Most divers have had the opportunity to explore a few reefs throughout our diving careers, and while it is plain that these are underwater oases for marine life, where concentrations of all types of creatures are at their heaviest, we might not understand much about the different ways in which reefs are formed. While there are a variety of reef types, some of the most common are also some of the most intriguing.
Reefs, Defined
Technically, a reef is any structure which sits at depths of six fathoms or less during low water. A fathom is just over 1.8 meters, or six feet. Geologists use various sets of factors to identify reef structure, including composition and deposit type, internal structure, and sedimentary features. Most reefs, particularly coral reefs, are held up by skeletal frameworks called macroscopic skeletons. Others are composed of sedimentary rocks, ore deposits, or bacterial mounds called stromalites, which are actually some of the oldest life forms on the planet.
Understanding Reef Structure
There are several common types of reef structure, and within those structures are several different habitat zones where we find distinctively different forms of marine life. Most of the reefs we enjoy today were formed during the last glacial period, and most are less than ten thousand years old. There are three main reef types:
- Fringing Reefs – these reefs extend directly from the shoreline, with flats that sometimes connect directly to the beach. Sometimes backed by lagoons, patch reefs, or seagrass meadows, they are the most common reef types found within the Red and Caribbean Seas.
- Barrier Reefs – surrounding all or part of a land mass, these reefs are separated from land by almost continuous channels.
- Atoll Reefs – usually quite circular, and often continuous, atolls normally encircle lagoons which once held long-gone central islands.
In addition to these reefs, there are several other commonly encountered structures. Here are a few examples of those you may see while diving.
- Table Reefs – much like atolls, these reefs are often circular, but they lack a central lagoon area. Instead, they have big flat surfaces.
- Cays – A cay is a low-elevation island that sits atop a coral reef, with a portion of exposed reef around it. Some are simple sand patches, while others host plant and animal life; some are even inhabited by people. Cays are common throughout tropical Indian, Atlantic, and Pacific Ocean environments, and can be found along the Belize Barrier Reef and Australia’s Great Barrier Reef. Many resort islands in places like the Maldives are built atop “private islands” that are actually cays.
- Patch Reefs – Some of the most common reefs, and normally found in conjunction with other reefs, these are usually found inside lagoons, bays, and other sheltered areas; they are usually circular and are normally surrounded by gardens of sea grass or sandy flats.
Reefs that extend from shorelines are usually based atop limestone bedrock, and feature several different zones, including the inner reef or reef flat; often peppered with chunks of coral, it often shelters high numbers of fish. Beyond that, the reef crest provides perfect conditions for coral growth and hosts and abundance of life, and further out, the outer reef either slopes away or drops off.
Reefs cover an estimated 109,800 square miles of the planet’s surface. Not only do they provide homes for countless species, they act as some of the world’s best dive sites. Essential to life on earth, these amazing structures deserve our utmost respect. Take only pictures, and leave nothing but your bubbles behind, and you’ll be playing a vital role in protecting them for the future.