<p><strong>No, they are not necessarily volcanic.</strong></p>

<p>Both of these terms, <a href="http://en.wikipedia.org/wiki/Coral_reef#Types" rel="noreferrer">along with many others</a> like <em>fringing reef</em> and <em>barrier reef</em>, 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.)</p>

<ul>
<li><p><strong>Cay</strong> &nbsp; A low island, often of sand or coral. <a href="http://en.wiktionary.org/wiki/cay" rel="noreferrer">See Wiktionary.</a></p></li>
<li><p><strong>Atoll</strong> &nbsp; A ring-shaped coral reef. <a href="http://www.oxforddictionaries.com/definition/english/atoll" rel="noreferrer">See Oxford English Dictionary.</a></p></li>
</ul>

<p>The name <em>Cay</em> is sometimes applied more generally to an island or group of islands. In some cases, as in <a href="http://en.wikipedia.org/wiki/Cay_Sal_Bank" rel="noreferrer">Cay Sal Bank</a>, Bahamas, the feature is actually an atoll: a ring-shaped reef. Cay Sal Bank is one of the largest atolls in the world, and <a href="https://www.livingoceansfoundation.org/publication/khaled-bin-sultan-living-oceans-foundation-2011-coral-reef-assessment-cay-sal-bank-bahamas/" rel="noreferrer">did not form on a volcano</a>. </p>

<p>Charles Darwin hypothesized in his monograph <em><a href="http://en.wikipedia.org/wiki/The_Structure_and_Distribution_of_Coral_Reefs" rel="noreferrer">The structure and distribution of coral reefs</a></em> that atolls form when a fringing reef around a volcanic islands grows as the volcano subsides:</p>

<p><img src="https://i.sstatic.net/pM4eR.png" alt="An atoll forming on a volcano"></p>

<p>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 <a href="http://dx.doi.org/10.1029/GM043p0061" rel="noreferrer">Keating et al.</a>, 1987, in <em>Seamounts, Islands, and Atolls</em>, AGU), or by island subsidence due to dissolution.</p>

CC BY-SA 3.0
2015-04-24T14:16:25.047Z
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
CC BY-SA 3.0
2015-04-24T14:24:50.703Z
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.)
Matt Hall
CC BY-SA 3.0
2015-04-24T14:30:16.607Z
@Fred this is the original text : http://www.livingoceansfoundation.org/assets/2013/08/KSLOF_Cay_Sal_Bank_Assessments_final.pdf p. 3 - it is a font encoding problem on the web article which is not present on the original pdf
marsisalie
CC BY-SA 3.0
2015-04-24T14:03:26.960Z
2015-04-24T14:03:26.960Z
Matt Hall
<p>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?</p>

<geology><volcanology>
Are coral cays and coral atolls just volcanic islands?
Aurelius
# Are coral cays and coral atolls just volcanic islands?

(Asked by: username_0 on 2015-04-24)

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?

**Tags:** geology, volcanology


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## 1 answers

### Answer by username_1 on 2015-04-24. Score: 11 | Accepted answer

**No, they are not necessarily volcanic.**

Both of these terms, [along with many others](http://en.wikipedia.org/wiki/Coral_reef#Types) 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.)

* **Cay**   A low island, often of sand or coral. [See Wiktionary.](http://en.wiktionary.org/wiki/cay)
* **Atoll**   A ring-shaped coral reef. [See Oxford English Dictionary.](http://www.oxforddictionaries.com/definition/english/atoll)

The name *Cay* is sometimes applied more generally to an island or group of islands. In some cases, as in [Cay Sal Bank](http://en.wikipedia.org/wiki/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](https://www.livingoceansfoundation.org/publication/khaled-bin-sultan-living-oceans-foundation-2011-coral-reef-assessment-cay-sal-bank-bahamas/).

Charles Darwin hypothesized in his monograph *[The structure and distribution of coral reefs](http://en.wikipedia.org/wiki/The_Structure_and_Distribution_of_Coral_Reefs)* that atolls form when a fringing reef around a volcanic islands grows as the volcano subsides:

![An atoll forming on a volcano](https://i.sstatic.net/pM4eR.png)

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.](http://dx.doi.org/10.1029/GM043p0061), 1987, in *Seamounts, Islands, and Atolls*, AGU), or by island subsidence due to dissolution.

> **Comments:**
> **username_2**: 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.  
> **username_1**: 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.)  
> **username_3**: @username_2 this is the original text : http://www.livingoceansfoundation.org/assets/2013/08/KSLOF_Cay_Sal_Bank_Assessments_final.pdf p. 3 - it is a font encoding problem on the web article which is not present on the original pdf  

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