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USGS Geology in the Parks


How to Construct Four Paper Models that Describe Island Coral Reefs

Educator's Guide

Coral landforms are unique because they are the only known landforms composed of living material. Coral attaches to any hard surface; it grows in large colonies and can develop reefs several thousand feet thick. Coral grows best in the western oceans, on the coastlines of continents and islands between latitude 250N and 250S where ocean temperatures are above 68°F. Coral grows where its food supply (plankton) is brought in by the west-blowing trade winds and where the water is clear and free of suspended sediment. The coral reef develops from coral animals that secrete lime to form their skeletons, and from algae, plants that also make limy encrustations. Coral develops as large colonies of individual animals. As the coral colonies die, new colonies are built upon them, and so a coral limestone evolves, made up almost entirely of strongly cemented limy skeletons. During strong storms, the coral is broken up and transported to form beaches, spits, and bars, which later are cemented into limestone.

Island coral generally grows on volcanic islands built up by shield volcanoes . Shield volcanoes can evolve to be the largest of the Earth's landforms. They are found on continents as well as in oceans. In the ocean environment, examples of island volcanos are the Hawaiian volcanoes. The Hawaiian volcanoes are huge; their submerged base is 100 mi (160 km) across and they rise more than 25,000 ft (7,620 m), to an altitude of 13,000 ft. (4,000 m) above sea level. The summit crater of the shield volcano is a wide, steep-sided crater that may be 2 mi (3.2 km) or more across and several hundred feet deep. Island shield volcanoes continue to grow by repeated lava flows both above and below sea level that issue from cracks or fissures and flow down gentle slopes.

These voluminous lava flows have low viscosities and spread out into thin sheets. With repeated lava flows, the island volcano grows, advancing a new shoreline. With time this shoreline is eroded by the ocean, and a wave-cut platform develops. As coral grows on this rocky wave-cut platform, it forms a fringing reef . Fringing reefs are coral reefs that are attached to the shore; they grow best where there is an abundant food supply and clean freshwater, and poorly near the mouths of streams where the water is muddy.

Gradually because the island is sinking, the reef grows upward along its outer margin and is called a barrier reef . Between the island and the barrier reef is a lagoon that is too deep to permit coral growth. Wave erosion along the growing edge of the coral produces fragments that are carried down the seaward slope as talus and toward the lagoon, where it forms a sediment apron along the lagoon margin. As the sinking of the island continues below sea level, the reef keeps growing, forming a ring of coral around the lagoon called an atoll . The upper surface of an atoll is a nearly flat surface called the reef flat, situated at or near low-tide level. Cracks are common on reef flats.

Crescent-shaped segments fall away, possibly owing to submarine slides on the slopes of the shield volcano, or to collapse of the coral because of oversteepening. Breaking off of large blocks of the coral reef by storms can result in tens of feet of relief on atolls, but the highest hills are sand dunes.

Continue to the first model: Shield Volcano
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