Andesite (pronounced: ann-da-site)

Andesite is a fine-grained, extrusive igneous rock. It is intermediate becuase it contains some minerals that are common to rhyolite, and some common to basalt. It is always aphanitic and gray. It can look very much like basalt to the unaided eye, but it is usually less dark or greenish in color. Andesite magma is viscous and forms thick lava flows or domes.

There are active Holocene volcanoes in Iceland that have erupted andesitic material, in particular Hekla. Most of the pumice ejected during recent eruptions has been andesitic (for example 1947-48, 1970, 1980, and 1991). During the Tertiary Period and Pleistocene Epoch andesite was extruded from central volcanoes. Andesite is common in many other parts of the world especially, in "the ring of fire" surrounding the Pacific. Here, it most commonly occurs as an oceanic plate subducted beneath another oceanic plate or a continental plate.

Information provided by Patrick M. Colgan, Northeastern University
http://www.casdn.neu.edu