5 March 2010

Watch your step: Field work on lava domes

Posted by Jessica Ball

I suppose I’ve left you all hanging long enough, so now it’s time to show off the first batch of photos from Guatemala. The trip started out in Guatemala City, where we loaded up our rental car and drove to Quetzaltenango (known as Xela or Xelaju to most people). From Xela we drove to a finca, or farm/plantation, and then spent three hours hiking through jungle, over landslide scars and down rocky riverbeds. It was a tough, messy hike, although we were lucky enough to have porters go first and cut a path through the brush with machetes. (This did have its drawbacks, though, since the average Guatemalan is shorter than me, and we had some pretty tall people in the group. There was a lot of stooping and some crawling, which isn’t all that fun when the foliage is covered in volcanic ash.)
Eventually, we made it to the campsite. How would you like to wake up to this view every morning?

This is the El Brujo dome, the westernmost and youngest dome in the Santiaguito dome complex. Don’t be fooled by the lack of scale in the photo – this shot was taken from a distance of ~ 400 m from the base of the dome, which is roughly 300 m high. “El Brujo” means “the wizard” or “the sorcerer” in Spanish (all the domes have somewhat odd names, and there’s not a lot of consensus on how they were chosen).The “back sides” of the western Santiaguito domes are abutted by an alluvial plain created from material washing off of Santa Maria and the domes. And this isn’t just sand and pebbles; this is serious lahar deposits, which you get to hike across every morning before you even get to the domes. The deposits range in grain size from ash and sand to boulders the size of small cars. There was, predictably, a lot of tripping on the way to Brujo.

Tripping also happened because we stopped to watch this every so often. Eruptions with your coffee, anyone?

After crossing the alluvial plain, we climbed up the pass between Brujo and El Monje (“the monk”). This was pretty much like climbing over lahar deposits, only at a thigh-busting steep angle. 

But once we reached the top, we were in a totally different world. El Brujo hasn’t been active for several decades, and in that time enormous amounts of ash have accumulated on the top and flanks of the dome. Probably because of the May-October rainy season in this part of Guatemala, much of the ash is covered in vegetation – moss, grass, shrubs, even a few stubby trees. It’s a cool landscape:

Enough ash collected in spots that there’s a kind of sandy alluvial plain in between El Brujo and El Monje. It would make a good camping spot as long as it didn’t get rained on.

The main attraction for me on the domes, however, was the fumaroles. And there were a lot of fumaroles – all down the divide between the two domes and well up onto their flanks, in fact. The fumaroles weren’t much hotter than boiling – I could and did spend a lot of time poking my face into them – and release mainly water vapor. If there were any other gases, they were in small enough amounts to be undetectable and non-irritating, which was definitely a bonus. No one likes to do field work with a mask stuck to their face all day!

I got a nice set of samples from these and other fumaroles on this part of the domes, and hopefully I’ll be able to find some clays in them. Clays in these fumaroles means that they could potentially be washing down through the void spaces and depositing in the domes,
or even forming there initially. The low temperature of the fumaroles is also an ideal condition for clay formation – once the temperature gets over 200 C, you start forming other alteration minerals.

That’s me on the right, contemplating how much I want a sample vs. the comfort level of sticking your face into the geologic equivalent of the spout on a boiling teakettle. I spent a lot of time with my glasses off to keep them from fogging up, which because of my nearsightedness required me to get even closer to the fumaroles. While I was pretty overheated by the end of that day of sampling, I did have lovely clean pores.
There were some great views of the other domes from El Brujo. This photo is looking roughly to the east at El Monje, La Mitad (“The Middle”), and El Caliente (“The Hot One”). The conical, flat-topped Caliente is the source of the regular eruptions that Santiaguito is known for (one of which appears in an earlier photo, and that I posted video of last year).  

Having seen and heard more eruptions from Caliente, I’m no longer convinced that calling them “Vulcanian” is the best description. They’re largely degassing explosions, with very little ash and almost no visible ballistics, and appear to be coming from a ring of small vents rather than a central larger one. 

We didn’t spend all our time scrambling around the domes; on the last day of field work some of us took time to examine the pyroclastic deposits and lava flows of Santa Maria, the main volcano that was responsible for the formation of the dome complex. Santa Maria last erupted in 1902, creating the huge crater that you see in the photo below. That eruption removed about 0.5 cubic km of material from the volcano’s flank, and ejected something like 5 cubic km of material over the surrounding area. More than 7,000 people were killed, thousands of acres of plantations (mostly coffee) were destroyed, and Guatemala experienced famine and unrest for some time following the eruption. 

This photo is a wonderful example of the “layer cake” model of a volcano – something that most volcanology classes will tell you is an inaccurate depiction of a volcano’s interior. Apparently Santa Maria didn’t get that message, because it’s easy to see the fabulous layering in this view of the 1902 crater. 

As we circled the flanks of Santa Maria, we encountered several outcrops of bedded pyroclastic deposits – in this case, several meters of air fall ash and pumice overlain by block and ash flow deposits (cobble to boulder-sized chunks of rock and pumice in an ashy matrix). These were under and overlain by massive lava flows, and they’re probably older than the 1902 eruption by a few thousand years at least. 

A chute made from massive andesite lava flows worn down by water. This would have been our route to get to the domes had we not chosen the jungle-and-landslide-scar path. It was fairly easy to climb up, but the thought of going downhill on polished lava with lots of ash underfoot and a 40-pound pack was somewhat terrifying.

On another lava flow closer to the El Monje dome, we found what my advisor described as “stretch marks” – places where the cooling lava flow pulled apart and left little stringy bits hanging in the crack (easiest to see next to the handle of the hammer and at the far right of the photo):

And, of course, we were rarely without views of the eruptions from Caliente:

In the next post I’ll have more photos of the 1902 and 1929 deposits, as well as some shots from our visit to the Santiaguito Observatory. And a little bit about the shaking we felt – or rather, didn’t feel.