Divergent plate boundary

In terms of plate tectonics , divergent boundaries are areas under tension where lithospheric plates are pushed apart by magma upwelling from the mantle. Lithospheric plates are regions of Earth's crust and upper mantle that are fractured into plates that move across a deeper plasticine mantle. At divergent boundaries, lithospheric plates move apart and crust is created.

Earth's crust is fractured into approximately 20 lithospheric plates. Each lithospheric plate is composed of a layer of oceanic crust or continental crust superficial to an outer layer of the mantle. Oceanic crust comprises the outer layer of the lithosphere lying beneath the oceans . Oceanic crust is composed of high-density rocks, such as olivine and basalt . Continental crust comprises the outer layer of the lithospheric plates containing the existing continents and some undersea features near the continents. Continental crust is composed of lower density rocks such as granite and andesite . New oceanic crust is created at divergent boundaries that are sites of sea-floor spreading .

At divergent boundaries, upwelling of magma along mid-ocean ridges (e.g., Mid-Atlantic Ridge) creates the tensional forces that drive the lithospheric plates apart.

Although initially formed from convection hot spots in the asthenosphere , rift valleys (e.g., Rift Valley of Africa ) can fuse or interconnect to form zones of divergence that ultimately can fracture the lithospheric plate.

Containing both crust and the upper region of the mantle, lithospheric plates are approximately 60 miles (approximately 100 km) thick. Lithospheric plates may contain various combinations of oceanic and continental crust in mutually exclusive sections (i.e., the outermost layer is either continental or oceanic crust, but not both). Lithospheric plates move on top of the asthenosphere (the outer plastically deforming region of Earth's mantle).

Divergent plate boundaries are, of course, three-dimensional. Because Earth is an oblate sphere, lithospheric plates are not flat, but are curved and fractured into curved sections akin to the peeled sections of an orange. Divergent movement of lithospheric plates can best be conceptualized by the movement apart of those peeled sections over a curved surface (e.g., over a ball).

At divergent boundaries, tensional forces dominate the interaction between plates.

Because Earth's diameter remains constant, there is no net creation or destruction of lithospheric plates and so the amount of crust created at divergent boundaries is balanced by an equal destruction or uplifting of crust at convergent lithospheric plate boundaries.

Evidence of symmetrical bands of rock with similar ages located on either side of divergent boundaries offer important evidence in support of plate tectonic theory. In addition, similar fossil and magnetic bands also exist in equidistant bands on either side of a divergent boundary.

See also Convergent plate boundary; Dating methods; Earth, interior structure; Fossil record; Fossils and fossilization; Geologic time; Hawaiian island formation; Mapping techniques; Mid-ocean ridges and rifts; Mohorovicic discontinuity (Moho); Rifting and rift valleys; Subduction zone