Plate tectonics may be younger than life, Earth’s oldest rocks suggest

A study of the world’s oldest rocks provides evidence that plate tectonic-driven recycling may not have begun until several hundred years after the formation of the Earth. The authors admit their findings aren’t conclusive, but if they’re right, they have big implications for what it takes to keep life going.

The fact that Earth is the only planet known to harbor life, and also the only one known to have plate tectonics, is not generally considered a coincidence. From available energy around hydrothermal vents to recycling old rocks at plate boundaries, crustal movements offer many benefits to life. Moving continents also played a key role in the distribution and spread of species.

The question of whether life actually requires plate tectonics, or just benefits from it, could be answered if we find that life established itself first. That’s one possibility, according to a new article in Science Advances, which notes that the earliest evidence of geological recycling dates back 3.8 billion years.

The oldest surviving terrestrial materials are zircons from Western Australia, however, Professor Ross Mitchell of the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS) told IFLScience: they are just minerals (not rocks) preserved as sediment grains in a much younger rock environment. The oldest known rocks are found in the Acasta Gneiss Complex in northern Canada. Mitchell is part of a team that studied them, looking for evidence of the changes that occur when subduction zones pull rocks into the Earth.

Our older samples show no signs of surface material recycling at 4.0 [billion years ago], IGGCAS Professor Li Xianhua said in a statement. And the first evidence we find for surface recycling into magmas isn’t until 3.8 [billion years ago].

The conclusions are based on the silicon and oxygen isotopes in the rocks. The first oceans were rich in heavy silicon. With no life, this sank to the bottom of the sea. Yet the rocks Mitchell and Li tested are not enriched with heavy isotopes as would be expected if the silicon were recycled through magma chambers to be ejected from volcanoes.

Over 4 billion years, the rocks have been so reworked that identifying their original silicon is a challenge, but the authors focused on the zircons within the rocks, whose resistance to change is demonstrated by the survival of their counterparts. Australian.

By measuring isotopes in rocks of somewhat varying ages, the authors found a shift to 3.8 billion years ago, which they suspect indicates the start of nearby plate subduction.

Unfortunately, to study such an ancient point in Earth’s history, the team depends on samples from only one location. Co-author Professor Allen Nutman of the University of Wollongong acknowledged that no subduction needed for a small area means no plate subduction on the planet at 4.0 [billion years ago].

Perhaps the shift at 3.8 billion years was local rather than worldwide. However, the authors believe that the possibility that plate tectonics activated globally at that point after the first life forms emerged should be explored.

Just two weeks before this study, another zircon scholar from South Africa’s Barberton Greenstone Belt found an absence of plate tectonics until even more recent times, about 3.3 billion years ago. On the other hand, a paper from earlier this year claims tectonics as early as 4.2 billion years ago.

The question clearly has a way to run, but once resolved it could affect the type of planets we prioritize in the search for life.

It was already surprising that these older rocks were preserved, Mitchell said. And now we learn that they also tell a tectonic story of formation.

The study is open access in Science Advances.