Barrier islands are long, straight, or curved, narrow strips of clastic particles (that is, sands and silts) adjacent to coasts that have been built up by waves. They are located where land and sea come together. Between the barrier islands and the coast are usually lagoons, bays, and marshes. Tidal inlets cross the islands and
connect these bodies of quiet water with the ocean. Barrier islands and their adjacent lagoons account for more than 10 percent of the world's shorelines. They are typically found adjacent to broad and low coastal plains like those found in the eastern United States. Cross-sections of barrier islands typically have a profile, starting from the sea, that consists of the shore face, beach, dune, back-island flats or marshes, coastal bay or lagoon, and mainland. The shore face extends from the place where storm waves affect bottom sediment to the low-tide zone. The exposed part of the island begins at low tide, the outer limit of the beach, and continues to the base of the dunes.
The beach is composed of two parts, an inner section with a uniform slope that extends to a drop-off or berm, and an outer section where the slope begins a steeper angle to low water. Many beaches are within the intertidal zone, but the inner sections may be affected by wave action only during storms. The ocean side of the island is constantly changing as a result of the interplay between waves, currents, and wind. The highest elevation of the island is occupied by one or more rows of dunes that are constantly reshaped by wind. The dunes are low and occur only if there is a supply of sand and if the wind blows from the same direction most of the time. Behind the dunes are low areas composed of sediments that have been washed through or over the dunes during storms and fine material carried in by the wind.
Some barrier islands are only beaches. Between the barrier islands and the mainland are lagoons, bays, and estuaries, where tidal currents are important processes. Barrier islands provide protection and defense for the mainland from high surf and storms. Barrier islands along the East Coast from Long Island to Cape Canaveral are composed of clastic deposits derived from the erosion of headlands and from the seafloor, but farther south in more tropical regions barrier islands are composed, in part or entirely, of carbonate sediment derived from shells of organisms.
Barrier islands are frequently broken by channels that form after storms. Where they occur, tides usually rush through them carrying sand and silt to form deltaic deposits in the lagoon behind the island. Severe storms and hurricanes can alter the shape of barrier islands, which becomes evident when lighthouses have to be moved as a result of the disappearance of the island. Overuse of the islands, including real estate development, often leads to the destruction of vegetation that binds the loose sand together, eventually subjecting them to increased erosion and in some cases disappearance.
Barrier islands are also very sensitive to changes in sea level. Subject to higher energy forces on the seaward side than on the landward side, barrier islands retreat landward over time, generally 0.5 to 2 m, but up to 20 m in some places. All around the world they have been repeatedly exposed and drowned during the Ice Age as sea level rose and fell during the retreat and advance of the glaciers, resulting in their shifting of position in response to the changes. In northern Canada, where glacial rebound has raised the land upward, barrier islands can be found as much as 100 m above the adjacent sea.
See also: Atolls; Beaches; Coastal Wetlands; Coral
Reefs; Estuaries; Lagoons; Oceans; Tides
Davis, Richard A., Jr., ed. 1994. Geology of Holocene Barrier Islands. New York: Springer Verlag; Ham-blin, W. Kenneth, and Eric H. Christiansen. 2000. The Earth's Dynamic Systems. Upper Saddle River, NJ: Prentice Hall; Pilkey, O. H. 1990. "Barrier Islands." Sea Frontiers 36, no. 6: 30-36; Plummer, Charles C., David McGeary, and Diane Carlson. 2002. Physical Geology, 9th ed. New York: McGraw-Hill
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