Category: volcano

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.

Four days in a row of hiking (since even though we came back on Sunday, we did another hike on Monday) have just about destroyed me. I’m limping around like an old lady today. Lots of very, very cool stuff was seen on the field trip. Yes, I took many pictures. No, I haven’t uploaded them yet. I’m working on it, though. And there’s lots of very amazing geology stuff to write about. I may never catch up, considering that school has apparently slowed me to a one-post-a-week crawl.

For the three day trip, we spent most of our time in the Raton Volcanic Field down in New Mexico, though on Sunday we did head back in to Colorado for the Spanish Peaks. (Which are a whole other cool thing to write about.) The Raton Volcanic Field (RVF from here on out) was and still is caused by the rifting near the Rio Grande River, where there’s hot, plastic mantle (asthenosphere) welling up to within 30 kilometers of the surface, which on a continent is a Very Big Deal. Normally, the asthenosphere minds its own business and stays at a depth of 100-200 km. At the Rio Grande Rift, it’s poking its steaming head above the Moho, which means there’s a lot of very hot rock where it really has no business being, and that makes for a lot of volcanic activity.

The RVF actually isn’t in the Rift Valley itself; it stands on the margins. The area is very topographically interesting; generally you have a lot of rolling plains there, but there are also stair-step like mesas and very prominent hills poking up from the landscape. Each of these prominent, conical hills is an extinct volcano. The mesas are caused by basalt lava flows that came from the volcano. So the basic process of the RVF is that a volcano pops up in a valley (where the crust has thinned a bit due to the rifting to the west), puts out a lava flow or two, there’s more rifting and a new valley created, and then the process repeats.

Now, most of the volcanoes in the RVF (with such prominent exceptions as Sierra Grande, which is a shield volcano) are cinder cones. Many of them are now covered with vegetation of some type, but I did see some prominent and presumably younger (since they still had their very distinctive shape) cinder cones that were completely naked. Naked cinder cones tend to erode down very quickly, since they’re basically made of layers of ash and other pyroclastic debris that aren’t well consolidated. As far as volcanoes go, cinder cones are fairly well understood. There are a lot of active cinder cones today, and one in Mexico even started its formation a little more than 60 years ago: Paricutin.

Capulin Volcano is one of the RVF cinder cones. It’s relatively young, between 58 and 62 thousand years old, and it is rather well vegetated. The vegetation layer has helped preserve the volcano’s shape, so it’s very distinct and pretty. The volcano itself is a national monument, and there are several extremely nice trails. One goes around the volcano’s rim, another goes down in to it, so you can look at the blocked-off vent that spewed all the ash and debris, and a third goes out on to the lava flow at the volcano’s feet. As is common, Capulin did put out a basaltic lava flow, but not from its central vent as we’ve come to expect from the normal images of composite and shield volcanoes. Since cinder cones are structurally weak due to their composition, most develop a vent at their base and that’s where the lava comes out.

Other than the simple OH MY GOSH COOL of begin able to walk on and down in to a volcano, there’s some very nifty geological stuff to be seen. At several of the road cuts on the volcano, you can see the layers of ash that make up the cone. They come in a lot of different colors and are fairly distinct. You can also see volcanic “bombs” all over the place. These are chunks of magma that got spewed into the air and solidified in distinct chunks. As you look over the lava flow at Capulin’s feet, there are several visible tumuli, which are dome-like features where hot lava welled up through the cooler, thin crust on the lava flow surface. Also, in the fields that cover most of the lava flow now, you can still see the ghost of pressure ridges, which are ripples preserved in the flow. These are also caused by the movement of hot lava under the cooler surface, causing deformation.

All this cool volcano stuff, and it’s only a four hour drive or so from Denver! I do have some pictures of Capulin that will hopefully be posted soon, but they’re not going to do the volcano much justice. Soon after we left, it started raining and then rained extremely hard for the next eight hours. So as you can imagine, while we were at Capulin it was extremely overcast. (And also shockingly cold.)