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Backyard Geology: The Valles Caldera

There’s still some geology left for me this semester – this coming Monday I get to start cutting my thin section from the kimberlite I picked up at Green Mountain. Eventually the thin section will be made in to a slide and I’ll be doing a petrographic analysis, taking a photo micrograph of it, and writing a paper. Which is fine. Kimberlite is super cool.

But the field component is definitely done. This last trip was another jaunt down to New Mexico, this time over by Los Alamos. We spent most of our time between the Bandelier National Monument and the Valles Caldera. It’s a very cool area. Our reason for being at Bandelier was to look at the Bandelier Tuff, as well as some other volcanic rocks in the area. The tuff was produced by the Valles Caldera blowing out about 1.2 million years ago.

The tuff starts pretty far away from the Caldera, as you might expect from the sort of massive volcanic explosion that would come from a caldera-forming eruption. At the first place where we examined it, I think we were at least 20 km away from the Bandelier National Monument, and the tuff and pumice layers were about 50 feet thick. The layering of the rocks in the area moving toward the caldera are pretty interesting. There are alternating layers of fairly unconsolidated pumice, tuff, ignimbrite. The tuff is basically pumice that has been partially welded back together by heat, and contains some phenocrysts. Sometimes the extremely well-welded ash units look eerily like basalt flows from a distance, which is very cool. By the time we got in to Bandelier National Monument to see the cliff dwellings, the tuff was about 500 feet thick.

The tuff and pumice makes for some pretty bizarre rocks. You normally expect rocks to be heavy, but the pumice feels almost as if it’s made of styrofoam. The cliff dwellings were actually cut in to the tuff layer, which is only slightly heavier and more solid than the pumice itself.

We also drove in to the caldera, which is a stunning area. It’s basically a massive, rolling plain covered with grass, which is surrounded entirely by a ring of large hills. The plain itself is dotted with smaller hills, which are actually obsidian domes that have formed at one time or another since the caldera collapsed. The biggest of the hills within the caldera is the resurgent dome. I do have some pictures (still need to pull them and the ones from the previous field trip off my camera) but for now, here’s a couple nice shots from Wikimedia Commons:
One of the domes in the Caldera
A couple more domes, during the winter

The pictures really can’t give you an idea of the scale of the place. You’ll just have to go there yourself, some day. Also, if you want a piece of Bandelier Tuff for yourself, you obviously cannot collect in the national park. However, there are several road cuts outside of the national monument where you can pull over and pick up large pieces of pumice and tuff, as well as some where you can find obsidian-like extrusions. It’s some very cool stuff.

Not far outside the caldera itself, there’s a picnic area where you can catch a trail up on to Battleship rock, which is made of ash deposits. The trail up to the rock is pretty tough. It gave my knees hell going back down particularly. But you do get a fantastic view from the top.

Also at that picnic area, you can catch a trail to the McCauley Warm Springs. It’s about a five mile round trip, and if you have knee problems like I do, I’d really recommend some walking sticks for this one. They make progress faster and much less painful. It’s a tough enough hike that there weren’t too many people in and out of the area, even on a beautiful and warm Sunday. The Springs themselves aren’t what you would expect. They’re meteoric hot springs, which basically means that rain water gets down in to the magmatically active zone via fissures and then is expelled to the surface. This means that they’re not too mineralogically strange – and don’t smell like sulfur, for example. (There are other sulfur-rich springs in the area which are hydrothermal in nature.) They’re also not as hot as you’d think – they’re more “warm” springs than hot springs. The temperature was like being in a very pleasant swimming pool, which is more remarkable than it seemed at the time considering that temperatures were getting down below 40 degrees F at night in the area. There’s a lot of algae growing in them, but the water’s warm enough that they certainly don’t smell like an active breeding ground for cyanobacteria. So it was a nice little excursion and a nice soak. There are also a lot of little fish that live in the springs. My feet got gently nibbled at a lot, which felt very ticklish and was quite amusing. I recommend having a beer (if you’re old enough) while relaxing in the springs.

Overall, an amazing experience courtesy of Giant Geological Features That Could Kill Us All.

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