ROCKS. - This is the name given to aggregates of one or more minerals, forming part of the lithosphere as independent geological units.
They may originate either from the consolidation of molten natural masses called magmas (eruptive or igneous rocks) or from the deposition of materials in suspension or in solution (sedimentary rocks) or through the transformation of eruptive or sedimentary rocks (metamorphic rocks).
Eruptive rocks are distinguished, on the basis of their conditions of emplacement and formation, into intrusive rocks (plutonic, abyssal), hypoabyssal rocks, for the most part vein rocks, and volcanic or effusive rocks. The most modern classifications of eruptive rocks, due to A. Johannsen, E. Tröger, P. Niggli, C. Andreatta, are based on the quantitative criterion of the percentage of the typical minerals present.
Sedimentary rocks may form through the deposition of fragments of pre-existing rocks or through the deposition of dissolved substances or through the accumulation of inorganic remains of organisms; thus we have clastic rocks or rocks of mechanical origin, rocks of chemical origin and organogenic rocks.
The complex of the actions that determine the mechanical fragmentation of rocks and the chemical transformations from which solutions or colloidal suspensions originate is called weathering: it is due in large part to atmospheric agents, but in many cases the materials that are destined to form sedimentary rocks may also form through endogenous actions.
Metamorphic rocks, also called crystalline schists because of their prevailing structural pattern, represent the product of the transformation and recrystallization, with possible change of phase, of eruptive material (orthoschists) or sedimentary material (paraschists) owing to changed chemical-physical conditions. Temperature, pressure and mechanical actions contribute to the transformation.
In close relation to the genetic process, rocks present characteristic structures. Thus intrusive rocks mostly present a granular structure; effusive rocks may present a porphyritic or vitreous structure; sedimentary rocks a clastic, granular, saccharoidal or concretionary structure. Metamorphic rocks offer a great variety of structures, including within this term also the meaning of "fabric": the granoblastic one is typical, as are the parallel structures with oriented elements of metamorphic rocks produced by dynamic metamorphism, or tectonites.
Bibliography4 works
New York 1933; A. Johannsen, A descriptive petrographie of the Igneous Rocks, Chicago 1937; F. W. Barth - C. W. Correns - P. Eskola, Die Entstehung der Gesteine, Berlin 1939.