How were the Andes formed?

• A. Subduction of oceanic crust beneath the continental crust
• B. Collision of oceanic and continental crust
• C. Uplift from mantle plume
• D. Accretion of volcanic arcs

Answer: (B)

Andes formation hinges on a subduction process at a plate boundary. The oceanic Nazca plate sinks beneath the South American continental plate, which compresses and thickens the margin and pushes it upward to create a long, high mountain belt. Water released from the subducting slab melts mantle rocks above it, producing magma that feeds the volcanic arc along the Andes. This combination of crustal shortening and magmatism explains the continuous, high mountain range and its volcanic activity. The other options point to different tectonic ideas—hotspot uplift from a mantle plume or adding pieces of crust by arc accretion—neither of which explains the sustained subduction-driven uplift that forms the Andes. Describing the process as a “collision” can be colloquial, but the precise mechanism is subduction of oceanic crust beneath continental crust.