Discussion
Which one of the following rock combinations is characteristic of volcanic arcs?
Answer: A
The combination characteristic of volcanic arcs is basalt-andesite-dacite.
Arc magmas form when water released from the subducting slab lowers the melting point of the overlying mantle wedge. They belong mainly to the calc-alkaline series, whose typical volcanic suite runs from basalt through andesite to dacite, with andesite the hallmark rock of arcs.
Basalt-trachyte-rhyolite is an alkaline or bimodal suite typical of continental rifts and ocean islands. Andesite-syenite-dacite mixes a plutonic rock, syenite, into a volcanic series and is not a natural suite. Trachybasalt-basaltic andesite-andesite is a potassic, mildly alkaline trend, more typical of intraplate or back-arc settings than of the main arc.
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