it can flow slowly over geological time, and only limited regions contain partial melt. Plates are also not necessarily moving because continents are actively pushed through the ocean floor. Instead, continents are carried as parts of lithospheric plates.
The movement of plates is ultimately connected to Earth's internal heat and gravitational energy and to processes such as slab sinking, ridge elevation and mantle flow. Their slow motion is responsible for many of the largest changes in Earth's surface, including the formation of mountains, ocean basins and volcanic arcs and the geographical rearrangement of continents.
