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: 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, 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, like an empty boat next to one with heavy cargo, pushing the vast High Plains far above sea level, according to the theory formulated by geologists Craig Jones and Kevin Mahan at the University of Colorado-Boulder.

Their work appeared on the website of the journal Geology and is a big deal for Denver, where 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, marking 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 as 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 and that 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 then 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, setting off chemical reactions that turned garnet and other dense minerals into mica and other less heavy minerals, making vast areas of the crust lighter.

Jones said the Earth's crust under the High Plains "floats higher" over the mantle, much like a plank of buoyant balsa wood 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, involving 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, who is also a fellow at the Cooperative Institute for Research in Environmental Sciences, 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 12 times


COMMENTS (19)
  • vincet-Koc
    3/23/2015 - 02:18 a.m.

    I never really thought about why it was called the mile high city, and never really understood why coloradians made such a big deal about it.

  • namelt-Che
    3/23/2015 - 01:56 p.m.

    The reason I think that it is a mile high is because it is actually a mile high. I think that it is pretty cool because it has the most earthquakes going on over there. But at the same time it is dangerous because of the earthquakes that happens over there.

  • samf-And
    3/24/2015 - 03:37 p.m.

    I think that they do because it is one of the most scientifically impressive things the city has to offer and is one of the reasons they have the tourists.

  • 9tylork
    3/25/2015 - 01:10 p.m.

    why is Denver called mile high i have had a couple questions in my life why Denver is called mile high and is it really a mile high

  • CharismaM
    3/30/2015 - 01:29 p.m.

    This was an interesting article to read. I've never really heard about this place before and it's seems like a nice place to visit. I also wonder if their theory is correct or if it's just another wrong guess.

  • EmmaBender
    3/30/2015 - 01:47 p.m.

    I think Denver is well know because of its name. I think it might have been caused by earthquakes in the earths surface. Which makes the plates move.

  • MadisonSch
    3/31/2015 - 08:30 p.m.

    This is quite interesting because I would have never guessed that water was the reason that cities and towns can be built so high. This is pretty cool

  • ShaniaWentz-Ste
    3/31/2015 - 09:54 p.m.

    Every town/city tries to find something to brag and boast about, whether it's a cheesesteak, a famous animal, or even a strange and unique name. However, not many towns can brag about their city being "a mile-high." I have always wanted to travel and vacation in Colorado, this being one of the many reason why.

  • Julian10
    4/08/2015 - 04:07 p.m.

    Geologists may finally be able to explain why Denver, the Mile High City, is a mile high. Scientist have found that chemicals in the earths water below the surface may have made the plates less dense. Because plates float on the Earth's mantle, parts of the Western United States might have risen. Denver, where the 5,280-foot elevation is a point of pride and a big part of the city's identity. 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.

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