The ring-shaped coral islands called atolls are a defining feature of the South Pacific, put on magnificent display in the recent Disney-Pixar film Moana. Atolls are known for being convenient harbors almost by default, rich habitats for marine life, and the source of the famous stark line between blue-water and green-water regions that surrounds Pacific islands. Atolls are also extraordinarily rare outside of the South Pacific, which is curious, because neither coral nor islands are similarly restricted. Other warm regions should have their own atolls, but they almost always don’t. So the question is…where are the Caribbean atolls?
It turns out this is a fairly involved question.
First, Some Background
Atoll formation is a well-described and reasonably well-understood process. Islands are almost never completely vertical, and therefore have nearby areas at depths suited to coral reefs. Corals grow around the islands at these depths, where light levels in particular are ideally suited to coral growth. As wind and water erode the island from above and the weight of the growing reef causes the island to sink, the coral grows upward and inward. In the most extreme cases, the original island is entirely submerged, leaving only the reef. The reef can then become an island in its own right if sea levels decrease or isostatic uplift wrenches the entire landmass upward, producing distinctive ring- or C-shaped islands called atolls.
Atolls cannot form without a pre-existing island to bury in coral, almost always a volcanic island. Underwater volcanoes form where subterranean magma leaks into the Earth’s crust. Magma becomes lava and then cools and solidifies on contact with air or water. Islands form when this mass of cooling lava breaks the ocean surface. If the lava does not emerge rapidly enough to produce land before the circumstances enabling its emergence shut down or move on, other geologic formations ensue, including mid-ocean ridges and seamounts. Islands following this pattern occur all over the world, distinct from islands that form as “microcontinents,” islands in river deltas, barrier islands, and other patterns of island formation.
Volcanic islands generally form in two ways. One is analogous to the way that volcanism-based mountain chains form on land, in which one plate’s heavy oceanic crust is pushed (subducted) beneath another plate’s crust. As the oceanic crust material (which is lighter than the mantle) melts and re-ascends through the second plate, it forms a chain of islands along the margin of the subducting plate. This pattern is often seen near large landmasses, typically via nearby oceanic crust subducting beneath the lighter crust of continental shelves, and is responsible for the Aleutian, Japanese, Ryukyu, and Philippine archipelagos. This is also the pattern responsible for the volcanic islands in the Caribbean Sea.
The other volcanic island pattern is via “hotspots.” Hotspots are locations in the Earth’s mantle that consistently push mantle material into the crust rapidly enough to build islands. At the mantle level, these locations are stable, but the crust is made of tectonic plates that move relative to one another. Thus, hotspots tend to produce island chains, in patterns that trace the movement of the tectonic plates that bear those islands. Hotspots are often associated with mid-ocean ridges, where mantle material pushes between two tectonic plates and forces them apart while forming new seafloor. These hotspots, effectively, represent sites where the mantle material penetrates the crust rapidly enough to form islands, rather than spreading apart as seafloor, and are responsible for Iceland and the Macaronese Islands. The hotspots in the Pacific Ocean are not associated with plate margins.
Coral versus Coral
This information makes the near-absence of coral atolls in the Caribbean, and the total absence of atolls along the volcanic Lesser Antilles, all the more puzzling. Although one set is a volcanic island arc formed due to plate interactions and the other formed due to a series of hotspots, both are volcanic islands in the tropics, surrounded by warm water and home to magnificent beaches and coconut palms. Both bodies are home to beautiful coral reefs and to tourism industries premised on same, and both are historically home to indigenous groups with impressive seafaring capabilities.
But, in all of these things, the South Pacific does them better, and there are reasons why.
In addition to an island base and tropical temperature, atolls require a number of other environmental conditions. Atolls’ necessary reefs form best when the surrounding ocean currents are gentle, so that erosion of the reef itself is slow relative to its growth and the subsidence of their central islands. So, Pacific atolls are mostly somewhat removed from the Pacific gyres and equatorial countercurrent. Corals are easily overwhelmed by seaweed, which grows much faster than coral in algae-favoring conditions. So, corals grow best when the water they inhabit is nutrient-poor, particularly in iron, so that most of the primary productivity of their ecosystem derives from their own symbiotic algae rather than from free-living plants.
All of these factors disfavor atoll formation in the Caribbean. A major ocean current, the Gulf Stream, passes through the Caribbean basin on its way northeast. The Caribbean Sea is mostly enclosed by land, with one major and numerous minor rivers across South America, Central America, and the Greater Antilles emptying into it, introducing both sediment and land-derived nutrients into the basin, where west Pacific rivers are almost wholly separated from the Pacific proper by non-atoll-forming volcanic island arcs and by Australia’s Great Dividing Range. The Caribbean, like the Amazon, receives windborne deposits of iron-rich dust lifted from the Sahara Desert, massively increasing its algal and seagrass productivity. All of these factors give the Caribbean Sea an impressive expanse of seagrass beds suited for manatee grazing, but change the physical incentives that Caribbean corals face.
Two additional factors work against atoll formation in the Caribbean Sea. The Isthmus of Panamá closed in very recent geologic memory. When it did, it reshaped ocean currents on a world scale, but it also cut off Caribbean corals from the center of coral diversity in the western Pacific. At precisely the same time that the Caribbean could no longer benefit from coral speciation in the Pacific, the altered ocean currents encouraged ice formation in the northern hemisphere and, in part, precipitated the last Ice Age. The Ice Age cooled the Caribbean Sea enough to prevent the same kind of reef-building as continued in the still-warm South Pacific. This difference complicates large reef formation in the Caribbean, and discourages some of the reef-building patterns that are common in the Pacific.
This array of factors mean that, despite both tropical regions featuring volcanic islands and being famous for their reefs, only one is defined by ring-shaped islands and inner lagoons. The Caribbean and the South Pacific are indeed different beasts altogether, subject to different geologic, geographic, and biological pressures that line the Caribbean with craggy volcanoes and the South Pacific with idyllic circles.
This, of course, adds to the surprise when it turns out that there are atolls in the Caribbean, scattered among the nearshore islands of Nicaragua and Venezuela. These islands are not volcanic in origin, with elongated, irregular shapes that give their atolls equally strange outlines. Nor, in their short lives, have these reefs had time to push their base islands beneath the waves, but they persist all the same in an environment hostile to the rest of their kind. Perhaps, eventually, they will add some coralline circles to the Caribbean’s geography, far from the volcanic rim islands where they would stand out less. More likely, anthropogenic climate change will do what it is already doing to the lowest atolls of Kiribati and the Maldives, and remove these peculiar formations from the world altogether by drowning them in deep water, pollution, and disease.
Until then, the Indo-Pacific has its lovely rings, jewels of Polynesia that it refuses to share with my island homeland. How rude.