Under the Rock Pile: The environment beneath us
We are surface creatures, living within a tenuously thin layer of light, atmosphere and water resting on a foundation of rock. We explore and monitor this dynamic, life-giving environment, and there is much left to learn. However, very little is known of the environment of rock buried beneath us.
As a youngster, I had the impression that it was a cold, dark place beneath the surface of the Earth. After all, weren’t all the caves and tunnels cold, dark places? So years later, it was a surprise to me that I discovered a hot, humid environment, while working as a miner on the construction of the new haulage tunnel at the Henderson Mine. We were penetrating rock at depths of about a mile underground. The conditions were alien, with temperatures approaching 120 degrees Fahrenheit and 100 percent humidity. Hot water would be seen issuing from the deep fractures, at times like a giant fire hydrant, under enormous pressure. With these oppressive conditions — and moisture condensing on our clothes and protective glasses — we took frequent breaks to an air-conditioned lunch room to avoid exhaustion and heat stroke. After about six months of supplying surface air to this deep tunnel, the temperature would cool, like taking a kettle off of a stove; and the once swamp-like environment became as cold as the caves.
What I was experiencing as a miner was something known as the geothermal gradient. Nearly all rocks are like ceramics, and as such, act as insulators, retaining the heat of their environment. This deep heat is generated from even deeper molten rock, and the natural radiation of the radioactive elements in the Earth; forming a thermal gradient toward the surface.
Within Summit County the near-surface geothermal gradient has been measured in numerous drill holes to be about 1.0-2.0 degrees Fahrenheit per 100 feet. Thus, at a mile depth, the temperature of the rock is anywhere from 50 degrees to 100 degrees Fahrenheit warmer than the average surface temperature. At a depth of 10,000 feet beneath the surface, the temperature under Breckenridge and Copper Mountain is estimated to be about 225 degrees Fahrenheit, and at Silverthorne and Frisco to be about 200.
The deepest drill hole in the world, the Kola superdeep borehole, located in Russia, penetrated 7.5 miles beneath the surface and encountered temperatures of 356 degrees. What was most surprising was that they also discovered 24 fossil species of plankton preserved by encasement of organic material within 2.2-billion-year-old metamorphic rock. In addition, they noted an abundance of water within the rock at these great depths.
There are areas in Colorado that have abnormally high geothermal gradients. According to the Colorado Geological Survey, at Princeton Hot Springs and at Pagosa Hot Springs, the gradient is closer to 5 degrees per 100 feet. Here, the temperature 10,000 feet beneath the surface is estimated to be 500 degrees to 600 degrees Fahrenheit. That’s hotter than most electric ovens.
What accounts for these high geothermal outputs? Yellowstone National Park gives us a clue. At Yellowstone, the surface water temperatures near many of the geysers are 200 degrees Fahrenheit. On June 7, 2016, a 23-year-old Oregon man fell into an acidic, boiling hot spring in Norris Geyser Basin after walking over 600 feet from the designated path and falling through the thin mineral crust. His body is believed to have been consumed by the corrosive waters and was never recovered. Only his flip flops were retrieved.
Seismic studies have shown that the source for highly elevated water temperatures at Yellowstone is a near-surface chamber of porous, sponge-like rock that has up to 15 percent molten magma filling its pores. An upper magma chamber, just 12-28 miles beneath the surface, is fed by a much larger and deeper chamber that is located 40 miles beneath the surface. In turn, these chambers of partially molten rock are heated by an even deeper mantle plume, called a hot spot. The mantle-derived plume extends down to over 400 miles beneath the surface. It is estimated that the upper magma chambers have enough volume that if they ever erupted, they would fill the Grand Canyon 11 times over, severely impacting our climate and the existence of life on Earth.
Few regions of high-geothermal gradients can be attributed to long-lasting hot spots, but the deep environment of our Rocky Mountains is a fascinating world that needs to be explored.
Alexander (Sandy) Gunow received his doctorate in geology from the University of Colorado. He also he worked as a miner at the Henderson Mine and eventually worked as an exploration geologist for the Climax Molybdenum Company. He is currently enjoying his retirement in the mountains near Silverthorne.
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