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atoll with view of beach and coral reef wall

Atolls are coral islands. They have beaches made of coral sand, and perhaps a few palm trees growing on little bars of coral rubble. The center of the atoll is a shallow lagoon. The island bit of the atoll is a narrow ring of land around the lagoon, with coral reefs all around the outside of the island. Atolls generally rise up out of very deep water. An atoll may be a simple ring of island, or it may be a ring of several islands around a lagoon.

In the early 1800s, once it was realized that corals were animals that grew their skeletons only in shallow water, atolls became puzzling features of the tropical oceans.

Observation: Coral reefs only grow in shallow water. Reef building corals have symbiotic algae that grow in their tissues and help them secrete a calcium carbonate skeleton. They can only grow in the upper 100 meters or so of the ocean where light penetrates. Corals are the main building stuff of coral reefs and are bound together and cemented by algae algae, which also require sunlight to grow. Since both the main building blocks (coral skeletons) and cement (algae algae) of coral reefs require light to grow, coral reefs can only grow in shallow water.

Observation: Atolls are coral islands, made entirely of coral that occur in deep water. Drill cores from coral reefs go down and down and down into coral with out reaching bedrock for a very long ways (often kilometers). Atolls are a little ring of living coral reef built on a kilometers high pile of dead coral.

These observations present a paradox - how could you get coral islands in deep water if coral reefs only grow in shallow water?

Charles Darwin figured out how coral atolls form. This idea was perhaps his major contribution from his voyage round the world as in the Beagle in the early 1800s.

We observe in the world several sorts of islands:

We observe two related processes:

Inference: Coral islands form a series. First an island is formed by a volcano, then corals start to grow as a fringing reef around the island. Over time the volcano dies and starts to erode and subside, while the corals grow up into a large barrier reef around the volcanic island. The old volcanic island continues to subside and sink beneath the sea, while the coral reef continues to grow upwards, maintaining a coral island at the sea surface. Given the slow growth of coral and lots of time, an old volcanic island can subside far below the surface of the sea, while a coral reef perched atop the island continuously grows upwards keeping the reef at sea level even while its volcanic roots subside deep beneath it.


This simple idea of how coral atolls form is a very clear example of the method of historical reasoning called staging. Staging is a very simple idea: Observe things in the world today. Line them up in a series and infer that they are all products of an ongoing process - just seen at different stages in the process.

Darwin had two key ideas about atolls - uniformatarian observation of uplift and subsidence and the staging observation of different forms of islands - with atolls found away from areas of active vulcanism.

Darwin seems to have essentially gotten it right. There have been two main amplifications to the idea of how atolls form since Darwin. First, sea level goes up and down with glacial advances and retreats - lots of islands got stuck out of water further during last glacial maximum - thus coastlines and profiles are not the simple result of continuous reef growth. Second, plate tectonics - explains areas of uplift and new volcanos and areas of atolls. Also explains chains of islands with active volcanos at end and subsided seamounts or atolls at other end (plate moving over hot spot)

Sources: Dana 1879, Darw in 1842
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Written by Paul J. Morris
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Date Created:5 Dec 1999
Last Updated: 5 Jan 2000