Plate tectonics may have begun 4 billion years ago, almost a billion years earlier than we thought, according to a new analysis of ancient rocks.
The claim has earned a mixed response from geologists. Many argue that Earth was too hot at the time for plate tectonics in its modern form.
Today, Earth鈥檚 crust is divided into , which move around over millions of years. Where two plates meet, one can be forced under the other and destroyed inside the planet, a process called subduction. Plate boundaries are prone to earthquakes and are dotted with active volcanoes.
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It is clear that plate tectonics has operated for hundreds of millions of years. But Earth is 4.5 billion years old, and for many years geologists have disagreed over when plate tectonics started, and what the planet was like beforehand.
There has been a growing consensus that plate tectonics started about 3.2 billion years ago. But according to Brian Windley at the University of Leicester in the UK, that is wrong. 鈥淚t really is a great misunderstanding of so many things,鈥� he says.
Instead, Windley and his colleagues argue that tectonics began much earlier, at least 4 billion years ago. The evidence of a shift 3.2 billion years ago, they say, merely reflects a change in the way the plates were behaving.
The team re-examined data from rocks laid down between 4 and 3.2 billion years ago. They argue that many of them contain evidence of mountain-building, but of a particular kind seen today in a few places, including Japan and the Caribbean. When two tectonic plates meet, if one gets subducted, the volcanic activity this generates can sometimes lead to the formation of a chain of volcanically active islands. 鈥淵ou get island arcs, like those in the western Caribbean today,鈥� says Windley.
Crucially, the rocks of these mountains are chemically distinct from those that form when continents collide 鈥� like the Himalayas, which were thrust up when India hit Asia.
Windley and his colleagues argue that before 3.2 billion years ago, the only mountains on Earth were formed by subducting oceanic plates. Big continents only started to form around 3.2 billion years ago, once the crust was thick enough. This explains the shift in the chemical make-up of rocks at that time, says Windley.
鈥淚 think the point they make, it may not be that there was a single transition at a single time, I think that鈥檚 fair enough,鈥� says Saskia Goes at Imperial College London. 鈥淚 think some of the discussion of 鈥榳hen did plate tectonics start鈥� depends on 鈥榳hat do you call plate tectonics鈥�.鈥�
鈥淧late tectonics requires rigid plates,鈥� says Nicholas Gardiner at the University of St Andrews, UK. 鈥淭he edges are where the action is but the internal parts are pretty stable.鈥�
However, billions of years ago, Earth鈥檚 interior was hotter, so the crust was probably less rigid. If it was divided into plates, he says, they wouldn鈥檛 have behaved as they do today. Instead most of the motion may have been vertical, as less dense rocks rose and denser rocks sank. Some plates may have been subducted, but overall the system would have looked different to today, he says.
鈥淭here are really strongly held views on either side of this debate,鈥� says Kathryn Goodenough at the British Geological Survey in Edinburgh, UK.
Some researchers, like Windley, have long argued for an early start to plate tectonics, while at the other end of the spectrum, others argue that modern plate tectonics began only 700 million years ago. 鈥淭he reality is, the situation is somewhere in the middle,鈥� says Goodenough.
One possibility is that plate tectonics started and stopped several times between 4 and 3 billion years ago, before getting going in earnest. A second study, published on 11 November, presents an argument along these lines: it states that plate tectonics took hold on a global scale 3.2 billion years ago, and that there were only isolated instances of subduction in earlier times.
Either way, the evidence is increasingly clear that there was exposed land as early as 3.5 billion years ago, says Gardiner. Sediments, which can only be produced when rocks on land are weathered, are known from that time. The oldest confirmed fossil organisms come from the same rocks, and life may have emerged in small bodies of water on the first land.
Journal references: Precambrian Research, ;听 Gondwana Research,
