Volcanism associated with convergent plate boundaries may also develop where slabs of oceanic lithosphere are sub ducted under continental lithosphere to produce a continental volcanic arc(Figure 9.36E). The mechanisms that generate We these mantle-derived magmas are essentially the same as those bu operating at island arcs. The major difference is that conti- Ki nental crust is much thicker and is composed of rocks having ua a higher silica content than oceanic crust. Hence, through the th assimilation of silica-rich crustal rocks, plus extensive mag- in matic differentiation, a mantle-derived magma may become Ca highly evolved as it rises through continental crust. Stated yo another way, the magmas generated in the mantle may change of from a comparatively dry, fluid basaltic magma to a viscous andesitic or rhyolitic magma having a high concentration of o volatiles asit moves up through the continental crust. The vol- a canic chain of the Andes Mountains along the western margin A of South America is perhaps the best example of a mature con- i tinental volcanic arc. Since the Pacific basin is essentially bordered by convergent r plate boundaries and associated subduction zones, it is easy to