Where does the oldest stone in the world come from?
How the oldest rock on earth was formed
The oldest rock on earth, around four billion years old, is found on the Acasta River in northwest Canada. It is gneiss - a so-called metamorphic rock that formed from another rock at high temperatures. How exactly the Acasta gneiss came about has so far been unclear. In the journal "Nature Geoscience", geoscientists working with Tim Johnson from Curtin University in Perth, Australia, are now proposing a possible scenario: The rock could have formed when large meteorites hit the still young Earth.
Gneiss is similar to granite, but is striking because of its banded pattern. Most of the deposits formed on tectonic plate boundaries. If one tectonic plate dips beneath another, the conditions necessary for it to emerge prevail. But with the Acasta gneiss it was evidently different. Because it has a different chemical composition than other gneisses. This suggests that the rock also formed differently. Johnson and his team used computer simulations to run through various scenarios for the origin of the Acasta gneiss. The scientists used the concentration of trace elements as a chemical indicator of how well simulation and reality match. Because depending on the path of origin, the rock contains different amounts of strontium or rare earth elements.
The researchers found that the Acasta gneiss could only have formed at temperatures of over 800 degrees Celsius and comparatively low pressure. The impact of a large meteorite on Earth is likely to create just such conditions. This scenario would be entirely plausible - after all, during the first 600 million years there were permanent meteorite impacts on the planet.
The outer layer of the early earth consisted of ferrous basalt rock. At that time, the meteorite impacts could have caused part of the rock in the top three kilometers of the earth's crust to turn into gneiss. According to the computer simulations, meteorites with a diameter of around 100 kilometers would be necessary to create the right conditions for the formation of the gneiss. Rock deposits like Acasta gneiss must have been very common at one point, Johnson says. But after four billion years and due to the advent of plate tectonics, hardly anything is left of it.
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