The Earth began to move 2.5 billion years ago
Rock analysis shifts the likely emergence of continental drift. Barry Keily reports.
Plate tectonics on Earth began around 2.5 billion years ago, new research reveals, contradicting existing theories.
Tectonics – the movement of large surface plates across the globe – is the geological driver responsible for continental drift, the creation of mountain ranges, earthquakes, and the constant slow recycling of minerals into, and out of, the planet’s crust.
Exactly when the phenomenon began in the 4.5 billion-year history of the Earth is a matter of considerable debate. One school of thought contends that it has been a feature of the planet’s geology almost since its formation, while another suggests that it emerged only comparatively recently, about one billion years ago.
The latest findings, published in the journal Nature, place the start of tectonics pretty much midway between the two estimates.
To arrive at their result, scientists led by geologist Robert Holder from Johns Hopkins University, US, turned to metamorphic rocks – a large class of minerals that are the result of tectonic recycling. Metamorphic rocks are so termed because they start life in one guise, but are then conveyed down to the planet’s crust, where the hot and pressurised conditions change them into another.
Quartzite, marble and anthracite are all examples of metamorphic rocks. By examining them it is possible to accurately measure the heat and pressure to which they were subjected, and the time period over which this occurred.
With a large enough sample across a wide enough distribution, therefore, it is possible to estimate heat flow patterns through the ancient Earth – and heat flow is very strongly associated with plate movements.
Holder and colleagues therefore gathered samples from 564 sites around the world. The oldest dated to about three billion years ago, but metamorphic creation kicked up in frequency about half a billion years later – indicating, the researchers conclude, the start of tectonics.
The findings – if confirmed by further analysis – are likely to force a rethink about the early history of the Earth.
“The framework for much of our understanding of the world and its geological processes relies on plate tectonics,” says Holder.
“Knowing when plate tectonics began and how it changed impacts that framework.”