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. They 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. It settled deep into the planet's mantle over millions of years.

Why?

"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. That 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. 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?

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COMMENTS (49)
  • D.F2001Rex
    3/19/2015 - 09:09 a.m.

    I think it is very cool that a city is one mile high. It is quite amazing how Denver is a mile above sea level. In the future I would love to visit Denver.

  • emmar1208
    3/19/2015 - 10:38 a.m.

    I think this theory makes the most sense about the Mile High City. The chemical reactions with the water. Its brilliant that they have come up with this idea!

  • rachelh10503
    3/19/2015 - 10:41 a.m.

    Denver might make a big deal out of it because maybe its an accomplishment to have the mile high status because no other place has that status.

  • Keegen
    3/19/2015 - 10:53 a.m.

    Although we are not 100% positive of why Colrado is a mile high one thing for sure is that it is a cool place and great for skiing.

  • Keegen
    3/19/2015 - 10:54 a.m.

    Although we are not 100% positive of why Colrado is a mile high one thing for sure is that it is a cool place and great for skiing.

  • morgang117
    3/19/2015 - 10:57 a.m.

    This is interesting. Scientists think the answer to Denver's mile high is water and chemical reactions. This could possibly ne the answer but scientists are still not sure.

  • christopherf-DiB
    3/19/2015 - 12:50 p.m.

    I think it's interesting that a chemical reaction with water deep underground causes the crust to become lighter and get pushed higher, it's an interesting theory however most people that are interested are geologists

  • JH2001skater
    3/19/2015 - 01:01 p.m.

    Denver makes a big deal about it because that is unquie to them and theyre are proud of it. They feel its an acomplishment that they are happy about

  • GD2000marinecorp
    3/19/2015 - 01:11 p.m.

    Denver makes such a big deal about its "mile high" status, because this city stands to 5,280- foot elavation above sea level and it's the point of pride, and identity of the city.

  • JaysonSGrey
    3/19/2015 - 02:33 p.m.

    Very interesting! Those are some good theories about why Denver is a mile above sea level! They make sense as well, and explains it very well!

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