3.24.2013

Evolution of a Coral Reef

Scientists have studied the structures of coral reefs for decades, trying to determine how they were formed. The theory that most present-day scientists accept was among the earliest proposed. Naturalist Charles Darwin first presented his ideas on reef evolution in the 1830s.

Darwin believed that coral reefs changed over long periods of time, evolving from fringing reefs to barriers and finally to atolls. He explained that the process began when the eruption of an active volcano in the ocean created a small island of lava. After the volcano became inactive, it cooled, leaving part of its surface (the island of lava) jutting above sea level. At first this tip of the volcanic mountain lacked life. Ocean waters carried immature coral animals to the mountain island’s rough, rocky shores. These young corals attached to
the volcano in the shallow waters and grew into adults with hard skeletons. As the corals grew and reproduced, they spread around the entire volcanic island, eventually creating a substantial fringing reef.

Meanwhile, the volcanic mountain began to sink gradually into the sea, taking part of the coral reef to deeper water. As the reef continued to develop, it grew in the direction of the water’s surface and the sunlight. At some point, the volcanic base sank to such depths that some of the coral animals on the island side of the reef could no longer receive enough light, and they died. As landward portions of reef disappeared, the body of water between the reef and the island increased in size, looking very much like a barrier reef by today’s standards.

Evolution of a Coral Reef
As time passed, the volcanic mountain continued to sink until even its tip was completely submerged below the surface of the water. However, coral kept on growing on top of the submerged reef. Eventually, all that could be seen above water was a ring of coral surrounding a lagoon. As the years passed, sand was trapped by the reef, creating beaches. This partial ring of coral became an inhabitable island, an atoll.

The atoll must constantly deal with the destructive forces that threaten it. Seawater and rain slowly dissolve its limestone base. Animals searching for food nibble at the reef to get the algae embedded in it, weakening its structure. Strong waves break apart pieces of limestone and wash silt, sand, and coral debris into the lagoon.

However, nature’s forces do not just erode the reefs; they also sculpt and remodel them. Wind and waves grind up the coral debris and sediment to form sand that finds its way to beaches on coral islands. The sandy shore provides a home for seeds that make their way to the beach by way of the wind and birds. Eventually plants and trees begin to grow. Only very hardy trees can gain a foothold on the side of the lagoon that receives the brunt of strong winds and high waves. A greater variety of plants, such as coconut and breadfruit trees, are found in the more protected interior portions of the lagoon.

The growth of trees on the new island helps to further develop its shape, and hold sand on its shores. A natural fertilizer, bird guano (excrement) enriches the soil as more and more bird species find their way to the new island. Eventually, a multitude of animal life inhabits the lagoon. Female turtles lay their eggs on the shore, bats feed on the fruit of trees, and small lizards dart across the debris-covered areas of the island. The atoll becomes an island that serves as the home to a large number of plants and animals.