Plate tectonics is a geological theory that seeks to explain the nature of the lithosphere, the Earth’s outer layer. The lithosphere is divided into a number of separate plates or blocks that move relative to each other. As they slide alongside each other, collide together, or move apart, they create and break up continents, open and close oceans, and change the Earth’s climate.
Earth's crust and upper mantle
The two parts of the lithosphere are an upper layer, the Earth’s crust, and a lower layer, the top part of the upper mantle. These act as a rigid unit between 100 and 150 kilometres thick. The Earth’s landforms are a continental crust between 25 and 50 kilometres thick that is mainly a granitic rock. The oceanic crust beneath the oceans is between 5 and 10 kilometres thick and mostly a denser basaltic rock. The lithosphere is fractured into plates that can contain both oceanic and continental crusts. They slide on a molten or semi-plastic upper mantle.
Convergent and divergent boundaries
The plates are pushed apart by the creation of new oceanic crust at mid-ocean ridges called divergent plate boundaries. Molten material rises from the mantle to fill the space, creating uplift and rugged topography. Ocean crust is consumed at subduction zones, also called convergent boundaries, where two plates collide. If oceanic crust collides with less dense continental crust, it plunges below the latter creating a deep ocean trench and melts as it sinks. In a collision of two oceanic crusts, the denser of the two crusts is subducted. Trenches are the deepest part of the oceans, such as the Mariana Trench in the western Pacific.
Continental collision and transform faults
When two plates carrying continental crust collide at a convergent boundary, the impact breaks, bends, and thickens the crust to form mountains, such as the Himalayas. Plates also grind alongside each other creating a zone of fracturing called a transform fault. The San Andreas Fault in California is an example. Transform faults also cause offsets at mid-ocean ridges and subduction zones.
Earthquakes and volcanoes
Any fracture in the Earth’s crust releases energy as earthquakes. The largest and deepest earthquakes occur at convergent boundaries. The source of the March 2011 earthquake in Japan was a subduction zone in the Pacific Ocean. Crustal melting at subduction zones creates material that is less dense than the surrounding mantle. Molten magma rises and erupts at the surface as a volcano. This process created volcanoes around the Pacific Ocean called the Ring of Fire.
Ocean circulation and climate change
Throughout geological time, lithospheric plate movement caused a number of cycles where a single continental crustal mass was created, fragmented, and fused back again. Oceanic crust underlay single and multiple expanses of water as the seas opened, closed, and reopened. Ocean current circulation carries heat around the Earth and is a primary driver of its climate. As the seas opened and closed, the circulation changed, and in turn, contributed to changing the Earth’s climate.