Atolls & Banks


Atolls occur throughout all the oceans. Atoll comes from a very specific administrative Maldivian term that has been inadequately used in a geomorphology context to designate Pacific Ocean annular-shaped reef structures, either entirely closed by a rim such as Taka and Utrik (#9) (Marshall) or very open to the ocean, such as Beautemps-Beaupré #10 or Ouvéa #11 in Loyalty Islands (New-Caledonia) who presents a fragmented rim (#30). A true atoll is North Mahe (#6), a Maldivian administrative entity with numerous rim and lagoon faros (Maldives #36). Faros are reefs with a central enclosed lagoon or depression. Actually, morphologically speaking, most atolls worldwide should have been called faros, or oceanic faros. Indeed, modern atoll present a faro-like annular-shaped ring with a central lagoon such as Rahsdoo Atoll (#7) (Maldives) or Fabre Atoll (#12) (New-Caledonia). The formation of faros is still a matter of discussion, but certainly, the processes that created faros in Maldives are different to the processes that created "atolls" worldwide, thus designating "atoll" as faro would be inappropriate.


Related to atolls, banks also occur throughout all the oceans. The definition of a bank is also problematic. The word "Bank" has been used to name platforms that show a very large diversity of morphology and structures, from extremely wide carbonate systems (#4 Great Bahamas Bank, #25 Lee Stocking Island, Bahamas), to narrow oceanic reefs (#19 Les Glorieuses, France, Indian Ocean) and to atolls (#14 Chinchorro Bank). Intuitively, banks should be almost entirely submerged when compared to atolls and other oceanic reefs. Reef formations were frequently named "banks" to refer principally to an area of shallow water. Lack of early clear geomorphological references and different administration likely explain the lack of consistency and homogenization. This is why Chinchorro Bank (#14) (Mexico) is called a bank while the structurally similar Lighthouse Atoll (#15) (Belize) is an atoll.

Some suggest using the presence/absence of emergent volcanic rock as supplementary criteria to classify oceanic reef formations. For instance, the superficial visible part of an atoll should be only carbonate structures. But Clipperton Atoll presents remnants of volcanic structures along its rim. North Hawaiian Islands include banks, atolls, reefs, reef-top islands and basaltic islands. Almost every reef formation of this archipelago is a specific case. North Western Hawaiian Islands (#18 Pear and Hermes Atoll) share many morphological features with other formations worldwide (#13 Alacranes Reef, Mexico) like lagoonal reticulated reefs.

In the relatively small Caribbean Seas (compared to the Pacific Ocean for instance), the diversity of atolls and banks is high. Mexico, Belize, Nicaragua, Venezuela and Colombia have significant coral surface areas on remote shallow oceanic reef formations. Alacranes #13 (Mexico) is rich in reticulated reefs, Lighthouse Atoll #15 (Belize) and Chinchorro Bank #14 (Mexico) share a similar structure, with a narrow lagoon. Glovers Atoll #16 (Belize) has a structure similar to many closed Pacific Ocean atolls with numerous small lagoonal patches. Los Roques Archipelago #17 (Venezuela) is a platform fragmented in numerous outer reefs surrounding the larger atoll-like formation. Most of these isolated "atolls" and "banks" are asymmetrical, revealing differential growth and erosion under different exposure to the past and present hydroclimate (swell, wind, rainfall, river plume dispersal).

Most of the modern area of these "atolls" and banks are sedimentary domain, seagrass-rich back-reef and lagoons. Modern coral dominated areas are actually very marginal on most of the images that we show here, though they can be accurately delineated. On some large banks, coral areas along the outer slopes represent an infinitesimal component in regards to the sediment and seagrass surface areas (#4 Great Bahamas Bank, #25 Lee Stocking Island, Bahamas).

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