félmasszív

Félmasszív is a term used in geology, particularly in petrology, to describe igneous rocks that have cooled at intermediate depths within the Earth's crust. These rocks exhibit characteristics that are a blend between those of extrusive (volcanic) rocks, which cool quickly at the surface, and intrusive (plutonic) rocks, which cool slowly deep within the Earth. The cooling rate for félmasszív rocks is moderate, leading to a texture that is typically porphyritic. This means that larger, well-formed crystals (phenocrysts) are embedded within a finer-grained groundmass. The groundmass itself can be a mixture of very fine-grained crystals and even some glassy material, indicating a slightly faster cooling than would be typical for a purely plutonic rock. Examples of félmasszív rocks include diorite, granodiorite, and some andesites and dacites. Their formation often occurs in environments such as the margins of larger intrusions or in volcanic settings where magma rises but does not erupt immediately. The study of félmasszív rocks provides valuable insights into the processes of magma ascent, emplacement, and cooling within the Earth's crust. Their mineral composition and textural features help geologists reconstruct the thermal and pressure conditions that existed during their formation.