May 25, 2022

A rock is a natural aggregate of minerals. Although rocks of different origins and different characteristics can be in close contact, their nature does not depend only on its origin, but also on the physical and environmental conditions in which it finds itself. The science that studies its formation is called petrology, while the one that describes it as petrography.

Description and training

Unlike minerals, rocks cannot be expressed or defined by formulas as they do not have a defined chemical composition; they are made up of several minerals, therefore fundamentally heterogeneous. Homogeneous rocks, on the other hand, contain a single type of mineral; in this particular case the distinction between rock and mineral becomes very subtle: generally the rock lacks true continuity (there is practically always the presence of impurities). Igneous rocks form when magma cools in the earth's crust or lava cools on the surface of the soil or on the seabed. Metamorphic rocks form when existing rocks are subjected to pressures and temperatures so great that they transform, which occurs, for example, when continental plates collide. Sedimentary rocks are formed by the diagenesis or lithification of sediments, which in turn are formed by erosion, transport and any sediment deposits of existing rocks.


Rocks are composed of mineral granules, which are homogeneous solids formed by an ordered chemical compound. The aggregate minerals that form the rock are held together by chemical bonds. The types and abundance of minerals in a rock are determined by how it was formed. Most rocks contain silicate minerals, compounds that include silicon oxide tetrahedra in their crystal lattice and account for about a third of all known mineral species and about 95% of the earth's crust. The proportion of silica in rocks and minerals is an important factor in determining their names and properties. Rocks are classified based on characteristics such as mineral and chemical composition, permeability, consistency of constituent particles and particle size. These physical properties are the result of the processes that formed the rocks. Over time, rocks can transform from one type to another, as described by a geological model called the rock cycle. This transformation produces three general classes of rock: igneous, sedimentary and metamorphic. These three classes are divided into many groups. There are, however, no hard and fast boundaries between the Allied rocks. By increasing or decreasing the proportions of their minerals, they pass through the gradations from one to the other; the distinctive structures of one type of rock can therefore be found gradually by merging with those of another. Thus the definitions adopted in the names of the rocks simply correspond to the points selected in a progressively graduated series. In geology, according to the lithogenetic process that gives them origin, rocks are divided into three main categories: Magmatic rocks, also called igneous or volcanic, are the rocks obtained from the solidification of magma, a molten mass generated in the depths of the lithosphere (therefore of endogenous origin) and containing volatile elements (water, carbon dioxide, acids, hydrogen) which give it fluidity and accelerate chemical reactions. These rocks, due to the various ways in which magma solidifies, are in turn divided into intrusive or plutonic, effusive or volcanic, hypoabyssal or phylonian igneous rocks: The former are formed inside the earth's crust or in the most superficial part of the earth's mantle and are characterized by a slow cooling that favors the cr