I visited Yellowstone National Park to do some geological fieldwork in 1992. With the benefit of twenty years hindsight, it has turned out to be possibly the best trip I have been lucky enough to undertake. Despite some of my meaner spirited class-mates calling it a holiday, I learned a great deal, and the experience of being in a totally different country was excellent.

The north gate of the park is a pretty imposing structure, with an inscription by Theodore Roosevelt. I didn’t take the photo below, which is from Wikipedia.

Roosevelt Gate

Arriving in Bozeman, Montana at night, I ended up dropped off up a mountain in the dark. What this meant was that I wasn’t at all prepared for the view when I opened the curtains.

Mt. Everts from the west

Closing the curtains and trying again didn’t make the view disappear, and it stayed the same as I emerged blinking into the sunlight.

Mt.Everts is essentially a large fault scarp, and the continuous layer on top was deposited during one of the Yellowstone volcano’s more recent eruptions. That layer can apparently be found on the opposite side of North America, in New York State, showing that the volume of rock and ash ejected was truly stupendous. More of that later.

In the northern Park, Mammoth Hot Springs is one of the first places on anyone’s itinerary. Some period buildings remain, and the administrative departments are housed in what was once the US Cavalry’s barracks. One of the things I love about Mammoth is that wildlife is not hindered by fences, and able to wander around everyone’s gardens.

Elk in the gardens of Mammoth Hot Springs

If you’ve never heard elk calling, you’re in for a treat. During the mating season, they tend to get a bit noisy. The sound is difficult to describe, kind of a combination of creaky gate and mad bugler. A quick check on Google will show you what I mean.

The hot springs that give Mammoth its name contain mainly calcium carbonate, in the form of travertine. It forms large terraces, which start off as tiny flat flakes of mineral, and these build up to give large structures.

Travertine flakes

Small travertine terraces

Travertine "stalactites"

Down at Mammoth Hot Springs, a remnant feeder pipe for a previous hydrothermal system is present, known as “Liberty Cap”. This formed in much the same way as a pipe furs up with deposits carried by water. If you’ve ever seen a kettle, or oil-pipe full of scale, this isn’t too different. Note the onion-skin structure below.

Liberty Cap fine detail

The whole thing is visible below, and would have been a pipe in the subsurface, feeding deposits similar to those higher up the mountain.

Liberty Cap

One of the most memorable things about my trip was the people that I encountered. Aside from being robbed in Minneapolis St Paul, virtually everyone was really friendly. The Park Service staff, particularly Dave and Kathy LaConte, Bob Lindstrom and Wayne Hamilton all made my life much easier while I was in the Park.

The scale of hydrothermal activity in Yellowstone is pretty significant. Geological studies show that there is a mantle “hot-spot” beneath the Park, and that continental drift has been moving North America over the heat source for a long time. It is believed that a similar mantle “plume” created the various Pacific seamounts in a chain ending with the Hawaiian Islands. Here’s a useful link –, showing a vast array of seamounts. Using the map to look at the Pacific Ocean, you can see the chain I’m talking about, stretching away north and west of Hawaii. Not all seamounts form from mantle plumes, but this tool is a useful illustration.

Anyway, back to the plume under Yellowstone. This plume has been responsible for a number of catastrophic volcanic eruptions through geological history. The National Parks Service have shown this far better than I could, and their graphic is reproduced below. The link to their site is here:

Eruption comparison (From NPS website)

Looking at the relative volumes of material ejected during the Mt. St. Helens and Krakatau eruptions, it is clear that when the Yellowstone caldera lets rip, it does so on a truly gargantuan scale. For non-geologists out there, a caldera is what you get when the top of a volcano collapses in on itself, a structure a bit like a basin. Good examples are Crater Lake, Oregon, and Olympus Mons. On Earth, calderas are not generally very large, a couple of miles in diameter or less. To give you an idea of scale, according to the US Geological Surgvey, the current Yellowstone Caldera measures approximately “28 by 47 miles”. This gives me the shivers. Basically the entire central section of Yellowstone National Park is a volcanic structure. I knew this before I did any field-work there, but the sheer size of it is awe-inspiring. On a trip to the southern end of the Park, we drove down the fault scarp for the caldera, and it stretched for miles to either side.

One thing is for sure, when that bad boy cuts loose, as it will eventually, the whole world will know about it. A drama that gives an idea of what a major eruption would look like is available through the BBC. Kind of tips 2012 and similar movies into a cocked hat.

Regardless of what lies beneath, Yellowstone National Park is well worth repeated visits. I for one would love to visit again in the depths of winter, when the Park is transformed by snow. The Indians called this part of the world “Big Sky country”, and I can see why. Standing among the grasslands, framed by mountains and deep clear blue sky above, I almost felt as if I could fall off the planet.


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