Barrier islands are a coastal landform and a type of dune system that are
exceptionally flat or lumpy areas of sand which form by wave and tidal action
parallel to the mainland coast. They usually occur in chains, consisting of
anything from a few islands to more than a dozen. They are subject to change
during storms and other action, but absorb energy and protect the
coastlines, as well as creating areas of protected waters where wetlands may
flourish. A barrier chain may extend uninterrupted for over a hundred
kilometers, excepting the tidal inlets that separate the islands, the longest
and widest being Padre Island of Texas. The length and width of barriers and
overall morphology of barrier coasts are related to parameters including tidal
range, wave energy, sediment supply, sea-level trends and basement controls.
Chains of barrier islands can be found along approximately thirteen percent of
the world's coastlines. They display different settings, suggesting that they
can form and be maintained in a variety of environmental settings. Numerous
theories have been given to explain their formation.
Formation Scientists have proposed numerous
explanations for the formation of barrier islands for more than 150 years.
There are three major theories: offshore bar, spit accretion, and submergence. No
single theory can explain the development of all barriers, which are
distributed extensively along the world's coastlines. Scientists accept
the idea that barrier islands, including other barrier types, can form by a
number of different mechanisms. There appears to be some general
requirements for formation. Barrier island systems develop most easily on
wave-dominated coasts with a small to moderate tidal range. Coasts are
classified into three groups based on tidal range: microtidal, 0–2 meter tidal
range; mesotidal, 2–4 meter tidal range; and macrotidal, >4 meter tidal range.
Barrier islands tend to form primarily along microtidal coasts, where they tend
to be well developed and nearly continuous. They are less frequently
formed in mesotidal coasts, where they are typically short with tidal inlets
common. Barrier islands are very rare along macrotidal coasts. Along with a
small tidal range and a wave-dominated coast, there must be a relatively low
gradient shelf. Otherwise, sand accumulation into a sandbar would not
occur and instead would be dispersed throughout the shore. An ample sediment
supply is also a requirement for barrier island formation. The last major
requirement for barrier island formation is a stable sea level. It is especially
important for sea level to remain relatively unchanged during barrier
island formation and growth. If sea level changes are too drastic, time will
be insufficient for wave action to accumulate sand into a dune, which will
eventually become a barrier island through aggradation. The formation of
barrier islands requires a constant sea level so that waves can concentrate the
sand into one location. = Offshore bar theory =
In 1845 the Frenchman Elie de Beaumont published an account of barrier
formation. He believed that waves moving into shallow water churned up sand,
which was deposited in the form of a submarine bar when the waves broke and
lost much of their energy. As the bars developed vertically, they gradually
rose above sea level, forming barrier islands.
= Spit accretion theory = American geologist Grove Karl Gilbert
first argued in 1885 that the barrier sediments came from longshore sources.
He proposed that sediment moving in the breaker zone through agitation by waves
in longshore drift would construct spits extending from headlands parallel to the
coast. The subsequent breaching of spits by storm waves would form barrier
islands. = Submergence theory =
William John McGee reasoned in 1890 that the East and Gulf coasts of the United
States were undergoing submergence, as evidenced by the many drowned river
valleys that occur along these coasts, including Raritan, Delaware and
Chesapeake bays. He believed that during submergence, coastal ridges were
separated from the mainland, and lagoons formed behind the ridges. He used the
Mississippi-Alabama barrier islands as an example where coastal submergence
formed barrier islands. His interpretation was later shown to be
incorrect when the ages of the coastal stratigraphy and sediment were more
accurately determined. Along the coast of Louisiana, former
lobes of the Mississippi River delta have been reworked by wave action,
forming beach ridge complexes. Prolonged sinking of the marshes behind the
barriers has converted these former vegetated wetlands to open-water areas.
In a period of 125 years, from 1853 to 1978, two small semi-protected bays
behind the barrier developed as the large water body of Lake Pelto, leading
to Isles Dernieres's detachment from the mainland.
= Boulder Bank = An unusual natural structure in New
Zealand may give clues to the formation processes of barrier islands. The
Boulder Bank, at the entrance to Nelson Haven at the northern end of the South
Island, is a unique 13 km-long stretch of rocky substrate a few metres in
width. It is not strictly a barrier island, as it is linked to the mainland
at one end. The Boulder Bank is composed of granodiorite from Mackay Bluff, which
lies close to the point where the bank joins the mainland. It is still debated
what process or processes have resulted in this odd structure, though longshore
drift is the most accepted hypothesis. Studies have been conducted since 1892
to determine the speed of boulder movement. Rates of the top-course gravel
movement have been estimated at 7.5 metres a year.
Ecological importance Barrier islands are critically important
in mitigating ocean swells and other storm events for the water systems on
the mainland side of the barrier island, as well as protecting the coastline.
This effectively creates a unique environment of relatively low energy,
brackish water. Multiple wetland systems such as lagoons, estuaries, and/or
marshes can result from such conditions depending on the surroundings. They are
typically rich habitats for a variety of flora and fauna. Without barrier
islands, these wetlands could not exist; they would be destroyed by daily ocean
waves and tides as well as ocean storm events. One of the most prominent
examples is the Louisiana barrier islands.
See also Island welding
Outer Banks Virginia Barrier Islands
New York Barrier Islands Texas barrier islands
Sea Islands Long Beach Island
Bald Head Island Notes
References Davis Jr., Richard A.; Fitzgerald,
Duncan M., Beaches and Coasts, United Kingdom: Blackwell Publishing, ISBN
0-632-04308-3 Morton, Robert A., Historical Changes in
the Mississippi-Alabama Barrier Islands and the Roles of Extreme Storms, Sea
level, and human activities, U. S. Geological Survey