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Geo-Joint: South Pacific Atolls

Posted on January 02 2018

As the island sinks, the coral forms a barrier reef, with the beginnings of a lagoon encircling the island.

Beauty is a subjective thing, but almost everyone agrees that the settings of Polynesia are among the most pleasant and soothing in all the world. The warm water helps, but the colors of the sea and sky, and the rich green plant life and colorful sea life all add to the beauty. It’s also nice not to have to dodge bears, wolves, and venomous snakes. So how did those places get there, and what arranged for those placid lagoons and, if you’re so inclined, the perfect surf?

How a coral atoll comes to be.

It’s no surprise that the specks of land out in mid-ocean are the tops of volcanoes that have risen from the sea floor. Depending upon depth and the rate of eruption, it can take from a few months to tens or hundreds of thousands of years to slowly build from a mound of lava on the bottom, to breaking the surface of the sea to form an island. Many, in fact most, would-be islands only make it part way to the surface before going dormant, which leaves behind what is called a seamount. If an ambitious volcano poking its head above the surface is to make a successful island, it must keep up the building once it gets to fresh air, lest waves, rain, and chemical erosion wear it back down to sea level, or below.

Once firmly established as a bona fide chunk of land, an island finds itself undergoing constant change and occupation. Erosion goes to work immediately by way of that churning sea and the heavy precipitation common to the tropics. But the breaking up of lava into smaller chunks of rock and sand makes rudimentary soil that pioneering species of plants like ferns can establish themselves upon. As these die and decay, richer soil is formed, which can support a wider range of the plants whose seeds will inevitably float ashore or be carried in by birds.

A fringing reef creates a skirt around an island’s shores in French Polynesia.

While all that is going on above the surface, or more likely even before all that goes on, sea creatures are busy scoping out the new lump of lava. Fish no doubt find its structure a good hiding place, and a place to find other hiding organisms to serve as food. When the rising seamount climbs to a depth where sunlight can reach it, coral larvae will find it a suitable place to set down and grow a polyp. The coral organisms themselves don’t need the sunlight so much as does the algae with which they have a symbiotic relationship. As the island grows, some of this coral will get fried by fresh lava, but as soon as it cools, the process starts again. At a certain point, the volcano blasts out its last dollop of molten rock, and goes dormant.

The lava island then is as big as it’s going to get, but processes continue to change its appearance and footprint. As corals of many shapes and sizes cement themselves to the edges of the island and grow, they form a fringing reef. As the eons roll by, the reef will widen. Meanwhile, the volcano, which is a relatively new weight upon the planet’s surface, plus the even more recent weight of the coral, begin to depress the ocean floor, and the whole affair starts to sink. The cooling brought by the cessation of volcanic activity also contracts the basement rock, adding to the settling action. This movement occurs at a very slow rate, so the living coral, though its base is heading deeper, has time to build its top level up and stay in the sunlight, the photic zone, and keep growing. As the central island mass slips lower, a moat of seawater forms between it and the growing ring of coral scrambling to stay near the surface.

Eventually, the island slips below the sea, and the coral ring left behind is an atoll, surrounding a calm lagoon.

At this point, the reef can be called a barrier reef, as it absorbs the wave energy heading for the shrinking island. Eventually, the island disappears below the surface, and a calm lagoon is left, encircled by the bowl of coral. The processes that made coral sand (wave erosion and parrotfish) continue on the free-standing reef. The uppermost parts of the barrier reef may be affected by sea level changes as well. If sea level falls due to the onset of a glacial period that ties up seawater, coral will be exposed and die, eroding into a very low surface structure. It can form a ring-shaped circle or oval supporting plants and animals, called an atoll. Gaps in the atoll’s ring can allow the tide to flow into the lagoon and provide nutrients to corals and fish living in its calm waters. Corals may grow new little reefs inside the lagoon, called patch reefs. The long road from nascent volcano to atoll can take 30 million years—the complex geology and ecosystem evolves slowly.

A hundred and seventy-five years ago, Charles Darwin theorized, after visiting the Society Islands in the South Pacific, that the processes just described were what created the fantastic beauty of the islands and atolls he encountered. He didn’t have sea-level change knowledge, but the rest of his observations explained the dynamics of the scene fairly well. He was doubted by some, who believed the visible coral was just a relatively thin layer growing on rock. But Darwin was proved right when drilling done on reefs in the 1950s showed that the coral material that built the atolls indeed went very deep.

A dropping sea level may expose shallow corals to the air, where they will die. Conversely, a rising sea accelerates the apparent sinking of an island, shortening the time available for coral to grow upward and keep its submersion from becoming too deep. The sea level rise brought on by climate change is also already threatening coral atolls that barely rise to a few feet above sea level. As with all the new circumstances involving climate change, it’s not the actual change in conditions that cause worry—the world has adapted to vastly different scenarios in its history—but the speed with which it is happening. It’s that speed, and the inability of plants and animals to adapt and keep up, that makes the disruption so damaging, and the survival of many species so tenuous. So enjoy those blissfully gorgeous tropical island locales while they’re still looking good—there’s change coming sooner than we’d like.


Here’s something to help you daydream about a tropical paradise, or to get a plan going! National Geographic’s Islands of the Pacific map from their December 1974 issue may be vintage, but the islands are all still there—just maybe a little shorter.




caption: How a coral atoll comes to be.
caption: A fringing reef creates a skirt around an island’s shores in French Polynesia.
source:  Wikimedia Commons:  FRED (CC by SA 3.0)
caption: As the island sinks, the coral forms a barrier reef, with the beginnings of a lagoon encircling the island.
source: Wikimedia Commons: Sergio Calleja (Life is a Trip) (CC by SA 2.0 Generic)
caption: Eventually, the island slips below the sea, and the coral ring left behind is an atoll, surrounding a calm lagoon.
source: Flickr: Jesse Allen, NASA (CC by 2.0 Generic)

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