All subduction zones have, at some distance in from the edge of the upper plate, arcs or chains of composite cone volcanoes. The subducting plate, as it goes down deep into the mantle, releases water. This changes the chemistry of the already hot rocks in the mantle and causes them to melt, forming magma. The magma is less dense than the solid rocks around it, so it rises upward, culminating in volcanic eruptions at the earth’s surface.

The volcanic arc at an ocean-continent subduction zone is not only a chain of volcanoes. The stress of plate convergence compresses the crust there, causing it to thicken through a combination of folds and thrust faults. Igneous intrusions and volcanic eruptions also thicken the crust there. Deep within the crust, the igneous intrusions solidify into batholiths of rocks such as granite, and the pre-existing rocks that are intruded by the batholiths are regionally metamorphosed into new rocks. The result is a high mountain range with granitic and metamorphic rock at its core, folded and faulted sedimentary and volcanic around its margins, and a chain of composite cone volcanoes distributed along the crest of the range.

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