Molten material is, so to speak, sweated off the subducting slab and begins to rise. A look at Bowen's reaction series shows that the last-formed silicates, quartz, K-feldspar, muscovite, etc., would be the first to melt. Because a mafic rock and an admixture of sediment rich in quartz, etc., are being melted, the resulting melt is a little less mafic and an andesitic melt is created.

This melt rises to create intermediate lavas in composite type volcanoes that occur near the subduction zone. One place where this happens is along the west coast of South America, where the Avenue of the Volcanoes running along the length of the Andes Mountains is a result. The Andes are part of the "Ring of Fire" that surrounds the Pacific.

Click here to go back to Chapter 1