Andesite
Intermediate volcanic rock (plagioclase + pyroxene/amphibole)
A fine-grained, intermediate volcanic rock common at subduction-zone volcanoes, between basalt and rhyolite in composition.
- Mohs hardness
- 6-7
- Color
- grey to greyish-brown, sometimes purplish or reddish
- Type
- igneous
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Overview
Andesite is a fine-grained volcanic rock of intermediate composition, sitting between basalt and rhyolite in silica content. It is composed mainly of plagioclase feldspar with pyroxene, amphibole, or biotite, giving it a grey to greyish-brown color.
It is the extrusive equivalent of diorite and frequently shows a porphyritic texture, with visible feldspar or dark mineral crystals set in a fine groundmass. Named after the Andes Mountains, it is the characteristic rock of subduction-zone volcanoes.
Andesite makes up much of the volcanic rock in continental arcs around the Pacific Ring of Fire.
Formation & geology
Andesite forms from intermediate magma erupted at the surface, typically at subduction zones where an oceanic plate sinks beneath another plate. Water released from the descending slab causes partial melting of the overlying mantle and crust, producing magma of intermediate silica content.
This magma erupts from stratovolcanoes and cools relatively quickly, yielding a fine groundmass that often encloses earlier-formed phenocrysts. Andesite dominates the volcanic arcs of the Pacific Ring of Fire, including the Andes, Cascades, and Japan.
How to identify it
Identify andesite by its fine-grained, medium-grey to brownish groundmass, often with visible white plagioclase or dark pyroxene/amphibole phenocrysts (porphyritic texture). Hardness is about 6-7.
It is lighter than basalt and darker than rhyolite, with an intermediate appearance. Unlike diorite, its groundmass is too fine to see individual grains throughout.
Distinguish it from basalt (darker, denser, often with gas vesicles) and from rhyolite (paler, more silica-rich). The grey color with scattered feldspar phenocrysts is characteristic.
Uses & significance
Andesite is used primarily as crushed stone for road construction, concrete aggregate, and railway ballast. As a durable building stone it has been used for centuries; notably, the Inca built structures from andesite, and it is used for paving and facing stone.
Its toughness makes it suitable for retaining walls and landscaping. It has little use in jewelry.
Andesite is also of major scientific importance for understanding subduction-zone volcanism and the growth of continental crust.
Frequently asked questions
What type of rock is andesite?
It is a fine-grained, intermediate volcanic (extrusive) rock, between basalt and rhyolite in silica content, and the volcanic equivalent of diorite.
Where does andesite form?
It forms at subduction-zone volcanoes, such as the Andes, Cascades, and the wider Pacific Ring of Fire.
How is andesite different from basalt?
Andesite has more silica and is lighter grey, while basalt is darker, denser, and richer in iron and magnesium.
Why is it called andesite?
It is named after the Andes Mountains of South America, where this volcanic rock is abundant.
Andesite guides
In-depth guides for identifying, valuing, and understanding Andesite.
Andesite identified by the community
Real specimens identified with Rock Identifier.
