Islands are land areas smaller than a continent that are completely surrounded by water. Islands range in size from islets (small islands) barely exposed at high tide, to vast landmasses almost the size of continents. Islands exist in all the ocean basins (the deep part of the ocean floor), along coastlines, and in freshwater lakes and rivers.
Islands come in many sizes and shapes, but they all share the same defining characteristics. There are more similarities than differences between a huge arctic island like Greenland and a small tropical one like Guam.
Islands are isolated. The water around them controls their climate and weather. The British Isles, which include Great Britain and Ireland, have a mild climate for their northern location because they lie in the path of the warm Gulf Stream current (a current is a steady flow of water on a prevailing direction).
The Galapagos Islands, located on the equator in the Pacific Ocean, are kept surprisingly cool due to the rising of cold water from the deep ocean. Islands have limited areas for catching rainwater or snow, and fresh water is generally scarce, particularly on small islands in arid (extremely dry) areas.
The majority of islands are remote and difficult for land plants and animals to reach. They often support unique ecosystems (groups of organisms that live in a particular environment) that have evolved (changed over time) with little influence from surrounding areas.
Geologic (natural Earth processes) activities form islands through various ways, including by raising a piece of seafloor above the water surface, or by separating an area of land from the edge of a continent. Volcanoes and corals (small crustacean animals that live in shallow parts of the ocean, building coral reefs with their shells) both construct mounds on the seafloor that can become exposed islands. Chunks of land can be separated from continents.
When the global sea level rises or falls because of melting and freezing of ice on the North and South Poles, islands can be exposed or submerged. Islands also sink below sea level under the weight of volcanic lava flows or ice, and rise when the rock is naturally worn away from their surfaces. Many islands form by a combination of these geologic processes.
Many islands are the exposed peaks of active and inactive seafloor volcanoes. An active volcano is one that has erupted in the past and is likely to do so again; an inactive volcano is no longer likely to erupt. Chains of volcanic islands, including the Aleutian Islands, Japan, the Philippines, and the Solomon Islands form the “ring of fire” that surrounds the Pacific Ocean.
These island chains, called volcanic arcs, lie along plate tectonic boundaries, where one massive plate of Earth’s outer layer is sinking beneath a second plate, in a process called subduction. (Plate tectonics is the theory that Earth’s rocky outer layer is broken into pieces, or plates, that move over time.) The islands of Indonesia and Borneo lie over a subduction zone in the northwestern Indian Ocean. Cuba, Hispanola, Puerto Rico, and the Lesser Antilles are volcanic arcs in the Caribbean Sea.
Other types of volcanoes also build islands and seamounts. (A seamount is an underwater mountain whose peak does not extend above the water surface. Most seamounts are volcanoes that never grew tall enough to become exposed islands, but some are former islands that have been submerged by rising sealevel or their own sinking.)
Sicily in the Mediterranean Sea, Catalina Island off southern California, and the remote Kerguelan Islands in the southern Indian Ocean are also volcanic islands. Some islands are created from hot spot volcanoes. A hot spot is a stationary heat source under a moving tectonic plate. As the plate moves over the hot spot, a line of volcanoes forms above it. The Hawaiian Islands, Iceland, and more than six hundred small islands in the southwest Pacific Ocean are hot spot volcanoes.
Erosion (wearing away of material by natural forces) and deposition by nearshore currents and waves shape sediment (sand, gravel, and silt) into islands along coastlines. These processes can form barrier islands, which are long, narrow coastal islands built up parallel to the mainland. Barrier island chains border the edges of continents that receive large amounts of sediment from rivers.
In the Gulf of Mexico, a strong current has built a continuous chain of barrier islands along the coasts of Texas and northern Mexico by dragging sediment away from the Mississippi River Delta. Tides cut narrow strips of water in the land that separate the sandy barrier strip into islands and carry water into the lagoons (a shallow area of water separated from the ocean by a coral reef or sandbar) between the islands and the mainland.
Barrier islands like North Carolina’s Outer Banks and South Padre Island in Texas are essentially large sandbars (long strips of sand) that are constantly being reshaped by currents, waves and winds. A large hurricane can completely destroy a barrier island—houses and all—and reshape the sediment into a new island within a few days.
Not all barrier islands and coastal islands are made of sand. The Florida Keys and barrier islands of northeast Australia are reefs that fringe continental shelves (the edge of land that slopes into the sea) away from major rivers where corals grow in clear waters.
Long Island, Nantucket, and the other islands off southern New England are huge piles of rock fragments and boulders left by glaciers (large masses of moving ice) that retreated at the end of the last ice age about twenty thousand years ago.
Some coastlines, like those of Maine, northern California, and Norway are rising from the ocean because of shifting tectonic plates on Earth. Islands off these rising coasts are fragments of sea cliffs and rock that resisted erosion toward the mainland.
Big islands: continental fragments
The movement of tectonic plates breaks the continents into pieces that move across the Earth surface. Many of Earth’s largest islands are blocks of continental crust that split from the main continents. Madagascar, for example, is a large island in the Indian Ocean that separated from the east coast of Africa. Greenland, New Guinea, and Tasmania are also broken continental blocks. India was a very large island until it collided with Asia about forty million years ago.
Some islands were at least partially constructed by animals. Many marine organisms including corals, mollusks, and gastropods have skeletons and shells composed of a mineral called calcium carbonate. Limestones and other rocks that form from the remains of these animals are called carbonates. The soft, white beaches of some of the world’s most beautiful islands— Tahiti, Hawaii, the Bahamas, Bermuda, and Bali to name a few—are composed of the carbonate remains of lagoon and reef species.
Corals are carbonate organisms that build elaborate community structures, called reefs, up from the seafloor. A reef is like an apartment building where each resident builds his own unit. When an individual animals dies, their “apartment” is vacant but still standing, and living corals continue to build the reef upward.
Carbonate species generally require clear, shallow water to thrive. Coastal carbonate islands, like the Florida Keys, only develop away from river deltas (where rivers meet the sea) where the water is too muddy. Carbonate islands away from coastlines usually have a volcanic or continental structure that provides an area of shallow water for light-loving species.
The islands of the Bahamas, for example, are the exposed spines of coral reefs that cover a large block of seafloor that was uplifted by plate tectonic forces about 200 million years ago. The Bahamian platform is sinking under the weight of its surfacing limestone layers, but the corals built the reefs up to the sunlight.
Volcanic islands like Hawaii and Tahiti are typically surrounded by a ring of reefs and a shallow lagoon that host numerous carbonate species. This explains, among other things, how islands of black volcanic rock can have white beaches.
Islands are remote. Island plants grew from seeds carried by birds. Some animals walked to islands on exposed land bridges when the sea level was lower during the ice ages, and others rode away from their home continent on a moving chunk of land. However they first arrived, island plants and animals live in closed ecosystems where they interact only with each other. (Some island ecosystems are, of course, more closed than others.
The difficulties of reaching a barrier island across a lagoon are much less than those of reaching a volcanic island at the center of a huge ocean basin.) Throughout most of Earth’s history, plants and animals that lived on islands are descendants of organisms that swam, floated, or rode there in the past. Today, humans sail, fly, and take man-made bridges to almost all of Earth’s islands, and they bring plants and animals with them.
Islands often are homes to extremely rare species that are unique to their island or group of islands. Charles Darwin’s (1809–1882) observations of the rare species of the Galapagos Islands inspired his theory of evolution in 1835. He theorized that new species evolve by combinations between individuals with different traits.
Organisms with the traits best suited to its environment will flourish and reproduce, causing these traits to be continuously passed to offspring. Individuals with traits less suited to their environment will not survive and eventually die out.
On islands, the number of individuals and species is limited and changes happen more rapidly. Furthermore, because new types of species arrive infrequently, the species that evolve on an island can be totally unique. The Galapagos Islands are home to Earth’s only swimming iguanas as well as huge tortoises that can live for two hundred years.
Island organisms also evolve to prey on and protect themselves from only the plants and animals on their island. They have no defense mechanisms from non-native predators and little immunity to foreign diseases. When species arrive from afar, island species can suffer. Humans have been particularly destructive to island species because they are predators themselves, and they import many non-native plants, animals, and diseases.
For example, humans brought snakes to Pacific Islands like Guam and Hawaii that decimated the endemic (native) birds and mammals. Humans then imported mongooses to kill the snakes, and the mongooses proceeded to kill not only snakes, but also a large number of the remaining island species. Today, many island governments and conservation groups are attempting to restore endangered island ecosystems.