The ring-shaped islands you see in the ocean are called coral atolls. They are created when coral reefs start to grow on a sinking island.
Have you ever seen an image like the one below? Most people tend to think they’re just regular islands, but that’s not entirely true.
These islands are called coral atolls.
If you look closely, the “island” has a little pool in the middle. These pools in the middle are called lagoons. The interesting thing is that this water is actually seawater that has been trapped over time! So how exactly does this water get trapped in the middle of the island?
Well, the process is far more complex than you may think and could take as long as 30 million years!
First, however, let’s talk about what those islands are!
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What is a coral atoll?
Scientists have a very disputed definition of what a coral atoll is. For our purposes, a coral atoll is an island composed of a reef on the outside and a lagoon in the center.
The reef has two parts: an outer reef and a reef rim. Looking at the picture above, you see an outer reef, which is greyish and rises above the water in typical cases. You can see a reef rim too, which lies inside. In typical cases, it extends a bit from the outer reef. The rims extend inward as a reef platform in smaller atolls, like the one pictured above.
The beautiful blue lagoon is what really sets an atoll apart from any regular island (in terms of natural beauty, at least). The reef shelters the lagoon from the ocean. This lagoon can also have small patches of reefs inside it.
So how are such atolls formed?
Why do these island form in the middle of the ocean?
Atolls are formed entirely by corals. The coral cannot form from the bottom of the ocean because sunlight does not reach there and they need sunlight to make their food. Initially, the coral reef forms around the island as described in the article here.
There are many theories on the formation of atolls. Daly proposed that atolls were formed during the end of the last ice age, due to rising sea levels, but one theory has gained the most prominence.
Charles Darwin (yes, the scientist who proposed the theory of evolution) also proposed a theory on how atolls were formed. He believed that, initially, volcanic eruptions formed an island. These volcanic islands acted as new homes for reefs to form.
Over time, the land starts to subside, slowly sinking deeper into the water and eroding. When the land starts to subside, the coastal reef turns into a barrier reef. A barrier reef is a coral reef that is separated from the coast by a large lagoon. This can be seen in the Great Barrier Reef of Australia, shown in the satellite image below!
Once the island starts to subside, that barrier reef grows inwards. At some point, the island goes under the water level and the coral grows over it. Now, unlike normal islands, which are composed of sand and rock, these islands are made of chalk!
Are these islands made of living coral?
When you think of coral, you normally think of hard structures shaped either like fans, branched-out trees or even platforms, like the examples in this picture below. But what you see there is actually only their skeletons!
Corals are formed by animals called polyps. Polyps are small animals that are actually related to jellyfish. Much like jellyfish, they are actually soft animals, but then how is coral so hard? Well, polyps secrete calcium carbonate (chalk) that entirely covers them. Over time, when these polyps die, their skeletons remain.
Now, coming back to atolls. Atolls are made of coral reefs, but not living coral reefs. Coral live for a very long time and can even reach 5,000 years of age, but remember that coral atolls can take as long as 30 million years to form!
That’s why coral atolls are not islands made of coral, but islands made of coral skeletons! Their skeletons are made of calcium carbonate, the same thing that makes eggshells and shellfish skeletons.
Even though the islands are made of dead reefs, that does not mean there are no living coral among them. The lagoons can have patches of living coral reefs that act as home to many species of fish and other marine life!
Atolls even host human life. Humans have lived on atolls for millennia; in fact, many famous islands like the Chagos archipelago, the Lakshadweep islands of India, and the island nation of Maldives are actually atolls… we just didn’t know it!