Science Puzzle
Why Is Iceland Getting Wider?
Iceland grows about 2 centimetres wider every year. At Thingvellir (Þingvellir) you can walk along a rift valley between two long walls of rock that are slowly drifting further apart, and divers can swim in a crystal-clear crack between them.
What is pulling Iceland apart, and why doesn't a gap simply open up across the island?
The Answer
Iceland sits right on top of the boundary between two of Earth's plates, which are moving apart. Molten rock rises to fill the gap and becomes new land.
Earth's outer shell is broken into giant plates. Down the middle of the Atlantic Ocean runs a long underwater mountain chain, the Mid-Atlantic Ridge, where the North American plate and the Eurasian plate are spreading apart at about 2 centimetres a year. Almost everywhere this seam lies deep under the sea. Iceland is the rare place where it rises above the waves, because a plume of extra-hot rock rising from deep below has built up so much volcanic rock there.
As the plates part, the pressure on the hot rock beneath them drops; some of it melts. That magma pushes up into the cracks, sometimes erupting as lava, and cools into fresh rock that welds onto both sides. So the island never splits open; it keeps adding new ground in the middle. The eruptions on Iceland's Reykjanes Peninsula in recent years are this process in action.
The whole Atlantic is widening the same way. Since Columbus crossed it in 1492, it has grown about 10 metres wider.
The principle: Sea-floor spreading. At a divergent plate boundary the plates move apart and rising magma fills the gap, creating new crust, so oceans like the Atlantic slowly grow wider.