Islands are landmasses surrounded by water that are smaller than continents. Their formation is the result of distinct geological and biological processes operating over millions of years. From the upwelling of molten rock to the accumulation of organic material, islands represent the Earth’s constant process of creation and transformation.
Islands Forged by Volcanic Activity
Some islands are born entirely from the ocean floor when magma rises and solidifies, creating new land far from continental margins. This magmatic upwelling occurs through two primary mechanisms that shape oceanic islands.
One type of formation happens far from plate boundaries at hotspots, which are fixed plumes of hot material rising from deep within the Earth’s mantle. As a tectonic plate moves over this stationary plume, magma repeatedly punches through the crust, forming a chain of volcanoes. The Hawaiian Islands exemplify this process, with the active volcano situated directly over the hotspot and the islands growing progressively older farther along the chain.
A different volcanic process creates island arcs along zones where two oceanic plates converge. In these subduction zones, one plate slides beneath the other. The resulting pressure and heat cause the sinking plate to release water into the overlying mantle, which causes the mantle rock to melt. The resulting magma rises to the surface, forming a curved chain of explosive volcanoes parallel to the deep ocean trench. The Aleutian Islands in Alaska are a prominent example of this formation.
Islands Separated by Shifting Continents
Continental islands are fragments of pre-existing, larger landmasses, not new creations. These islands possess continental crust, which is thicker and less dense than the oceanic crust underlying volcanic islands. Their existence is primarily due to the movement of tectonic plates and shifts in global sea level.
The process of continental rifting can tear a large landmass apart, isolating a fragment as a major island. Madagascar, for example, was once nestled between Africa and India in the supercontinent Gondwana. It separated from Africa and later from India through rifting events that physically separated the island from its neighboring continents.
Other continental islands are separated due to the flooding of low-lying coastal areas. During glacial periods, massive ice sheets lock up ocean water, causing sea levels to drop significantly and exposing continental shelves, often creating temporary land bridges. As the ice melts during warmer periods, the sea level rises, flooding the lower coastal terrain and isolating higher ground that was once part of the mainland. The British Isles were connected to continental Europe by a land bridge that was submerged by rising sea levels around 8,000 years ago.
Islands Built by Living Organisms
A unique class of islands is constructed almost entirely by living organisms, primarily corals. These biological structures form the foundation of tropical islands, often starting where initial volcanic activity provided a base in the open ocean.
The formation begins when tiny marine animals called coral polyps secrete calcium carbonate to build hard, protective skeletons. Coral reefs start as fringing reefs growing directly along the coastline of an island. As the underlying volcanic island slowly subsides or the sea level rises, the coral continues to grow upward to stay within the shallow, sunlit water it needs to survive.
This upward growth, separated from the sinking landmass by a deep lagoon, transforms the structure into a barrier reef, such as the Great Barrier Reef. If the central volcanic island completely sinks beneath the ocean’s surface, the continuous ring of coral remains, encircling a central lagoon. This final ring-shaped structure is called an atoll.
Islands Shaped by Sediment and Erosion
A final type of island formation involves the constant motion of water, which deposits and rearranges loose material near coastlines. These islands are often transient features, constantly subject to the forces of erosion and deposition.
Barrier islands are long, narrow accumulations of sand that run parallel to the mainland shore, typically separated from the coast by a lagoon or bay. They form when wave action and ocean currents repeatedly deposit sediment in shallow offshore areas. Strong longshore currents transport sand along the coast, building up underwater bars that eventually break the surface. The Outer Banks off the coast of North Carolina is a well-known example of this type of island chain.
Another type of sediment-based island forms at the mouth of large rivers, resulting in deltaic islands. When a river carrying large amounts of eroded material meets the still water of the ocean, the flow slows down dramatically, causing the suspended sediment to settle out. This deposition builds up the river delta. The river often splits into multiple channels called distributaries, with islands forming between these channels. The Mississippi River Delta region features numerous such islands, built from the river’s massive sediment load.