Andesite: Definition, Composition & Uses

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  • 0:00 Definition of Andesite
  • 1:02 Andesite Composition &…
  • 2:02 Uses of Andesite
  • 2:49 Lesson Summary
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Lesson Transcript
Instructor: Heather Pier

Heather has taught high school and college science courses, and has a master's degree in geography-climatology.

Learn about andesite, an extrusive igneous rock that forms as a result of volcanic activity. Andesite is fine-grained and intermediate in color and chemistry. It has some uses in the construction industry.

Definition of Andesite

Andesite is a fine-grained extrusive igneous rock, forming when volcanic magmas erupt and crystallize outside of the volcano. An extrusive igneous rock is a rock that was formed at the volcanic surface of the earth. Like many things in geology, andesite is named for where it is found: the Andes Mountains of South America. But unlike many things in geology, which are of one extreme or the other, andesite is comfortably entrenched in the middle ground of the igneous rock world.

It is considered intermediate in terms of its color (shades of gray and brown) and its chemistry (around 50 to 60 percent silica), and generally uniformly fine-grained with some slightly larger crystals called phenocrysts within it. Its magmas aren't highly explosive or highly viscous. It isn't as dark as basalt or as silica-rich as rhyolite, and it doesn't have extreme variations in grain size that some pumices have. Andesite is a comfortably average igneous rock.

Andesite Composition and Formation

Andesites form from intermediate-grade magmas, on the outside of volcanoes. This quick cooling after exiting the volcanic vent leads to its fine-grained texture, as the crystal grains don't have the necessary time to grow into larger crystals that an intrusively formed rock would have. They form in subduction zones, where one tectonic plate is sliding under another, as is the case with the Andes Mountains, and also along volcanic island arcs.

In terms of magma composition, they are chemically similar to diorite, their intrusive cousin. They contain a mix of darker colored minerals, such as pyroxene and olivine, and lighter colored minerals, including plagioclase feldspar and roughly 50 to 60 percent silica. Andesite will often contain phenocrysts, which are mineral crystals that are slightly larger in size than the overall grain size of the rock as a whole. These form when certain mineral grains have slightly more time inside the volcano to cool and crystallize before being ejected.

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