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The Origin of Barrier Islands

Stuart Bright

Dr. Chatelain

Geol. 1122K – B

22 Nov. 2010

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Stuart Bright

Dr. Chatelain

Geol 1122K – B

22 Nov. 2010

The Origin of Barrier Islands

According to the National Oceanic and Atmospheric Administration of the Coastal

Services Center, barrier islands are “elongated accumulations of sand that are separated from the

mainland by open water in the form of estuaries, bays, or lagoons.” These rare formations that

parallel the coast account for a mere fifteen percent of the entire amount of coastlines on the

globe today. Although they are found on every continent except Antarctica, the majority of them

are found along the border of the Atlantic Ocean. However, the northern coast of the Pacific that

stretches from Alaska to California also contains many barrier island formations. Barrier islands

are only able to subsist if their origin is along shallow marine or coastal, nearshore continental

shelve environments. These environments must be gently sloping and relatively shallow, or else

the island forming mechanisms would not be able to produce withstanding structures. Being so

highly valued among the biosphere today, it is essential to understand the origin, the importance,

and the future of these naturally occurring phenomena.

In order to fully understand the importance of barrier islands, it is necessary to first

become familiar with the theories of origin. And since they are constantly under change, it is

also relevant to distinguish the difference in environments at the time of origin and at the time of

current existence. As Dr. Lindley S. Hansen of Salem State College puts it, barrier islands are

“form[ed] in wave dominated or mixed energy environments” at their time of formation, and

they are “wave and wind built landforms” as they currently stand. This means that as easily as

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they may be formed (and from a number of different techniques), they may just as easily be

broken down and eroded back into loose sediments. By forming in a high energy location and

also being influenced by a high energy environment, barrier islands often are able to relocate at

an annual rate of movement that is clearly visible to the human eye as seen in Figure 1 below.

Figure 1. In sequence from left to right, these pictures of Upham Beach, FL show the effects of erosion and struggle to maintain beach nourishment techniques after a year.

This environment is also the factor for the next two points of discussion, in terms of importance

and what exactly lies in the future for barrier islands. But first, it is time to look into the theories

of origin.

One of the initial formation theories, presented by Elie de Beaumont in 1845, was a

model known as the Offshore Bar Theory. It suggested that through high energy wind and

waves, “shallow water churned up sand which was deposited in the form a submarine bar when

the waves broke and lost much of their energy” (FitzGerald and Buynevich 44). This offshore

accumulation would then eventually break the sea level, and it would continue to form in parallel

chains. However, this theory was later by Mckee and Steward in 1961 as they showed that such

a formation would not be able to withstand growth above sea level. This theory also raised the

question as to why we don’t see them forming along the coast in various stages today?

In 1885, a second formation theory derives from Gilbert and Fisher known as the Spit

Accretion Theory. The belief was that the material or sediment used to build up barrier islands

did not originate from offshore locations, but instead it came from alongshore sources that were

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agitated to create longshore transport buildups. These protruding formations were then breached

by a storm creating a channel, and the resulting isolated extension became the barrier island.

This process included coastal erosion from “up-stream” wave breaking locations, and deposition

of this newly eroded sediment. The Spit Accretion Theory is widely accepted today, and it has

the capability to easily explain why barrier islands form in elongated chain formations parallel to

the coastline.

The last, current theory encompasses a little portion of multiple ideas. One of the main

ideas is from Hoyt in 1967. He claimed that barrier islands are the result of onshore dunes

created by landward wind, and that these dunes were then, in fact, submerged as the sea level

rose above their base. The landward area of the now separated dunes would then flood as a bay

or lagoon. This theory, along with others that include the melting of glaciers and submergence

of coastline following a subduction, includes a drastic change in the sea level. When coupled

with the ever present sedimentation from rivers and beach erosion, it is apparent as to how

barrier islands may continue to form. This theory, as well as the other two models, is shown

below in Figure 2.

Figure 2. The left diagram depicts de Beaumont’s Offshore Bar Theory, the middle is Gilbert’s Spit Accretion Theory, and the right is Hoyt’s Submergence Theory.

Figure 3A (above) represents Jekyll Island’s tidal flat mud, while 3B (below) portrays a flooded flat.

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The next topic at hand corresponds to the importance that barrier islands play in terms of

their impact on the biosphere. Depending on the amount of inhabitable space, ranging from

hundreds of square miles to just 100 meters wide, barrier islands host varying conditions for

wildlife. The barrier natural habitats include, but are not limited to, the following: beaches,

dunes, barrier flats, and salt marshes. Beach habitats are areas consisting of low elevation on the

ocean side of the island. They are heavily influenced by salt water by being located in the

intertidal zone, and they maintain a small amount of wildlife – mostly scavenging birds or

burrowing/filter feeders. Dune habitats are more abundantly filled with creatures, because they

receive and are able to hold more fresh water from rainfall. During storms, dunes are often

susceptible to flooding, making them a prime location to find a plethora of crabs. They also

produce maritime forests, which can inhabit small mammals and snakes. Barrier flats, which are

low energy areas that are flooded daily, contain many

rich, decomposing plants and animals in the sediment.

As seen in Figures 3A and 3B, these muddy wetlands

are home to crabs, small invertebrates, and filter

feeding bacteria. Salt marshes are located on the

sound or channel side of the island. Similar to those

found on the coastal lowlands, salt marshes contain

much of the same inhabitants and flood plains. It is

evident to conclude that barrier islands are host to vast

array of naturally occurring wildlife. Likewise, they are

also extremely important for residential and commercial

use as tourism and firms seek new areas of growth.

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The importance of man-made development on barrier islands can only be labeled with

one word – dangerous. As it has been made clear in the past, developing such dynamic

ecosystems as barrier islands can drastically alter the way nature is meant to exist. From trying

to preserve the islands with jetties to filling in wetlands after dredging, man’s effects do nothing

but speed up the erosion process and endanger the animals. Figure 4 shows just what happens

when humans attempt to interject for their

own good. The hopes were to preserve a

jetty for water-way transportation, but in

successfully doing so, the natural erosion

process was halted on the north side and

accentuated on the south side. The

permanent destruction resulted in the south

portion being offset by nearly half a mile

toward the coastline. It is critical to

understand the effects that man-made structures can have on naturally occurring barrier islands.

The last and most important feature of barrier islands is their ability to protect the

mainland from storms and hurricanes. The barrier island does exactly what its name entails, and

that is to act as the first line of defense for the coastline. Although these events do greatly

reshape, erode, and even eliminate sections of barrier islands, they are also built back up through

gradual wave flow and shoreline deposition. It requires storms of extreme force like hurricanes

to damage the coastline where barriers are present, and even then, most of the wave impacts have

been absorbed and washed over the barriers, resulting in primarily wind and rain damages only.

It is vital to preserve as many barrier islands as possible and deem them prohibited of

Figure 4. Fenwick island of Ocean City, MA is separated and offset from Assateague island after a jetty system is developed.

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development for the shear purpose of environmental protection.

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Works Cited

1. http://w3.salemstate.edu/~lhanson/gls214/gls214_barrier_isl.htm

2. http://science.howstuffworks.com/environmental/conservation/issues/barrier-island3.htm

3. http://www.csc.noaa.gov/beachnourishment/html/geo/barrier.htm

4. http://www.2fla.com/barrier_islands.htm

5. FitzGerald, Duncan M., and Ilya V. Buynevich. "BARRIER ISLANDS." Encyclopedia of Sediments

& Sedimentary Rocks (2003): 43-47. Academic Search Complete. EBSCO. Web. 21 Nov. 2010.