Lithospheric Plates (World of Earth Science)
Lithospheric plates are regions of Earth's crust and upper mantle that are fractured into plates that move across a deeper plasticine mantle.
Earth's crust is fractured into 13 major and approximately 20 total lithospheric plates. Each lithospheric plate is composed of a layer of oceanic crust or continental crust superficial to an outer layer of the mantle. Containing both crust and the upper region of the mantle, lithospheric plates are generally considered to be approximately 60 mi (100 km) thick. Although containing only continental crust or oceanic crust in any one cross-section, lithospheric plates may contain various sections that exclusively contain either oceanic crust or continental crust and therefore lithospheric plates may contain various combinations of oceanic and continental crust. Lithospheric plates move on top of the asthenosphere (the outer plastically deforming region of Earth's mantle).
The term "plate" is deceptive. Remembering that Earth is an oblate sphere, lithospheric plates are not flat, but curved and fractured into curved sections akin to the peeled sections of an orange. Accordingly, analysis of lithospheric plate movements and dynamics requires more sophisticated mathematics that account for the curvature of the plates.
In geological terms, there are three types of boundaries between lithospheric plates. At divergent boundaries, lithospheric plates move apart and crust is created. At convergent boundaries, lithospheric plates move together in collision zones where crust is either destroyed by subduction or uplifted to form mountain chains. Lateral movements between lithospheric plates create transform faults at the sites of plate slippage.
At each of the unique lithospheric plate boundaries there are specific geophysical forces that are characteristic of the plate dynamics. At transform boundaries there are shearing forces between the lithospheric plates. At divergent boundaries, tensional forces dominate the interaction between plates. At subduction sites, compression of lithospheric plate material dominates.
The dynamics of plate tectonics, driven by deeper thermal processes, stress and cause elastic strain on lithospheric materials. Resulting fractures of rock in the lithosphere cause a release of energy in the form of seismic waves (i.e. an earthquake).
Because Earth's diameter remains constant, there is no net creation or destruction of lithospheric plates.
In contrast to the technical definition of lithosphere used by geologists, many geographers use the term lithosphere to denote landmass. This is a distinct concept as the geological definition of lithosphere may include sections containing oceanic crust completely submerged beneath Earth's oceans. Using the geographical definition, Earth is approximately 71% hydrosphere (a region covered by water) and 21% lithosphere (a region of land).
See also Dating methods; Earth, interior structure; Hawaiian Island formation; Mantle plumes; Mapping techniques; Mid-ocean ridges and rifts; Mohorovicic discontinuity (Moho); Ocean trenches; Rifting and rift valleys; Subduction zone