Australian rocks suggest early Earth may not have had plate tectonics
Richard Lovett US Science Writer
The ancient Earth did not require plate tectonics to form rocks similar to those associated with today’s tectonic processes, a team of Australian and American scientists report in a paper published in Nature.
It is the latest salvo in one of modern geology’s most enduring debates, regarding when in our planet’s history modern-style plate tectonics commenced.
Today, the motions of the Earth’s crust are responsible for everything from volcanoes and mid-ocean ridges to mountain-building and earthquakes.
When that process began is open to debate, but the majority of geophysicists probably believe it was about 3.0 to 2.8 billion years ago, says Michael Brown, an applied metamorphic petrologist at the University of Maryland, College Park, Maryland. “That wouldn’t be be everybody,” he says, “[but]…that would be a consensus of a majority of earth scientists at present.”
Prior to three billion years ago, he adds, many geophysicists believe the Earth went through a prolonged period in which its crust was single solid shell, punctuated here and there by volcanoes, but without today’s moving plates.
But a problem with that theory is the presence of “greenstone terranes” such as the East Pilbara Terrane in Western Australia, which contains some of the Earth’s oldest known rocks, formed as far back as 3.5 billion years ago. (The Earth itself is roughly 4.5 billion years old.)
Now eroded into low hills, this region, some 1200 kilometres north of Perth, is composed of a mix of basalt and Tonalite-Trondhjemite-Granodiorites (TTGs) – rocks that are chemically similar to granites, often seen as a signature of modern-style plate tectonics.
In the modern world, granites are formed near where slabs of seabed basalt are subducted into the mantle, at places where plates collide.
As they descend, these slabs begin to melt. The molten rock then rises back to the surface to produce magma. Some of this magma reaches the surface, forming volcanoes. Some solidifies underground as granite, later exposed when erosion strips away the softer overlaying rocks.
But it doesn’t have to be that way, Brown and the Australian team, led by Tim Johnson of Curtin University in Perth, say.
Using models of how rocks melt and crystalise at different temperatures and pressures, the scientists took into account the fact that the ancient Earth’s mantle was hotter than today’s, probably by about 250°C. That means that rocks would melt, rise, and recrystallise at different depths—and therefore pressures—than they would today.
Based on this, the scientists found, it is possible for TTGs to be formed that are chemically very similar to modern granites, without any need for plate tectonics.
The process involves repeated eruptions of basalt onto the surface of the crust—though at that time, Brown notes, these would have been undersea eruptions. “You have to imagine the surface of the Earth as mostly water,” he says. Other than scattered volcanoes, “everything was under water.”
As layer after layer of lava erupts, the accumulating weight of rock starts to press underlying layers downward. There, they heat up, re-melt, and move upward again. Some erupt as more seabed lava. Some only gets partway, solidifying as TTGs.
“You have to do this process more than once to get the right composition,” Brown says, but in the East Pilbara Terrane there appears to have been a lot of time for the process to cycle over and over again. That’s because geological records indicate the region was volcanically active for 300 million years, allowing repeated geological cycles. Nowhere on the modern earth have volcanoes been active in a single place so long a time.
Is this representative of the entire ancient earth? Who knows? The farther we look back in time, the fewer surviving rocks we have to look at, making it difficult to determine what the entire planet looked like.
The point, Brown says, is that scientists have to be careful not to presume that ancient rocks were produced by the same processes as modern ones with similar geochemistry. “It may be that we can reproduce a similar geochemical fingerprint in a different geodynamic and tectonic regime,” he says.
Andrew Gleadow, a professor of geology, geochronology, and tectonics at the University of Melbourne, who was not part of the study team, thinks the new study has made a “strong case” that the granitic rocks in the East Pilbara Terrane could have been produced without plate tectonics. “The key conclusion that gives rise to their work is that this melting could only have occurred at temperatures of 850-900°C under a high geothermal gradient,” he says.
Not that this proves that plate tectonics didn’t exist 3.5 billion years ago, he adds, but it does demonstrate that they are not required to explain the chemical composition of the East Pilbara Terrane granitics.
That won’t end the debate about whether subduction was operating on the early earth, he says, but it’s definitely an “important contribution to the ongoing debate.”