JANUARY/FEBRUARY 2020. DISCOVER 83Early Start for Our
First Continents
BY ERIK KLEMETTIESSAY
i
We’re all here thanks to continents.
Among rocky planets in the solar sys-
tem, only ours has masses of less-dense
rock that rise above surrounding crust.
Yet our planet wasn’t born with them.
We know that these land masses are a direct
consequence of plate tectonics, when slabs of
crust, continental and oceanic, interact as they
move across the planet’s melted mantle. But we
don’t know when or how quickly the continents
formed — it’s one of the most challenging ques-
tions about Earth’s early history. Some geologists
believe most of the continents popped up in the
last billion years. Others think they have been
forming slowly and steadily since the planet took
shape some 4.6 billion years ago. Still other schools
of thought suggest the land masses formed in fits
and starts as pieces of them collided and then
broke apart.
The evidence needed to solve this mystery
is hard to find. Geologists typically analyze
samples of Earth’s oldest-known crust — dating
to the Archean Eon, from 4 billion to 2.5 billion
years ago — to try to determine when the first
continents formed. But little of that material has
survived.
Some researchers consider 4.4-billion-year-old
Australian zircon crystals — the oldest preserved
building blocks of rocks — as evidence of very
early continental crust, but theirs is a controversial
conclusion.
Derrick Hasterok of the University of Adelaide
and his colleagues took a new approach to solvingthis mystery. They collected a huge amount of geo-
chemical data from almost 25,000 samples to map
out how much heat has been produced by rocks in
Earth’s crust in the last 4 billion years. The conti-
nents are made of granite, which is enriched with
radioactive elements like potassium, uranium and
thorium. As these elements decay, they produce
heat at known rates.
In July, the team reported in the journal
Precambrian Research that they’d identified a “heat
deficit” in the models of our planet’s early history
that could only be solved by more granite existing
in the past than previously thought.
The team’s results suggest that continental crust
might have formed half a billion years earlier than
most current models suggest. These early con-
tinents would have been unstable, due to more
abundant radioactive elements present at the time.
The elements could have produced four times the
heat than levels seen later in the geological record,
making the first continents prone to melting and
reworking — and thus less likely to be preserved.
Much more than the ages of rocks is at stake. If
the new model is correct, and the first continents
emerged earlier than thought, it means plate
tectonics was already in motion at the time. The
tectonic engine has been a driving force on the
planet, creating carbon dioxide-belching volca-
noes and influencing ocean and wind currents,
for example. Without that influence on climate,
Earth may have remained a lifeless planet. And, if
the dynamic process began much earlier than we
SCI thought, it’s possible that so did the story of life.ENCE
PHOT
O^ LIBRAR
YGeologists have
long debated when
continents first
formed. A new model
suggests they took
shape half a billion
years earlier than
thought, during our
planet’s turbulent
childhood — which
could mean life
started earlier, too.