Barrier Islands

Currently existing barrier islands are geologically young features that formed during the Holocene epoch (approximately the last 10,000 years). During that time, the rapid rise in sea level associated with melting glaciers slowed significantly. Although the exact mechanisms of barrier island formation are not fully understood, the decreasing rate of sea level rise allowed the islands to form.

Several conditions must be met in order for barrier islands to form. First, there must be a source of sand to build the island. The 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. Second, the topography of the coastline must have a broad, gentle slope. This condition occurs from the coastal plains of the mainland to the edge of the continental shelf along the Atlantic and Gulf Coasts. Finally, the waves, tides and currents in the area of a future barrier island must be strong enough to move the sand. Waves must be the dominant of these three water movement mechanisms.

Several explanations for barrier island development have been proposed. According to one theory, coastal sand is transported shoreward as sea level rises and, once sea level stabilizes, waves and tides work the sand into a barrier island. Another possibility is that sand is transported from shoals. Some barrier islands may form when A barrier island in the Cape Hatteras National Seashore Park, off the coast of North Carolina. JLM Visuals. Reproduced by permission.

low-lying areas of spits, extensions of beaches that protrude into a bay, are breached by the sea. Finally, barrier islands may form from sandy coastal ridges that become isolated and form islands as sea level rises.

Once formed, barrier islands do not persist as static landforms. They are dynamic features that are continuously reworked by wind and waves. Changes in sea level also affect barrier islands. Most scientists agree that sea level has been gradually rising over the last thousand years, and this rise may be accelerating today due to global warming. Rising sea levels cause existing islands to migrate shoreward, and barrier islands off the Carolina coast are thought to have migrated 40-50 mi (64-80 km) during the Holocene epoch.