andesiittinen

Andesiittinen is a Finnish term used in geology to describe rocks, magmas, and volcanic activity with an intermediate silica content, between basaltic and rhyolitic compositions. In technical usage, andesitic rocks typically contain about 57–63 percent SiO2, though exact boundaries can vary among sources. The adjective andesiittinen applies to both the rocks themselves (andesite) and the magmas that crystallize to form them.

Formation and occurrence: Andesitic magmas mainly form in subduction-zone settings. Fluids released from the subducting oceanic crust lower the melting point of the overlying mantle and crust, producing intermediate-composition magmas through partial melting, ambiguous fractional crystallization, and crustal assimilation. This process is central to many volcanic arcs.

Petrology and textures: Common minerals in andesitic rocks include plagioclase (often calcium-rich) together with pyroxene and/or amphibole, and magnetite may be present. Andesites are frequently porphyritic, featuring larger phenocrysts in a finer-grained groundmass. The term is used for volcanic rocks and, less commonly, to describe plutonic equivalents such as diorite to granodiorite in some contexts.

Geological significance: Andesitic volcanism characterizes many convergent plate boundaries and is associated with major volcanic arcs, including the Andes, the Cascades, and many island arcs around the Pacific. The relatively higher magma viscosity compared with basalt tends to produce more explosive eruptions, pyroclastic activity, and related hazards.

Notes: The label andesiittinen is descriptive of composition; precise chemical boundaries vary, and some rocks near the basalt–andesite–dacite sometimes fall outside strict categories.