Ancient rocks show the earliest signs of tectonic activity on earth

The rocks in Australia preserve the evidence that the plates of the crust of the earth moved 3.5 billion years ago, an observation which repels the beginnings of the tectonics of hundreds of millions of years.

Today, about eight vast rigid rock plates on the surface of the planet, plus a few smaller plates, are pulled or pushed along a softer layer of rock below. When the edges of these plates slip or slide each other, sudden geological events can occur, such as earthquakes, as well as more progressive processes, such as the formation of mountain chains.

But geologists do not agree on the number of plates that were there, when they started to move and how they moved. Some researchers claim to have found evidence from further away than 4 billion years ago, when the planet was much hoter, while others say that the highest evidence is more recent, against 3.2 billion years.

Most of these evidence is indices of the chemical composition of rocks, which geologists can use to deduce how these rocks have moved in the past. However, there are few records on how the first plates may have moved to others, which is considered to be the strongest proof of tectonic plates.

Now, Alec Brenner at the University of Yale and his colleagues say they have found unambiguous evidence of queries relating to the plate about 3.5 billion years ago in the Craton of East Pilbara in Western Australia. The researchers followed how the magnetic field of the rocks, which was aligned on the magnetic field of the earth, moved over time, similar to the way in which a compass burned in the rock would change its direction to the needle as the ground moved.

Brenner and his team are dated for the first time rocks by analyzing the radioactive isotopes they contain, then proved that the magnetization of the rocks had not been reset at some point. By following the way in which this magnetization had evolved, they could show that the whole rock region had migrated over time, at a rate of tens of centimeters per year. Then they compared this with rocks that had been dated and followed using the same technique in the Greenstone Barberton belt in South Africa, which has not shown any movement.

“It means that there must have been a sort of plate limit between these two [regions] To take into account this relative movement. This is the movement of the plaque, definitionally, “Brenner told the Goldschmidt conference in geochemistry in Prague, in the Czech Republic, on July 9.

“The Pilibara, about 3.8 billion years ago, spends middle latitudes up to very high latitudes, in fact in the geomagnetic pole area, and probably near the place where Svalbard’s latitude is today, within a few million years. Brenner.

“If two plates move compared to each other, there must be a lot of things that are also happening between the two,” said Robert Hazen at the Institution for Science in Washington DC. “It cannot be a completely local thing.”

But there is a scope for different interpretations of what causes this movement, says Hazen. This is partly because there is a generalized uncertainty on the speed at which the plaque moved, and the data could adapt to several different theories of what the interior of the earth looked at that time.

At the very least, the observation involves the existence of a tectonic border, explains Michael Brown at the University of Maryland. However, he says that the movement of the rocks seems clearly different from what we understand as a tectonics of the plates today. “Essentially, the Pilbara [plate] Steam at higher latitudes and stops dead, which is unusual in any tectonic context of plaque. »»

Brown maintains that this corresponds to a theory that the crust of the earth at the time was made up of many smaller plates which were pushed by columns of hot rock, called plumes, which escape from the more melted coat. The surviving remains of these small plates, which, from this point of view, Brenner and his team would have sampered, are useful to indicate that there was a movement, but because they are only a small proportion of the crust, they might not be representative of the way the earth moved, says Brown.

Brenner and his team also found evidence that the direction of the Earth’s magnetic field overthrew 3.46 billion years ago, 200 million years before the next more recent. Unlike today’s magnetic field, which reverses about 1 million years, the magnetic field then seemed less frequently, at a rate of tens of millions of years. This could involve “very different energy and driving mechanisms,” said Brenner.

What was the magnetic field of the earth at this stage of its development is also very debated, says Hazen, partly due to the lack of magnetic data. “I think it moves the bar,” he says. “It is a really important discovery of a reversal that is early. It tells you something about the geodynamics of the nucleus that has not been nailed.”