In the beginning, there was Ur
by Elizabeth
Zubritsky
John Rogers charts the eons-old dance of
the continents.
As John Rogers sees it, three billion years is as far back as you can
go in studying continents. Before that, they didn't exist - not by his
definition.
Traditionally, geologists determined the age of a continent based on
the oldest exposed rock, which is some variety of granite. Because
granites constitute the foundations of continents, geologists assumed
that any granite found must date back to the continent's formation. But
Rogers, Kenan professor of geology, pointed out that granites were
forming and being reworked into the crust before the crust was steady
enough to sustain a continent.
"I kept saying that a whole pile of granites that is continually
being destroyed is not a continent," Rogers says. "It's just a
temporary pile of rock."
Rogers began defining continents more strictly - as large, stable
platforms that other rocks could settle on - and it changed the way he
determined a continent's age. Instead of using the oldest rock, he used
the oldest stable rock. Then he saw a pattern that nobody else ever had:
Continental rock in the ancient supercontinent Pangea was grouped by age.
Pangea was the last supercontinent, a giant conglomeration of all the
continents. It formed 300 million years ago when the tectonic plates
that make up Earth's crust modified and rearranged themselves, bringing
the continents together. Today, six large plates and more than a dozen
small ones float on a mobile layer in the mantle, a zone of rock that
surrounds the Earth's core. Some geologists speculate that currents flow
in the mantle and carry the tectonic plates like rafts on a river, but
nobody knows why they would assemble into one large landmass.
Geologists think supercontinents break up because they trap an
immense amount of heat. Eventually, the pressure reaches a critical
point and literally blows the supercontinent apart. When Pangea broke up
200 million years ago, the pieces became the continents we know today.
By re-examining data that he and other people have collected, Rogers
calculated the ages of various parts of Pangea. He saw that one area
contained three-billion-year-old rock. Another consisted of rocks
two-and-a-half billion years old, and two others, two-billion-year-old
material. Rogers reasoned that these areas must have emerged as
successive continents, which must have remained intact until Pangea
broke up.
"It would have been highly unlikely, statistically impossible,
for these areas to form as continents, to split up and wander around the
Earth for several million years, and to come together as Pangea with the
areas that are the same age next to each other," Rogers says.
Based on his conclusion that these early continents stayed together,
Rogers charted their gross movement and development. In January 1996
the Journal of Geology published his paper, "A History of
the Continents in the Past Three Billion Years."
Rogers says Ur was the first continent,
formed three billion years ago, followed by Arctica half a billion years
later. Another half a billion years passed before Baltica and Atlantica
emerged. The four continents roamed separately until, about
one-and-a-half billion years ago, Arctica and Baltica collided with what
is now eastern Antarctica to form Nena.
When Nena, Atlantica, and Ur came together one billion years ago, the
supercontinent Rodinia was born. After 300 million years, the three
landmasses separated for about 400 million
years, then came back together in a new configuration, Pangea.
The continents gained mass over time, Rogers says, because lighter
material in the mantle has been slowly making its way to the surface, a
process called "gravitational segregation." Through all of
this, the original continents remained intact. But when Pangea came
apart, Ur, Arctica, and Atlantica split up too. Parts of Ur went to
Africa, Australia, and India, while Arctica became Canada, Greenland,
and part of Siberia. Atlantica was divided between South America and Africa.
Why would Ur have survived for so long, only to be torn apart when
Pangea broke up? Rogers must wait for more data to answer that question,
but he expects to find evidence that supercontinents, in general, break
into large chunks rather than small pieces. And that will be his next
challenge to geologists' theories about continents.
Don't Push the Earth Too Far
Once you realize how long the Earth has been in the making, you begin
to think it deserves some respect. In the forthcoming book, People
and the Earth, John Rogers and Geoffrey Feiss, professor of geology
and associate dean of the School of Arts and Sciences, examine how geological changes influence the way we live and vice versa.
Rogers and Feiss take a geologist's view of topics such as global
warming, the United States' dependence on foreign oil, and the
exportation of hazardous waste to Third World countries.
The authors emphasize the need for wealthy nations to recognize the
impact of their consumption and to understand the needs of
less-developed nations. Ten percent of the Earth's population uses
two-thirds of the resources, Feiss says.
"Can the Earth go on if the rest of the world achieves the same
standard of living?" Feiss asks. "If not, what does this mean,
for us and for them?"
One of the book's themes is that all organisms, especially humans,
affect their environment. "What's new is the scale, rapidity, and
efficiency with which people can do it," Feiss says.
And we may not anticipate or recognize the impact we have.
"Economists call it 'the effect of unintended consequences,'"
Feiss says. "Because the Earth is a complex system and because we
don't understand the interdependencies, we might think we're doing
harmless things when they actually have disastrous consequences.
"We want people to think through these policy issues,"
Feiss says, "to consider the scientific realities and not to push
the Earth too far."
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