A study of the world’s oldest rocks provides evidence that plate-driven recycling may not have begun until hundreds of years after Earth formed. The authors admit that their findings are not conclusive, but if they are correct, they will have major implications for what it takes to sustain life.
In fact, Earth is the only planet that has life and is the only planet that is known to have bricks, generally not I thought it was a coincidence. From the energy available around hydrothermal vents to the recycling of old rocks at plate boundaries, the movement of Earth’s crust provides many benefits for life. Continental migration also plays an important role in the spread and propagation of species.
The question of whether life really needs plate tectonics, or just benefits from it, can be solved if we find life that established itself first. That’s a possibility, according to a new paper in Science Advances, which finds that the first evidence for geological recycling comes from 3.8 billion years ago.
The world’s oldest surviving objects are zircons from Western Australia, however, Professor Ross Mitchell from the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS) told IFLScience: “They are just minerals (not rocks) preserved as sedimentary grains. Very soft rocks.” The oldest rocks are found in the Acasta Gneiss Complex, in northern Canada. Mitchell was part of a team that studied these, looking for evidence of changes that occurred when subduction zones pulled rocks into the Earth.
“Our oldest samples show no signs of recycling the 4.0 raw materials [billion years ago],” Professor Li Xianhua of IGGCAS said in a statement. “And the first evidence we found for surface recycling into magmas was not until 3.8. [billion years ago].”
Conclusions are based on silicon and oxygen isotopes in the rock. The early oceans were rich in heavy silicon. When there is no life, this has sunk to the sea floor. But the rocks Mitchell and Li tested were not as rich in heavy isotopes as would be expected if silicon was recycled through magma chambers to be erupted by volcanoes.
Over the course of 4 billion years, the rock has been reworked, identifying its original silicon is a challenge, but the authors focused on zircons within the rock, resistance to change is shown to survive their Australian counterparts.
Measuring isotopes in rocks of different ages, the authors found a change at 3.8 billion years ago, which they suspect indicates the beginning of a nearby plate subduction.
Until 2.5 billion years ago, seawater was saturated with silicon, leaving the seafloor saturated with heavy silicon isotopes. But before 4 billion years ago, this was not included in the granites, but after 3.8 billion years ago they were.
Photo credit: Prof. Group. LI Xianhua
Unfortunately, to study such a starting point in the history of the world, the team depends on samples from only one place. Professor Allen Nutman of the University of Wollongong admits that “the subduction required for a small area does not mean that there is no subduction on the planet at 4.0. [billion years ago].”
Perhaps the change at 3.8 billion years was local, rather than global. However, the authors think that the possibility that plate tectonics changed globally at that point, after the first life forms appeared, should be explored.
Just two weeks before this study, another investigator from the Barberton Greenstone Belt, South Africa, found that there was no plate tectonics until recently – about 3.3 billion years ago. On the other hand, documents from earlier this year make the case for tectonics beginning 4.2 billion years ago.
The question clearly has a way to proceed, but the agreement when influencing the arrangement of the planets that we prioritize in the search for life.
“It’s amazing that these oldest rocks have been preserved,” Mitchell said. “And now we’re learning that they also tell the story of tectonic age as well.”
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