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When the Earth started to crack

Depiction of tectonic plates with cracks through which magma emerges (in red)

Visdia / Getty Images

Tectonic plates — the large blocks of rock that make up the Earth’s surface, responsible for earthquakes and the formation of mountain ranges — were likely formed 3.2 billion years ago. Although it is not known exactly when the Earth stopped being a ball of hot rock and assumed its current appearance, the surface began to crack about 1.3 billion years after the planet was formed, probably in response to the cooling of a magma ocean, the molten layer below the outermost, solid layer. A team from Curtin University in Australia reached the conclusion by examining the isotopic composition of uranium, thorium, and lead from ancient deposits in the country. The proportions of each were used to identify when the Earth’s layers began to mix, indicating formation of the plates. Around 3.2 billion years ago, the Earth changed, assuming its current form like a layered cake, with a core, mantle, and crust. Gravity pulled the heavier chemical elements deeper while the lighter ones rose to the surface. Thus, rocks from the crust migrated to the mantle and rocks from the mantle rose to the crust, leading to the current composition of the tectonic plates (Live Science, July 26, Earth-Science Reviews, August).