What is an island?
An island is a body of land completely surrounded by water and smaller than a continent. That simple definition hides enormous variety, from bare rocks poking above the waves to Greenland, which at over 2.1 million square kilometres is the largest island on Earth. A cluster or chain of islands is called an archipelago, and the world holds hundreds of thousands of islands, ranging from crowded, city-covered islands to remote specks that no human has ever settled. The study of how islands form and how life colonises them is one of the richest and most influential fields in all of geography.
The line between an island and a continent is one of convention rather than science. Australia is treated as a continent while Greenland is called an island, even though both are simply large land masses ringed by ocean; the difference is a matter of agreed size and tradition, not a natural law. What geographers can define precisely is how a given island came to exist, and that origin shapes its landscape, rocks, soils and wildlife more than almost anything else. For this reason the most useful way to understand islands is to group them by how they formed, which is where the story of island geography really begins.
How islands form
Islands arise through several distinct processes, and each leaves a lasting imprint. Continental islands sit on the same shallow shelf as a nearby mainland and are built of the same continental crust; they were once joined to the continent and became separated when sea levels rose or the land subsided. Great Britain was connected to mainland Europe by dry land until rising seas flooded the Channel a few thousand years ago, and Madagascar split from Africa long before that, carrying its own cargo of life into isolation.
Oceanic islands, by contrast, rise from the deep seafloor and were never part of a continent. Most are volcanic, built where magma erupts through the ocean floor and piles up eruption by eruption until it breaks the surface. Some form over hotspots, fixed plumes of heat that stay put while the tectonic plate drifts slowly over them, producing a chain of islands that grows older along its length, as in Hawaii. Others rise along volcanic arcs where one plate dives beneath another, as with Japan and Indonesia. Coral islands add a third route, built not by fire but by living organisms whose limestone skeletons accumulate into reefs, while barrier islands form from sand deposited and reshaped by waves and currents along a coast.
Types of island
Geographers group islands into a handful of main types that follow directly from these formation processes. Continental islands, such as Britain, Greenland and Madagascar, share a geological past with their parent landmass and so often begin with similar rocks and, at first, similar plants and animals. Volcanic or oceanic islands, such as Hawaii and Iceland, are freshly built of erupted rock rising from the deep sea, and they never had a land connection to any continent.
Coral islands and atolls form a distinctive tropical class. Corals grow in warm, shallow seas, and where a volcanic island slowly sinks beneath a growing reef, the reef can be left as a ring of coral enclosing a lagoon, the landform known as an atoll. The Maldives and many Pacific nations are made almost entirely of such low-lying coral islands, few of them rising more than a metre or two above the tide. Barrier islands are long, narrow ridges of sand lying parallel to a coast and constantly reshaped by the sea, while smaller categories include river and lake islands built from deposited sediment, and the growing number of artificial islands that people construct for ports, airports and even whole new city districts.
Key characteristics of islands
Islands share a set of characteristics that flow from their isolation and their surrounding sea. Size and remoteness are the two most important, because together they largely determine how much life an island can hold: larger islands offer more habitats and support more species, while more distant islands are harder for plants and animals to reach. This relationship lies at the heart of the theory of island biogeography, one of ecology's most powerful ideas. Elevation is another key trait, ranging from the towering volcanic peaks of high islands to the barely emergent coral flats of low islands, and it strongly influences climate, freshwater supply and vulnerability to the sea.
The maritime setting gives islands their own climate signature, generally milder and wetter than continental interiors at the same latitude, because the surrounding ocean moderates temperature and feeds moisture to the land. Freshwater is often a limiting characteristic, especially on small or low islands, where rainfall collects in a delicate lens of groundwater floating on denser seawater. Islands also tend toward high endemism, hosting species found nowhere else, and toward distinctive human cultures shaped by isolation and the sea. Taken together, these traits make islands both biologically precious and unusually sensitive to disturbance.
Notable islands worldwide
The largest islands anchor the subject in familiar places, each with a telling figure. Greenland is the world's largest island at over 2.1 million square kilometres, most of it buried under a thick ice sheet up to three kilometres deep. New Guinea is second, a mountainous, extraordinarily biodiverse island north of Australia whose interior remained little known to outsiders until modern times. Borneo, the third largest, spans about 743,000 square kilometres and is shared by three countries beneath a canopy of tropical rainforest, while Madagascar, the fourth largest, covers roughly 587,000 square kilometres and is renowned for wildlife found nowhere else on Earth.
Smaller islands illustrate other themes. Iceland, at about 103,000 square kilometres, sits astride the Mid-Atlantic Ridge and is one of the world's most volcanically active places. The Maldives, spread across hundreds of coral islands, has an average elevation of only around 1.5 metres, making it among the most climate-vulnerable nations on the planet. The Galapagos, a volcanic archipelago on the equator, is famous as the living laboratory where Charles Darwin gathered evidence for natural selection. Each shows a different facet of how origin and isolation shape a place.
Ecology and life of islands
Islands are natural laboratories of evolution because their isolation filters which species arrive and then lets those pioneers evolve on their own path. Only a few organisms ever cross open water to reach a remote island, arriving by flight, by swimming, or by drifting on rafts of vegetation and on the wind. Freed from many mainland predators and competitors, they often diverge into forms found nowhere else, and the proportion of such endemic species on isolated islands can be extraordinarily high. Madagascar, cut off for tens of millions of years, is home to lemurs and thousands of unique species, and the finches and giant tortoises of the Galapagos helped inspire the theory of evolution itself.
Isolation also drives some famously strange patterns. On islands, large animals sometimes shrink over generations, as with extinct dwarf elephants, while small animals can grow unusually large, trends known as island dwarfism and gigantism. When a brand-new volcanic island rises sterile from the sea, life assembles in a rough order that ecologists have watched unfold in real time on islands such as Surtsey off Iceland: wind and currents deliver seeds, spores and insects, birds carry seeds and nutrients, pioneer plants stabilise the ground, and gradually a full community builds from these chance arrivals. This same isolation, though, makes island species fragile, and a disproportionate share of the world's recorded extinctions has occurred on islands.
Importance of islands to people
Islands hold a significance far greater than their share of the land surface. They shelter concentrations of unique species, which makes them global priorities for conservation, and they have shaped human history as stepping stones for migration, exploration and trade. The settlement of the vast Pacific by seafaring peoples, navigating thousands of kilometres of open ocean by the stars, swells and birds, ranks among the great feats of human achievement, and island positions have long given their holders control over sea routes and strategic advantage in war and commerce.
Economically, island nations control enormous areas of surrounding ocean and its fisheries, minerals and energy through their exclusive economic zones, so even a tiny island can command a vast maritime territory. Many islands depend on tourism drawn by their beaches, reefs and distinctive cultures, and on fishing that sustains both livelihoods and identity. Culturally, island communities have developed rich and distinctive traditions, languages and ways of life, shaped by the sea that both connects and separates them from the wider world. For all these reasons islands punch far above their geographical weight.
Threats, change and conservation
Islands face some of the most acute environmental threats on the planet, and low-lying coral islands face an existential one. Rising sea levels and stronger storms driven by a warming climate put whole nations such as the Maldives, Tuvalu and Kiribati at risk of disappearing beneath the waves within a few generations, contaminating scarce freshwater with saltwater and eroding the little land that exists. Even where islands survive, coral reefs that protect and build them are being bleached and killed by warming, acidifying seas, removing a natural defence exactly when it is most needed.
Island ecosystems are equally imperilled. Because island species evolved without many predators, introduced animals such as rats, cats, goats and snakes, along with habitat clearance, have driven countless island endemics to extinction, and invasive species remain a leading threat today. Conservation responses include eradicating invasive animals to let native wildlife recover, establishing reserves and marine protected areas, and international support for the small island states least responsible for climate change yet most exposed to it. Protecting islands means guarding both their irreplaceable biodiversity and the communities bound to these fragile pieces of land in the sea.
Key facts
- An island is land surrounded by water and smaller than a continent; a group is an archipelago.
- Continental islands sit on a continent's shelf; oceanic islands rise from the deep seafloor.
- Most oceanic islands are volcanic, built at hotspots or along plate boundaries.
- Atolls are rings of coral left after a volcanic island sinks beneath its reef.
- Greenland is the largest island on Earth at over 2.1 million square kilometres.
- Species richness rises with island size and falls with distance from the mainland.
Frequently asked questions
What is the difference between an island and a continent?
The distinction is one of convention and size rather than a strict scientific rule. Continents are the largest land masses, and anything smaller that is surrounded by water is called an island, which is why Australia is treated as a continent but Greenland as an island.
How do volcanic islands form in the middle of the ocean?
Magma erupts through the seafloor and piles up over many eruptions until it finally breaks the surface. Many form over hotspots, fixed plumes of heat that create a chain of islands as the tectonic plate drifts slowly over them across millions of years.
Why do islands have so many unique animals?
Isolation means only a few species reach an island, and once there they evolve separately from their mainland relatives, often becoming distinct species found nowhere else. This high level of endemism makes islands biologically precious but also extremely vulnerable to disturbance.
What is an atoll?
An atoll is a ring-shaped coral island enclosing a lagoon. It forms when a volcanic island is fringed by a coral reef and then slowly sinks, leaving the upward-growing reef behind as a low circle of coral land around the vanished peak.
Why are island nations so threatened by climate change?
Many islands, especially coral ones, sit barely a metre or two above sea level, so rising seas, stronger storms and saltwater intrusion into freshwater threaten their very existence. Warming also kills the coral reefs that build and protect them, compounding the danger.
