Andesite


Andesite is named after the Andes mountains in South America. 

It is an extrusive igneous rock that forms from explosive stratovolcanoes.





This means the rock is reasonably silica-rich and the magma hasn't contained much volatiles.

It can be fine grained, porphyritic or amyglaloidal. 

 andesite
 An Andesite volcano.
By brewbooks via Flickr.com

Its minerals are mainly andesine and oligoclase, but some biotite or pyroxene also tends to be present.

 polished green andesite
 
By Miguel Vera León via Flickr.com

It does not contain any quartz, unlike the closely related rhyolite.

 light brown andesite 
 
By James Brooks via Flickr.com

If a magma of the same composition would form an intrusive igneous rock, it would be diorite.

 andesite volcanic rock 
 By brewbooks via Flickr.com

Andesitic volcanoes tend to be on or near subduction zones, something that helps to find ancient subduction zones.

 andesite rock 
 By Ian Geoffrey Stimpson via Flickr.com 

Some more known of such volcanoes are Mount Fuji in Japan, and Krakatau in Indonesia.













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