<p>its the same way atolls form, sea level changes and tectonic plates move while the corals keep growing up as the land sinks. Any spot high enough for coral to survive can slowly grow in this way. they can end up hundreds of meters above the sea floor by the slow gradual process of growing coral.</p>
<p>I find a simple <a href="https://www.youtube.com/watch?v=vrD1O5hex6Y" rel="nofollow noreferrer">animation</a> makes it easier to follow.</p>
<p>Cays are just trapped sand in the coral reef, the sand can come from the erosion of nearby  islands, volcanic, or even erosion of the coral itself. This is why most cays occur near other land masses or in volcanically active areas.</p>
<p>Just as land can sink it can also rise, pushing those deposits high above the waterline, note these cays are very rare. Also note the parts far above the water are dead ancient corals not living ones, although there may be leaving coral around the base. there are ancient fossil corals in the Himalayas after all.</p>

CC BY-SA 4.0
2020-11-05T03:34:35.620Z
2020-11-05T03:40:44.340Z
2020-11-05T03:40:44.340Z
John
<p>From reading about <a href="https://en.wikipedia.org/wiki/Cay" rel="nofollow noreferrer">Cays on Wikipedia</a> it seems they are formed from coral reefs covered with sand or gravel.  There are however some cays scattered around the world which are hundreds of feet off of sea level at high tide.  How did these cays form, or are they mis-classified islands?</p>

CC BY-SA 4.0
2020-11-03T00:02:35.653Z
The Virgin Islands are volcanic cones that haven't eroded away yet. They're the "before" picture in [this animation](https://oceanservice.noaa.gov/education/tutorial_corals/media/supp_coral04a.html).
Spencer
CC BY-SA 4.0
2020-11-03T21:25:12.160Z
@Spencer if you want to submit that as an answer I'll accept it, it answers my question.
CoryG
<geology><geography><geomorphology><islands>
How are tall cays formed?
CoryG
# How are tall cays formed?

(Asked by: username_0 on 2020-11-02)

From reading about [Cays on Wikipedia](https://en.wikipedia.org/wiki/Cay) it seems they are formed from coral reefs covered with sand or gravel. There are however some cays scattered around the world which are hundreds of feet off of sea level at high tide. How did these cays form, or are they mis-classified islands?

**Tags:** geology, geography, geomorphology, islands

> **Comments:**
> **username_1**: The Virgin Islands are volcanic cones that haven't eroded away yet. They're the "before" picture in [this animation](https://oceanservice.noaa.gov/education/tutorial_corals/media/supp_coral04a.html).  
> **username_0**: @username_1 if you want to submit that as an answer I'll accept it, it answers my question.  

---

## 1 answers

### Answer by username_2 on 2020-11-05. Score: 2 | Accepted answer

its the same way atolls form, sea level changes and tectonic plates move while the corals keep growing up as the land sinks. Any spot high enough for coral to survive can slowly grow in this way. they can end up hundreds of meters above the sea floor by the slow gradual process of growing coral.

I find a simple [animation](https://www.youtube.com/watch?v=vrD1O5hex6Y) makes it easier to follow.

Cays are just trapped sand in the coral reef, the sand can come from the erosion of nearby islands, volcanic, or even erosion of the coral itself. This is why most cays occur near other land masses or in volcanically active areas.

Just as land can sink it can also rise, pushing those deposits high above the waterline, note these cays are very rare. Also note the parts far above the water are dead ancient corals not living ones, although there may be leaving coral around the base. there are ancient fossil corals in the Himalayas after all.


---

