# Are coral cays and coral atolls just volcanic islands?

Are coral cays and coral atolls just a different type of volcanic island? When I hear: "volcanic island" I think at places like Moorea or Hawaii, but ultimately, coral cays and coral atolls lay their foundation on the residuals of a volcano, right?

No, they are not necessarily volcanic.

Both of these terms, along with many others like fringing reef and barrier reef, are just morphologic — to do with shape. They tend not to imply anything genetic — to do with origin. (The separation of description and interpretation is an important principle in observational sciences like geoscience.)

The name Cay is sometimes applied more generally to an island or group of islands. In some cases, as in Cay Sal Bank, Bahamas, the feature is actually an atoll: a ring-shaped reef. Cay Sal Bank is one of the largest atolls in the world, and did not form on a volcano.

Charles Darwin hypothesized in his monograph The structure and distribution of coral reefs that atolls form when a fringing reef around a volcanic islands grows as the volcano subsides:

Indeed, Darwin turned out to be correct that atolls often have a volcanic origin, but Cay Sal Bank is a counter-example. I think it's fair to say that most people would assume that origin though, and lots of Internet definitions , so it's notable if an atoll formed some other way, e.g. on a non-volcanic seamount (see Keating et al., 1987, in Seamounts, Islands, and Atolls, AGU), or by island subsidence due to dissolution.

• just a query about something in your reference for the Cal Sal Bank. It states "an area of approximately 122,000 kmÇ ". Do you know what the unit kmÇ stands for? This is the first time I've seen it. The C has a cedilla under it. – Fred Apr 24 '15 at 14:16
• That has to be some weird character encoding bug for km$^2$ I think. (Note that it's not the area of Cay Sal Bank, but of the entire archipelago.) – kwinkunks Apr 24 '15 at 14:24
• @Fred this is the original text : livingoceansfoundation.org/assets/2013/08/… p. 3 - it is a font encoding problem on the web article which is not present on the original pdf – fre0n Apr 24 '15 at 14:30