Sea-floor spreading

In 1962 Harry Hess proposed that ocean floors are always being made and destroyed. Along a long underwater mountain chain in the middle of an ocean, the mid-ocean ridge, molten rock from the asthenosphere rises into a crack, cools and becomes new ocean floor. The new floor moves away on both sides, a few centimetres a year, so the further a rock is from the ridge, the older it is. Where the old floor reaches a continent it sinks back into the mantle (subduction), making a deep trench, so the Earth does not grow. That is why no ocean floor is older than about 200 million years, while continents hold rocks billions of years old. The Atlantic is widening this way, and so is the Red Sea, a young ocean opening between Africa and Arabia.