Islands are areas of land surrounded by water or, in some classifications, relatively dry land surrounded by water or low wetland. They range from tiny rocky outcrops to enormous land areas comparable in scale to continental regions. USGS defines an island as a tract of land smaller than a continent surrounded by water of an ocean, sea, lake or stream, while its geographic naming system includes related forms such as atolls, cays, keys and island groups.
There is no single universal minimum area that separates an island from an islet or other small landform in every context. The number of islands on Earth is also difficult to determine precisely because definitions, mapping scales, tides, vegetation and satellite resolution affect what is counted.
Islands form through several major geological and geomorphological processes. Volcanic islands develop when magma reaches the surface beneath or near the ocean, with repeated eruptions building volcanic cones or larger volcanic structures. Tectonic processes can expose or create land through uplift, faulting and plate movement. Continental islands are fragments of continental crust that became isolated by changes in sea level, tectonic separation or erosion.
Coral islands develop through biological and geological processes associated with reef growth, especially around subsiding volcanic foundations in tropical oceans. Depositional islands can form from sediment transported by rivers, waves and currents. Because their origins differ, islands vary greatly in rock type, elevation, age, climate, ecology and shape.
Volcanic islands provide important examples of the connection between Earth's Interior and surface geography. At volcanic hotspots, magma can rise from deep within the mantle and accumulate beneath the ocean, eventually building land above sea level. At convergent plate boundaries, volcanic island arcs can form where one oceanic plate is subducted beneath another. Mid ocean ridge systems can also produce volcanic islands where tectonic plates diverge, although much of the ridge remains below sea level. Volcanic islands can grow through successive eruptions, while erosion, landslides and subsidence can reduce their size. The geological history of an island can therefore involve both construction by volcanic activity and destruction by surface processes.
Coral islands and atolls illustrate a different pathway to island formation. Reef building organisms grow in warm, shallow marine environments where conditions permit carbonate production. Around a volcanic island, a fringing reef can develop near the coastline. As the volcanic foundation subsides or sea level changes, reef growth can continue upward and outward, potentially producing a barrier reef and eventually an atoll surrounding a lagoon.
Not every reef island follows exactly this sequence, and carbonate islands can have complex origins involving sediment accumulation, reef growth, storms and sea level changes. Coral islands are generally low lying, making their physical environment particularly sensitive to wave action, storms, erosion, sea level change and changes in reef health.
Islands have distinctive ecological characteristics because water surrounding them limits movement and gene flow. Geographic isolation can promote endemism, meaning species occur naturally in a particular island or island group and nowhere else. Island populations may undergo evolutionary changes because of limited resources, unusual predators, founder effects and other processes. Islands can also be highly vulnerable to introduced species because organisms that evolved without particular predators or competitors may lack effective defences. Human activities have therefore produced major ecological changes on many islands through species introductions, habitat alteration, hunting and development. Island biogeography examines how island size, isolation, colonisation and extinction influence patterns of biodiversity.
Islands can be grouped into archipelagos, which are collections of islands that may have geological, geographical or historical relationships. Archipelagos can form from volcanic island chains, tectonic structures, coral reef systems or fragmented continental land. Some island groups extend across enormous distances and contain islands with different ages and geological origins. The spatial arrangement of islands affects transportation, ecology, climate exposure and human settlement.
Channels and straits between islands influence currents and navigation, while shallow shelves can connect islands during periods of lower sea level. The concept of an island therefore includes not only the individual landform but also its relationship with surrounding water, neighbouring islands and the larger continental or oceanic setting.
Islands have substantial human significance. Many have supported distinctive cultures because geographic separation can promote linguistic and cultural divergence while also creating strong maritime connections. Islands can serve as locations for settlements, ports, agriculture, tourism, conservation areas and scientific research. Their limited land and freshwater resources can create particular planning challenges. Coastal hazards can be especially significant on low lying islands because storms, erosion and changing sea level directly affect settlements and infrastructure. Larger islands may contain substantial mountains, rivers, forests and agricultural regions, whereas small islands may have very limited terrestrial resources. The practical characteristics of an island therefore depend strongly on its size, elevation, climate, geology and degree of connection to larger land areas.
A common misconception is that all Islands are small, tropical or volcanic. Islands occur in polar, temperate, tropical and subtropical environments and can be volcanic, continental, coral, sedimentary or formed through other processes. Greenland, for example, is geographically an island despite its enormous size, while many small islands are no more than exposed rock. Another misconception is that the boundary of an island is always obvious and permanent. Tidal changes, storms, erosion, sediment deposition, vegetation and changing water levels can alter the apparent extent of small islands. USGS notes that the true number of islands distributed across the planet remains difficult to determine despite increasingly detailed satellite imagery and geographic information systems.
