The most essential part of an island arc is the volcanic center, consisting of a line of volcanic islands comprising volcanic and pyroclastic debris, forming a linear chain about 60-70 miles (100-110 km) above the subducting slab. In island arcs the volcanic rocks are generally of several different types called volcanic series. These include a tholeiitic series, consisting of tholeiitic basalt, andesite, and less common dacite. The calc-alkaline series has basalts rich in alumina, abundant andesite, dacite, and some rhyolite. A third series, the alkali series, includes a sodic group dominated by alkali olivine basalt, alkalic andesite, trachyite, and alkalic rhyolite. A rarer assemblage of volcanic rocks consists of the shoshonite group, comprising of shoshonite, latite, and leucite-bearing magmatic rocks.
The magmatic rocks in island and continental-margin arcs are built on a basement or substrate made of older oceanic crust, or on the edge of a continent. Some arcs change along strike, being built on the edge of a continent in one location and extending off into the ocean, such as the Aleutians, which are built on North American basement in the east and extend to an oceanic island arc in the west. A similar transition exists in the arc system in Indonesia that extends from Bali to West Papua (Irian Jaya) to sumatra. There is a general correlation between the type of magma that erupts in arc and the type of underlying crust. Arcs built on oceanic crust are dominated by tholeiitic-type volcanic series, whereas arcs built on thicker continental basement are more evolved and have more of the calc-alkaline and sho-shonitic series erupted in their volcanic centers.
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