Volcano Vocab #5: Caldera

Part of my research this summer involves visiting Los Alamos to learn how to work with a computer model; in addition to one of the world's greatest research laboratories, northern New Mexico also hosts the Valles caldera, a major volcanic center north of Albuquerque. (Pretty much everything around me is volcanic, which means that whenever I drive or bike anywhere, I'm always staring at the scenery and going "holy crap, that's amazing!")

Caldera ("cal-dare-uh") is a Spanish word meaning "cauldron", and it describes a type of large, bowl-shaped volcanic structure. Calderas are created by collapse of the roof of a magma chamber after the chamber's contents have been removed, either in effusive or explosive eruptions. They're technically not craters, which are smaller and usually located on the summit of a volcano, but much larger features that form when a volcano expels the contents of a big magma reservoir and then collapses. (If activity continues after this happens, it can even create new stratovolcanoes within the caldera.)

The eruptions that form calderas are big - hundreds to thousands of cubic kilometers of material is involved. Collapse features that form over magma chambers that large are often not recognized as volcanic features until they're seen from the air, because they're simply too large to distinguish from the ground. On the volcanic explosivity index (VEI), caldera-forming eruptions top the chart - and in the case of some, are too big to even show on the chart:

VEI figure from the USGS Volcano Hazards Program Photo Glossary.

To give you an idea of what this translates to in reality, here's part of the Valle Grande in the Valles caldera:

This meadow is only a fraction of the whole caldera, and you can't even see the far walls because they're hidden behind the resurgent and smaller domes. Here's a map of the whole caldera:

View Larger Map

The Valle Grande is the light green patch in the southeast; the hills in the background of the photo are the Redondo Peak resurgent dome and smaller lava domes, which are pretty common post-caldera-eruption features. Resurgent domes are thought to be related to rebounding of the caldera floor, possibly due to new magma intrusion; the lava domes represent later eruptions through fractures. (If you want to know more about the specific geologic history of the Valles Caldera, Garry Hayes over at Geotripper has a great post from last year.) Some well-known examples of calderas in the United States are Yellowstone in Wyoming, Crater Lake in Oregon, Long Valley in California, but there are plenty of others:  Krakatau and Tambora in Indonesia, Santorini in Greece, and Colli Albani and Campi Flegrei in Italy, to name a few. 

I'll leave you with a photo from the rim of the Colli Albani caldera in Italy, with the Faete stratovolcano to the left of center:

A Guatemalan Lago Como: Lago de Atitlán and its volcanoes

Aldous Huxley described Lake Atitlán as "Como with the additional embellishment of several immense volcanoes. It really is too much of a good thing." I completely disagree on the "too much" part, because Atitlan is stunning (and, in my opinion, the addition of volcanoes gives it a leg up on Lago Como!)

At the end of our field work, and after a brief trip to the Santiaguito Volcano Observatory (more on that in the next post!) we took the Pan-American Highway (CA-2) through the mountains to Lake Atitlán. Atitlán is a ~20 km wide caldera in the Guatemalan highlands that was formed by the Los Chocoyos explosive eruption around 85,000 years ago (although activity began 11-12 million years ago). Three stratovolcanoes (Atitlán, around 10,000  years old; Tolimán, a bit younger than 40,000 years; and San Pedro, which stopped erupting 40,000 years ago) have grown on the southern margin of the caldera, partially infilling it and jutting out into the caldera lake. The last recorded activity was pyroclastic flows, lahars and explosive eruptions in the 1850s at Volcán Atitlán, although historical records for this volcano date back to the 15th century; the volcano is still considered active. The lake itself is supposed to be the deepest in Central America, and may be 340 m deep or more. An earthquake that occurred in 1976 seems to have breached the lake at some place, and the lake level has been slowly dropping since then. (Ole Nielsen at olelog has a great post with more in-depth info on the caldera, and Wikipedia's article isn't too shabby.)

Here's a Google Map to get you oriented. Volcán San Pedro is the farthest to the left, Tolimán is the northernmost of the paired cones and Atitlán is the southern one.

View Guatemala Volcanology in a larger map

We had time for visits to San Pedro La Laguna and a boat ride to Santiago Atitlán, and it was a great chance to do some relaxing sightseeing. I've seen some articles recently about the bad aspects of visiting Atitlan - they claim that the lake smells, is sludgy and brown, surrounded by robbers and banditos and all sorts of other nasty things. Well, I don't know what lake they were visiting, but I found very little of that to be the case. True, the lake is contaminated with bacteria from sewage and farm runoff, and I wouldn't recommend drinking the water, eating fish from it or going swimming, but there wasn't a bit of sludge in sight when I was there:

The water was perfectly clear the whole time we were on the boat, and you could see the bottom for a long ways out. (There have been some nasty algal blooms recently, which no doubt account for the warning about staying out of the water, but we didn't see anything visibly yucky on the way from San Pedro to Santiago.)

As for getting to the lake, there is one road (I'm not certain which) where bandits have pretty much made a business out of stopping vehicles and robbing people. It's fairly easy to avoid this road, however, and we had no problems despite being a bunch of gringos in a big shiny SUV. (No problems with people, anyway. The last bit of the road into San Pedro, which is basically switchbacks down the inside wall of the caldera, is crazy curvy, and not a bit of fun if there are buses on it with you. You can make the turns, but the buses can't, and it's impossible to see around the corners on the switchbacks. Drive really slow and honk a lot to let the bus drivers know you're there, because otherwise they'll run you off the road.)

In San Pedro, we stayed at the Hotel Mikasa, which is very nicely appointed (hot showers without exposed electrical wires!) and has a great European-Spanish restaurant on the roof (I definitely recommend the paella.) San Pedro is a bit of a hippie town - there are a lot of transplanted gringos and massage parlors (real ones) and solar-powered hot tubs - but it's pretty quiet, and has some interesting shops. And a great view first thing in the morning:

Santiago Atitlán has a much bigger population of the native Mayan groups, but it seems to be a major stop on the tourist route, because the main road is pretty much lined with shops and crawling with loud, clueless Americans. (It took me ten minutes of waiting to get a photo without a tour group in it!) The shops have some beautiful textiles, however, and there seem to be quite a few local artists who sell their work.

The boat ride was fun, although if there had been any more people in there I think the sides would have been about an inch above the water. Talk about a low rider! The boat service is fairly reliable, though, and only costs about five dollars for a two-way trip. 

It was fun to go shopping there, but I'm sure we would have seen much more interesting things if we'd had more than an afternoon. Alas, we had to drive back to Guatemala City later that night, so we didn't have the opportunity for exploration. The views on the boat ride, however, totally made up for the short trip:

My recommendation is that Lago de Atitlán is definitely not to be missed. The smaller towns like San Pedro and Santiago Atitlán are difficult to get to but worth the trip, and the larger ones (like Panajachel) have a lot to offer as well. And they're all a boat ride away from each other!