The Seafloor - The Changing Earth: Exploring Geology and Evolution - page 235

Geo-inSight

Reefs: Rocks Made by Organisms

Reefs are wave-resistant structures composed of the skeletons
of corals, mollusks, sponges, and encrusting algae. Most reefs
are characterized as fringing, barrier, or atolls, all of which
actively grow in shallow, warm seawater where there is little

or no influx of detrital sediment, especially mud. Reef rock is
a type of limestone that forms directly as a solid, rather than
from sediment that is later lithified. Ancient reefs are
important reservoirs for hydrocarbons in some areas.

Vertical growth of coral

Vertical growth of coral

Plate moves into deeper water

Fringing reef Barrier reef Atoll

1. Three stages in the evolution of a reef. A fringing reef forms around a volcanic
island, but as the island is carried into deeper water on a moving plate, the
reef is separated from the island by a lagoon and becomes a barrier reef.
Continued plate movement carries the island into even deeper water. The
island disappears below sea level but the reef grows upward, forming an atoll.

2. Underwater views of reefs
in the Red Sea (left) and
in Hawaii.

3. The white line of
breaking waves marks
the site of a barrier reef
around Rarotonga in the
Cook Islands in the
Pacific Ocean. The
island is only about
12 km long.

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