Geography
The Mechanisms Behind the Formation of Atolls and Guyots
Quick fact
Charles Darwin first proposed the subsidence theory of atoll formation in 1842, correctly explaining how a fringing reef around a volcanic island becomes a barrier reef and finally an atoll as the island sinks.
Why this is interesting
You've seen pictures of tropical atolls—perfect rings of coral around a blue lagoon. But did you know the same process that creates these rings also produces flat-topped underwater mountains called guyots?
Read the full explanation
Understanding The Mechanisms Behind the Formation of Atolls and Guyots
Imagine a volcanic island born from the ocean floor. Over millions of years, the island cools and slowly sinks (subsides) under its own weight. Meanwhile, coral reefs grow upward, staying near the sea surface. Initially, corals form a fringing reef hugging the shore. As the island subsides, the inner part of the reef becomes separated from the land by a lagoon, creating a barrier reef. Eventually, the island disappears entirely below the surface, leaving only the ring of coral—an atoll. If the island subsides faster than corals can grow, or if volcanic activity stops, the top of the island may erode flat by wave action before sinking deeper, forming a flat-topped seamount called a guyot. Guyots are thus remnants of ancient volcanic islands that never developed a full atoll structure.
A deeper explanation
The key mechanism is the balance between vertical crustal movement (subsidence) and the rate of coral growth. Atolls form when subsidence is slow enough that reef-building corals can keep pace, maintaining their photic zone habitat. Darwin's theory was later confirmed by drilling through atolls and finding volcanic rock beneath thousands of feet of coral limestone. Guyots, on the other hand, represent islands that were eroded to sea level—often during periods of lower sea level—and then subsided below the surface before coral growth could rebuild a reef cap. Their flat tops are ancient wave-cut platforms, now submerged. Plate tectonics drives the subsidence: oceanic crust cools and thickens as it moves away from mid-ocean ridges, causing the entire volcanic edifice to sink. This process explains the chain of atolls and guyots in the Pacific, such as the Emperor Seamounts and the Marshall Islands.