Types of reefs
There have been many attempts to classify different types of reefs. All lack general agreement because there is continual variation from one reef type to another, also because they can be classified according to their geological history, their shape, their position relative to land masses and by their carbonate compostion. In principle, these types of classification can be merged into three broad categories: ‘fringing reefs’, ‘barrier reefs’ and ‘atolls’ (Darwin’s original groups).
Fringing reefs are mostly close to coastlines, are usually unconsolidated wherever protected from wave action, and usually have a high component of non-carbonate sediment. Barrier reefs are off-shore and are composed of wave-resistant consolidated limestone. Atolls are usually (but not necessarily) a wall of reefs enclosing a central lagoon. As the shape of both barrier reefs and atolls is largely determined by the bathometry of the substratum, there are all manner of intergrades between them. Likewise, ‘fringing reefs’ gradually become ‘barrier reefs’ with increasing distance from the shore. Reefs which do not conform to any of these descriptions are commonly called ‘platform’ reefs.