Science Puzzle
Why Are the Hawaiian Islands in a Line?
The Hawaiian Islands lie in a line across the middle of the Pacific Ocean. At the south-east end, the Big Island has volcanoes that are still erupting. Each island further to the north-west is older and more worn down, and beyond the last island the line carries on as a chain of drowned volcanoes stretching thousands of kilometres.
Most volcanoes sit on the edges of the plates that make up Earth's surface. Hawaii is in the middle of the Pacific Plate, thousands of kilometres from any edge. So why are the islands there at all, and why are they in a line, oldest at one end?
The Answer
They sit over a hotspot, a plume of hot rock rising from deep inside Earth that stays in roughly the same place. The Pacific Plate slides north-west over it, carrying each volcano away so a new one can form behind it.
Where the plume reaches the bottom of the plate, rock melts and magma punches up through the sea floor, building a volcano. Hawaii's volcanoes are enormous: measured from the sea floor, Mauna Kea is taller than Everest. While a volcano sits over the hotspot it keeps erupting and growing. But the plate is moving north-west at about 7 to 10 centimetres a year, roughly twice as fast as your fingernails grow, so after a few hundred thousand years the volcano is carried off the hotspot. It goes quiet. Rain and waves wear it down, and the cooling plate beneath it slowly sinks, taking the island down with it.
Meanwhile a fresh volcano starts to grow over the plume. One is growing already: Kamaʻehuakanaloa, once called Lōʻihi, sits on the sea floor south-east of the Big Island, its top about 1,000 metres below the waves. The whole chain is a record of the plate's journey, like a sheet of paper slid slowly over a candle and scorched in a line of spots. There's even a bend in the chain from about 47 million years ago, when the plate changed direction.
The principle: Hotspots. A plume of hot rock that stays put beneath a moving plate builds a line of volcanoes, the youngest over the plume and the oldest carried furthest away.