What is the difference between basalt and andesite?
http://en.wikipedia.org/wiki/Andesite Andesite is formed at accretionary plate margins. Intermediate volcanic rocks are created via several processes: Dehydration melting of peridotite and fractional crystallization Melting of subducted slab containing sediments Magma mixing between felsic rhyolitic and mafic basaltic magmas in an intermediate reservoir prior to emplacement or eruption.  Via fractional crystallisation Andesitic magma in island arc regions (i.e. active oceanic margins) comes from the interplay of the subducting plate and the mantle wedge, the part of the overriding plate above the subducted plate. Water in the subducted oceanic crust ‘boils off’ from the slab by dehydration of hydrated minerals such as amphibole, zeolites, chlorite etc, which are formed in oceanic crust during hydrothermal circulation at the mid-ocean-ridge spreading centre. As these minerals are subjected to Barrovian Facies Seque