Andesitic Magma

The average composition of the continental crust is andesitic, or somewhere between the composition of basalt and rhyolite. Laboratory experiments show that partial melting of wet oceanic crust yields an andesitic magma. Most andesites today are erupted along continental margin convergent boundaries where a slab of oceanic crust is subducted beneath the continent. 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 nonsubduc-table. Andesite forms above places where water is released from the subducted slabs, and it migrates up 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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