Definition (Encyclopedia of Global Warming)
Barrier islands are narrow strips of land that are oriented parallel to the mainland and that lie on average a few kilometers offshore. Barrier islands are largely composed of sand-sized grains of sediment plus some finer sedimentary material and organic debris. They are separated from the mainland by an intervening body of water called a lagoon or bay. Barrier islands therefore have two shorelines, one facing the open ocean and the other facing the mainland. The side facing the open ocean is typically a much more energetic shoreline, experiencing higher waves and storm surges. The ocean-facing side is typically much straighter than the mainland-facing side, which is a low-energy coastal area.
Barrier islands may have several origins, but the most common is thought to be related to the post-ice age rise in sea level that commenced about eighteen thousand years ago. This sea-level rise inundated the world’s coastal areas, and many barrier islands probably formed from coastal sand ridges and dune fields that were surrounded by the rising sea. Once the barrier islands were established, sand that was eroded from land and washed along the shore by wave currents helped build and maintain the islands.
Barrier islands are nourished—that is, restocked with sand—by a process called longshore drift. Longshore drift is the movement of sand down the seaward coast of a barrier island, resulting from the effects of wind waves that...
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Significance for Climate Change (Encyclopedia of Global Warming)
Barrier islands are significantly affected by climate change. As global sea levels rise, barrier islands are at risk of being inundated or eroded away, as has been seen in many areas of the world. The loss of barrier islands or a diminution in their width and continuity is a direct effect of sea-level change in many places. Increased storm activity, a result of climate change in general and of changes in sea surface temperatures in particular, affects barrier islands because they are at the front line when a sea storm comes ashore. Storm wave energy is spent on barrier islands, and the result can be dissection of the barrier islands or their complete destruction.
Barrier islands are integral parts of typically fragile coastal ecosystems, which are home to many plant and animal species. If climate change causes the loss of barrier-island habitats for coastal species, these organisms will need to adapt or perish.
Barrier islands require a continual sand supply to persist. This supply can be interrupted in many ways, including through climate change. Climatic changes that affect local runoff, for example, might reduce the flow of rivers that bring sand to the coastline and nourish the barrier islands there. There are other ways to lose sand supply, including human intervention.
The potential loss of barrier islands represents the possibility that areas of mainland currently sheltered by barrier...
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Further Reading (Encyclopedia of Global Warming)
Morton, Robert A. Historical Changes in the Mississippi-Alabama Barrier Islands and the Roles of Extreme Storms, Sea Level, and Human Activities. Reston, Va.: U.S. Geological Survey, 2007. Extensive study of natural and anthropogenic effects upon the Mississippi-Alabama barrier islands; invaluable resource for understanding the diversity of influences impinging upon these islands.
Pilkey, Orrin H., and Mary Edna Fraser. A Celebration of the World’s Barrier Islands. New York: Columbia University Press, 2003. Very well illustrated popular-science book, coauthored by one of the world’s foremost experts on barrier islands, Pilkey. Discusses the natural history of barrier islands and their modern peril.
Pilkey, Orrin H., Deborah Pilkey, and Craig A. Webb. The North Carolina Shore and Its Barrier Islands. Durham, N.C.: Duke University Press, 2000. Scientific and ecological guide to one of the classic barrier island coastlines of the world, North Carolina and its Outer Banks. Provides an outstanding introduction to barrier-island studies and issues in a changing global environment.
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Barrier Islands (World of Earth Science)
A barrier island is a long, thin, sandy stretch of land, oriented parallel to the mainland coast that protects the coast from the full force of powerful storm waves. Between the barrier island and the mainland is a calm, protected water body such as a lagoon or bay. Barrier islands are dynamic systems, constantly on the move, migrating under the influence of changing sea levels, storms, waves, tides, and longshore currents. In the United States, barrier islands occur offshore where gently sloping sandy coastlines, as opposed to rocky coastlines, exist. Consequently, most barrier islands are found along the Gulf Coast and the Atlantic Coast as far north as Long Island, New York. Some of the better known barrier islands include Padre Island of Texas, the world's longest; Florida's Santa Rosa Island, composed of sugar-white sand; Cape Hatteras of North Carolina, where the first airplane was flown; and Assateague Island near Maryland, home of wild ponies.
Barrier islands are young in geologic terms. They originated in the Holocene Epoch, about 4,000,000 years ago. During this time, the rapid rise in sea level, associated with melting glaciers from the last ice age, slowed significantly. Although the exact mechanisms of barrier island formation aren't fully understood, this slowdown of sea level rise allowed the islands to form.
In order for barrier islands to form, several conditions must be met. First, there must be a source of sand to build the island. This sand may come from coastal deposits or offshore deposits (called shoals); in either case, the sand originated from the weathering and erosion of rock and was transported to the coast by rivers. In the United States, much of the sand composing barrier islands along Florida and the East Coast came from the Appalachian Mountains. Next, the topography of the coastline must have a broad, gentle slope. From the coastal plains of the mainland to the edge of the continental shelf, this condition is met along the Atlantic and Gulf Coasts. Finally, the forces of waves, tides and currents must be strong enough to move the sand, and of these three water movement mechanisms, waves must be the dominant force.
Several explanations for barrier island development have been proposed. According to one theory, coastal sand was transported shoreward as sea level rose, and once sea level stabilized, wave and tidal actions worked the sand into a barrier island. Another possibility is that sand was transported to its present location from shoals. Barrier islands may have formed when low-lying areas of spits, extensions of beaches that protrude into a bay as a result of deposition of sediment carried by longshore currents, were breached by the sea. Finally, barrier islands may have formed from sandy coastal ridges that became isolated from low-lying land and formed islands as sea level rose.
Once formed, barrier islands are not static landforms; they are dynamic, with winds and waves constantly reworking and moving the barrier island sand. Changes in sea level also affect these islands. Most scientists agree that sea level has been gradually rising over the last thousand years, and this rise could be accelerating today due to global warming. Rising sea level causes existing islands to migrate shoreward.
Barrier islands do not stand alone in a geologic sense. A whole system of islands develops along favorable coastlines. The formation of these islands allows other landforms to develop, each characterized by their dominant sediment type and by the water that helps form them. For example, each barrier island has a shoreline that faces the sea and receives the full force of waves, tides, and currents. This shoreline is often called the beach. The beach zone extends from slightly offshore (subtidal, or underwater) to the high water line. Coarser sands and gravels are deposited here, with finer sands and silts carried farther offshore.
Behind the beach are sand dunes. Wind and plants (such as sea oats) help form dunes, but occasionally dunes are inundated by high water and may be reworked by storm surges and waves. On wide barrier islands, the landscape behind the fore-dunes gently rolls as dunes alternate with low-lying swales (marshy wet areas). If the dunes and swales are well developed, distinct parallel lines of dune ridges and swales can be seen from overhead. These differences in topography allow some soil to develop and nutrients to accumulate despite the porous, sandy base. Consequently, some barrier islands are host to trees (which are often stunted), bushes, and herbaceous plants. Other narrower or younger barrier islands may be little more than loose sand with few plants.
On the shoreward side of the main body of the island is the back-barrier. Unlike the beach, this zone does not bear the full force of ocean waves. Instead, the back-barrier region consists of a protected shoreline and lagoon, which is more influenced by tides than waves. Occasionally, during storms, water may rush over the island carrying beach and dune sand and deposit the sand in the lagoon. This process, called rolling over, is vital to the existence of barrier islands and is the method by which a barrier island migrates landward. Characteristic sand washover fans in the lagoons are evidence of rolling over. Because the back-barrier region is sheltered, salt marsh, sea grass, and mudflat communities develop. These communities teem with plant and animal life and their muddy or sandy sediments are rich with organic matter.
Finally, barrier islands are characterized by tidal inlets and tidal deltas. Tidal inlets allow water to move into and out of bays and lagoons with rising and falling tides. Tidal inlets also provide a path for high water during storms and hurricanes. As water moves through an inlet, sand is deposited at both ends of the inlet's mouth, forming tidal deltas. Longshore currents may also deposit sand at the delta. Eventually the deltas fill in with sand and the inlet closes, only to appear elsewhere on the barrier island, usually at a low-lying spot. The size and shape of the inlet are determined by various factors, including the size of the associated lagoon and the tidal range, or the vertical height between high and low tide for the area. A large tidal range promotes the formation of numerous inlets, thereby creating shorter and wider barrier islands referred to as drumsticks. In addition, the larger the lagoon and the greater the tidal range, the deeper and wider the inlet due to the large quantity of water moving from ocean to lagoon and back. Deep, wide inlets occur where the main source of energy shaping the coastal area is tides or tides in conjunction with waves. In contrast, wave-dominated areas form long barrier islands with narrow bays and narrow, shallow inlets.
See also Beach and shoreline dynamics; Gulf of Mexico; Offshore bars; Tropical cyclone; Wave motions