An island is a body of land surrounded by water. Continents are also surrounded by water, but because they are so big, they are not considered islands. Australia, the smallest continent, is more than three times the size of Greenland, the largest island.

There are countless islands in the ocean, lakes, and rivers around the world. They vary greatly in size, climate, and the kinds of organisms that inhabit them.

Many islands are quite small, covering less than half a hectare (one acre). These tiny islands are often called islets. Islands in rivers are sometimes called aits or eyots. Other islands are huge. Greenland, for example, covers an area of about 2,166,000 square kilometers (836,000 square miles).

Some islands, such as the Aleutian Islands in the U.S. state of Alaska, are cold and ice-covered all year. Others, such as Tahiti, lie in warm, tropical waters. Many islands, such as Easter Island in the South Pacific Ocean, are thousands of kilometers from the nearest mainland. Other islands, such as the Greek islands known as the Cyclades in the Aegean Sea, are found in closely spaced groups called archipelagoes.

Many islands are little more than barren rock with few plants or animals on them. Others are among the most crowded places on Earth. Tokyo, one of the world’s largest cities, is on the island of Honshu in Japan. On another island, Manhattan, rise the towering skyscrapers of the financial capital of the world, New York City.

For centuries, islands have been stopping places for ships. Because of isolation, many islands have also been home to some of the world’s most unusual and fascinating wildlife.

Island Formation

There are six major kinds of islands: continental (1), tidal (2), barrier (3), oceanic (4), coral (5), and artificial (6).

Continental islands (1) were once connected to a continent. They still sit on the continental shelf. Some formed as Earth’s shifting continents broke apart.

Scientists say that millions of years ago, there was only one large continent. This supercontinent was called Pangaea. Eventually, slow movements of the Earth’s crust broke apart Pangaea into several pieces that began to drift apart. When the breakup occurred, some large chunks of land split. These fragments of land became islands. Greenland and Madagascar are these type of continental islands.

Other continental islands formed because of changes in sea level. At the peak of the most recent glacial period, about 18,000 years ago, ice covered large parts of the Earth. Water was locked in glaciers, and the sea level was much lower than it is today. As glaciers began to melt, the sea level rose. The ocean flooded many low-lying areas, creating islands such as the British Isles, which were once part of mainland Europe.

Some large continental islands are broken off the main continental shelf, but still associated with the continent. These are called microcontinents or continental crustal fragments. Zealandia is a microcontinent off Australia that is almost completely underwater—except for the island nation of New Zealand.

Continental islands may form through the weathering and erosion of a link of land that once connected an island to the mainland. Tidal islands (2) are a type of continental island where land connecting the island to the mainland has not completely eroded, but is underwater at high tide. The famous island of Mont Saint-Michel, France is an example of a tidal island.

Barrier islands (3) are narrow and lie parallel to coastlines. Some are a part of the continental shelf (continental islands) and made of sediment—sand, silt, and gravel. Barrier islands can also be coral islands, made from billions of tiny coral exoskeletons. Barrier islands are separated from shore by a lagoon or a sound. They are called barrier islands because they act as barriers between the ocean and the mainland. They protect the coast from being directly battered by storm waves and winds.

Some barrier islands form when ocean currents pile up sand on sandbars parallel to coastlines. Eventually the sandbars rise above the water as islands. Aits, or islands in rivers, form in this way. The same currents that formed these barrier islands can also destroy or erode them.

Other barrier islands formed during the most recent ice age. As glaciers melted, the sea level rose around coastal sand dunes, creating low-lying, sandy islands. The Outer Banks, along the southeastern coast of the United States, are this type of barrier island.

Still other barrier islands were formed of materials deposited by Ice Age glaciers. When glaciers melted, they left piles of the rock, soil, and gravel they had carved out of the landscape. These piles of debris are called moraines. As flooding occurred along coasts after the glaciers melted, these moraines were surrounded by water. Long Island, New York, and Nantucket, Massachusetts, are both barrier islands formed by glacial moraines.

Oceanic islands (4), also known as volcanic islands, are formed by eruptions of volcanoes on the ocean floor. No matter what their height, oceanic islands are also known as “high islands.” Continental and coral islands, which may be hundreds of meters taller than high islands, are called “low islands.”

As volcanoes erupt, they build up layers of lava that may eventually break the water’s surface. When the tops of the volcanoes appear above the water, an island is formed. While the volcano is still beneath the ocean surface, it is called a seamount.

Oceanic islands can form from different types of volcanoes. One type forms in subduction zones, where one tectonic plate is shifting under another. The island nation of Japan sits at the site of four tectonic plates. Two of these plates, the Eurasian plate to the west and the North American plate to the north, are associated with continental shelves. The other two, the Philippine plate and the Pacific plate, are oceanic. The heavy oceanic plates (the Pacific and the Philippine) are subducting beneath the lighter Eurasian and North American plates. Japan’s islands are some of the most actively volcanic in the world.

Another type of volcano that can create an oceanic island forms when tectonic plates rift, or split apart from one another. In 1963, the island of Surtsey was born when a volcanic eruption spewed hot lava in the Atlantic Ocean near Iceland. The volcano was the result of the Eurasian tectonic plate splitting away from the North American plate. This tiny island is one of the world’s newest natural islands.

Another type of oceanic island forms as a continent shifts over a “hot spot.” A hot spot is a break in the Earth’s crust where material from the mantle bubbles or rushes up. The crust shifts, but the hot spot beneath stays relatively stable. Over millions of years, a single hot spot formed the islands of the U.S. state of Hawaii. Hawaii’s “Big Island” is still being formed by Mauna Loa and Kilauea, two volcanoes currently sitting over the hot spot. The newest Hawaiian island, Loihi, also sits over the hot spot, but is still a seamount about 914 meters (3,000 feet) beneath the Pacific.

Coral islands (5) are low islands formed in warm waters by tiny sea animals called corals. Corals build up hard external skeletons of calcium carbonate. This material, also known as limestone, is similar to the shells of sea creatures like clams and mussels.

Colonies of corals may form huge reefs. Some coral reefs may grow up in thick layers from the seafloor, until they break the water’s surface, creating coral islands. Other organic and inorganic material, like rock and sand, helps create coral islands. The islands of the Bahamas, in the Atlantic Ocean and Caribbean Sea, are coral islands.

Another kind of coral island is the atoll. An atoll is a coral reef that begins by growing in a ring around the sides of an oceanic island. As the volcano slowly sinks into the sea, the reef continues to grow. Atolls are found chiefly in the Pacific and Indian Oceans.

Artificial islands (6) are made by people. Artificial islands are created in different ways for different purposes.

Artificial islands can expand part of an already-existing island by draining the water around it. This creates more arable land for development or agriculture. The Nahua people of 14th-century Mexico created their capital, Tenochtitlan, from an artificial island in Lake Texcoco. They expanded an island in the swampy lake and connected it to the mainland through roads. Aqueducts supplied the city’s 200,000 residents with freshwater. Mexico City sits on the remains of Tenochtitlan.

Artificial islands can also be created from material brought in from elsewhere. In Dubai, companies dig (dredge) sand from the Persian Gulf and spray it near shore. Dubai’s huge artificial islands are shaped like palm trees and a map of the world. A new island complex, the Dubai Waterfront, will be the largest man-made development in the world.

Many island chains are combinations of different kinds of islands. The island nation of Seychelles is made of both continental granite islands and coral islands.

Island Wildlife

The kinds of organisms that live on and around an island depend on how that island was formed and where it is located. Continental islands have wildlife much like that of the continent they were once connected to. The critically endangered island fox, native to the six Channel Islands off southern California, is much like the grey fox of the North American mainland, for instance.

Isolated oceanic and coral islands, however, have plant and animal life that may have come from distant places. Organisms reach these islands by traveling long distances across the water.

Some plant seeds may travel by drifting in the ocean. The seeds of coconut palms, for instance, are encased in durable, buoyant shells that can float significant distances. The seeds of red mangrove trees often float to new locations along a coastline.

Other plant seeds travel to islands on the wind. Many lightweight seeds, such as fluffy thistle seeds and the spores of ferns, can drift long distances in air currents. Still other plant seeds may be transported to islands by birds—dirt stuck on their feet or feathers, or released in their droppings.

Birds, flying insects, and bats all reach islands by air. Many are blown long distances by storm winds.

Other creatures may ride to islands on floating masses of plants, branches, and soil, sometimes with trees still standing on them. These land rafts are called floating islands. Floating islands are usually torn from coasts and swept away during storms, volcano eruptions, earthquakes, and floods.

Floating islands can carry small animals hundreds of kilometers to new homes on islands. Snakes, turtles, insects, and rodents find shelter in tree branches or among plant leaves. Some of the best travelers are lizards, which can survive a long time with little freshwater.

People create their own artificial floating islands. The Uros people are native to the area surrounding Lake Titicaca, in Peru and Bolivia. The Uros live on 42 large floating islands constructed of reeds and earth. The islands can be anchored to the bottom of the lake using stone and rope.

Because plants and animals living on islands are isolated, they sometimes change to adapt to their surroundings. Adaptive radiation is a process in which many species develop to fill a variety of different roles, called niches, in the environment.

The most famous example of adaptive radiation is probably the evolution of the finch species of the Galapagos Islands. This group of birds is called “Darwin’s finches” because the scientist Charles Darwin was the first to study and document their adaptations. With no competition or threats from other species, the birds adapted to eat different foods. Their beaks reflect the different roles they play in the Galapagos Islands ecosystem: a finch with a large beak eats hard-shelled fruits and nuts, while a thin-beaked finch gets its nutrition from cactus flowers.

Lacking predators, some island creatures become enormous. This is called island gigantism. Also on the Galapagos Islands, giant tortoises developed from smaller ancestors over millions of years. Scientists believe the first tortoises probably came to the islands from South America on floating islands. Gradually, the animals grew larger in body size because there were few competitors for the plants they ate. Today, the tortoises may weigh as much as 250 kilograms (551 pounds).

Scalesias, plants related to sunflowers, gradually grew larger on the Galapagos Islands, too, because there were few insects or rodents that ate the flowers. Eventually, scalesia trees grew to be 6-9 meters (20-30 feet) tall. Scalesias are called the “Darwin’s finches of the plant world.”

The isolated populations on islands can lead to smaller, as well as larger, species. This process is called insular dwarfism. The critically endangered Sumatran tiger is only found on the island of Sumatra, Indonesia. It is significantly smaller than its Asian cousins, because it has less land to roam, smaller prey to feed on, and must move quickly in the jungle.

The isolation of many islands may protect some animals on them from predators and other dangers that exist on mainlands. Relatives of some animals long extinct in most parts of the world still survive on islands.

One of the most remarkable of all creatures was discovered in 1913 on the island of Komodo, Indonesia. Rumors of fantastic animals on Komodo had persisted over the years. When scientists arrived to investigate, they were astounded to see what looked like a dragon. The creature was a gigantic lizard nearly 3 meters (10 feet) long. Soon, more of these enormous reptiles were discovered, some even larger. Called Komodo dragons, they were found to be relatives to the Earth’s most ancient group of lizards. The isolation of the island of Komodo had preserved them.

People can accidentally or intentionally introduce organisms to island habitats. These organisms are called introduced species or exotic species. Ships delivering goods, for example, may unintentionally dump exotic algae into the water with their ballast. Ships carrying food cargo may accidentally carry tiny, hidden spiders or snakes. Island residents also bring pets with them. Some of these pets are released into the wild, either accidentally or on purpose.

Islands and People

How the world’s most remote islands were first discovered and settled is one of the most fascinating stories in human history. The vast Pacific Ocean is sprinkled with many small islands, such as the Marquesas, Easter Island, and the Hawaiian Islands. These islands are far from the coasts of the Americas, Asia, and Australia. When Europeans began exploring the Pacific islands in the 1500s, they found people already living there. We now know these people as Polynesians. Where did these people come from?

Most scientists say the ancestors of these Pacific island inhabitants originally came from Southeast Asia, probably around Taiwan. (The famous scientist Thor Heyerdahl disagreed. He said Polynesians migrated to the Pacific islands from the west coasts of North and South America. Heyerdahl successfully sailed a wooden raft, the Kon-Tiki, from Peru to Raroia, French Polynesia, in 1947. Although this proved the migration was possible, linguistic and genetic evidence suggest it is unlikely.)

Beginning around 3,000-4,000 years ago, groups of early Polynesians set out in great oceangoing canoes on voyages over thousands of kilometers of ocean. Sailing without compasses or maps, they discovered islands they could not have known existed. Their most famous expeditions took them east, as far as the Hawaiian Islands and Easter Island. Recent evidence suggests these early people also sailed west, across the Indian Ocean. They were probably the first people to inhabit the African island of Madagascar.

Archaeologists who study Polynesian culture say the ancient Pacific people were excellent sailors who navigated by the stars. Many sailors still use celestial navigation. Ancient Polynesians also knew how to interpret winds and ocean waves. Some of their voyages were probably accidental, and occurred when storms blew canoes traveling to nearby islands off course. Other voyages were almost certainly intentional.

Europeans visited and colonized remote islands beginning in the 1500s. They sometimes caused harm. For example, they brought devastating diseases unknown to islanders, who had no resistance to them. Many island people perished from diseases such as measles. Island populations such the Taino (in the Caribbean, probably the first Native Americans encountered by Christopher Columbus) shrunk to near-extinction.

On their ships, Europeans also brought animals—including cats, dogs, rats, snakes, and goats. These invasive species preyed on native island plants and animals. They also took over native species' niches and destroyed the natural ecological balance of the islands. The so-called Jamaican monkey, for example, was native to the Caribbean but went extinct after Europeans colonized the area.

Since the days of the early explorers, islands have been important as places for ships to take on supplies and for their crews to rest. Later, islands became part of ocean trade routes, linking distant parts of the world. Islands became particularly important to seafaring thieves known as pirates. Islands from the Bahamas (in the Atlantic Ocean) to Madagascar (in the Indian Ocean) became notorious as pirate bases. The rule of law did not always reach these remote places, and the rugged terrain made finding pirate hideouts difficult for law enforcement.

Like stepping stones, islands have helped people migrate over vast expanses of ocean from one continent to another. During World War II, Asian battles were fought in the “Pacific theater” of the war. Instead of attacking Japan directly, Allied powers (led by the United States, the United Kingdom, and the Soviet Union) chose a strategy of “island hopping.” Allied forces “hopped” from one small Pacific island to the next, establishing military bases and air control. The battles of Guadalcanal and Tarawa were important battles in the island-hopping campaign.

Today, millions of people live on islands all over the world. Some even own them—islands are available for purchase just like any other piece of real estate. There are many island nations. Island nations can be part of an island (such as Haiti and the Dominican Republic, which share the island of Hispaniola), one island (such as Madagascar), or many islands (such as the Philippines).

Islands and Climate Change

Some low-lying coral islands may be threatened by climate change. Global warming has led to rising sea levels, while rising sea temperatures have led to coral bleaching—the process of destroying the coral on which many new islands form. Some scientists believe rising sea levels put low-lying islands at greater risk for damage from tsunamis, floods, and tropical storms.

The island nation of Maldives is particularly threatened by sea level rise, for example. Maldives is a chain of 26 atolls in the tropical Indian Ocean. All the atolls of Maldives are low-lying, and some uninhabited areas are even covered by shallow water at high tide. Tsunamis and storms regularly erode at the fragile coral islands. Extreme weather, such as storms and cyclones, has become more frequent and is often associated with climate change. The delicate beaches of Maldives are eroding at a fast pace.

Maldivians are protecting their islands. They are addressing current threats by dredging sand from the ocean floor. This sand has fortified the coastline and elevated some structures to several meters above sea level. However, Maldivians are also preparing for the worst. Leaders have considered migrating the entire Maldivian population to Australia, Sri Lanka, or India if sea levels continue to rise.

Climate change can also threaten island economies. Tourism is an important industry for many island nations. Bleached and dying coral, invasive algae and jellies, and beach pollution reduce the number of tourists who want to dive or snorkel among the coral reefs. More than 80% of the economy of the Virgin Islands, in the Caribbean Sea, relies on tourism, for example. A change in the water quality, coral reef ecosystems, or pristine beaches would devastate the livelihood of islanders. This would impact not only residents of the Virgin Islands, but other nations, as economic refugees migrated to countries with more stable economies.

Islands are now valued by people as homes for rare and endangered wildlife. Many islands where people once destroyed native species by hunting them or destroying their habitats are now maintained as national parks and wildlife refuges. On some of these island preserves, such as the Galapagos Islands, scientists conduct research to learn more about wildlife and how to protect the animals from further harm.

The Galapagos ecosystems, both terrestrial and marine, are a example of human impact on islands. More than 100,000 people visit the protected islands of the Galapagos every year. Tourists flock to see the indigenous wildlife, such as marine iguanas, giant tortoises, and blue-footed boobies. Scientists come to study the unusual wildlife and the way it evolved.

The population of the Galapagos has grown to accommodate tourists and scientists. Thousands of people have migrated to the islands illegally in search of a more stable economic livelihood. Tourists and Galapagueños (most in the service industry) have stressed the environment with development such as clearing land for housing, industry, and agriculture; the need for sophisticated energy and sewage systems; and increased demand for freshwater.

The marine ecosystem of the Galapagos is also threatened by human activity. Although the islands prohibit some forms of fishing, fisheries such as marlin and tuna thrive in the area. Overfishing, however, threatens the population of these large, predatory fish and the livelihood of the people who depend on them for food and trade.

The international community, through the United Nations and many non-governmental agencies, work with Galapagueños and the government of Ecuador to successfully manage the ecosystems of the Galapagos and develop their economy.