Four different ways
How Does a New Island Appear?
Islands form by volcanoes building up, coral growing, land being cut off, or sea level dropping.
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Step 1 of 5
Some islands are volcanoes that kept going.
An underwater volcano erupts repeatedly, each flow adding height, until eventually the pile breaks the surface and stays there.
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Step 2 of 5
Some are built by living things.
Coral grows towards the light over thousands of years, and its accumulated skeletons build a solid platform that can reach the surface.
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Step 3 of 5
Some are just high ground that got cut off.
When sea level rises, hilltops become islands. Many around Britain were ordinary hills during the last ice age.
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Step 4 of 5
And some appear when the sea drops.
Falling sea level or land slowly rising after ice melts can lift a shallow bank clear of the water and turn it into new land.
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Step 5 of 5
Which story it is shows in the shape.
Volcanic islands are steep and cone-like, coral islands are flat rings, and cut-off islands look exactly like the nearby mainland.
Surtsey rose out of the sea off Iceland in 1963 and is now studied as a natural record of how life colonises new land.
The short version
Islands form in four quite different ways — volcanic building, coral growth, being cut off by rising seas, or the sea dropping — and the shape usually reveals which.
Try it yourself
Look at a satellite view of Hawaii and then the Maldives. One is steep and volcanic, the other flat coral rings.
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