How are barrier islands formed? theories and facts

Barrier islands are formed by different mechanisms. There are various theories that are used in the explanation of barrier island formation but none can explain the formation of all the different types. One theory can describe the process of formation of a certain type of barrier island but may not fit the description of another. We will discuss the 3 major theories of barrier island formation and then describe a few other theories that may be important. But before we know how they are formed, let us also know what they are.

What are barrier islands?

A barrier island is a deposit of sand that is formed parallel to the coast of the mainland. They often occur as chains of islands and less as a single island. A barrier island can be moved, eroded, or even disappear largely as a result of wind or tidal waves which continuously alter the position, size, and shape of these barrier islands.

How are barrier islands formed?
Great Barrier Island of New Zealand (Aotea Island)

Facts and Characteristics of barrier islands

  1. They are constantly changing and not stable because the same forces that form these barrier islands also alter them.
  2. Barrier islands often occur as chains and less as a single island.
  3. The islands are parallel to the coast, and therefore protect the coastal settlements from the impact of harsh weather such as flooding or storm.

How are barrier islands formed?

Barrier islands might form in many ways: they may extend out from an existing island, be formed by the deposition of sediment over water currents, or when waves push against a shoreline. Barrier islands are constantly changing due to erosion from weather and tides, but they are very long-lasting features with an average lifespan of 10,000 years.

3 major theories for the formation of barrier islands

The major theories for the formation of barrier islands are outlined below. The three theories are presented in order of which they were introduced but there is no real consensus on which theory is most correct.

In the Wave-dominated barrier island formation, the question of “What causes a limit of outward growth?” still needs answers.

For the Tectonically-controlled barrier island formation, the evidence includes barrier islands on some continental plates that do not contain evidence of shoreline erosion such as South America and Africa.

While the Rising sea level/Isostatic rebound theory, does not explain “What causes isostatic rebound?”

Wave dominated barrier island formation

The first theory for the formation of barrier islands is that they are formed by longshore currents, waves, and tides. This includes both subaerial (land) processes, such as wave erosion and deposition, and subaqueous (underwater) processes, such as the rise and fall of sea levels. At certain times in geologic history, conditions may favor the crossing of shorelines by rivers or tidal currents during periods when sea level is relatively high compared to the land along a given coast. When water levels recede due to either tectonic or climatic causes, this process stops abruptly leaving behind barriers separated from the mainland by creeks or be filled with sediment. The major question for this theory is whether or not there should be a limit to the outward growth of barrier islands, and if so, what causes it.

Tectonically-controlled barrier island formation

The second major theory is that barriers are formed by tectonic forces along convergent plate boundaries. This includes both active and passive margins. In an area where two plates move toward each other, a mid-ocean ridge may form from volcanic activity due to rising magma from deep inside the earth’s core. Over time, lava flows harden forming a new ocean floor which pushes older ocean crust further down into the mantle. In areas where two plates collide, one plate may form a subduction zone as one plate slides beneath another causing deeper sections of the ocean floor to be pushed downward. Evidence for this theory includes the discovery of barrier islands on some continental plates that do not contain evidence of shoreline or tidal erosion such as South America and Africa.

Rising sea level/Isostatic rebound theory

The third major theory is that barriers form by eustacy (rising seas) and isostasy (balance of forces within earth’s crust) during periods where sea levels rise higher than previously and new land is formed from sediment deposition. It is theorized that ice ages cause an overall lowering of sea level due to large amounts of water being tied up in glaciers at the poles. This creates greater land areas across the earth’s surface, including land bridges across continents. When ice sheets begin to melt due to climatic changes, the water returns to the oceans causing sea-level rise around the world. Some land areas gain elevation rapidly through isostatic rebound, others slowly or not at all.

Pending questions on the theories of how barrier islands are formed

The three theories are presented in order of which they were introduced but there is no real consensus on which theory is most correct.

In the Wave-dominated barrier island formation, the question of “What causes a limit of outward growth?” still needs answers.

For the Tectonically-controlled barrier island formation, the evidence includes barrier islands on some continental plates that do not contain evidence of shoreline erosion such as South America and Africa.

While the Rising sea level/Isostatic rebound theory, does not explain “What causes isostatic rebound?”

This short video below answers the question based on the first theory of wave action.

Video explaining how barrier islands are formed by waves.

FAQs on how barrier islands are formed

Are barrier islands formed by erosion?

Yes, barrier islands are typically formed by erosion.
Erosion occurs when waves hit the coast and pull material closer to the ocean, creating sandbars. Over time, the sandbars can grow in size and eventually become barrier islands.
If waves hit the island perpendicularly, they can easily erode the island back into the sea; but if waves hit the island at an angle or break on it, erosion becomes difficult and sand can pile up instead, creating a bigger island.

What are barrier islands primarily made from?

Barrier islands are made from sand or gravel that has been deposited by water or wind. These types of islands can be found on all continents except Antarctica. They are found on all coasts but are more common on low-energy shorelines.

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