Matthew
MatthewHi Matthew,
It's kind of sad if your books don't have much information about andesitic volcanism. It is one of the most common types that there is. For starters "andesite" is named for the Andes mountains in S. America since many of the volcanoes there erupt andesite. Andesite is a type of lava that is more viscous than basalt but not as viscous as dacite or rhyolite. It is usually a dark gray color and often has large phenocrysts of feldspars, pyroxenes, and amphiboles. These indicate that the magma has had a chance to cool somewhat between the time of its formation and eruption. Where does the actual andesite come from? Some geologists argue that there is actual "primative" andesite formed in the Earth's mantle, but that is not generally accepted. The very strong association of andesite with subduction-zone volcanism argues that somehow subduction is related to the production of this magma. What may happen is this: the plate that is being subducted is forced into hotter and hotter regions of the mantle. This causes the volatile materials (mainly water that is trapped in sediments and chemically-altered basalts) to be liberated. These gases then migrate upwards through the plate that is over-riding. The addition of gases is known to initiate melting (to form magma) and this magma, combined with the gases themselves as well as with parts of the shallowest parts of the overlying plate, all mix together to form andesite. I realize that this is not a very clear picture, but not everybody agrees on what the exact picture is.
As far as volcanic activity is concerned, andesitic eruptions are moderately explosive and should be watched from a distance. Many times thick, slow-moving lava flows are produced, but andesitic pyroclastic flows are not uncommon either. The volcanoes are usually quite steep and cone-shaped.
Hopefully this will help to fill in some of the gaps in your books.
Sincerely,
Scott Rowland
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