Staying Afloat
How Anegada defies a death by drowning
When Anegada was named after the Spanish word meaning “flooded” or “drowned,” the island may have looked different from the way it looks today. Anegada’s extensive salt pond and wetland systems create the impression that the Spanish translation of the island’s name still holds true. But is Anegada really flooded or drowning? Doesn’t its low elevation of only eight metres make it vulnerable to the potential impacts from climate change such as erosion and island flooding resulting from sea-level rise and the increased frequency and intensity of storms? Some people would jump to the conclusion that, yes, the island will eventually flood; but based on Anegada’s geologic history and the oceanographic processes that shape it, the island isn’t going to give up quite so easily.
Anegada is entirely different from the rest of the Virgin Islands and Puerto Rico, all of which make up the Puerto Rican/Virgin Islands (PR/VI) platform. This platform is part of the earth’s crust that is actually a tectonic microplate which fits like a jigsaw puzzle between the North American and Caribbean plates. All the islands on this microplate, except for Anegada, are volcanic in origin and formed roughly 80 million years ago. Anegada, on the other hand, formed as part of a massive coral reef system between 119,000 and 130,000 years ago. During this time, the climate was warmer than it is today, and sea levels were higher, somewhere in the range of 2.5 metres to 15 metres higher in the Caribbean.
Over the next 100,000 years or so, the climate became cooler, and the sea level lowered to a maximum of 120 metres below current levels, exposing the entire PR/VI platform, a time when you could actually walk to Puerto Rico from Anegada. The coral reef that formed the north-eastern part of the PR/VI platform had died off and began transforming into a solid slab of limestone from years of exposure to sun, wind, rain and the many other elements of nature. Then about 20,000 years ago, sea levels in the Caribbean started rising again and eventually reached the current levels about 4,000 to 6,000 years ago. Since the sea levels started rising, a fringing coral reef along the northern side of Anegada, the patch reefs to the south, and a barrier reef to the southeast of the island formed into what is collectively known as the Horseshoe Reef and covers an area approximately 133 square kilometres.
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The island of Anegada today is divided into two distinct geologic formations that geographically split Anegada in half. The eastern side of the island is made up of the exposed slab of coral rock where you can still see corals embedded in the ground. This side of the island is geologically named the Anegada Limestone Formation.
The western side of the island is a bit more mysterious in its formation, since it is made up of large sand dunes that almost completely surround seven interconnected ponds, making up one of the largest remaining mangrove wetlands in the Caribbean. This side of the island is geologically named the Anegada Ridge Plain Formation.
From a bird’s-eye view, the dunes form distinct ridges that run parallel to the shore, but how they formed required extreme activity (storm or tsunami) to push sand onshore.
The interior ponds on the western side of Anegada (in the Ridge Plain Formation) were once all connected to the ocean, but at least one major event caused a large amount of sand to block the northern entrances. According to the preeminent authority on tsunamis, Dr Brian Atwater and his team proved that the western Anegada ponds changed from a marine state to a hypersaline (or very salty) state sometime between 1650 and 1800, most likely from a trans-Atlantic tsunami that occurred after the Lisbon earthquake of 1755. This tsunami would have been capable of moving a massive amount of sand onshore to block the ponds.
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Instead of the tsunami completely destroying Anegada, a series of at least seven dune ridges continued to form on the northern side of the island, most likely from high energy waves from hurricanes capable of moving sand landward. Even the mangroves along the south-eastern side of the island are moving out seaward, indicating a stable shoreline. While the creation of sand dunes and continuing growth of new mangroves show how resilient the island really is to natural events, this resiliency is based on the island’s current ability to adjust to both prevailing conditions and extreme events. However, in order for this to occur, the nearshore coastline naturally displays alternating areas of erosion and accumulation of sand, in some cases nearly 300 metres of change. If these nearshore areas are blocked or built upon by man, adjacent land or even areas further along the coast can be severely impacted.
Jumping to the conclusion that the island is just going to flood and eventually sink with sea level rise is merely a misunderstanding of this dynamic reef island’s ability to aggressively adjust to change. If we don’t understand how the island formed and continues to evolve, decisions to build too close to the shore could actually inhibit the island’s ability to continue the fight against tsunamis, hurricanes and the changing climate of the future. Only then can we blame ourselves for allowing such an amazing island to drown and live up to its name.
Common Anegada Misconceptions
Anegada is completely flat
The highest point is eight metres. The western side of the island gently slopes to the south and the western side of the island has a series of ridges and depressions up to four metres.
Anegada is an atoll
The island neither formed as a reef around a subsiding volcano (the island is too young) nor is the island circular in shape with a shallow lagoon on the inside and deep around the outside. (The southern side is a gently sloping bank).
Anegada is sinking
The island has the capability to adjust its shape according to changes in oceanographic processes and weather.
There’s nothing of real biological importance on Anegada, since it all looks the same
Anegada contains five species found nowhere else in the world: two butterflies, two plants and an iguana.
