ROCCE. — 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 natural molten masses called magmas (igneous or eruptive rocks), from the deposition of materials in suspension or in solution (sedimentary rocks), or through the transformation of igneous or sedimentary rocks (metamorphic rocks).
Igneous rocks are distinguished, on the basis of their conditions of emplacement and formation, into intrusive rocks (plutonic, abyssal), hypabyssal rocks, mostly vein rocks, and volcanic or effusive rocks. The more recent classifications of igneous rocks, due to A. Johannsen, E. Tröger, P. Niggli, and C. Andreatta, are based on the quantitative criterion of the percentage of the typical minerals present.
Sedimentary rocks may be formed by the deposition of fragments of pre-existing rocks, by the deposition of dissolved substances, or by the accumulation of inorganic remains of organisms; thus there are clastic rocks, or rocks of mechanical origin, rocks of chemical origin, and organogenic rocks.
The complex of processes that bring about the mechanical fragmentation of rocks and the chemical transformations from which solutions or colloidal suspensions originate is called weathering: it is due largely to atmospheric agents, but in many cases the materials destined to form sedimentary rocks may also be produced by endogenous processes.
Metamorphic rocks, also called crystalline schists because of their predominant structural pattern, represent the product of the transformation and recrystallization, with a possible change of phase, of igneous (orthoschists) or sedimentary material (paraschists) as a result of altered chemical and physical conditions. Temperature, pressure, and mechanical action contribute to the transformation.
In close relation to the genetic process, rocks display characteristic structures. Thus intrusive rocks generally have a granular structure; effusive rocks may have a porphyritic or a glassy structure; sedimentary rocks may have a clastic, granular, saccharoidal, or concretionary structure. Metamorphic rocks exhibit a great variety of structures, the term here also including the meaning of “texture”: characteristic are the granoblastic structure and the parallel structures with oriented elements of metamorphic rocks produced by dynamic metamorphism, or tectonites.
(by courtesy of the hand of G. Pallani)
ROCCHETTO - Pleated spool with lace.
Nuova 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, Berlino 1939.