Changes

{{merge from|Continental drift}}

{{merge from|Continental drift}}

'''Plate tectonics''' is a theory of [[geology]] that has been developed to explain the observed evidence for large scale motions of the Earth's crust. The theory encompassed and superseded the older theory of [[continental drift]] from the first half of the 20th century and the concept of seafloor spreading developed during the 1960s.

'''Plate tectonics''' is a geological model of the movement of [[continental plate]]s around the surface of the [[Earth]].

 

The outermost part of the [[Earth]]'s interior is made up of two layers: above is the [[lithosphere]], comprising the [[crust]] and the rigid uppermost part of the [[mantle]]. Below the lithosphere lies the [[asthenosphere]]. Although solid, the asthenosphere has relatively low [[viscosity]] and shear strength and can flow like a liquid on geological time scales. The deeper mantle below the asthenosphere is more rigid again. This is, however, due not to cooler temperatures but to high pressure.

 

The lithosphere is broken up into what are called tectonic plates—in the case of Earth, there are seven major and many minor plates (see list below). The lithospheric plates ride on the asthenosphere. These plates move in relation to one another at one of three types of plate boundaries: [[convergent]], [[divergent]], and [[transform]]. [[Earthquakes]], [[volcanic activity]], mountain-building, and oceanic trench formation occur along plate boundaries. The lateral movement of the plates is typically at speeds of 0.66 to 8.50 [[centimeters]] per year.

== Developing the Theory ==

== Developing the Theory ==