What are the primary types of rocks in the volcanic arc, and how do they differ chemically from rocks formed at spreading centers?

• A. Ultramafic rocks with low silica.
• B. Mafic basalts typical at spreading centers.
• C. Andesitic to rhyolitic rocks with intermediate to high silica; they contrast with mafic basalts at spreading centers.
• D. Sedimentary rocks formed by deposition.

Answer: (C)

In volcanic arcs, magmas form mainly from fluids released by the subducting slab, which induces melting of the overlying mantle. The melts that result are silica-rich and evolve into intermediate to felsic compositions, producing rocks such as andesite to rhyolite. This silica enrichment and the potential for crustal assimilation or fractional crystallization give arc rocks a noticeably higher silica content than the basalts typical of spreading centers. Rocks at spreading centers come from direct mantle melting and are mafic with lower silica. So the best description is that the primary arc rocks are andesitic to rhyolitic with intermediate to high silica, unlike the mafic basalts at spreading centers.