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.)