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.