Barrier Islands: Formation, Shape & Location

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  • 0:04 What are the Barrier Islands?
  • 0:44 Barrier Islands Formation
  • 2:29 Islands' Change Over Time
  • 4:05 Barrier Islands Impact
  • 4:42 Lesson Summary
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Lesson Transcript
Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson, we'll define barrier islands and learn how they form. We'll also look at how these types of islands, unlike other islands, can suddenly change shape or shift location over time. Lastly, we'll cover why these land masses are important to coastal areas.

What Are the Barrier Islands?

Picture the North Carolina shore, a classic East Coast beach. White sand and choppy waves make the Outer Banks, small islands off the coast, an ideal vacation spot. Tall grasses line a wooden path, transporting you to calming beaches dotted with lighthouses just off the shore. These islands are known as a barrier islands.

Barrier islands are a type of island made up of long stretches of sand that are parallel to the mainland. They form estuaries and lagoons that are essential for sea life. We'll look at how barrier islands form and how they continue to change over time. Lastly, we'll look at the impact barrier islands have on coastal environments.

Barrier Islands Formation

There are three main theories that explain how barrier islands form. The earliest theory was proposed by Elie de Beaumont in 1845. The offshore bar theory proposes that individual barrier islands formed when waves stirred up sediment from the sea floor. When the waves broke, their energy was used to deposit sand into a bar along the sea floor. Over many years, the sand accumulated vertically until a sandbar, and eventually, a barrier island was formed. However, there were some flaws with this theory. For example, studies done in tanks showed that bars wouldn't grow above sea level. Other opposing theories followed.

Grove Karl Gilbert, an American geologist, proposed an alternate theory in 1885 called the spit accretion theory. Instead of sediment coming from offshore sources like Beaumont suggested, Gilbert reasoned that the sediment that created barrier islands could come from the mainland coast. He suggested that breaker waves stir up sediments to form a structure called a spit, a narrow landmass similar to a barrier island but attached to the coast at one end. Over time, storms would break up the spit's connection with the mainland, forming a barrier island.

Lastly, William John McGee suggested in his 1890 submergence theory that barrier islands could be formed by submerging mainland coastal structures. The Mississippi Delta area along the Louisiana coast has been changed over time into barrier islands through this mechanism. The waves formed beach ridge complexes, which protrude out of the water. This, combined with the sinking of the mainland marsh, has formed large stretches of open water. With the beach ridges sticking out, a protected area was formed near the coast, creating barrier islands nearby.

Islands' Changes Over Time

Unlike other landmasses, which exhibit almost imperceptible changes over time, barrier islands are constantly being shaped by wind, waves, and currents. Longshore currents are currents that run parallel to the shore. The angle at which the waves hit the beach from these currents determine how sediment will build up on the island. For example, if the winds come from the northwest, sand builds up on the south end of the island. The process of sand building up to create new island structures is called accretion, like the accretion theory proposed by Gilbert.

Waves not only build up islands through accretion, they can also tear them apart, even multiple times a year. For example, in Georgia, when soft summer waves hit the shore, they bring sediment from the sea, allowing the island to increase in size. But heavy storms in the winter bring high energy waves, which remove sediment from the island and cause it to decrease in size.

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