Rocks

Rocks present several aspects and various compositions. In order to simplify their classification, scientists have classified them into three broad types based on their formation processes. These rocks, while stable in appearance, can transform and change categories over a very long period of time.

The rock cycle illustrates the origin of the three major types of rock. Magma, coming from the internal mantle of the Earth, is responsible for the formation of the Earth's crust. Magma is therefore the starting point of the rock cycle. It is also the point of arrival since all the rocks will end up, after several tens of thousands of years, melting again into magma.

During the cycle, the magma circulating beneath the Earth's crust cools (4) and then forms igneous rocks (or magmatic rocks). These igneous rocks, exposed to environmental agents, break down into small particles that will be carried by wind, water, and other erosion agents (1). The accumulation of these particles will cause the formation of sedimentary rocks. Igneous and sedimentary rocks, under the effect of elevated temperatures or high pressures (3) generated by the movements of the Earth's crust, are liable to transform into metamorphic rocks. It is worth noting that metamorphic rocks can also undergo erosion (1) and produce the particles that underlie sedimentary rocks. Finally, all the rocks, when buried near magma, melt and return to magma (2). The cycle is then complete and can restart.

Igneous rocks are the most common rocks in the Earth's crust. The word “igneous” comes from the Latin word igneus which means “comes from fire." Thus, magma, a viscous liquid formed from molten rocks and containing gases dissolved at very high temperature, is the source of igneous rocks. The magma is carried to the Earth’s surface where it will be cooled and then solidified. The crystals form in a disorderly manner, without any particular orientation. The rocks resulting from this process are said to be igneous.

Depending on whether the cooling takes place outside or inside the Earth's crust, two types of igneous rocks can be distinguished: intrusive igneous rocks and extrusive igneous rocks.

When the magma cools slowly and completely inside the Earth, it is referred to as intrusive igneous rocks, also called plutonic igneous rocks. Slow solidification of the magma will give rise to rocks with crystals that are easily visible to the naked eye (large crystals).

Examples of intrusive igneous rocks

Volcanic eruptions are manifestations of a spectacular and very rapid rise of magma from the Earth’s depths. The magma then cools very quickly and the resulting rocks have a fine texture, free from crystals visible to the naked eye. These rocks are said to be extrusive igneous rocks.

Occasionally, small holes can be observed in some extrusive igneous rocks, such as pumice stone. These small holes were formed by the rapid degassing of the lava projected during the volcanic eruption, thus forming bubbles.

Examples of extrusive igneous rocks

As the name suggests, Sedimentary rocks are formed by sediments, which are rock fragments (igneous or metamorphic) formed by erosion and are carried away. Sedimentary rocks often form in stratigraphic layers, which tells us about the history of the Earth. These rocks sometimes contain fossils.

According to their formation process, two types of sedimentary rocks are distinguished: detrital sedimentary rocks and chemical sedimentary rocks.

This type of sedimentary rock is formed from sediments accumulated in layers or strata. This type of formation is common in water where suspended sediments are eroded, carried away, compacted, and cemented: shells, rock fragments (igneous or metamorphic), or animal fragments (shells, bones, etc.), sand, clay , etc.

Examples of detrital sedimentary rocks

These rocks are also formed by the accumulation of sediment. However, the sediment that accumulates comes from the precipitation of substances found in water. These sediments of chemical origin come mainly from the evaporation of seawater, rich in mineral salts.

Examples of chemical sedimentary rocks

Under high temperature and pressure, igneous rocks and sedimentary rocks can transform into metamorphic rocks. When the temperature is hot enough to allow recrystallization, new crystals form. The rearrangement can also lead to the formation of uniform bands of crystals in the rock.

There are two types of metamorphic rocks depending on the process involved: regional metamorphism modifies the rocks in place due to the pressure between two tectonic plates (these rocks are sometimes said to have a banded structure), whereas contact metamorphism involves the heat released by the magma in the Earth's crust (these rocks will not have a banded structure).

To correctly identify a rock, the main characteristics that distinguish igneous rocks, sedimentary rocks, and metamorphic rocks must be known. The categories of the same type of rock must also be differentiated. For instance, being able to recognize the differences between an intrusive igneous rock and an extrusive igneous rock is essential.

Once the type of rock (igneous, sedimentary, or metamorphic) and its category have been determined, simply consult directories with photos to find the exact name of the specimen.

It can then be useful to create a table with the main characteristics of the specimen to be identified (type, category, dominant colour, other colours, etc.) and to attach a photo.