Middle-aged spread (Image: Wild Wonders of Europe/Lundgren/Naturepl.com)

Our planet has calmed down in its middle age. Earth was at its most active 1.1 billion years ago, when all the continents collided into one huge supercontinent, and has been getting calmer ever since. It’s evidence that the planet has a kind of lifespan.

Earth’s surface layer, the crust, is divided into tectonic plates that constantly jostle against each other. When two plates collide, they can be forced up into mountains, or one plate can slide under another and be destroyed.

This tectonic activity has been going on for at least 3 billion years, but nobody knows whether Earth has been getting more or less active over time. To find out, Martin Van Kranendonk of the University of New South Wales and Christopher Kirkland of the Geological Survey of Western Australia set out to reconstruct the history of plate tectonics.


First, they looked at 3200 rock samples from around the world and measured how much zirconium and thorium they contained. These elements are more common in rocks formed in tectonically active periods. They then measured the oxygen isotope content of 1200 other samples of rock. These isotopes are also affected by crustal recycling.

The results suggest that tectonic activity increased from 3 billion years ago – Earth’s toddler years – to a peak around 1.1 billion years ago, and then fell. During the peak, all the continents collided to form a vast supercontinent called Rodinia, spanned by a mountain range that dwarfed the Himalayas.

Van Kranendonk thinks several factors led Earth to become more active. A key factor was that the tectonic plates became larger and thicker, so their collisions were more violent. As Earth cooled, the tectonic plates moved much more slowly and so tectonic activity fell.

Adrian Lenardic of Rice University in Houston, Texas, suspects the lower tectonic activity seen in Earth’s first few billion years is down to heat: the rocks were hotter and more flexible, so plates were harder to break. That meant they stayed still for long periods of time while stress built up, before lurching forwards. If that’s correct, the early Earth was riven by earthquakes far more violent than any in human history.

Kent Condie of the New Mexico Institute of Mining and Technology in Socorro warns about the small sample size. “If those samples are not representative, the whole argument falls to pieces,” he says.

So does this mean our planet is now slowing to a halt? “Eventually the Earth is going to run out of internal energy,” says Lenardic, but we don’t know how long it will take for tectonics to stop. Steven Shirey of the Carnegie Institution of Washington in Washington DC thinks the planet’s internal heat will keep tectonics going until it is eaten by the sun in 7 billion years’ time.

Journal reference: Geology, DOI: doi.org/md8

This article appeared in print under the headline “Earth’s billion-year youthful rampage”