By Alexandra Witze, Science News
For all its glorious views, the Colorado plateau remains an ugly mystery to geologists. They can’t figure out why and how it rose thousands of feet over the millions of years it took to carve spectacular natural wonders like the Grand Canyon and Monument Valley.
The answer may lie deep beneath the plateau’s chiseled landscape, a study in the April 28 Nature suggests. Hot rock welling up from below invades the plateau, causing blobs to drip off the bottom.
“It looks kind of like a lava lamp,” says team leader Alan Levander of Rice University in Houston.
Geologists have wondered about the high plateau ever since early explorers stood on the edge of the Grand Canyon and peered down at the Colorado River through 1,500 meters of glorious layer-cake rock.
The plateau roughly covers the Four Corners area where Utah, Colorado, New Mexico and Arizona come together. About 2 kilometers above sea level, the plateau behaves as a single, mostly undisturbed chunk of crust even as tectonic forces crumple the landscapes on either side.
To the east, the Rocky Mountains thrust toward the sky; to the west, the Basin and Range region wrinkles in long ridges of mountain and valleys. But something, mysteriously, has kept the Colorado plateau high and intact.
Most theories focus on the uppermost layers of the Earth’s innards: the crust; the “lithospheric mantle” below that, which moves with the crust as a relatively hard outer shell about 150 kilometers thick; and even deeper, the “asthenosphere,” which flows like a fluid.
Levander’s team probed these hidden realms by studying how seismic waves traveled through the Earth. The data come from a major project called the USArray, in which geologists blanket the continent, in strips moving from west to east, with a dense network of seismometers.
By studying the waves’ progress, the scientists spotted a weird feature sloughing about 200 kilometers down, just north of the Grand Canyon. This blob, they say, is part of the crust and lithospheric mantle that peeled off to founder in the planet’s depths.
Blame the asthenosphere. When it can, this less-dense fluid layer rises from below. Where it infiltrates the stiffer crust above, the fluid freezes, weakening the lithosphere and eventually chiseling chunks of rock away. Over time, more and more blobs fall off, allowing the rest of the plateau to rise upward like a floating cork.
Geologists have previously spotted other places where blobs might once have dripped, Levander says, such as along the Idaho-Oregon border. But in the Colorado plateau, he says, “it looks as if we’ve caught one of these as it happened.”
Though the plateau drips have probably been happening for the past 25 million years or so, he says, they really took off about 6 million years ago—allowing the plateau to rise in earnest and ancient rivers to begin to carve the Grand Canyon.
Other researchers have proposed versions of the drip idea before, but without the detailed seismic observations of the Earth’s guts.
Some scientists, though, aren’t yet convinced. “The tendency is to say we have what looks like a drip and therefore it is a drip,” says geophysicist Mousumi Roy of the University of New Mexico in Albuquerque. “We need to be really careful about that interpretation.” Two years ago, Roy and her colleagues described an alternate theory to explain plateau uplift, in which the lithosphere warmed but did not drip.
In particular, she says, the new work assumes that rock infiltrating from below would make the lithosphere more dense by rearranging atoms such as iron into minerals with less space between the atoms. But this “refertilization” process is messier in real life than in theory, Roy argues. The chemical changes aren’t well understood, she says, and probably don’t lead to chunks of lithosphere getting more dense and breaking off.
The drip theory can’t explain all of the plateau’s uplift, Levander notes. He and his colleagues are now looking for drips elsewhere across the country.