Difference Between Atolls And Coral Reefs

“In every outthrust headland, in every curving beach, in every grain of sand there is the story of the earth.” —Lost Woods: The Discovered Writing of Rachel Carson

In A Nutshell

Both coral reefs and atolls are highly valuable and incredibly delicate underwater ecosystems. A coral reef consists of a limestone structure upon which a wide variety of marine organisms—including coral—make their home. An atoll is a very specific kind of coral reef; it is a circular reef that forms around the mouth of an underwater volcano. Coral reefs support thriving ecosystems, while atolls are at the end of their life spans and are frequently referred to as “underwater deserts” because of their lack of life.

The Whole Bushel

A coral reef is essentially a huge, diverse underwater community. Many are built on limestone deposits that have been left behind by living—and decaying—creatures, although they can also form around man-made structures such as sunken ships. Over the course of hundreds, if not thousands of years, coral will begin to grow on these underwater structures. Although they can easily be mistaken for rock, coral are extremely simple organisms that have remained mostly unchanged over the course of the last 400 million years. The coral forms a backbone for a habitat for other marine life, from fish and plankton to algae and other marine plants.

An atoll is a very specific type of coral reef. They are circular coral reefs that surround a lagoon completely or partially, depending on how long and well-developed the reef is. They may also have an island or a handful of small islands in the center.

An atoll is formed around an underwater volcano. When the volcano erupts, it creates piles of lava on the sea floor that will eventually rise above the water level to create the center island. Corals will begin to congregate around the volcanic eruption. These hard and stony corals will leave behind hard limestone exoskeletons, forming the basis of the atoll; this is also similar to the method in which many coral reefs are formed, but the difference is in the presence of the volcanic activity.

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As thousands and millions of years pass, the volcano that originally created the center island of the atoll will fall dormant. In many cases, the movement of the water will erode the lava deposits, and the island will sink back beneath the ocean. Even as this center is eroding, the coral continues to grow and attract more and more marine life forms to the ecosystem. Eventually, the center island becomes an underwater structure known as a “guyot.” The reef around it, also called a “barrier reef,” begins to change. The fresh ocean water on the outside of the ring continues to deliver new organisms and nutrients, feeding the outside while the inside of the ring starts to go stagnant and begins to die.

It’s this slow decay of the inside of an atoll that changes the color of the water, giving it the distinctive teal that is so noticeable from above.

Once this decay starts, the inside of the reef becomes vulnerable to the same destructive forces of erosion that have already eaten away at the volcanic center. Dead corals are broken down, their limestone skeletons forming a hard, fine sand that builds up on the reef itself and on underwater structures. When enough coral is broken down, the levels of the built-up skeletal sands reach above the water and form the small islands that are characteristic of an atoll.

Coral reefs are characterized by their diversity; some estimates say that as much as 25 percent of all known marine life can be found in the world’s coral reefs. In contrast, the coral reefs of an atoll are dead and decaying in the center of the ring and sometimes in their entirety. Depending on how long they’ve been around and how far into their life span they are, these coral reefs can be so abused by the tides and the water that they are uninhabitable.

The water around a coral reef is very rich in nutrients and micro-organisms, making it possible for the thriving ecosystems they support. An atoll has very nutrient-poor soil, and because they can no longer support much life, they are little more than a dead zone in the ocean. Large atolls are known to support human life, but the humans that live there are forced to be extremely careful with their resources. Both are presented with a huge risk with the rising ocean levels.

Show Me The Proof

National Geographic: atoll
Coral Reef Alliance: What is a coral reef?
NOAA’s Coral Reef Information System (CoRIS)

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