ROCKS cycle

Rocks are materials composed of one or more minerals (natural substances, inorganic in nature, with a homogeneous structure and a defined chemical composition). Although they are inert objects, rocks are not immutable far from it, they undergo constant alterations and transform, giving rise to other different types of rocks over thousands of years. This is a closed process in which, constantly, some rocks are destroyed and others are formed.

Geologists refer to the transformation of one type of rock into another as the rock cycle or lithological cycle. In this lesson from a TEACHER we will see a rock cycle summary.

Before starting with the summary of the rock cycle, we are going to know, first, how the three main ones are formed types of rocks depending on its origin:

Igneous rock formation

The first stage of the rock cycle begins when the magma it works its way to the surface of the Earth's crust (the outermost solid part of the Earth).

Magma is a matter in a semi-fluid state, which is found under the surface of the earth's crust. It is composed of silicates (minerals that contain silicon) and contains gases and minerals in solid form inside. Magma is at very high temperatures (between 7000 and 1200ºC). On its way to the surface of the earth's crust, magma

Igneous rocks are of two types, depending on whether the cooled magma reaches the earth's surface or not:

• Extrusive rocks (lava): are the igneous rocks that are deposited on the crust terrestrial through volcanic activity. They are also called volcanic rocks or simply lava, which is the name given to solidified magma on the earth's surface. These rocks are formed in relatively rapid magma cooling processes.
• Intrusive or plutonic rocks: are the igneous rocks that do not reach the surface and they are deposited in the cracks or cavities of the earth's crust. They are formed in very slow magma cooling processes inside the earth's crust. The slow cooling of magma, in the process of formation of these rocks, allows a better development of mineral crystals (ordered atomic or molecular structures, in which a repeating pattern is observed that extends to form a network three-dimensional). The crystallization of minerals that form rocks intrusive, allows observing the different minerals that form the rock with the naked eye, as in the case of granite. These rocks can reach the surface once formed, by the lifting of the earth's crust or by erosion processes of the most superficial layers.

Metamorphic rock formation

Metamorphic rocks are formed from other rocks in solid state. Metamorphic rocks have a laminated appearance (they are formed by overlapping mineral sheets) that reflect the high pressures supported in their formation process.

They are formed by changes caused in pre-existing rocks, due to a significant increase in the Pressure and the temperature that takes place in the deep areas of the earth's crust. They can also originate from processes of tectonic activity (movement of the plates that make up the earth's crust), which generate friction or horizontal pressure.

A final process of formation of metamorphic rocks is that of contact metamorphism, when the changes that originate the metamorphic rock are due to the proximity of a mass of magma inside the earth's crust. Once formed, metamorphic rocks can follow the same processes that affect sedimentary rocks:

1. They can rise to the surface due to the erosion of the upper layers or due to the elevation of the earth's crust. Once they have reached the surface, they can be incorporated into the erosion-transport processes.
2. Descend to even deeper areas where the high pressure and temperature will melt the rock turning it from new in magma, which over time, will again give rise to igneous rocks, thus closing the cycle of rocks.

Sedimentary rock formation.

The process of formation of sedimentary rocks begins when the rocks on the earth's surface undergo erosion and transport processes. Natural agents (changes in temperature, action of water, action of wind), break the rocks into small fragments (erosion) that are transported and accumulate in the so-called sedimentation basins. Sedimentation basins are areas of the earth's surface that are surrounded by higher ground. They are, therefore, places where materials from erosion and transport processes accumulate, due to the force of gravity.

The most important sedimentation basins are the ocean bottoms, where sediments accumulate that withstand great pressure due to the weight of the water column above them.

The accumulated sediments are compacting little by little and undergoing the lithification process (rock formation process) that receives the name of diagenesis and, through which, sedimentary rocks originate. The diagenesis process is the set of physical, chemical and biological processes that give rise to sedimentary rocks. The main processes of diagenesis are:

• Compaction: reduction of sediment volume.
• Cementation: filling of the pore spaces, joining the particles together. It is one of the most common diagenic processes.
• Recrystallization: changes in crystal size, orientation, or shape; without changes in its composition.
• Replacement: one mineral is replaced by another occupying the same space that the mineral that was initially present had occupied.
• Dissolution: the selective dissolution of one of the minerals that form the sediment takes place.
• Autogenesis: development of new minerals within the sediment.

Once formed, sedimentary rocks can follow three different paths:

1. Emerge to the earth's surface and re-enter the erosion-transport cycle.
2. Sink, descending into the deep areas of the earth's crust, where pressure changes and temperature, cause changes in the minerals that will lead to the formation of rocks metamorphic.
3. Descend below the earth's crust to the upper mantle, and melt to rejoin the magma. Thus closing the rock cycle.

As we have seen in the previous section, the rock cycle is not exactly a cycle in the that one type of rock moves successively to another, forming a closed process that ends where it started. Conversely, the rock cycle includes numerous transformation processes in which the rocks can give rise to more than one different type of rock.

There are three basic transformation processes, which we have already commented and which we summarize below, but they can occur in different types of rock so that the possible transformations between types of rock are multiplied.

Transition to igneous rocks

Rock formation from the cooling of magma on its way to the earth's surface. Once formed, these rocks can be transformed into metamorphic or sedimentary rocks, in the same way that can sink back below the earth's crust to melt again and join the magma.

Transition to metamorphic

Rock formation from pre-existing rocks subjected to extreme conditions of pressure and temperature. Therefore, this process can affect any type of rock.

They are large rock masses associated with two types of geological processes:

• Tectonic plates
• Contact metamorphism

Transition to sedimentary

It occurs in all those rocks, igneous, metamorphic or sedimentary that are in contact with the earth's surface, subjected to the process of erosion-transport giving rise to sediments, which accumulate in sedimentation basins, where they undergo physical and chemical transformations to form rocks sedimentary.

In summary you can see below what the rock cycle is, in the diagram that we show you below, showing all the possible transitions between the different types of rock and the processes that they provoke them.

Edward J. Tarbuck, Frederick K. Lutgens. (2013). Earth Sciences: An Introduction to Physical Geology. Madrid: Pearson, cop.