Earth Science: Earth – Islands and Atolls
Earth’s largest island is Greenland, with an area of over 830,000 square miles. The smallest islands are just dots in the ocean. Many islands are part of a continent; others owe their existence to volcanic eruptions. Islands are categorized into continental and oceanic islands. Continental or shelf islands were once connected to the mainland.
Rising sea levels or subsiding continents have caused former mainland areas to become flooded. This explains, for instance, the island location of Great Britain or Madagascar. The long and low-humped islands off the Swedish coastline are peaks of flooded mainland areas. Dune islands are formed by wind and oceanic currents.
Oceanic islands have never been part of other landmasses. They are usually of volcanic origin. Where tectonic plates are pulling apart and then drifting away from each other, mid-oceanic ridges develop that sometimes protrude above sea level as islands, such as Iceland or the Azores Volcanic island arches, such as the Aleutians, are formed when one oceanic plate is pushed underneath another and then continues to subside (subduction).
Islands are also formed as a result of hot spots, where magma rises from the Earth’s lower mantle. When the oceanic crust breaks apart, the volcanic cone that has been created is moved and along with it the island protruding above sea level. A new volcanic cone is formed above the stationary hot spot, leading to the birth of a new island. This way, long island chains can develop, with volcanoes still active at the geologically youngest end of the chain. The Hawaiian Islands have been formed this way. In tropical waters, reef-building corals often build their colonies along rocky island coast- lines in the surf zone.
When the seafloor subsides due to subduction of the respective crust plate, the continuously growing reef moves further away from the coast. When the subsiding seafloor eventually submerges the entire island below sea level, the former reef continues to grow as a ring-shaped wall and then appears as an atoll. If the atoll sinks faster than the corals grow, the coral polyps end up at a depth where they can no longer survive due to the lack of sufficient light. The reef dies and the atoll disappears.
DROWNING ISLANDS
The rise of the sea level due to climate change endangers the existence of numerous Pacific islands. Based on estimates, atolls protruding only a few feet above sea level, such as Kiribati, Tuvalu, or the Marshall islands are threatened with becoming totally sub merged within the next few decades.
Continuous melting of Ice masses would also have fatal consequences for the Maldives In the Indian Ocean. Moreover, numerous Caribbean islands would be affected by the concurrent occurrence of cyclones.
BASICS
ARTIFICIAL ISLANDS More and more over-crowded areas are relocating their infra- structure onto water.
Whether in the form of an airport in Osaka, a city section in Amsterdam or an exclusive tourist island in Dubai, the trend toward artificial islands seems unstoppable.