What is the Rock cycle, and how is it defined?

The rock cycle is a continuous process in which rocks are formed, transformed and recycled over time, which is essential to maintain the stability of the earth's crust. It is an example of how natural processes work in a complex and constantly changing system.

All existing rocks on the planet participate in the cycle, and throughout the process they transform from one type to another. In this way, the materials of the earth's crust (the upper layer of the planet, made up of solid rock) are continuously recycled and transformed. The rock cycle has occurred on Earth since the formation of the planet, about 4.5 billion years ago.

rock formation

The first step of the cycle is the formation of rocks by different geological processes. According to the formation process, three types of rocks can be distinguished: igneous, metamorphic and sedimentary.

igneous rocks are those that are formed by solidification of magma under the earth's surface (igneous intrusive or plutonic rocks) or by the solidification of lava expelled by volcanoes on the earth's surface (volcanic or igneous rocks extrusive). Plutonic rocks, which form below the surface, can be exposed by tectonic movements or by erosion of the upper layers.

Once they have reached the surface, a cycle of rupture and degradation of the rocks begins, by the processes of weathering and erosion. The result of these processes is the breakdown of rock fragments, called sediments, such as gravel, sand, clay or silt.

sedimentary rocks They are formed by the accumulation and lithification of sediments accumulated at the bottom of bodies of water such as rivers, lakes or oceans. Lithification is a long and complex process that transforms accumulated sediments into sedimentary rocks, which are the most common rocks on the earth's surface.

metamorphic rocks They are formed when pre-existing rocks are subjected to high temperatures and pressures, as occurs during the formation of mountains (for collision or subduction of tectonic plates) or in areas close to subterranean magma deposits (known as magma chambers). The metamorphism process can affect and transform any type of pre-existing rock into metamorphic rocks; Thus, igneous and sedimentary rocks can become metamorphic.

Rock transformations

Once the rocks have been formed, they can be transformed by the geological processes that we have discussed:

• Igneous rocks, by weathering, erosion and lithification, become sedimentary rocks and metamorphism converts igneous rocks into metamorphic rocks.

• Metamorphic rocks can also erode and contribute sediment to the formation of sedimentary rocks.

• Sedimentary rocks, due to erosion, contribute to generate more sedimentary rocks, but they can also undergo metamorphism, thus becoming metamorphic rocks.

• If the metamorphism of the rock is very intense, such as that which occurs at the subduction edges of the tectonic plates, the metamorphic rock melts and becomes magma, which, by solidification, will give rise to igneous rocks, restarting the cycle.

In the rock cycle, rocks are transformed from one type to another, and the final fate of all rocks is to be transformed back into magma, by processes such as tectonic subduction. Subduction occurs when two tectonic plates collide and one plunges under the other, driving rocks to great depths where the temperature is high enough to form magma.

The edges of the plates where subduction occurs are called "subducting edges." An active subduction rim is west of South America, off the coast of Peru and Chile. This is the boundary between two tectonic plates: the South American plate and the Nazca plate. At the edge, the Nazca plate plunges under the South American plate. This subduction is related to the origin of the Andes mountain range and to the intense volcanic activity of the entire area.

The magma formed can be stored in magma chambers and cool slowly to form igneous or plutonic rocks. be expelled to the surface, through volcanoes, and form volcanic igneous rocks, contributing newly formed rock to the cycle.

The rock cycle and plate tectonics

The rock cycle is important because it is the process that maintains the stability of the earth's crust. Subduction rims are known as "destructive rims" because at them rock melts and turns into magma. In other words, at destructive edges the old crust is destroyed and recycled. There are plate edges where the plates are continually pulling apart, and magma flows continuously through the space between them. These edges are known as “building edges” because new oceanic crust is continually being formed from them. The largest building edges form in the oceans and are called mid-ocean ridges. Each of the planet's great oceans has a great ridge.

On continents, building plate edges are rarer, and cause the continent to break up, as it happened with the ancient supercontinent of Pangea, in which all the current continents were joined. The breakup of Pangea gave rise to the opening of the Atlantic Ocean and the supercontinents of Gondwana (South America, Africa, Antarctica and Australia) and Laurasia (North America and Eurasia), which later also fragmented, giving rise to the configuration of continents current.

References

Source: e. J.Tarbuck (2005). Chap. 4, 7 and 8. Earth sciences. An Introduction to Physical Geology., Ch. 2. (8th ed.). Editorial Pearson Education S. TO.