The average composition of the continental crust is andesitic, a composition that falls between that of basalt and rhyolite. Laboratory experiments show that partial melting of wet oceanic crust yields an andesitic magma. Remember that oceanic crust is dry, but after it forms it interacts with seawater, which fills cracks to several miles' (kilometers') depth. Also the sediments on top of the oceanic crust are full of water, but these are for the most part nonsubductable. Andesite forms above places where water is released from the subducted slabs, and it migrates into the mantle wedge above the subducting slab, forming water-rich magmas. These magmas then intrude the continental crust above, some forming volcanic andesites, others crystallizing as plutons of diorite at depth.
Continue reading here: Solidification Of magma
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