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Barrier Islands

Barrier islands

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 4,0006,000 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

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barrier island

barrier island An elongated ridge that may extend from a few hundred metres to 100 km along a coast forming a segmented barrier-bar complex, and found between two tidal inlets. Barrier-island systems have a lagoonal area on their landward side, and often have wind-blown dunes and vegetation on the exposed (seaward) side of the barrier. There are three main hypotheses to explain the origin of barrier islands: (a) the building up of submarine bars; (b) spit progradation parallel to the coast and segmentation by inlets; and (c) submergence of subaerial coastal beach ridges by a rise in sea level. Barrier islands are most common in areas of low tidal range.

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"barrier island." A Dictionary of Earth Sciences. . Encyclopedia.com. 26 Jun. 2017 <http://www.encyclopedia.com>.

"barrier island." A Dictionary of Earth Sciences. . Encyclopedia.com. (June 26, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/barrier-island

"barrier island." A Dictionary of Earth Sciences. . Retrieved June 26, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/barrier-island

barrier island

barrier island An elongated ridge that may extend from a few hundred metres to 100 km along a coast, forming a segmented barrier-bar complex and found between two tidal inlets. Barrier-island systems have a lagoonal area on their landward side, and often have wind-blown dunes and vegetation on the exposed (seaward) side of the barrier. There are three main hypotheses to explain the origin of barrier islands: (a)the building up of submarine bars;(b)spit progradation parallel to the coast and segmentation by inlets; and(c)submergence of subaerial coastal beach ridges by a rise in sea level. Barrier islands are most common in areas of low tidal range.

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"barrier island." A Dictionary of Ecology. . Encyclopedia.com. 26 Jun. 2017 <http://www.encyclopedia.com>.

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"barrier island." A Dictionary of Ecology. . Retrieved June 26, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/barrier-island-0

barrier island

barrier island A segmented barrier-bar complex found between two tidal inlets. Barrier-island systems have a lagoonal area on their landward side, and often have dunes and vegetation on the exposed barrier. There are three main hypotheses to explain the origin of barrier islands:
a. the building up of submarine bars;

b. spit progradation parallel to the coast and segmentation by inlets;
and
c. submergence of subaerial coastal beach ridges by a rise in sea-level. Barrier islands are most common in areas of low tidal range.

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"barrier island." A Dictionary of Plant Sciences. . Encyclopedia.com. 26 Jun. 2017 <http://www.encyclopedia.com>.

"barrier island." A Dictionary of Plant Sciences. . Encyclopedia.com. (June 26, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/barrier-island-1

"barrier island." A Dictionary of Plant Sciences. . Retrieved June 26, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/barrier-island-1