The terrestrial surface, the lithosphere, is divided into plates that move at a rate of about 2 to 20 cm per year, driven by convection currents that take place under it, in the asthenosphere.
There are seven large main plates in addition to other smaller ones. Some of the plates are exclusively oceanic, such as that of Nazca, at the bottom of the Pacific Ocean. Others, the majority, include continental crust that protrudes from sea level forming a continent.
The outermost solid part of the planet is a layer about 100 km thick called a lithosphere that is formed by the crust plus the upper part of the mantle. In the oceanic areas the crust is thinner, from 0 to 12 km and formed by basaltic type rocks.
The crust that forms the continents is thicker, up to 40 or 50 km and composed of crystalline rocks, similar to granite. The continental crust is the coldest and most rigid layer on Earth, so it deforms with difficulty.
The asthenosphere, located immediately below the lithosphere is formed by materials in semi-fluid state that move slowly. Temperature differences between a warm interior and a cooler external zone produce convection currents that move the plates.
These plates form in the ocean ridges and sink into subduction zones. In these two edges, and in the areas of friction between plates (faults), there are great tensions and magma outflow that cause earthquakes and volcanoes.
The continents, being embedded in mobile plates, do not have a fixed position and shape, but are moving on the plate to which they belong.
The oceanic part can be introduced under another plate until it disappears into the mantle. But the continental portion of a plate cannot, because it is too rigid and thick. When two continents, dragged by their plates, collide with each other, they end up merging with each other, while a large mountain range rises in the crash zone.
Pangea and plate movements
In the history of the Earth there were times when most of the continents were reunited, after colliding with each other, forming the great supercontinent Pangea. The last time this happened was at the end of the Paleozoic and early Mesozoic.
During the Mesozoic, Pangea was disintegrating. First it was divided into two large continental masses: Laurasia to the north and Gondwana to the south, separated by an equatorial ocean called Tethys. During the Mesozoic, about 135 million years ago, the Atlantic Ocean began to form as America separated from Europe and Africa.
The displacements of the continents and the climatic and sea level changes they have caused have had a great influence on the evolution that living beings have followed on our planet. In places that have remained isolated from the rest of the mainland for a long time, such as Australia or Madagascar, surrounded by sea for more than 65 million years, very special life forms have evolved. Another example is the difference in flora and fauna between North and South America, isolated for tens of millions of years and united only about 3 million years ago.
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