What are reefs built by corals and other organisms on sinking volcano islands called?

Reefs built by corals and other organisms on sinking volcanic islands are known as atolls. These formations occur when a volcanic island gradually sinks beneath the sea surface, typically due to subsidence, while coral growth continues around the island's perimeter. Over time, as the island erodes and sinks, the coral reef maintains its position near the surface, eventually forming a ring-shaped structure, or atoll, that encloses a lagoon.

Atolls are critical marine ecosystems that provide habitats for a diverse range of marine life. The formation process underscores the interaction between geological processes (like volcanic activity and subsidence) and biological processes (like coral growth) that shape these unique features of the ocean. The term specifically refers to those reef formations that develop in this manner, distinguishing them from other oceanic features, such as oceanic ridges, seamounts, and deep-ocean trenches, which have different origins and characteristics.