Wed. Aug 21st, 2019

Does the clash between tectonic plates always cause mountain ranges? | Science



When there is a collision between the plates, mountain ranges are always formed. But the interaction between the plates can be of another type besides the collision or shock. Although to understand what happens between them it is better that we begin by explaining what the plates are. What in geology we call tectonic plates are the pieces in which the outermost and most rigid layer of our planet is divided. That outer layer, rocky, is called lithosphere and is formed by the crust and the upper part of the mantle below. The lithosphere of our planet has a varied thickness as oceanic or continental. The regions emerging from the earth's surface correspond to areas with continental lithosphere that is thicker, up to 150 kilometers or more. While the oceanic lithosphere is thinner and denser, that is why it is submerged. This terrestrial lithosphere is broken into pieces, the so-called tectonic plates, which are in continuous motion with respect to each other and form a kind of puzzle. It is at the edges of those moving parts where the mountain ranges are formed, most of the largest earthquakes and active volcano alignments occur.

The pieces that make up that puzzle on the surface are seven large plates that can be made up of oceanic or continental lithosphere or pieces of both. There are also dozens of other smaller plates that accommodate large movements. Small plates tend to move faster than large ones, and all these movements are what we call plate tectonics. There are three main types of relative movement at the edges between the plates: convergence, the two plates approach; the divergence, the plates are separated; and the time when they slide next to each other.

Convergence is, for example, the case of the Himalayan mountain range, where two continental masses collide that have generated the highest mountains on our planet. Also our Pyrenees mountain range, although much smaller, has been generated by the same process. If the convergence occurs in a boundary between oceanic lithospheres or one oceanic and another continental, what happens is that one of them, always the oceanic, gets under the other. We call this action subduction and it usually causes the formation of volcanoes. That is what happens in the so-called Pacific arc of fire, where the arches of islands are generated. When the subduction is due to the convergence between a continental and an oceanic mass, as happens in the Pacific margin of South America, then a mountain range of mountains and volcanoes such as the Andes.

In the divergent limits, the plates separate from each other and in the hollow that originates from that separation the mantle goes towards the surface, begins to solidify and when it consolidates it creates new oceanic crust that fills the crack. Mountain ranges are also formed in this case, with a pronounced valley that marks the crack in the middle. An example is the Atlantic dorsal that forms an alignment of mountains thousands of kilometers long from south to north and is submerged in the middle of that ocean. That separation that occurs in the Atlantic dorsal causes the coasts of the eastern continents to move away from the coasts of the western ones. The speed of the separation depends on the zones, but it can reach a couple of centimeters a year which is approximately the speed at which our nails grow.

The third type of relative movement at the edges of plates occurs when they move in parallel, sliding side by side. This parallel slide can be in the opposite direction or in the same direction but at different speeds and also usually generates mountain alignments. An example of a current limit between two continental masses It is the famous San Andres fault in California, causing earthquakes of great magnitude. The current limits also occur at the bottom of the oceans between two oceanic lithospheres.

The movements of the plates and the formation of mountains like the ones we see today are processes that have existed for about a billion years. For these plates to exist, the first thing that was needed was for the Earth's crust to cool and solidify and therefore, it is possible that in the early stages of our planet since its formation some 4.5 billion years ago there were no plates or reliefs mountainous as we observe today.

Joaquina Álvarez Brown She is a doctor in Geology, head of the Department of Structure and Dynamics of the Earth and Crystallography of the Institute of Earth Sciences Jaume Almera of the CSIC

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