What best describes an oceanic-oceanic boundary?

• A. A boundary where two continental plates collide, forming mountains.
• B. A boundary where two oceanic plates slide past each other.
• C. A boundary where plates move away from each other within a subduction zone.
• D. A convergent boundary where two oceanic plates collide, forming island arcs and deep ocean trenches.

Answer: (D)

At an oceanic-oceanic boundary the plates collide in a convergent setting and one oceanic plate is subducted beneath the other, creating a deep ocean trench at the boundary. As the subducted slab sinks and melts, magma rises to form volcanoes, producing volcanic island arcs. This combination of a trench plus volcanic island arcs is the hallmark of oceanic-oceanic convergence, along with associated earthquakes from the moving slabs.

For context, hitting a boundary where continental plates crash together forms mountain belts, not trenches or island arcs. If two oceanic plates slide past each other, the boundary is transform, with lateral motion and earthquakes but typically no large trenches or island arcs. Divergence, where plates move apart, occurs at mid-ocean ridges and builds new ocean floor, not subduction.