Richard A Lovett
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.”
