By Sid Perkins
The chemical composition of ancient crystals now found in Australian rocks bolsters the notion that tectonic plates may have jostled across Earth’s surface more than 4 billion years ago.
![Rocks in the Jack Hills of Western Australia hosted zircon crystals that contain tiny mineral inclusions, such as the one denoted by the arrow in the false-color inset. The zircons and inclusions are more than 4 billion years old and contain evidence suggesting an early start for tectonic activity on Earth.](https://i0.wp.com/www.sciencenews.org/wp-content/uploads/2008/11/9284.jpg?resize=300%2C243&ssl=1)
Scientists call the first 600 million years of Earth’s history the “Hadean eon” because of the presumably hellish temperatures on the freshly coalesced and largely molten planet. Also, radioactive isotopes, which generate heat inside Earth as they decay, were much more common then than they are now, says Mark Harrison, a geologist at the University of California, Los Angeles.
Few intact rocks survive from that period (SN: 10/11/08, p. 12), but previous studies suggest that the rate of heat flow from Earth’s interior during the Hadean eon was between three and five times higher than it is today, he notes.