Content provided by Becky Oskin, OurAmazingPlanet
The age of the Grand Canyon is a puzzle, because the Colorado River has washed away many of the clues.
So for 150 years, geologists have pondered the processes shaping the
canyon, one of the world's great wonders and a living laboratory for
understanding Earth history.
The gorge's rugged beauty, with its sheer cliff and steep slopes, looks
young. And the general scientific consensus, updated at a 2010
conference, holds that the copper-colored Colorado River
carved the Grand Canyon beginning 5 million to 6 million years ago.
Many strong lines of evidence support this theory, including a pile of
gravel and limestone pancaked with lava at a place called Muddy Creek.
This geologic layer cake, at the western mouth of the canyon, locks down
the Colorado River from exiting the canyon before 6 million years ago.
However, recent advances in dating techniques have upended the notion of a uniformly young Grand Canyon.
The new approach determines when erosion uncovered rocks in the canyon.
The big picture: there were two ancestral canyons, one in the west and
one in the east. And the western canyon may be as old as 70 million
years.
The latest sally is a study reporting samples from the western Grand
Canyon were close to the Earth's surface 70 million years ago. The
evidence suggests the western Grand Canyon was cut to within 70 percent
of its current depth of 3,200 feet (1,000 meters) long before the
Colorado River existed. The results appear today (Nov. 29) in the
journal Science.
"Our data suggests that there was in fact a large canyon present for
most of the Grand Canyon by about 70 million years ago in its western
segment, and that canyon was carved to nearly modern depths," said
Rebecca Flowers, lead study author and a geology professor at the
University of Colorado, Boulder. "In the eastern canyon, the canyon was
higher, and lowered into its modern configuration sometime after 20
million years ago."
This much older western "paleocanyon" was incised by an ancient river flowing west to east.
This Cretaceous river carved the western Grand Canyon to within a few
hundred meters of its modern depth, and the eastern Grand Canyon to a
higher level.
When combined with rock sample ages Flowers collected in the eastern Grand Canyon during this study and in 2008,
the overview gives the Grand Canyon a complicated history. However, the
research can fit into the constraints presented by the Muddy Creek
barrier and other evidence supporting a young canyon, Flowers told
OurAmazingPlanet.
"The presence of the [Muddy Creek] detritus represents the integration
of the river system," Flowers said. That is, the Muddy Creek simply
represents the Colorado River appropriating the paleocanyons and created
a single drainage 6 million years ago.
Geologist Richard Young, who has studied the Grand Canyon for nearly 50
years, said scientists have considered the idea of two Grand Canyon
precursors — one west, one east — since the research community's first
symposium in 1964.
"We agreed that there were two canyons, one in the west and in the
east, we don't disagree on that," he said. The problem is that Dr.
Flowers wants to make the western canyon very old, Young told
OurAmazingPlanet. (Related: Grand Canyon in Pictures)
"It really looks like they're onto something, but it's hard to make
sense out of it," said Young, a professor at the State University of New
York in Geneseo. "It's really good work and it's really interesting, so
obviously there's something we're missing in the story. I'm sure we're
going to be talking about it forever," he said.
Recent work by geologist Karl Karlstrom supports the idea for a
paleocanyon in the east. "We showed very conclusively that there was a
paleocanyon in the eastern Grand Canyon that was carved between 25 and
15 million years ago," said Karlstrom, a professor at the University of
New Mexico in Albuquerque.
But Karlstrom is a strong advocate for a Grand Canyon quickly carved
by the Colorado River starting 6 million years ago, not older rivers
coming from the west. The western Grand Canyon region was cut across
nearly at right angles by one or more paleocanyons with rivers that
flowed north around 70 million years ago, but these paleorivers did not
follow the modern course of Grand Canyon, Karlstrom said.
"The best answer is that Grand Canyon was carved by the west-flowing
Colorado River in the last 5 to 6 million years and that earlier
paleocanyons were likely re-used and deepened once the river found its
present path," he said.
had a radically different appearance 70 million years ago. Most of the
region's famed dinosaur fossils come from the Jurassic, and the
canyon-cutting identified by Flowers and colleague Ken Farley of Caltech
began in the Late Cretaceous.
Seen from the air, the flat Colorado plateau might be recognizable, but
the rainbow-hued pillars and monuments of national parks such as
Arches, Zion and Bryce had yet to take shape. Close to the west rose a
volcanic arc similar to today's Andes — the precursor to California's
Sierra Nevada Mountains. A wrinkled belt called the Sevier mountains was
northwest of the plateau. To the east was the Western Interior Seaway.
Rivers flowed out of mountains generally heading northeast into the
ocean.
The infant Rocky Mountains didn't start their rise in the east until
about 10 million years later, though this timing is debated. The Basin and Range
province, which built the classic Southwest monuments and valleys
immortalized in film and art, began tearing apart 20 million years ago.
Rivers crossing the Colorado Plateau reversed their course, flowing east
to west, around this time.
"We know the river systems must have evolved dramatically during this
time. The controversial part of it is how they evolved," Flowers said.
Copyright 2012 OurAmazingPlanet, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.
