What makes the Mile High City a mile high? (Thinkstock)
What makes the Mile High City a mile high?

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Geologists may finally be able to explain why Denver, the Mile High City, is a mile high. It's water.

A new theory suggests that chemical reactions, triggered by water far below the Earth's surface, could have made part of the North American plate less dense many millions of years ago. That's when the continents we know today were still forming.

Because plates float on the Earth's mantle, parts of the Western United States might have risen. Think of it like an empty boat next to one with heavy cargo, pushing the vast High Plains far above sea level. The theory was formulated by geologists Craig Jones and Kevin Mahan. They work at the University of Colorado-Boulder.

Their work appeared on the website of the journal Geology. It is a big deal for Denver. The 5,280-foot elevation is a point of pride and a big part of the city's identity. At Coors Field, where the Colorado Rockies play baseball, a single row of purple seats interrupts about 50,000 green ones. It marks the mile-high line in the grandstand.

Geologists have long been puzzled by how the High Plains could be so big, so high and so smooth. The plains descend gently from roughly 6,000 feet to 2,000 feet above sea level, and stretch for thousands of square miles, from the Texas Panhandle to southern Montana and from western Kansas to the foothills of the Rocky Mountains in Colorado.

It's well established that much of the West was still at sea level 70 million years ago. Tectonic shifts don't fully explain the High Plains' altitude. The lifting began long after the ancient Farallon oceanic plate was shoved deep under a vast part of western North America and it settled deep into the planet's mantle over millions of years.


"Crustal hydration," Jones and Mahan theorize.

They suggest that water that had been locked in minerals in the Farallon plate was released because of pressure from the overlying rock and heat emanating from the Earth's core. The water then rose into the continental plate. That set off chemical reactions that turned garnet and other dense minerals into mica and other less heavy minerals, which made vast areas of the crust lighter.

Jones said the Earth's crust under the High Plains "floats higher" over the mantle. It is much like a plank of buoyant balsa wood that rises higher in the water than a plank of dense pine.

Few geological formations appear so uniform on such a vast scale as the High Plains. The only other known location in the world that's similar is in southern Africa, Jones said. The prevailing theory there is different. It involves some other source of buoyance, Mahan said.

The composition of rocks found in the High Plains is strong evidence in favor of the hypothesis, Jones said. But it needs more testing and that was one reason for publishing it.

"Do we think this is 'the' answer? No. Could it be 'an' answer? I suppose it's possible," said Jones. He is also a fellow at the Cooperative Institute for Research in Environmental Sciences. It is a partnership of CU and the National Oceanic and Atmospheric Administration.

Not knowing why Denver is a mile high is a little awkward for Colorado geologists. Jones recalls having to tell a British TV producer a few years ago that he couldn't explain it.

"We probably need to figure this one out guys, because it's kind of embarrassing," Jones said.

Critical thinking challenge: Why does Denver make such a big deal out of its mile high status?

Assigned 16 times

  • BreeE-Bru
    3/19/2015 - 01:06 p.m.

    The main idea of "What makes the Mile High City a mile high?" is how it's water helps the land be higher than the surrounding plains. I know this because in the story

  • Eric0221-YYCA
    4/25/2016 - 09:25 p.m.

    The researchers might have been curious about what makes Denver, Mile High City, to be a mile high which they wanted to research on what is making Denver to be a mile high from any other elevations in the Western United States. The scientists might have wanted to find out what is making Denver to be a mile high from sea level which people might have wanted to find out what is making it a mile high. The scientists might have been able to find out about what had made Denver to be a mile high which scientists might have been able to find out what is making it a mile high. People might have been able to find out that the tectonic plate had made Denver to be made into a Mile High City which people didn't find out yet.
    Critical Thinking Question: Why does Denver make such a big deal out of its mile high status?
    Answer: I know why Denver make such a big deal out of its mile high status because people wanted to find out what had been making Denver into a mile high status.

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