Why does andesite have small crystals

Nevertheless, you can still see the black hornblende crystals set against the fine-grained gray groundmass. This is another andesite like the one above: gray groundmass with black hornblendes. E-mail C. Jones with comments or corrections.

Free courses. All content. Andesite Updated Thursday, 28th September A brief description of the nature of andesite. Copyright: The Open University How was it formed? Andesite is the fine-grained extrusive equivalent of diorite.

Get closer to geology. Geology toolkit Discover the unique landscape of the British Isles with our interactive Geology Toolkit, featuring a geology timeline, rock analyser, rock cycle, landscape features and safety tips. Take part now Geology toolkit. Activity Level: 1 Introductory.

Copyright information. Publication details Originally published : Wednesday, 27th September Last updated on : Thursday, 28th September 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.

Igneous rock composition chart: This chart shows that andesite is typically composed of plagioclase, amphiboles, and micas; sometimes with minor amounts of pyroxenes, quartz, or orthoclase. 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.

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. Stratovolcanoes: Pavlof Volcano right and Pavlof Sister Volcano left are a pair of symmetrical stratovolcanoes built of andesite flows and tephra on the Alaska Peninsula.

Pavlof Volcano is one of the most active volcanoes in Alaska. Photo by T. Miller, United States Geological Survey. 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. 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. If you are interested, there are some links below to help you better understand how andesite relates to other rock types.

Rock Primer. Igneous Rock Site Introduction to the Beginner ,. Exercise 14 - Tempe Butte.