Rocks in northwestern Canada contain traces of the primary crust of the Earth, which emerged 4 billion years ago at the time of the formation of the planet in the gas and dust disk of the solar system, according to an article published in the journal Science.
“The discovery of the remains of the Earth's primary crust has proven to be a daunting task for us. Our new technique allows us to find traces of it in those layers of rocks that have turned into simply 'the oldest crust of the Earth' as a result of various forms of geological activity during the time that our planet has existed,”says Richard Carlson of the Carnegie Institute of Science at Washington (USA).
Today geologists are actively discussing when tectonic processes started in the bowels of the Earth, when its first continents arose, and when our planet acquired its own magnetic shield that protects life from cosmic rays and the solar wind.
There is no consensus on this score. In recent years, theorists and practical geologists have found a lot of evidence in favor of the fact that the movement of tectonic plates could be launched almost immediately after the birth of the Earth, and almost a billion years after its birth. The former is supported by traces of an ancient magnetic field in zircon crystals formed 4 billion years ago, while the latter is supported by many other geological factors related to the properties of the rocks where these stones were found.
The answer to this question is extremely difficult to obtain, says Carlson, for the reason that until now scientists believed that there were no traces of the primary crust left on Earth that covered the planet after its formation 4.5 billion years ago. It turned out that this is not the case.
Carlson and his colleague Jonathan O'Neil (Jonathan O'Neil) from the University of Ottawa (Canada) drew attention to the fact that the primary crust of the Earth must have had one unusual feature that distinguished it from all other rocks on the planet - a special ratio of the proportion of isotopes of neodymium.
As scientists explain, in all rocks of the Earth there are small amounts of two isotopes of neodymium - neodymium-142 and neodymium-144. Neodymium-142 accumulated in the early solar system as a result of the decay of samarium-146, an extremely unstable element, all traces of which should have completely disappeared from the earth's rocks about 500 million years after the planet's formation.
Samarium-146, in turn, was unevenly distributed throughout the bowels of the planet, thanks to which scientists today can determine where certain rocks were deposited at the time of the birth of the planet, comparing the proportions of neodymium-142 and neodymium-144, as well as neodymium-144 and stable samarium-147. In general, the less neodymium-142 the rocks contain, the closer they were to the Earth's surface at the time of its formation.
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Guided by this idea, scientists analyzed the concentration of these isotopes in the supposedly ancient, but not primary rocks of the Earth's crust, lying in Canada, Greenland, Australia, South Africa and several other parts of the Earth. It turned out that it is in the north-west of Canada, in the vicinity of the Hudson Bay, that there are deposits of granite and other rocks, the proportion of neodymium in which corresponds to the values characteristic of the primary crust of the planet.
The study of samples of this crust has already yielded one extremely interesting result - its rocks, as scientists note, spent about 1.5 billion years on the Earth's surface before they plunged into the bowels of the planet. This indicates that the movement of tectonic plates on Earth could not have begun immediately after the formation of the planet, as a large number of scientists believe today. To be sure about this, as Carlson and O'Neill point out, more data from other parts of the earth is needed.
