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)
  • MichaelaJ-3
    3/20/2015 - 11:25 a.m.

    The Mile High City is a very cool sounding place and I'd love to go there once. I like how they marked the mile high elevation in the stadium using the seats. The Mile high city is called that because of water!!!

  • MichaelaJ-3
    3/20/2015 - 11:25 a.m.

    The Mile High City is a very cool sounding place and I'd love to go there once. I like how they marked the mile high elevation in the stadium using the seats. The Mile high city is called that because of water!!!

  • MichaelaJ-3
    3/20/2015 - 11:25 a.m.

    The Mile High City is a very cool sounding place and I'd love to go there once. I like how they marked the mile high elevation in the stadium using the seats. The Mile high city is called that because of water!!!

  • AustinC-Kut
    3/22/2015 - 01:58 p.m.

    The city of Denver is one mile above sea level. It's unusual for a city to be 1 mile above sea level which interested geologists to learn why. I was interested because I never knew there was a city 1 mile high. It has always been a mystery but recently geologists have found that the North American plate is less dense than the other areas of the earth's mantle. Geologists believe a chemical reaction caused this area to be less dense. Denver floats higher above other cities because the earth's mantle below it is less dense.

  • ad2000stealth
    3/23/2015 - 08:43 a.m.

    Denver makes such a big deal out of its "mile high" status because the city is actually a mile above sea-level and the fact that Denver is a mile high above sea-level, is a big part of what makes the city what it is now.

  • MCS2001softball
    3/23/2015 - 08:47 a.m.

    I think that the idea of bououncy in the earth, something that weighs thousands of kilograms, is a bit crazy. Just thinking that there is a possibility of the tectonic plates floating is crazy enough, and I think it's really special to be a mile-high city.

  • reaganspiva
    3/23/2015 - 02:10 p.m.

    Denver makes a big deal about being a "mile high" city. Why you might ask? Well, it has to do with the elevation and Denver has an elevation of 5,280 ft. (1 mile) above sea level.

  • EmilyJ-Kut
    3/23/2015 - 03:01 p.m.

    I did not know that because of chemical reactions, triggered by water far below the Earth's surface, could have caused Denver to be 1 mile higher than the other states. It's kind of cool because when other away sports teams come to play baseball, football etc. they have a disadvantage because of the air pressure that they are not used to.

  • ryanha-And
    3/23/2015 - 03:44 p.m.

    I learned that the mile high city is Denver Colorado and its all because of water, the precipitation is very dense and if it wasn't the wold could be very different then it is now.

  • MiaM-Kut
    3/23/2015 - 03:53 p.m.

    i really like this article because it shows what type of things happen under us. Tectonic plates basically make our world spin. How could tectonic plates raise a mile high?

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