If you’ve visited the Mammoth Hot Springs terraces in Yellowstone National Park more than once, you may have noticed it looked different than when you first saw it.
Recently several people contacted us or posted on social media that some of the hot springs were shutting down. One person visiting the park even stopped me in a parking lot to tell me about it. So I asked the scientists what was going on.
Yellowstone National Park geologist Jeff Hungerford said, “You will see change within a week or two. You might watch a feature come alive and next week it will be gone.”
So the changes people are seeing are not that unusual. But it concerns some because visitors love the hot springs so much. Standing on a boardwalk next to Pallet Spring, Dalton Brookie of Colstrip, Montana said, “It’s not something you get to go to in your backyard. I’m from northern Montana and you don’t get to see it up there very often. We do have hot springs, but nothing, nothing quite like this.”
So, what exactly is happening? After all, with only a casual look, you can clearly see where terraces formed, dried up, then sprang back to life just a few feet away. Scientists say both geology and biology are at work on the terraces.
It starts with where the water here comes from.
“The waters start in the Gallatins, they go downslope,” said Dr. Bruce Fouke of the University of Illinois Urbana-Champaign. He has studied the hot springs at Mammoth for more than thirty years. He said after the water runs off as rain or snow melts, it flows underground along fault lines in the earth’s crust.
He added, “And then they go upwards of a kilometer or so underground.”
At that point, the water flows through soft limestone rock from a half-billion-year-old seabed.
“And then they hit another fault system and they come back to the surface,” said Fouke.
It’s a long journey.
“The average length of time it takes for the water to get from the Gallatins into Mammoth is something on the order of 400 to one thousand years,” said Fouke.
But what causes the water to stop flowing?
Fouke calls it the hot springs midlife crisis and breaks out laughing. He could barely contain himself while adding an apology. He said, “Forgive me for getting so excited, but I just love all this so much you know.”
It turns out the prevailing theory about the minerals in the water clogging the small, little finger-sized spring pathways is not right at all. That’s because this water is slightly more acidic when it’s under pressure beneath the surface and that means the minerals stay dissolved. Fouke said the springs never have the ability to clog up.
So what does stop it?
Fouke said, “It builds and it builds, and that all sounds good, but the mid-life crisis is they get overweight.” And he laughed again.
Thanks to gravity, the fast-building travertine rocks just collapse. Eventually, the water finds another crack in the rocks and works its way back to the surface.
“And so that can take place over the course of three years, five years, seven years, something on that kind of order. It’s not hundreds of years,” said Fouke.
That’s really fast. Fouke said it’s one million to one billion times faster than travertine accumulates in places like caves, lakes, and oceans. Plus, the geologic process is getting help that makes it grow even faster.
Fouke explained, “The microbes that live in the hot water, the thermophiles, the heat-loving microbes at mammoth, as they metabolize, those proteins have the ability to make calcium carbonate grow super-duper fast.”
So, that makes Mammoth pretty unique, even in Yellowstone. It's the only place in the park whose hot springs have the right chemical and biological makeup to create the travertine ledges in this way.
In all, Mammoth accumulates about a ton of new travertine deposits, every day. Fouke says the only thing that keeps the travertine from overrunning the entire park is that the actual spring flow is quite small compared to the size of Yellowstone.
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