Coral Reef Types

Credit: Blane Perun
Coral reefs are divided into four types: fringing, barrier, atoll, and patch. The largest Barrier Reef is the famous Great Barrier Reef in Australia.

Barrier Reef

Barrier reefs are similar to fringe reefs, but they lie farther from shore and contain a relatively deep lagoon. Some reefs are as far as 100 km from shore, and 2,000 km in length.

Barrier reefs are among the most prevalent and well-known types of coral reefs in the world. Among the most significant locations where they can be found is also the Great Barrier Reef, the largest known conglomeration of coral reef formations in the world.

Commonly found in parallel formation compared to the shoreline, separated from the shoreline by a large or medium sized lagoon, and located far off shore, a barrier reef is usually pierced by a number of channels that provide access to the lagoon or body of water that comes between them and the shore of the island or continent they surround.

While somewhat similar to fringing reefs, barrier reefs are separated by a much deeper body of water from the shore, so that they never start growing directly from the shoreline itself.

Also, a most important characteristic of any barrier reef formation is that it forms extensive linear complexes that mostly thrive in the tropical Atlantic and Pacific Ocean. Also, it is worth mentioning that barrier reef formations are not as prevalent and commonly found as fringing or atoll reefs.

Newer studies have recently shown that coral reefs are not formed through a continuous process, and this seems to be even more pronounced in the case of barrier reef formations. Reefs such as the Belize Barrier have formed alongside deep trenches or bodies of water such as the Cayman Trough, that have allowed for localized coral growth and provided stability for the thriving of large numbers of fish and coral species that end up populating the rather narrow area of the barrier reef in question.

Similarly, most barrier reefs are kept in a state of constant formation through the slow movement of plate tectonics and the presence of storms and waves which cause the reefs to grow over time, while allowing the lagoon they protect to become a safe haven that is ripe for the presence of a significantly rich biodiversity.

Larger barrier reef formations often present a complex development process facilitated by a number of factors, including plate tectonics, the slow shift in sea level growth over time, specific currents facilitating the shaping of the reef and in some cases even the presence of land masses that used to be larger hills or volcanoes.

Among the most important barrier reef formations is the Great Barrier Reef – the world’s most imposing and sizable coral reef structure, being composed of about 3000 individual coral reefs. The Great Barrier Reef is located in the Coral Sea, and is actually the largest structure in the world that was built by living organisms, being easily visible from outer space.

Other well-known barrier reef formations also include the Belize Barrier Reef, the New Caledonia Barrier Reef and much of the Great Maya Reef which encompasses the Belize Barrier Reef and a sizable region stretching out about 1000 km from the Yucatan Peninsula to Guatemala.

Barrier reefs are among the most significant types of reefs in the world due to their protective properties and ability to use storms for fueling their own growth and enhancing their ability to keep larger waves away from the shoreline.

Coral Reef Atoll

Atolls are rings of reef, with steep outer slopes, that surround a central lagoon . They are found far from land, and mainly in the west Pacific. How they are formed was a major puzzle. Charles Darwin, famous for the theory of evolution, solved this puzzle by postulating that atolls formed around subsiding islands. If the upward growth of the coral keeps pace with the downward subsidence of the original volcanic island, an atoll is formed.
An atoll reef is essentially a type of reef that was formed surrounding a body of water such as a lagoon. These types of ring shaped coral formations often form tiny islets that protect the lagoons and the central island behind it.

Formed through a complex process initiated by underwater volcanoes, atolls develop slowly, over millions of years, and are now among the most prevalent types of coral reef formations in the world.

Seamounts, or underwater volcanoes, play an important role in the formation of atoll reef formations. These ring shaped corals are formed after a volcanic island is built up gradually by cooling lava on the seabed. Once the island has formed, breaking the surface of the water, the island is then ready for the next stage in which the slow growth of atolls can begin.

That being said, the stages of an actual atoll reef’s development are as follows:

  1. The first step is made by tiny animals known as corals that begin to form the reef structure around the island. These are known as hard corals, due to their ability to form hardened, limestone-based exoskeletons that maintain the structural integrity of the reef over time.
  2. Fringing reefs are the first that form, remaining relatively close to the island’s shoreline, and being maintained close to the surface of the water. A shallow strip of water known as a lagoon forms between the island and the reef structure.
  3. The atoll reef is then enhanced to form a structure similar to a barrier reef, as the seamount slowly erodes over the course of millions of years, and subsides into a flat-topped structure known as a guyot.
  4. Finally, the reef is pushed farther out from the shoreline, and the lagoon grows in size. As waves and storms pound the coral formation over time, pieces and parts of the reef are dislodged, and the sand is later deposited by waves to form the small islets that are the major characteristic of what makes atolls unique.

One of the main characteristics of an atoll reef is that the entire process of its formation may take as long as 30 million years to complete. This is significantly longer when compared to many other types of reefs, being accounted for by the reefs’ unique structure and form.

Atolls are also roughly circular in all cases, being formed around a seamount, and in many places in the Pacific, they can also occur mid-ocean. A few examples of these types of reefs can be found in the Chagos and Maldives island groups, as well as the Cocos Islands and Seychelles.

Another important trait of atoll reef formations is that they are quite difficult to spot. While still keeping the shoreline relatively protected over time, they have also presented significant problems to ships sailing to volcanic islands, many of them having become shipwrecked due to being unable to avoid the shallow, but sharp reef structures and islets hidden by the waves.

Atoll reef formations are significantly more well-known and prevalent than barrier reefs and other common types of reefs, being among the unique characteristics that set volcanic islands apart from other land masses.

Fringing Reef

These are the simplest and most common kind of reef. They develop near the shore throughout the tropics. Some hard substrate is necessary for the polyp to establish, but after that the corals can make their own hard bottom. Note in Figure 5 the different aspects, including the inner reef flat and outer reef slope.

Found mostly near the coastlines of islands and continents, fringing reefs are the most frequently spotted type of coral reef that we can study. Separated from the shoreline through shallow and narrow lagoons, they are also distinguished from atoll and barrier reef formations through their low slope reef flat and their slower growth, as well as their unique development which makes them use up a greater amount of the space between the reef slope and the surface.

One of the interesting facts about fringing reef formations is that they can often stretch out for hundreds of yards from the shoreline, and the lagoons or backreef zones that they are associated with are extremely shallow and narrow. In fact, if a fringing reef grows very close to the shoreline, it may present no backreef zone whatsoever.

Although fringing and barrier reefs are quite similar, there are a few important differences one has to note, that always set them apart. Unlike barrier reefs, fringing formations are much closer to the shoreline – even the varieties that grow somewhat farther away. Also, they do not present any significantly deeper growth areas, while the development of barrier reef is less consistent, the reefs sometimes having small areas that are far deeper than the rest of the formations.

Sea levels determined by plate tectonics or glaciation are among the main factors conducive to the growth and development of fringing coral reefs. These reefs will often grow either at the same rate as sea level growths (keep-up reefs) or slightly slower at the beginning (catch-up reefs). Catch-up fringing reef formations also accelerate their growth over time, however.

Fringing corals are formed from a reef flat that is relatively low sloped, and a portion found farther out at sea known as the reef slope. Fringing reef slopes are extremely steep, and often descend to the farthest depths that still allow for the growth of corals.

Most fringing reef formations are developed either vertically or expanding away from the shoreline. Some also develop sporadically, while others can be found alongside muddy sediments or at a greater distance from the sea shore, forming barriers that are fed by storms, and often end up forming larger lagoons.

Darwin believed that fringing reefs became the first types of reefs to develop around most landmasses, and they are also easily found near or farther off from the shorelines of many islands and continental regions today.

Some of the most well-known fringing reef formations are seen in the Bahamas or certain locations in the Caribbean. These reefs can, however, also be found in places like the Red Sea, and most can easily be spotted in the tropics, near the shoreline, where they are known as the most common varieties of coral reef formations to be found. In fact, many of the reefs found in the Great Barrier Reef (more than 700 of the 3400 reef formations) are actually fringing reef formations.

Fringing reefs are considered to be among the most fascinating types of coral reefs ever studied, and their widespread development places them among the most important coral reef formations in the world.

Patch Reef

Patch reefs are small reefs that usually occur within lagoons behind barrier reefs or within an atoll.

As the name would suggest, a patch reef is a small reef formation that is isolated and often found in a small area surrounded by water – usually within a small or open body of water, and in some cases even completely submerged underwater.

Sometimes, they also form in the middle of a lagoon or close to barrier reefs and atolls. A few common examples of patch reefs include the reefs occurring off the north coast of St Croix (in the Caribbean) and reefs found on the Florida Reef Tract.

A patch reef has many important qualities that set it apart from most other types of coral reef formations. It is, first of all, important to note that they are most commonly found in shallow waters. Depths of around 3-6 meters (or 10-20 feet) are quite common.

These reefs, in most cases, are also surrounded by a halo of sand that extends to seagrass beds. These are determined by the distance that herbivorous fish decide to forage while keeping in close proximity to the reef, and so each patch reef differs depending on the types of fish and marine creatures that thrive in their generally defined area.

These types of reefs are formed through a unique process set into motion by a coral larva that settles outside the plankton, attaching itself to a harder portion that will later become the center of the reef.

Over time, the larva develops into a moderately sized coral colony that becomes the essential platform for other coral formation after its death. Perishing due to damage caused by either severe storms or predators, the colony leaves behind hardened limestone that is then used as a settlement area by other coral larvae.

Over the course of many hundreds of years, this process continues to repeat itself, as the reef keeps growing and expanding both in height, and later on, in size as well, after it reaches the surface of the water.

Star and brain corals are most prevalent in patch reefs, their majestic and robust structures settling on exposed dead corals to repopulate every little part of the reef with new life.

Other fascinating marine species, large and small, can also be found on these reefs. Sponges, mollusks and various types of worms can commonly be viewed here, excavating the dead corals to create crevices that later provide refuge for small fish and invertebrates.

Reef fish are very prevalent on most types of patch reefs as well, some of the most common varieties of fish species including bluehead, angel fish, redband parrotfish, surgeonfish and damselfish.

You can also commonly observe many other marine creatures thriving in these reefs. These normally include green morays, spiny lobsters, squirrelfish and herbivore fish that find refuge on patch reefs during day time, and feed on the seagrass close by during the night.

Most patch reef formations are safe havens for a variety of other marine creatures as well – especially in the case of reefs formed within lagoons that have been protected by barrier reefs for thousands and even millions of years, allowing marine life in the region to thrive.

Whale in Ocean