Andesite

Andesite is the name of a family of fine-grained, extrusive igneous rocks that are usually light to dark gray in color. They have a mineral composition that is intermediate between granite and basalt. Andesite is a rock typically found in volcanoes above convergent plate boundaries between continental and oceanic plates

An Extrusive Igneous Rock

Andesite is typically found in lava flows produced by stratovolcanoes above subduction zones. Because these lavas cooled rapidly at the surface, they are generally composed of small crystals. The mineral grains are usually so small that they cannot be seen clearly without the use of a hand lens or other magnifying device.

Some specimens that cooled rapidly contain a significant amount of glass, while others that formed from gas-charged lavas have a vesicular or amygdaloidal texture.

Mineral Composition

Andesite often weathers to various shades of brown. In the field, weathered specimens must be broken to properly see their color and their mineral composition. Classroom specimens usually do not require breakage.

Andesite is rich in plagioclase feldspar and amphibole minerals. Quartz and pyroxene minerals may be absent or present in small quantities. Small amounts of mica will be present as biotite or muscovite. Andesite usually does not contain olivine. The accompanying chart titled "Generalized Composition Ranges of Common Igneous Rocks" illustrates the mineral composition of andesite.

Where Does Andesite Form?

Andesite and diorite are common rocks of the continental crust above subduction zones. They generally form after an oceanic plate melts during its descent into the subduction zone to produce a source of magma.

Source: geology.com