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.


"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?

Assigned 92 times

  • paytonj-And
    3/19/2015 - 03:33 p.m.

    I learned that Denver, Colorado or The Mile High City is a mile high, because of water, chemical reactions made North Americas tectonic plates less dense.

  • ksadat-5
    3/19/2015 - 03:54 p.m.

    According to recent studies by Geologists, Denver, or the "Mile High City," is actually a mile high. The reason Denver is a mile high is because of their water. Millions of years ago, water deep below the earth's surface triggered a chemical reaction. The chemical reaction left the certain part of the North American plate less dense. Plates float on the earth's vast mantle. So since the Denver area was less dense, it was pushed up.

    Personally, I think this is an interesting article. I have never heard of Denver being called the "Mile High City," but I find it cool that Denver is actually a mile above sea level. Not only is this cool, but it all happened because of a chemical reaction caused by water, millions of years ago.

  • Colby N Turquoise
    3/19/2015 - 05:31 p.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. It seems odd that just having an elevation of one mile can make a city such a big deal. That makes me wonder if where I lived was a mile high, would it be popular?

  • Eric0221-YYCA
    3/19/2015 - 08:53 p.m.

    I think that this must be very scientific because Two Geologist knew how the Mile High city that what makes it a mile is because of water from the Ocean 70 million years ago which could look cool if you look at Mile High city, but it looks like a problem. Well if people think that Mile High city was formed by an ancient earthquake but its actually formed by water from the ocean.

  • AJ_Slater
    3/19/2015 - 09:17 p.m.

    In the city of Denver, Colorado at certain parts they reach above a mile above sea level. This is because of the way the North American tectonic plate collide with the pacific it created the rockies. This makes Denver so high up. This is cool because of how much pride they take in to being a mile high.

  • LucasU-5
    3/19/2015 - 09:38 p.m.

    The article is about the very famous city known as Denver which was nicknamed the Mile High City because of its very large elevation level known to be approximately one mile or 5,280 feet. The reason that the city is a mile high was discovered by two scientists that attended the University of Colorado in Boulder, Colorado. Denver is so proud of the line that shows one mile above sea level that they changed one whole row of seats at the Coors Stadium to purple to honor that level.

  • AhaanJ-Jac
    3/20/2015 - 12:45 a.m.

    Wow! I think that this cool that water makes the mile high city a mile high. It is interesting to know that Earth's crust "floats higher."

  • RS2000BLUE
    3/20/2015 - 08:39 a.m.

    I think they make such a big deal about it because denver is in colorado and there really isnt anything cool about colorado. so i think they are doing this to get tourists

    • JB000blue
      3/20/2015 - 01:04 p.m.

      That is pretty cool really even though I still have no idea what the heck there talking about in this artical reading. *Wow thats really sad*.

  • JJ01blue
    3/20/2015 - 08:43 a.m.

    This artucle is about a city in denver that is called the mile high city. The reason for it's called the mile high city is because of the crustal hydration that took place alongtime ago

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