Formation of mountain ranges

Although we have already seen the different boundaries between tectonic plates, as a review, we will see the main geological processes that occur when two tectonic plates converge. It is in the subduction zones where the destruction of the lithosphere occurs, the lithosphere created in the ridges returning to the mantle. Here, the incoming lithosphere has cooled, has more sediment, and is denser, so it sinks under the other plate.

Several situations are distinguished:

• Collision between two oceanic plates. One plate subducts under the other, creating an oceanic trench. Sometimes the subducing plate partially melts and, together with the sediments, volcanoes appear that create arch islands. In these areas there is great volcanic and seismic activity, such as the Pacific Island arches (Philippine Trench and Mariana Trench).

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• Collision between oceanic plate and continental plate. Since the oceanic plate is denser than the continental one, it subducts under it in an oceanic trench. The accumulated sediments on the subducting layer, fold and originate an elongated pericontinental mountain range. Volcanoes are also produced by the fusion of the subducting plate and earthquakes, due to the great stresses that accumulate. Example, the Andes.

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• Collision between two continental plates. When two mixed plates collide, the oceanic lithosphere disappears until the continents collide, forming an intracontinental mountain range formed with the sediments that had accumulated on the oceanic lithosphere, between the two continents. First, the sediments form an accretion prism and then, as the ocean shrinks, they fold, emerge, and form the ridge. They can melt but, as the continental crust is thicker, they do not come out or form volcanoes, remaining inside the mountain ranges as plutonic rocks. The stresses created are very strong and the seismic activity is intense, such as the Himalayas, Alps and Pyrenees.

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