Islands come in two broad families. Continental islands are pieces of a continent cut off by rising seas or rifting, such as Greenland, the largest at 2,166,086 km², Borneo and Great Britain. Oceanic islands were never attached to a continent. Within that family, volcanic islands are built by eruptions over hotspots (Hawaiʻi), mid-ocean ridges (Iceland) or subduction zones (Japan); coral islands and atolls grow on sinking volcanoes (Kiritimati); and barrier islands are sand bars piled up by waves (Padre Island, 182 km long). Rivers and lakes form their own islands, such as Bananal in Brazil and Manitoulin in Canada. Some places are islands only at high tide, and people have built artificial islands from Bahrain to the Netherlands. Many of these types look alike from the air, so the clearest way to tell them apart is to set each against its nearest look-alike.
Continental vs Oceanic: Greenland and Borneo Against Iceland and Hawaiʻi
The deepest divide is what an island is made of and where it sits. A continental island stands on the continental shelf and is built of the same light, thick continental crust as the nearby mainland. An oceanic island rises from the deep ocean floor and is made of dense volcanic rock, or of coral sitting on top of it.
How continental islands are cut off
Some were separated by the sea rising over low ground. Great Britain was joined to mainland Europe by Doggerland, a low plain stretching across what is now the southern North Sea to the coasts of France, Belgium, the Netherlands, Germany and Denmark’s Jutland peninsula. Rising seas after the last ice age drowned it between 10,000 and 7,000 years ago. Borneo, the third-largest island at 748,168 km², sits on a shallow shelf that was also dry land during the ice ages.
Others were split off by plate tectonics. Madagascar broke away from Africa about 180 million years ago, during the break-up of Gondwana, and from the Indian subcontinent about 90 million years ago. That long isolation is why more than 90% of its wildlife is found nowhere else. Greenland, though it belongs politically to the Kingdom of Denmark, is geologically part of North America, and 81% of it lies under an ice sheet.
Why the difference matters
A continental island starts with the plants and animals of its mainland and keeps them, at least for a while. An oceanic island starts empty, and everything that lives there had to fly, float or swim across open sea. That is why oceanic islands such as the Galápagos produce so many species found nowhere else, and why Charles Darwin’s ideas about evolution grew out of oceanic islands rather than continental ones.
Hotspot, Ridge or Arc: How Volcanoes Build Islands From Hawaiʻi to Japan
Volcanic islands look similar, a cone of dark rock rising from the sea, but they form in three different settings, and each leaves a different pattern on the map.
Hotspot islands: a line of ages
A hotspot is a fixed plume of hot mantle beneath a moving plate. The Hawaiian Islands are the exposed peaks of the Hawaiian–Emperor seamount chain, built as the Pacific Plate drifts north-west over the hotspot at about 51 km per million years. The result is a line of islands that get older, smaller and more eroded towards the north-west. Only the two south-eastern islands have active volcanoes: Haleakalā on Maui, and Mauna Loa, Kīlauea and others on Hawaiʻi Island, which covers 10,430 km² and holds 63% of the archipelago’s land. Mauna Kea, also on Hawaiʻi Island, rises to 4,207 m above sea level. The next island is already under way: the Kamaʻehuakanaloa Seamount is expected to break the surface in 10,000 to 100,000 years.
Ridge islands: new crust breaking the surface
At a mid-ocean ridge, plates pull apart and magma wells up to fill the gap. Usually this happens kilometres underwater, but Iceland sits on the Mid-Atlantic Ridge directly above a mantle plume, and so much magma has erupted there that it forms an island of 101,826 km². The ridge also produced Surtsey, off Iceland’s south coast. Its eruption began 130 m below sea level and broke the surface on 14 November 1963. When it ended on 5 June 1967, the island covered 2.7 km²; waves had cut it to 1.3 km² by 2012, and it is expected to survive above sea level until at least 2100.
Island arcs: curved chains above subduction zones
Where one oceanic plate dives beneath another, water released from the sinking slab melts the mantle above it, and volcanoes rise in a curved line parallel to a deep-sea trench. These are island arcs, and they come with frequent earthquakes. The Aleutian Islands, a chain of 14 large and 55 smaller volcanic islands stretching west from Alaska, are a textbook arc. Japan is a more complex case. Its archipelago of 14,125 islands sits above two subduction zones, where the Philippine Sea Plate and the Pacific Plate dive beneath it. Yet it was once attached to the coast of Eurasia. Back-arc extension pulled it away and opened the Sea of Japan around 15 million years ago, so Japan mixes continental and arc origins.
So the clue is in the shape. A straight line of islands ageing in one direction suggests a hotspot; an island sitting on a submarine mountain range suggests a ridge; a curved chain beside a trench suggests an arc.
Atoll, Reef Island or Barrier Island: Bora Bora, Kiritimati and Padre Island
These three are the most often confused, because all are low, sandy and fringed by beaches. The difference lies in what they are made of and what lies beneath.
Atolls: coral on a drowned volcano
An atoll is a ring of coral reef, often topped by small islands, around a central lagoon. Charles Darwin explained it in 1842 in The Structure and Distribution of Coral Reefs, having worked out the idea during the voyage of HMS Beagle before he had even seen a coral island. A reef first grows as a fringe around a volcanic island. As the extinct volcano slowly sinks, the coral keeps growing upwards towards the light, so the fringe becomes a barrier reef separated from the island by a lagoon. When the volcano disappears completely, only the ring of reef remains. The lagoon is not the old crater; it is the space where the island used to be.
Bora Bora, in French Polynesia, shows the middle stage: the eroded remains of an extinct volcano, rising to 727 m at Mount Otemanu, surrounded by a lagoon and a barrier reef. The final stage was confirmed in 1952. On Enewetak Atoll, a borehole sunk by the United States Atomic Energy Commission passed through 1,410 m of coral and only then hit volcanic rock. There are about 440 atolls in the world, most of them in the Pacific. By land area, Kiritimati in Kiribati is usually listed as the largest, at about 312 km², with a further 328 km² of lagoons. Most atoll islands are less than 5 m high, which makes them among the most exposed places on Earth to rising seas.
Barrier islands: sand piled up by waves
A barrier island has no coral and no volcano. It is a long, narrow ridge of sand and dunes, built parallel to the mainland by waves and tides, with a lagoon or sound behind it. Padre Island, on the Gulf of Mexico coast of Texas, is the longest in the world at 182 km, yet only about 3 km wide. Barrier islands shift constantly as storms move sand, and they shield the mainland by absorbing wave energy before it reaches the shore.
The test is simple. If the sand is broken coral and shell sitting on a reef far out at sea, it is a coral island. If it is quartz sand lying just off a continental coast, it is a barrier island. A barrier island whose inlet closes and joins it to the shore becomes a barrier peninsula.
River, Lake, Tidal and Artificial Islands: Majuli to the Flevopolder
Not every island sits in the sea. Rivers split around sediment and create fluvial islands, most of them low and temporary. A few are huge and permanent. Bananal Island in Brazil’s Tocantins state, formed where the Araguaia River forks around it, is 320 km long and covers 19,162 km², making it the largest river island without an ocean coastline. Majuli, between the Brahmaputra and Subansiri rivers in Assam, India, is recognised by Guinness World Records as the world’s largest inland river island, and in 2016 it was declared a district, a status no Indian island had held before. Lakes hold islands too: Manitoulin Island in Lake Huron, Canada, covers 2,766 km² and is the largest lake island in the world, large enough to contain more than 100 lakes of its own.
A tidal island is joined to the mainland at low tide and cut off at high tide. Mont-Saint-Michel in Normandy is a well-known example, a rock that was dry land in prehistoric times before rising seas surrounded it.
Artificial islands have a much longer history than their modern image suggests. People in the Langa Langa and Lau lagoons of Malaita, in the Solomon Islands, built about 60 islands on the reef by diving for rocks and dropping them one at a time, and Al-Sayah island in Bahrain, at least 1,200 years old, is described as the world’s first permanent one, created around a water spring. Modern versions such as the Palm Islands in Dubai and the Pearl-Qatar, a 13.9 km² island off Doha built from 2006, were made by land reclamation. The Flevopolder in the Netherlands, drained in two stages in 1955 and 1968, covers 970 km² of land and is by some definitions the largest artificial island of all.
The legal status of these islands differs from natural ones. Under the United Nations Convention on the Law of the Sea, an artificial island does not count as an island and generates no territorial sea or exclusive economic zone of its own. That rule is one reason the legal effect of land reclamation on features in the South China Sea is contested among the several governments that claim them.
For more on the coral rings described above, read our guide to what an atoll is; to see continental islands at the top of the size rankings, look at the largest islands in the world, and for the sand ridges along the Texas coast and beyond, see what a barrier island is.