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.
