All the rocks that exist on Earth can be divided into three main types: igneous rocks, metamorphic rocks and sedimentary rocks. This classification is not based on a particular mineralogical composition but is established on the basis of the genesis processes, which are very different from one type to another. Each type has very specific characteristics that allow them to be differentiated.
Igneous rocks – sometimes called endogenous rocks or igneous rocks – result from the cooling and crystallization of minerals from a magma.
A magma is a liquid produced by the partial (i.e. incomplete) melting of rocks in the mantle or crust. It contains a fluid part and a crystallized part, in proportions which will depend on the temperature and pressure conditions. The chemical composition of the magma, which varies according to the nature of the molten rocks, the degree of partial melting and the environmental conditions (hydration, for example), will greatly influence the composition of the magmatic rocks that will result from it. But this final mineralogical composition will also strongly depend on a process which takes place within the magma chamber and which is called fractional crystallization.
Indeed, all the minerals will not crystallize at the same time. Each mineral species has a temperature of liquidus/solidus quite specific. With the progressive cooling of the magma in the magma chamber, certain minerals will begin to crystallize very early while others will remain in solution in the liquid phase. Thus, the crystals of the different mineral species can appear gradually. It is this process that will give rise to different magmatic rocks, from the same magma.
Another process occurs during crystallization, and will influence the final chemical composition of igneous rocks. Certain chemical elements will indeed tend to remain preferentially in the liquid phase during crystallization. These are the so-called incompatible elements (like K, Rb, Cs, Ba, Sr, U Th), whose ionic radius is large, which prevents them from easily integrating a crystal lattice. Conversely, there are also so-called compatible elements (such as Cr and Ni), which will easily integrate the crystalline structure of the minerals during crystallization.
It is all of these processes that take place during crystallization that will give rise to the wide variety of igneous rocks. Conversely, the study of the composition of the rocks makes it possible to go back to the conditions of the environment at the time of their formation and thus to trace the source and the evolution of the magma.
Within magmatic rocks, we thus differentiate between plutonic rocks, which will crystallize slowly at depth, and volcanic rocks, which will crystallize quickly on the surface. Depending on their rate of crystallization, magmatic rocks will present very different textures. This ranges from the porphyritic texture, which has very large crystals, to volcanic glass, which is the result of extremely rapid cooling of a magma that does not allow the minerals time to crystallize (quenching effect).
Typical examples of igneous rocks: gabbro (plutonic rock), granite (plutonic rock), basalt (volcanic rock), andesite (volcanic rock).
Sedimentary rocks (or exogenous rocks) result from the deposition and accumulation of sedimentary particles which can be very diverse and have many origins. Sedimentary particles come from the mechanical erosion and chemical alteration of source rocks (we speak of the source of the sediments). Sedimentary particles can thus be found in the form of more or less large clasts (pebbles, rock debris, shells, etc.), grains (minerals or fragments of minerals), but also in the form of particles in solution, which will precipitate under certain conditions.
The deposit environment is variable: sediments can be deposited in a continental environment, in a lake or a river, or in a marine/ocean environment. Once deposited, generally in the form of strata (overlapping layers), the loose sediments will undergo a process of lithification called diagenesis, which is linked to the increase in pressure and temperature following the burial of deep layers. At the end of the diagenesis, an indurated sedimentary rock is obtained, the nature of which will depend both on the origin of the elements which compose it, but also on the diagenesis process.
Unlike igneous rocks which form at a high temperature (650 to 1,200°C), sedimentary rocks therefore form at low temperatures.
Depending on the nature of the sediments (chemical composition of the rock), the mechanisms of diagenesis and the deposit environment, a classification can be established, which does not however fully reflect the complexity of the sedimentary rocks. Considering the mechanisms of genesis, three main categories can be defined:
1. Detrital rocks
Detrital rocks are the most abundant. We speak of detrital terrigenous rocks if they consist of at least 50% of debris from the erosion of other rocks (magmatic, metamorphic or sedimentary). We speak of biogenic detrital rocks if the debris is of biological origin, such as shells.
Within this family, we can differentiate pyroclastic rocks, which are formed by the accumulation of volcanic debris such as ash.
Typical examples of detrital sedimentary rocks: sandstone, conglomerate, clays, silt, tuff…
2. Biogenic rocks
Biogenic rocks come from the accumulation and transformation of organic matter (petroleum coal), or mineral parts of living organisms (chalk). Bioconstructions, such as corals, are also part of this category of sedimentary rocks. We speak more specifically of biochemical rocks to describe the rocks resulting from the precipitation of chemical elements under the influence of a biological metabolism (limestone rock for example).
3. Chemical rocks
When there is precipitation of chemical elements in solution, we will speak of chemical rocks. When the precipitation occurs under evaporative conditions, we speak of evaporitic or saline rocks, such as gypsum, rock salt or phosphates.
Metamorphic rocks form the third and last major family of rocks. Unlike the other two types of rocks, which originate under relatively well-defined thermodynamic conditions, the pressure/temperature field of formation of metamorphic rocks is much broader.
Metamorphic rocks come from the transformation and re-assembly of pre-existing rocks (protolith) under varying conditions of pressure and temperature. The protolith can belong to any type of rock: magmatic or sedimentary. Metamorphism, the process that governs the transformation of a protolith, is explained by the fact that minerals are stable only in a well-defined range of temperature and pressure. However, it can happen, especially during tectonic movements, that rocks are driven to very great depths. As pressure and temperature increase, minerals will become unstable and react with each other through various mechanisms. A new rock will thus form. It can then be exhumed on the surface through new tectonic movements.
From the composition of a metamorphic rock, it is possible to retrace quite finely the pressure/temperature path followed by the protolith and to reconstruct the tectonic history of a region. Metamorphic rocks are thus differentiated according to their composition into several metamorphic facies, which define pressure-temperature ranges relating to the appearance of certain key minerals.
During their transformation, metamorphic rocks can undergo significant deformations, which will structure the rock in various ways depending on the stresses undergone. This is called schistosity or foliation.
Typical examples of metamorphic rocks: schists, gneiss, marble…
We see that the three types of rocks are not independent and that the interconnections are numerous. An igneous rock can be eroded and produce a sedimentary rock, which itself can evolve into a metamorphic rock. A metamorphic rock can undergo an episode of partial melting and produce magma, which will give rise to new igneous rock. This form of cyclicity highlights the great principle of recycling to which the rocks of the earth’s crust are subject, in connection with plate tectonics.