andesite classification

Andesite is field O2 in the TAS classification. Andesite is an aphanitic (fine-grained) to porphyritic (coarse-grained) igneous rock that is intermediate in its content of silica and low in alkali metals. It has less than 20% quartz and 10% feldspathoid by volume, with at least 65% of the feldspar in the rock consisting of plagioclase.

Classification By Mineral Abundance. Igneous rocks can be divided into four categories based on their chemical composition: felsic, intermediate, mafic, and ultramafic. The diagram of Bowen's reaction series ( Figure 4.6) shows that differences in chemical composition correspond to differences in the types of minerals within an igneous rock.

Classification. According to modal composition projected within the QAPF discrimination diagram for volcanic rock (Streckeisen, 1978), the andesite project within basalt field. However, the andezit has higher SiO2 content (> 52 wt. %) compared to that in basalt with less than 52 wt. % SiO2.

Andesite is a fine crystalline intermediate extrusive rock. It is commonly grey and porphyritic. It can be found in the Andes Mountains and in some island arcs (see Chapter 2). It is the fine grained compositional equivalent of diorite. Mafic Composition: Gabbro

TAS classification of fine-grained volcanic rocks with andesite just after basalt andesite. More classifications Based on silica content, andesite falls into the intermediate category, i.e., between acidic/felsic and basic/mafic.

Diorite and andesite likewise refer to intrusive and extrusive intermediate rock. Gabbro and basalt are the intrusive and extrusive names for mafic igneous rock. Peridotite is ultramafic and intrusive and komatiite as the fine-grained extrusive equivalent.

Andesite: Typically found in volcanic arcs associated with subduction zones. Plutonic or Intrusive Igneous Rocks. Plutonic or intrusive igneous rocks form when magma cools slowly beneath the Earth's surface, leading to the formation of large crystals. Examples of intrusive igneous rocks include:

The composition of andesite is classified as "intermediate" among igneous rocks, implying that its silica content is in the range of 52-63 percent. Its texture ranges from aphanitic to porphyritic. [1] The mineral assembly is typically dominated by plagioclase, with pyroxene and/or hornblende.

There are three subdivisions of this rock family: the quartz-bearing andesites, or dacites, sometimes considered to be a separate family; the hornblende- and biotite-andesites; and the pyroxene-andesites. The dacites ( q.v.) contain primary quartz, which may appear in small blebs or crystals or only as minute interstitial grains in the groundmass.

The formal definition of andesite is problematic. Many authors have classified igneous rocks based upon their chemical and mineralogical compositions. However, none of these classifications are in perfect agreement. For a fine-grained rock like andesite, these classifications are impossible to use precisely when in the field or the classroom.