The Koyal Group Info Mag articles
- A zircon crystal embedded in sandstone found on a sheep ranch in Australia is
the oldest piece of the Earth’s crust to be discovered, shedding new light on
our planet’s formation.
The zircon, described in the
journal Nature Geoscience, is about 4.4 billion
years old and much smaller than a single grain of rice. But the tiny crystal
carries an outsize significance: It is evidence that by that point in its
history, Earth had gone from a superheated ball of molten rock to a congealed
surface eventually capable of supporting life.
“One of the main goals of the space program is to understand if there’s
life elsewhere in the universe,” said John Valley, a University of
Wisconsin professor who led the study, collaborating with scientists in Australia,
Canada and Puerto Rico.
By studying how the conditions of
life came together on our planet, scientists believe we will learn what to look
for on other planets.
But the earliest rocks and first
evidences of life have been subject to dispute over the years. Some scientists,
for example, maintain that the earliest evidence of life is about 3.8 billion
years old and found in Isua, Greenland. Skeptics, however, note that no fossils
were found in the Greenland rock. They point instead to 3.5 billion-year-old
evidence of life found in rocks in Pilbara, Australia.
That’s no small difference — 300
million years.
The age of the zircon described
by the Valley team, however, does not appear to be in dispute. The Valley team
used a new technique called atom-probe tomography, which allowed them to
confirm the accuracy of the crystal’s age. The new instrument, made in
Wisconsin, is so sensitive that researchers were able to identify the atomic
number and mass of each atom in the sample.
“I think they have shown unequivocally, beyond a shadow of a doubt,
that this grain is that old,” said Samuel Bowring, an expert in the early
history of the Earth and a geology professor at the Massachusetts Institute of
Technology. Bowring was not involved in the new study.
“It’s only one grain, mind you,” he added, “but it’s very significant.”
Jim Mattinson, a professor
emeritus in the department of earth science at University of California, Santa
Barbara, said zircons have been found previously that were about the same age
as the one in the current paper, but the earlier discoveries were met with
skepticism.
“This paper drives a nail into that coffin (of doubt),” Mattinson said.
“We’re really getting back as far as we can go in the Earth’s geologic
records.”
Zircon crystals are composed
mainly of the elements zirconium, silicon and oxygen. Small amounts of uranium
also appear in zircon.
The uranium decays at a set rate,
forming lead. Because of these characteristics, scientists can use the lead and
any remaining uranium in a zircon crystal to calculate the age.
Zircon is found embedded in
younger rock. Valley found the zircon used for the current study in sandstone
collected in the arid Jack Hills of western Australia, a region known to
contain some of the oldest pieces of the planet’s crust.
“The oldest rock in Australia was collected not far from where we were
working,” Valley said.
Dating of the zircon helps
clarify an early chapter in the Earth’s history. Scientists have theorized that
one of the crucial early events occurred when an asteroid roughly the size of
Mars struck a glancing blow to the Earth, vaporizing the mantle and crust. Dust
from the collision merged to form the moon.
The enormous energy from the
collision transformed the surfaces of the Earth and moon into oceans of molten
rock. Both subsequently cooled. Zircon was one of the minerals formed when the
planet cooled.
Although minerals also were
formed as far back in history, what makes zircon so valuable to geologists is
its ability to endure. Zircon is a very hard mineral with stable chemistry able
to survive extreme temperatures.
“We like to say that zircons are forever,” Valley said. “They really persist in the rock record.”
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