The surface and even the atmosphere of our planet is being shaped under the enormous influence of plate tectonics. But these slow movements of the earth's crust did not begin immediately. It is assumed that they could have been launched by a Late Heavy Bombardment or even a single impact of a massive asteroid that fell on a fairly thin section of the lithosphere. The time of this event also remains unknown.
Estimates range from one to four billion years ago, although plate tectonics most often date back to about 2.8 billion years ago. New work by Harvard geologists dates the start of tectonics to a much more ancient period of 3.2 billion years. This is reported in a paper by Alec Brenner and coauthors published in Science Advances.
The authors investigated the Pilbara Craton, the oldest continental crustal basement that reaches the surface in western Australia. It is here that the oldest minerals on the planet are found, whose age reaches 3.5 billion years. Back in 2017, Brenner's team collected 235 samples of ancient basalt in the craton.
420 km section of the Pilbara craton; areas of ancient basalts are highlighted in green / © Alec Brenner, Harvard University, GSWA
Such rocks contain ferromagnetic inclusions that, like the arrows of tiny compasses, orient themselves along the lines of the Earth's magnetic field while it remains molten. Cooling and freezing, the stone fixes the directions of the “arrows”, which allows us to find out their initial position even after billions of years. This work was carried out by scientists, carefully dating each sample and tracing changes in their orientation over hundreds of millions of years.
It was found that already between 3.5 billion and 3.2 billion years ago, the craton was displaced in the horizontal plane at a speed of about 2.5 centimeters per year, approximately like modern continents. In principle, other explanations of this process are also possible: for example, local (regional) movements of the crust or wandering of the true magnetic pole of the Earth. However, the authors consider the most likely the most obvious explanation, concluding that plate tectonics could have started only 1.3 billion years after the formation of the planet itself.