How Old Is the Grand Canyon?
The Grand Canyon may be 10 million years older than
we thought
by Liz Olson
Until recently, the Grand
Canyon, one of the most spectacular and mysterious natural wonders of
the world, was thought to be about six million years old. With improved
dating techniques, geologists now believe that it actually started to form
some 17 million years ago.
Geologists estimated the Grand Canyon's age
at six million years by using a technique called uranium-lead dating. The
Grand Canyon walls are lined with a sedimentary rock called Redwall Limestone.
Redwall Limestone can be seen where water levels have dropped, leaving
behind deposits of calcium carbonate. Their results showed that six million
years ago, a river started from the west and another from the east, carving
a mile-deep canyon until they met in the middle and formed what is now the
Grand Canyon and the Colorado River.
Dating the Grand Canyon
Uranium-lead dating allows researchers
to determine the age of mineral deposits hundreds of millions of years old. Over time
uranium, which can be found in many
mineral deposits, decays into lead. Uranium has a half life of 4.5 million
years, which means that only half the original amount of uranium in a rock
is left after 4.5 million years. Geologists can collect mineral deposits and
find their age by measuring the amount of lead found in the rocks. The
amount of lead tells how long the uranium has been decaying. The mineral
deposits collected from the Grand Canyon walls reveal when and how fast
water levels dropped.
New Research
In 2008, Victor Polyak
led a team of geologists from the University of Mexico to conduct new research on the Grand
Canyon. They also tested minerals that line the canyon walls at different
heights in order to track water levels, which allowed them to find the rate
at which the canyon deepened. Polyak and his team, however, rafted, hiked,
and climbed to access caves that dot the canyon's walls. Many of the caves
were difficult to reach—some were hundreds of feet high. In the caves
they found mineral deposits that contain larger amounts of uranium.
What are mammillaries?
The uranium-rich mineral deposits are
called mammillaries. Mammillaries are
mineral deposits that form at the top of the water table and provide a
timeline of the river and canyon’s history. Mammillaries, often called
"cave clouds," form just under the surface of water that has a high
calcium-carbonate concentration. They leave behind formations, often
ring-shaped as seen in trees and stalagmite, on existing rocks.
New
birth date for the Grand Canyon
Polyak and his team collected
mammillaries from nine sites in the Grand Canyon that were at a much higher
elevation than the current water level and then used uranium-lead dating to
find the age of the Grand Canyon. They found that the Grand Canyon eroded
much slower on the western end than was previously thought, suggesting it is
much older than six million years. Findings also showed that instead of one
large river, small streams caused the erosion and they joined to make one
large river, the Colorado River which runs through the Grand Canyon today, stretching 277 river miles long,
up to 18 miles wide, and a mile deep.
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