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How Denver Became the Mile-High City

For a long time geologists have been trying to figure out why the High Plains, where Denver is located, turned out to be so high, yet also level and smooth enough that a city could be built there. It may have had something to do with water. Continue reading →

Denver's mile-high elevation has long been a big part of the city's identity. That's evidenced by how at Coors Field, where the National League's Colorado Rockies play baseball, a single row of purple seats is marked to denote the spot where the terrain is precisely 5,280 feet in altitude.

For a long time, though, geologists have been trying to figure out why the High Plains, where Denver is located, turned out to be so high, yet also level and smooth enough that a city could be built there. But now, in a just-published article in the journal Geology, University of Colorado-Boulder geologists Craig Jones and Kevin Mahan and colleagues offer a new theory. They think that chemical reactions triggered by a flow of deep water could have caused a portion of the North American tectonic plate to become less dense, and in turn rise far above sea level.

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"The High Plains are perplexing because there is no deformation - such as major faults or volcanic activity - in the area to explain how this big, vast area got elevated," lead author Jones said in a press release. "What we suggest is that by hydrating the lower crust, it became more buoyant, and the whole thing came up."

According to the new theory, this all happened between 45 and 75 million years ago, when the subducting Farallon plate, which is under the Pacific Ocean, slid under the North American plate, bringing with it a lot of water trapped in minerals. Due to the pressure and heat caused by the plate movement, the water was released, and moved up through the mantle. That action caused the water to hydrate minerals in the lower crust, converting dense ones such as garnet into lighter mica and amphibole. That, in turn, made the crust more buoyant.

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"If you get rid of the dense garnet in the lower crust, you get more elevation because the crust becomes more buoyant," Jones explained. "It's like blowing the water out of a ballast tank in a submarine."

More research will be needed to substantiate the theory, the scientists caution. But it could explain other phenomena, such as why Wyoming turned out to be higher than Montana.

The view from Denver from the Colorado State Capitol Building. On its 15th step are the engraved words: "One Mile Above Sea Level."

Artist Nickolay Lamm has depicted a number of U.S. cities as they would look under 12 feet of sea-level rise. That projection, based on data from the organization Climate Central, is right in the middle of several forecasts that report that Antarctic glaciers are starting to collapse. Above, Boston Harbor at 12-feet of sea-level rise.

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With 12 feet of additional ocean, water rises up the steps to the the Jefferson Memorial.

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Liberty Island is mostly submerged in this view of New York Harbor.

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Seawater runs through Ocean Drive in Miami, a major thoroughfare in South Beach.

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Harvard's campus in Cambridge, Mass.

The Citadel in Charleston, S.C.

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