Curving chain of volcanic islands and seamounts almost always found paralleling the concave edge of a trench.

• A. Island Arc
• B. Hydrothermal Vent
• C. Seamount
• D. Shelf Break

Answer: (A)

That pattern reflects the formation of an island arc at a subduction zone. When an oceanic plate dives beneath another plate, it melts in the mantle and the magma rises to the surface on the overriding plate, creating a line of volcanoes. These volcanoes build into a curved chain that runs roughly parallel to the trench, following the geometry of the plate boundary. The concave edge of the trench marks where the subducting plate is dipping, so the volcanic arc curves to mirror that boundary.

Hydrothermal vents occur along ridges and hot spots along the seafloor and do not form long, continuous chains of islands. Seamounts are underwater mountains that may originate from volcanoes but aren’t organized into a curved arc parallel to a trench. A shelf break is a continental feature at the edge of the continental shelf, not a volcanic arc.