Islands; volcanoes, coral atolls, and coral reefs

How Are Islands Formed?
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Islands form in several ways. The most common events that lead to island formation are volcanic activity and continental drift. Islands also form due to erosion, buildup of sediment and coral that grows enough to penetrate the surface of the water.
Volcanic islands are created as tectonic plates move over a lava flow. The lava flows up and cools, gradually building on itself to create islands that eventually break the surface of the ocean. The Hawaiian islands are a result of this process.
Continental drift is when continents break apart; a process that occurs over many hundreds of thousands or millions of years. This creates strings of islands between landmasses and along the edges of landmasses.
Erosion also creates islands. The strip of land connecting the lower portion of a peninsula to a land mass can erode, turning the peninsula into an island.
Buildup of sand and sediment create islands in some areas, including the Outer Banks of North Carolina and the barrier islands along the Georgia coast including St. Simon’s Island.
Similarly, coral reefs continue to grow from the sea floor toward the surface of the water and sometimes peak above the surface, forming a new island. Coral islands are most common in warm areas such as the Caribbean Sea.
How do volcanic islands form?
Volcanic islands form over hot spots, which occur when tectonic plate movement allows magma from the Earth’s core to erupt. As the volcano continues to erupt and cool, an island is formed.
The surface of the Earth is made of moving pieces of land called tectonic plates. When the plates collide, a process known as subduction occurs. Subduction happens when the lighter continental plate or crust pushes across the dense oceanic crust. The collision pushes the oceanic crust into the Earth’s hot layer, called the mantle. The crust melts and forces its way back up to the surface to form volcanoes that, in turn, allow the flow of magma to erupt. The magma flows out of the newly formed volcano in the form of lava and cools as it comes into contact with ocean water. In time, the volcano becomes large enough to break the surface of the ocean and islands form from the cooled lava.
Tectonic plates move regularly and as such, so do the volcanoes that form. When undersea volcanoes become extinct before breaking the surface, they form mounds that are known as seamount chains. When moving seamounts encounter active hotspots, volcanoes can become active above them to produce linear hotspot tracks. As the active volcanoes move, island chains, like Hawai’i and the Galapagos are formed.
What Is A Submerged Continent?
A submerged continent is a continent or a large landmass that is mainly under water. There are two known submerged continents, the Kerguelen Plateau and Zealandia. The rising levels of the sea are thought to have caused the sinking of the submerged lands.
The Kerguelen Plateau
The Kerguelen Plateau is a submerged continent located in the southern part of the Indian Ocean. The plateau is one of the Large Ingenious provinces of the world with an area of approximately 480,000 square miles. It is raised above the adjacent oceanic basins. The plateau is said to have originated from the Kerguelen hotspot either before or after the split of Gondwana. The plateau is believed to have once been a part of a continent 100 million years ago before it broke off and sunk. The landmass is thought to have submerged around 20 million years ago and now is at 3,300 feet to 6,600 feet below sea level. Some parts of the plateau are above sea level, and they are the Heard and McDonald Islands and the Kerguelen Islands. During the southern summer season, the southern part of the Kerguelen Plateau becomes home to several whale species that migrate to the area.
Zealandia
Zealandia is a continental crust that broke off from Australia about 85 million years ago and is partially beneath the Pacific Ocean. Bruce Luyendyk came up with the theory and name of Zealandia in 1995. The landmass is thought to have once been completely submerged about 23 million years ago, but today around 93% of the land surface is below sea level. Zealandia has an area of about 1,900,000 square miles and is thought to be the biggest existing micro-continent. In 2017 scientists argued that Zealandia has the characteristics of a continent and should be considered as such rather than a micro-continent. The total land area of Zealandia above water is 110,678 square miles, New Zealand and several neighboring islands make up 93% of the land area with an area of 103,471 square miles. New Caledonia and its surrounding islands make up 7% of the continent’s land area at 7,172 square miles; the remainder of the area is made up of Elizabeth and Middleton reefs, Norfolk Island, and Lord Howe Island. The Zealandia population is estimated at five million.
The Search for Lemuria
Lemuria was once, in the 1930s, theorized as a lost continent located either in the Pacific or Indian Oceans. This theory portrayed Lemuria as a land bridge, but this has been invalidated. When the theories of plate tectonics and continental drift gained acceptance by scientists, the theory about Lemuria was refuted. Some of the lands thought to have existed before getting submerged and lost include Sundaland, Dvaraka, Beringia, Maui Nui, Doggerland, Part of Malta, Strand, New Moore Island, Jordsand, Verdronken Land van Reimerswaal, Sarah Ann Island, Dunwich, Ferdinandea, and Ravenser Odd.

What is a Submerged Continent?
A submerged continent is a continent or a large landmass that is predominantly under water. There are two known submerged continents, the Kerguelen Plateau and Zealandia. The rising levels of the sea are thought to have caused the sinking of the submerged lands.

How Do Coral Reefs Form
Coral reefs begin to form when free-swimming coral larvae attach to submerged rocks or other hard surfaces along the edges of islands or continents. As the corals grow and expand, reefs take on one of three major characteristic structures —fringing, barrier or atoll. Fringing reefs, which are the most common, project seaward directly from the shore, forming borders along the shoreline and surrounding islands. Barrier reefs also border shorelines, but at a greater distance. They are separated from their adjacent land mass by a lagoon of open, often deep water. If a fringing reef forms around a volcanic island that subsides completely below sea level while the coral continues to grow upward, an atoll forms. Atolls are usually circular or oval, with a central lagoon. Parts of the reef platform may emerge as one or more islands, and gaps in the reef provide access to the central lagoon.
In addition to being some of the most beautiful and biologically diverse habitats in the ocean, barrier reefs and atolls also are some of the oldest. With growth rates of 0.3 to 2 centimeters per year for massive corals, and up to 10 centimeters per year for branching corals, it can take up to 10,000 years for a coral reef to form from a group of larvae (Barnes, 1987). Depending on their size, barrier reefs and atolls can take from 100,000 to 30,000,000 years to fully form.
Was Darwin Wrong About Coral Atolls?
Charles Darwin sparked more than one controversy over the natural progression of life. One such case involved the evolution of coral atolls, the ring-shaped coral reefs that surround submerged tropical islands. Coral reefs are actually huge colonies of tiny animals that need sunlight to grow. After seeing a reef encircling Moorea, near Tahiti, Darwin came up with his theory that coral atolls grow as reefs stretch toward sunlight while ocean islands slowly sink beneath the sea surface. (Cooling ocean crust, combined with the weight of massive islands, causes the islands to sink.)

A century-long controversy ensued after Darwin published his theory in 1842, because some scientists thought the atolls were simply a thin veneer of coral, not many thousands of feet thick as Darwin proposed. Deep drilling on reefs finally confirmed Darwin’s model in 1953. But reef-building is more complex than Darwin thought, according to a new study published May 9 in the journal Geology. Although subsidence (shrinking of a land mass) does play a role, a computer model found seesawing sea levels, which rise and fall with glacial cycles, are the primary driving force behind the striking patterns seen at islands today. “Darwin actually got it mostly right, which is pretty amazing,” said Taylor Perron, the study’s co-author and a geologist at MIT. However, there’s one part Darwin missed. “He didn’t know about these glacially induced sea-level cycles,” Perron told OurAmazingPlanet.
What happens when sea-level shifts get thrown into the mix? Consider Hawaii as an example. Coral grows slowly there, because the ocean is colder than waters closer to the equator. When sea level is at its lowest, the Big Island builds up a nice little reef terrace, like a fringe of hair on a balding pate. But the volcano — one of the tallest mountains in the world, if measured from the seafloor — is also quickly sinking. Add the speedy sea-level rise when glaciers melt, and Hawaii’s corals just can’t keep up. The reefs drown each time sea level rises. The researchers also found that one of the few places in the world where sinking islands and sea-level rise create perfect atolls is the Society Islands, where Darwin made his historic observations.