Island, Reef & Atoll Formation: Steps & Stages

Instructor: Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

Special features form in oceans where the water is relatively warm, and volcanic activity was present at some point in time. This lesson will go through the process involved in forming islands, reefs, and atolls.

Atolls and World War II

Over four days in late November, 1943, United States military forces fought a battle on Betio Island, in the Tarawa Atoll, which is roughly located in the middle of the Pacific Ocean. Battles on small islands like these were part of the United States' campaign of island hopping. This referred to the gradual accumulation of land bases closer and closer to Japan. This would not have been possible if it weren't for the thousands of years of geologic and biologic activity that created these islands.

Types of Islands

Volcanic Islands

In weak areas in the earth's crust, magma often rises to the surface. Magma is molten rock below the surface, and lava is molten rock that has reached the surface. When an underwater volcano, or, 'seamount,' erupts, lava accumulates, and eventually a volcanic island forms. The Pacific Ocean is known for this activity, which is why it has been dubbed the ring of fire. The islands of Hawaii, for instance, are volcanic islands in the Pacific Ocean.

Continental Islands

Due to earthquakes, lowering sea levels, or erosion, parts of a large land mass can become separated from the mainland. A new island is formed.

Tidal Islands

These are segments of land that are still part of the mainland, but are underwater at high tide. At low tide they're above and surrounded by water -- an island.

Barrier Islands

Barrier islands are formed from the accumulation of sediments or coral rocks, which we will learn about in the next section. Ocean waves transport all sorts of sediments that accumulate, and eventually reach the point where they break the surface of the water. Glaciers have also created islands of sediments. They pick up sediment when they advance. When the glacier melts, that material is left behind. If it is near a body of water, an island is formed. Long Island, New York is an example of a barrier island.

Coral Islands

The oceans are teeming with life, and among them are organisms called hermatypic corals. These corals secrete calcium carbonate as an external skeleton made of rock. A single coral polyp can divide, generating multiple polyps that live in large groups. These polyps live in warm, shallow waters, and attach themselves to the sea floor. If sea level drops, the coral rock is exposed, and we have a coral island.

Artificial Islands

Human beings can piled up sediment in a body of water, and create an artificial island. Sand from the Persian Gulf has been pumped close to the shores of Dubai creating artificial islands.

Forming an Atoll

Corals, which we learned about earlier, form rings around the underwater portions of volcanic islands. If the volcanic activity creating a volcanic island stops, no new above-water territory is formed. The coral colonies' growth doesn't stop, though, and the volcanic island starts to sink due to the increasing weight of the coral colonies. This process is called subsidence. In addition, erosion from the constant wave action, wind, and rain also decrease the height of the island above the water, giving it a flat surface called a guyot.


Subsidence and coral growth
atoll

Even though the volcano may not be growing anymore, the coral polyps continue building vertical colonies reaching toward the surface. Theses coral colonies serve as a reef barrier, creating an atoll. The atoll has a central lagoon that is protected from the open ocean. Different ecological systems develop in the atoll due to the lagoon's still waters.


Atafu Atoll in South Pacific Ocean
Atafutrim

A way to remember the order of the atoll formation is the acronym VC-SEA, pronounced 'vic-sea'.

  • V is for volcano
  • C is for coral growth
  • S is for subsidence
  • E is for erosion
  • A is for atoll formed

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