plate tectonics

The lithosphere is divided into tectonic plates that move, albeit very slowly, in relation to each other – converging, diverging, riding over/under one another, and sliding past one another.

Tectonic plate movements are responsible for the Pacific "Ring of Fire" and for the enormous mid-ocean ridge systems, as well as for the rock cycle, magmatism, and continental motion from the late Archaean to the present.  Tectonic Plate Boundaries 

▼ accretion : collision : compression : convergence : divergence : earthquakes : extension : faulting : folding : magmatism : mid-ocean ridges : orogeny : rifting : Ring of Fire : Plates – Cocos, Juan de Fuca, Nazca, North American, Pacific : thrust faulting : transform-boundary fault : transform fault : shear zone : sinking : subduction : subduction zone : volcanoes : volcanic island arcs▼

Illustration of the Main Types of Plate Boundaries

Phenomena associated with the four main types of plate boundaries include:

Convergence between arc-continent (ocean-ocean), ocean-continent, or continent-continent plate boundaries. Collision (convergence) generates the compressional forces associated with accretion, folding and shear zone faulting (thrust), orogenic mountain building, subduction, magmatic emplacement, regional metamorphism, seismic activity, and volcanism.

Divergence of plates generates spreading centers where plates are separating or rifting apart under tensional stresses. New crust is created by magma welling up from the mantle in continental rift zones (c-c) or at mid-ocean ridges (o-o).

Subduction zones and associated submarine trenches occur where denser oceanic plates sink below less dense continental plates in the special case of subduction at converging arc-continent or ocean-continent plate margins. In subduction zones, a cool slab of oceanic lithosphere sinks beneath an accretionary sedimentary prism, which, in turn, often lies beneath a forearc basin. At subduction zones, the oceanic lithosphere, layers of marine sediments, and trapped water are recycled into the deep mantle. Subduction zones give rise to a range of magmas in magmatic arches.

Transform (strike-slip) fault boundaries, or transform boundaries occur along the margins of plates that are sliding past one another, producing shallow earthquakes as accumulated stresses are released suddenly. Transform boundaries are large faults or fracture zones that connect two spreading centers (divergent plate boundaries) or, less often, trenches (convergent plate boundaries). Most transform faults occur on the oceans, though the San Andreas fault zone in California is an example on land.


The Pacific 'Ring of Fire' (image at right) is a concentration of volcanic island arcs, oceanic trenches, and Cordilleran stratovolcanoes (and associated earthquakes and tsunamis), which encircles the Pacific Basin.

The Nazca and Cocos Plates are subducting beneath the South American plate. The Juan de Fuca and a section of the Pacific Plate are being subducting beneath the North American Plate (Aleutian Islands, Kamchatka—Kurile Islands arcs). The San Andreas Fault zone is a transform fault that offsets a portion of the East Pacific Rise under southwestern United States and Mexico.


The exact mechanism by which radioactive heating and resultant convection currents (image below left) is coupled to plate motion is not yet fully elucidated. Nevertheless, plate tectonic theory revolutionized understanding of important geological processes.

 Tectonic Plate Boundaries