3.6 billion years ago, massive meteor impacts broke the thick crust that was still going on. Thus the first continents arose and the continental drift began.
This is the result of analyzes of ancient crystals as thin as a human hair. These crystals come from a piece of continental crust more than three billion years old in Western Australia. This region, called the Pilbara Craton, is one of the oldest untouched pieces of continental crust on Earth. Geologist Tim Johnson from Curtin University in Australia and colleagues subscriber This week’s results are in the magazine temper nature. This confirms the theory put forward in the early 1960s.
Continent formation and plate tectonics are complex
Paul Mason Geologist
Although Utrecht University geologist Paul Mason, who was not involved in the study and has just returned from the Pilbara Craton for field work, calls it an elegant analysis and finds the results robust, he doesn’t expect this to be the last discussion about the formation of the first continents. “The formation of the continent and plate tectonics is complex. Researchers have studied the composition of the continent in a few places. Although we expect meteorites to strike all over the world, this was not necessarily the starting signal for the formation of all continents around the Earth. It is not necessary to start from all over the world. one place at a time.”
Earth is the only planet known where giant individual continents and oceanic plates are constantly sliding over each other, colliding and diving under each other. They do this at roughly the same rate as nail growth.
Geologists know how the new continental crust is formed today. When an oceanic plate slides under another oceanic plate or continent (subduction), the submerged plate slides deeply into the warm interior of the Earth. There the rocks melt and their chemical composition also changes. Granite is made up, which is the rock from which many kitchen worktops are made. Granite is less dense than basalt, which consists mainly of ocean crusts. Because of this low density, large granitic islands, or the foundations of young continents, float to the surface.
But how the first continental crust formed has been a question that has puzzled geologists for decades. The young Earth had no continents yet. The planet was completely covered by a continuous oceanic crust submerged in water. So the way continents are formed today, via diving plates, was not possible back then.
The current hypothesis is that the first continents arose through intra-Earth processes. But the alternative idea is that the first continent arose by external influence, through the impacts of meteorites – a theory put forward in the 1960s. Calculations of archaeological digs on the Moon and Mars show that the young Earth must have been heavily bombarded by meteorites. In any case, there were many impacts 3.9 to 3.5 billion years ago. That’s when the oldest continents were formed. Evidence for an alternative idea is still lacking.
But now Johnson and his colleagues are bringing in the evidence. So they went to the Pilbara Craton. They analyzed 26 ancient stones. These stones contained very thin crystals of zircon. Geologists have been able to estimate the ages of the crystals by slowly decaying isotopes of uranium to form the isotopes of lead in the crystals. Some were 3.6 to 3.4 billion years old.
Johnson, from Australia: “And what happened to those ancient crystals – and thus to the piece of the ancient continent – we can see from the oxygen isotopes in the crystal.” Isotopes are variants of the same element: they have the same number of protons, but a different number of neutrons. “Due to different conditions, the ratio of isotopes in zircon changes.”
Isotope analysis of the oldest zircons shows that meteorite impacts melted Earth at the impact sites. Granite is formed by melting, just like by subduction. Then the granite appeared through the lower density.
“Ancient zircons had oxygen isotopes that could only be present in them when rocks melted from the surface,” Johnson says. “So the analysis on the Pilbara Craton shows that melting started from above, as you would expect from a meteorite impact.”
A version of this article also appeared in the August 11, 2022 newspaper
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