In the field of Earth sciences, which brings together all the branches of science that are concerned with studying the solid Earth, the atmosphere and the hydrosphere, as well as the relationships established between them, geology or, better said, the geological sciences study the solid Earth in order to reconstruct the most important phases of the formation of our planet and, in this way, to provide a more precise answer and therefore a more detailed image of the current constitution of the Earth .
The last step of the geological sciences has been to abandon the purely theoretical and descriptive scientific research to face application problems that extend from the field of oil and mining industry to construction, and from the attention to the environment to the prevention of natural catastrophes, etc. Therefore, they have received an important impulse purely applied disciplines such as geotechnics (which studies, for example, the mechanical, physical and chemical properties of the terrain), applied geology, applied geophysics and hydrogeology. All these branches have turned the figure of the geological scholar of nature and the adventurous researcher into that of the professional scientist increasingly involved in the practical problems of daily life.
Among the different disciplines that are related to the geological sciences, there are some in particular that provide the indispensable basic knowledge: the mineralogy is responsible for the study of minerals, defining their characteristics and properties, and cataloging them in different groups; Petrology and petrography study and classify the magmatic and metamorphic rocks and the lithogenetic processes (of rock formation) that have generated them; sedimentology deals instead with sedimentary rocks, detailing the phases and modalities of their formation; paleontology analyzes fossils and their chronological and environmental meaning according to the principles of the evolution of life forms; stratigraphy studies the disposition of rocks in space and time to establish the order of succession of major geological events; structural geology deals with the geometric arrangement of rocks and the deformations suffered by the earth's crust; the geomorphology examines the different forms of the terrain and tries to explain the causes that have originated them; geophysics or terrestrial physics studies the physical properties of the Earth, its variations and the effects they produce in order to investigate the deep structure of the planet and understand the mechanisms and distribution of earthquakes.
The geological sciences, whose mission is to reconstruct the history of our planet, need to have a time scale that serves as a point of reference for the dating of the different events and to establish the sequence in which they happened. As it will be seen, there is a relative chronology that indicates the order that certain events occupy in time, ordering them in a sequence from the oldest to the most recent, and an absolute chronology that, on the contrary, establishes the age of an event indicating the years since its formation. Because the geological time is enormous with respect to the human, criteria different from those used by history have been adopted, since the latter can count on the direct testimonies of the men of the past.
The relative chronology is based on the knowledge acquired by stratigraphy and paleontology. The numerous information collected is studied following perfectly defined principles: the principle of actualism and the stratigraphic principles or criteria.
The principle of actualism states that all geological processes that occur today occurred in the same way and according to the same laws also in the past, only the extremely long time necessary for the completion of some geological processes prevents the man an easy interpretation of the same, since the capacity of direct observation of the human being is much more limited in time. According to this fundamental principle, the present is the key to the past. It was the British James Hutton (considered the father of modern geology), towards the end of the 1700s, and Charles Lyell, who collected this idea in his work Principles of Geology in the nineteenth century, the first to formulate such a principle.
The stratigraphic criteria are the following: criterion of superposition, by virtue of which, in a succession of sediments, those that occupy the lower positions are older than those that cover them, therefore a stratum older than the one that has superimposed and more recent than the precedents; This principle is also valid for lava flows, that is, the most recent ones are superimposed on the oldest ones.
It is important to indicate that sometimes the strata are not horizontal or have suffered deformations that have altered their original disposition; it is possible to differentiate the sedimentary structures (thanks to the sedimentology) that allow to recognize the base and the roof of the strata. The criterion or principle of correlation is that by which it is possible to attribute the same age to rocky strata, even very distant from each other, that contain the same fossils, remains of organisms equal and co-equal with each other; The correlation is made by comparing fossils of a series of strata of known age with those of unknown age. There is also a criterion that postulates that when a geological phenomenon disturbs a series of rocks, this process is evidently subsequent to the formation of the most recent disturbed rock and preceding the oldest disturbed rock (for example, in a series of rocks traversed by a fault it can be said that the fault is more recent than the younger rock traversed and older than the older rock not affected by the fault).
