Grand Canyon Mystery

Geologists debate the age of the planet’s deepest canyon, with the answer to ancient riddles hanging in the balance


Many geologists believe it took the Colorado River just 5 million years to carve the Grand Canyon. But a recent study argues the canyon is really 55 million years old, which means dinosaurs may have once stomped along the rim. The study based its conclusions on the decay of radioactive elements in rocks that cools near the surface just as the dinosaurs died out.

Many geologists believe it took the Colorado River just 5 million years to carve the Grand Canyon. But a recent study argues the canyon is really 55 million years old, which means dinosaurs may have once stomped along the rim. The study based its conclusions on the decay of radioactive elements in rocks that cools near the surface just as the dinosaurs died out. |

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So I’m sitting on a river-smoothed swirl of Vishnu Schist in the bottom of the Grand Canyon trying to reset my imagination as the Colorado River roars past.

I pat the rock. Gotta be 4.6 billion years old if it’s a day. I kid you not.

I change my perspective — tilt my head to take in the cliff edge of the inner basin, a mile deep in the heart of the planet’s biggest canyon.

I can’t see the distant rim of the canyon beyond the layer upon layer of rock. Here in the bottom, the granite and schist formed miles beneath the surface shortly after the planet had cooled sufficiently to spawn the first, single-cell life forms.

I struggle for a moment to comprehend that figure: 4.6 billion years.

No good. Can’t do it. Plumb eludes me.

All right. Something easier.

How about the age of the canyon itself?

How long did it take to gouge out this mile-deep tear in the crust of the earth, with its bewilderment of side canyons?

Well, now — that’s an unexpectedly complicated question.

For awhile there, the geologists had it all worked out.

Maybe 5 million years, they said.

That answer relied on lots of evidence that suggests some titanic shift in the earth took place at that time, causing the inexorable uplift of a big chunk of North America now dubbed the Colorado Plateau. That upward movement continues today at about the rate your fingernails grow and stretches from the Mogollon Rim to the tips of the Rocky Mountains. As the land rose, the ancestral Colorado River ripped into the rising mass of rock, carrying unimaginable volumes of rock down into the Gulf of California, which gaped open at about the same time.

That theory for a long while stood as a stunning example of geological violence. It suggests uplift and erosion could have created this network of canyons in roughly the time it took to evolve all the difference between chimps and geologists — since the DNA evidence suggests that humans and chimps shared a common ancestor some 5 to 8 million years ago.

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Tom Brossart photo

New evidence suggests that the river that started carving the Grand Canyon once flowed north into Utah, until the south-flowing Colorado River captured it.

In geological terms, that’s a relative eye blink.

Interesting theory. But now it turns out the picture actually looks a little more complicated.

Two recently published studies suggest that the Grand Canyon may actually be 55 million years old — about 10 times older than the commonly accepted theory suggests.

Moreover, the river that carved the deep canyon that came before the Grand Canyon probably flowed north for millions of years, depositing much of the rock removed from the canyon in a vast, inland lake or sea — akin to the modern Great Salt Lake on a grand scale.

I pat the smooth, ancient schist — with its veins of granite and quartz and its strange, eroded fluting — sand blasted in the solid rock by eons of flooding. If the latest studies concerning the age of the canyon hold true, perhaps dinosaurs trod the rim of that ancestral Grand Canyon.

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Tom Brossart photo

New evidence suggests that the river that started carving the Grand Canyon once flowed north into Utah, until the south-flowing Colorado River captured it.

The mind-warping findings emerged from a comparison of rocks in the canyon bottom and rocks on the high, flat rim, made by researchers from the California Institute of Technology and the University of Colorado at Boulder and published earlier this year in the Geological Society Bulletin.

The surprising findings rely on the use of the steady decay of radioactive elements in a phosphate mineral known as apatite to create tiny geologic clocks.

Turns out, apatite always includes trace amounts of uranium and thorium, which decay by spitting out helium atoms at a steady rate. The mineral forms far beneath the surface, but the clock-like decay doesn’t set in until the mineral cools below a certain temperature. So the mineral that forms at great depths cools as uplift and erosion brings it closer to the surface. Once it rises to a certain depth, the radioactive clock starts ticking.

Now, here’s where it gets weird.

The apatite samples up on the rim and the apatite samples near the bottom both approached the cool surface at about the same time — roughly 55 million years ago.

Now, that sounds completely strange — how could the bottom of the canyon and the canyon rim both be about the same?

See, geology does that to you: Messes with your mind.

Near as I could tell by poring over the papers, it goes something like this:

Some 55 million years ago, a great canyon had formed at more or less the present location — although at that time the river most likely flowed north into an interior sea rather than south into the not-yet-formed Gulf of California.

Once the Colorado Plateau began its uplift, erosion went to work on both the canyon bottom and the canyon rim — removing a great mass of rock perhaps a mile deep from both rim and bottom as the plateau rose.

As Caltech’s Brian Wernicke put it, “because both canyon and plateau samples resided at nearly the same depth beneath the earth’s surface 55 million years ago, a canyon of about the same dimensions of today may have existed at least that far back, and possibly as far back as the time of the dinosaurs at the end of the Cretaceous period 65 million years ago.”

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Tom Brossart photo

The Grand Canyon is much younger than the rocks it contains. The canyon itself is somewhere between 5 and 55 million years old, but the oldest rocks in the bottom are 4 billion years old.

Another study fills in an additional piece of the puzzle. Turns out that 55 million years ago a big river flowed north from Arizona into Utah. The 400-mile-long river carried sediment made from volcanic bedrock from the current Mojave Desert into Utah, where it deposited what’s now referred to as the Colton Formation, according to a study by Carnegie Institution lead author Steven Davis published recently in the journal Geology. Co-authors of the study included William Dickinson and George Gehrels from the University of Arizona and Jon Spencer, with the Arizona Geological Survey.

The researchers dubbed this ancient river the California River and speculate that it might have flowed for some 20 million years, during which time it could have cut a big chunk of the Grand Canyon.

Eventually, some additional shift caused the river to reverse course and flow to the south, until it found its present path to the Gulf of California. It is possible that some other, smaller river system that drained off to the south chewed back into the rising edge of the Colorado Plateau and eventually captured the ancestral Colorado River, forcing it to finally head south.

I sit on the ancient rock, trying to let that perspective seep into the dense granite of my mind.

The sun has set already, a chill is gathering. But I still cannot quite wrap my mind around it. If I squint mentally and let go, sometimes I get just a flash — like the white belly of the darting swift that veers crazily across the river’s surface, hunting bugs.

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Tom Brossart photo

The easily eroded sedimentary layers in the Grand Canyon recall vanished seas.

It’s no use, of course — a creature with a lifespan measured in years cannot possibly comprehend the difference between a 5-million-year-old canyon and a 55-million-year-old canyon — much less a 5-billion-year-old rock.

But I can dimly sense the tremble of the earth — imagine the uplift of a continent.

I cannot say why, exactly — but I glean some comfort from the thought. This place does not take notice of me, does not depend on me. It dreams in cycles that take no notice of our whole history of a species.

That’s humbling — but comforting too. It is not all up to us after all.

And then I notice the first bat of the evening, replacing the flashing swift with his own dark flutter.

The first bats go back about 55 million years, back to the time when the granddaddy of the Grand Canyon formed.

Perhaps they can explain it all to me, if I listen closely enough.

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