How is porphyritic andesite formed?

Porphyritic texture — andesite: This is an extrusive igneous rock. The magma from which it formed cooled slowly for a while deep below the surface (forming the large crystals), then finished cooling very quickly when it was ejected at the surface, forming the fine-grained groundmass.

What is porphyritic andesite composition?

Andesite most commonly denotes fine-grained, usually porphyritic rocks; in composition these correspond roughly to the intrusive igneous rock diorite and consist essentially of andesine (a plagioclase feldspar) and one or more ferromagnesian minerals, such as pyroxene or biotite. …

How are andesites formed?

Andesites are typical rocks of island arcs and may either form by fractional crystallization processes or by mixing between a mafic and a felsic magma.

How are Phenocrysts formed?

Porphyrys are formed by a two-stage cooling of rising magma. Second, the magma cools rapidly at shallower depths having been injected upward or extruded by a volcano, allowing for formation of small crystals in the groundmass.

Is porphyritic andesite intermediate?

Andesites are typically medium gray and porphyritic, with phenocrysts of white to light gray plagioclase and/or black augite, hornblende, and/or biotite. Some andesites are reddish and some are vesicular. If you find quartz, you are probably looking at a rhyolite.

How strong is diorite?

Diorite, being a mix of minerals, varies in its properties, but generally it is hard (its dominant minerals having hardnesses of around 6 on the Mohs scale). This makes it difficult to carve and work with, but also allows it to be worked finely and take a high polish, and to provide a durable finished work.

What kind of rock is porphyritic andesite?

What is porphyritic andesite? Andesite ( /ˈænd?sa?t/ or /ˈænd?za?t/) is an extrusive igneous volcanic rock of intermediate composition, with aphanitic to porphyritic texture. Characteristic of subduction zones, andesite represents the dominant rock type in island arcs.

How are andesite and diorite igneous rocks formed?

Diorite is a coarse-grained igneous rock that forms when the magma remains below Earth’s surface and cools slowly. Slow cooling facilitates the growth of large mineral crystals in the rock. Andesite is a fine-grained rock that forms when the magma erupts onto the surface and crystallizes quickly.

How are porphyritic rocks formed in the crust?

Porphyritic rocks are formed when a column of rising magma is cooled in two stages. In the first stage, the magma is cooled slowly deep in the crust, creating the large crystal grains, with a diameter of 2mm or more.

Where does andesite form in the Earth’s crust?

Andesite can also form away from the subduction zone environment. For example, it can form at ocean ridges and oceanic hot spots from partial melting of basaltic rocks. It can also form during eruptions at continental plate interiors where deep-source magma melts continental crust or mixes with continental magmas.