What are island arcs primarily composed of?

Island arcs are primarily composed of basalt due to their formation in subduction zones, where oceanic tectonic plates converge. When an oceanic plate sinks beneath another tectonic plate, it leads to the melting of the subducting plate and the overlying mantle, creating magma that is rich in iron and magnesium. This magma typically rises to the surface, resulting in the formation of volcanic islands that are largely made up of basaltic rock. The characteristic properties of basalt, such as its dark color and mineral composition, are indicative of the volcanic activity associated with these geological features.

Granite, limestone, and shale are associated with different geological processes and environments. Granite, for instance, is a plutonic rock formed from the slow crystallization of magma beneath the Earth's surface, typically associated with continental crust rather than oceanic settings. Limestone is primarily formed from the accumulation of marine organisms and is not typically associated with volcanic activity. Shale, a fine-grained sedimentary rock, forms from the compaction of silt and clay and is also distinct from the processes that lead to the formation of island arcs.