Last weekend I was in Montana, for a wedding. We were near Bozeman and the north entrance to Yellowstone National Park, surrounded by high country, open meadows and mountain ranges with names like the Crazies, the Absarocas, the Gallatins, and the Bangtails; a landscape so dissimilar to our home territory of coastal Maine. The air was different, the sun was different, it was a nice change of pace from the fog and damp we left behind.
We had one day after the wedding to explore before flying home, and we chose to make a lightening fast tour of the northern part of Yellowstone National Park, a place I have longed to see but never had visited. I especially wanted to soak in a hotspring; hotsprings being one of the few things Maine lacks that would make it perfect in my mind. Maine formed as the result of plate tectonics and volcanic activity, but the last of this was 200 or more million years ago. Yellowstone, in contrast, is located over a hot spot, a place where the Earth’s crust is particularly thin (3-5 miles in this case). A plume of magma is able to come unusually close to the surface of the Earth as a result, and can influence conditions on the surface. Conditions I was hoping to participate in, like a hot spring. The relatively recent nature of Yellowstone’s volcanic activity (a little over half a million years ago), the size and scope of this activity (described in the literature as “cataclysmic” and “massive”), and the thinness of the crust have ensured the profusion of geothermal features that make Yellowstone unique.
Hot springs result from a special set of circumstances. Water is a requirement, a spring is simply a spot at the surface where water emerges. Proper plumbing is key, the underlying geology must be fissured in such a way that all cracks lead to that one spot where the water is funneled out. Heat is also requisite, and the heat in this case comes from the interior of the Earth, resulting from the radioactive decay of elements like uranium and thorium in the mantle and core, elements trapped there since the earth formed 4.6 billion years ago. Here in Maine our main impediment to having hot springs is the plumbing. We have plenty of water, and there is certainly heat below the crust under our feet. The problem is the crust is too thick, and while it may be cracked, those cracks aren’t connected in such a way as to allow water to sink and get heated and then make it all the way to the surface.
In Yellowstone, precipitation and the melting alpine snow pack drain into the bedrock , which in much of the park is made up of rhyolite, a extrusive volcanic rock that gives us a clue about the nature of Yellowstone’s significant volcanic past. Many minerals are dissolved into this water as it flows deep below the surface, and surprisingly there are bacteria deep in the Earth’s crust that make use of these minerals. The most notable are the anaerobic hydrogen sulfide producing bacteria, which use the sulfur in the water in their metabolism, and produce the smelly hydrogen sulfide gas that is associated with many hot springs.
The spring I went to was called the Boiling River, and while it has a dramatic name, it is a fairly mild mannered thermal feature, by Yellowstone standards. The spring simply emerges out of the ground and forms a short water course before it drains into the Gardiner River. You soak at the narrow interface on the edge of the river where the hot spring water meets the cold river water. You remember the age old question about having your feet in the oven and your head in the ice box? Its kind of like that when you first get in. I would just get in a comfortable spot and the turbulence of the river’s flow would send a burst of cold river water into my lap, like a peal of laughter from the mountains. The characteristic orange mineral build up lined the edge of the spring’s flow, and the rocks were covered with blue green mats of heat loving extremophile bacteria. A perfect last quarter moon hung in a bright blue sky, and we were slightly veiled by the steam that rose around our heads. As I soaked I reveled in this heat that was not from the sun or the woodstove or any other earthly (as we know it) source, but from the deep mystery that is the interior of this rock we ride around the sun. Elements from an exploded star, coalesced into a planet. Elements that are still active, 4.6 billion years later, still releasing energy, pumping heat into water, water I was sitting in as it emerged from a crack in the ground. I couldn’t stop grinning, even though there were tears in my eyes.
And now I am returned home smelling a bit like sulfur, dreaming of alpine meadows and steaming rivers, and basking in the Montana afterglow.
The National Park Service Yellowstone website: http://www.nps.gov/yell/naturescience/geothermal.htm
Nice overview from a Canadian website associated with Banff National Park: http://www.mountainnature.com/geology/HotSprings.htm#Origin
Some nice local knowledge from a park gateway community business owner: http://www.pineedgecabins.com/yellowstone-guide-book.htm *Note: I have never stayed at this place and have no affiliation with the business. This is not any kind of endorsement, I just like the “guidebook” they have on their website.