Lagoons: How Sinking Islands and Sandbars Make Sheltered Waters

A lagoon is a body of water walled off from a larger one by a natural barrier — a sandbar, a barrier reef, a coral reef. The name comes from the Italian laguna, meaning "pond" or "lake." But not every place called a lagoon is one, and the two real kinds form in very different ways and on very different timescales.

A NASA satellite image of the lagoons and reefs of New Caledonia, a French-governed archipelago with the world's third-largest coral reef structure. Credit: NASA.

Names can mislead

Lagoons are well defined geographically, but the word is sometimes used for a larger area that contains lagoons, or for something that is not a lagoon at all. Laguna Madre on the Texas Gulf Coast is actually made up of smaller bays and lagoons. Laguna Beach in Southern California is a beach, not a lagoon.

Atoll lagoons: made by a sinking island

An atoll lagoon forms when an island sinks completely beneath the water, leaving a ring of coral that keeps growing upward. At the center of the ring is water, often deep. Coral growth and water together make the lagoon. Forming an atoll can take as long as 300,000 years.

Coastal lagoons: young and exposed

Coastal lagoons form along gently sloping coasts. They are generally shallower than atoll lagoons and are usually separated from the ocean by an island, reef or sand bank, with an inlet connecting them to the sea most of the time. Sea level rise, the amount of sediment available and the tidal range all help shape them.

Coastal lagoons are younger and more changeable than atoll lagoons, and because they sit exposed on the shore, their "lifespans" may be shorter.

Sources

Based on "What is a lagoon?" from NOAA's National Ocean Service, a work of the United States government in the public domain.