February 2010 dome collapse deposits at Soufriere Hills

If you ever want to visit a post-apocalyptic wasteland, someplace that's been run over by pyroclastic flows would be a great choice. On February 11 of this year, a partial dome collapse on the northeastern flank of the Soufriere Hills lava dome produced spectacular pyroclastic flows, surges, and a 50,000 ft (~15 km) high ash plume. The pyroclastic flows extended the eastern coastline significantly in the area of the old Bramble Airport, and surges were observed flowing out over the ocean on the eastern side of the island.

Here's a photo from the edge of the collapse deposits, below the Jack Boy Hill overlook. In the distance of this photo, you can (just) see a chimney stack. This is one of the only visible structures left in the whole area; even the old Bramble Airport (which would have been visible to the left of the chimney) is now completely buried.

The chimney stack is part of an old sugar mill, and I'm pretty sure that what's visible isn't the whole chimney. To give you a sense of scale, here's yours truly standing next to the stack. 

Walking on pyroclastic deposits isn't difficult, but it's not the most pleasant hike I've taken. Ash is nasty stuff, especially when you're kicking it up whenever you walk somewhere. In addition, these deposits are still(!) quite hot; a few inches down is enough to make it uncomfortable to stand in one place too long, and digging less than a foot down, they become hot to the touch.

Here's what the deposits look like in cross-section. Those dark streaks are degassing structures, which are cut off by the most recent deposits on the top of the sections. 

As I mentioned before, the Montserrat Volcano Observatory scientists brought us on this hike; in the khaki hat and olive shirt is Dr. Paul Cole, the Director of the Observatory. We're examining one of the boulders that was transported downslope in the collapse - and while it looks pretty big, it's actually one of the smaller boulders that we saw. The largest were the size of small houses! (Definitely not something you want to get hit by, which is why it's a good thing that the people on Montserrat pay attention to the exclusion zones.)

On several of the boulders, we saw examples of marks that are interpreted to have been created by the impact and scraping of one boulder against another during transport. They could be described as slickensides, except here they're glassy surfaces that were created very quickly during an impact, rather than slowly during the scraping of a fault surface. (The study I linked to mentions that in the largest marks, frictional melting formed pseudotachylite, which is basically glass.)

Things like the friction marks above, and this next photo, remind you of just how dangerous pyroclastic flows are. Aside from giant boulders smashing into each other, you also get stronger-than-hurricane-force blasts, poisonous gases and extremely high temperatures. To give you an idea of what that does to the landscape, here's a view of the end of a pumice flow lobe that carried trees with it. All the wood here is carbonized; my best guess is that temperatures of around 400°C (or higher!) were involved. That's about twice as hot as your kitchen oven will go.

Did I mention the force involved in pyroclastic flows? Here's an example: a tree limb thicker than my arm that was snapped in half and then pretty much welded into that position. 

Another interesting feature just beyond the pumice lobe were these pit craters, formed when the flow buried a water source, which was then heated to steam and exploded up through the new material.

I think the water in question was probably part of this drainage (this is looking roughly to the west), which we walked down on our way back to the vehicles. While these deposits do retain their heat, there doesn't seem to be enough time to really weld them together, so they're pretty easily washed away by precipitation, and form these sorts of drainage channels.

The channels do make it much easier to see cross-sections, though! There are at least five (probably more) different deposition events represented here, with a lovely pumice-filled channel right in the middle. 

I'll try to get to a post on how to distinguish different types of deposits next (and hey, maybe some annotated photos!)

UPDATE: Claire Howard (one awesome reader) sent in some photos of the factory chimney at Trants from May 1995, 6 months before the beginning of the eruption. Turns out that chimney was a lot taller than it is now. Enjoy - and thanks to Claire!

Looking backward: Past eruptions at Volcán Santa Maria

On our way to visit the Santiaguito Volcano Observatory, Gustavo Chigna of INSIVUMEH (the Guatemalan equivalent of the USGS) was kind enough to take an afternoon off and show us some of the older deposits near Santiaguito. Our first stops were at an exposure of the air-fall deposit from the October 24, 1902 eruption of Volcán Santa Maria. This eruption was a devastating one, stripping the land for more than 50 km around the volcano, burying villages and fincas (plantations) in more than 3 meters of ash, mud and rock, and killing more than 7,000 people (the exact number will probably never be known). The area had already experienced months of earthquakes prior to the eruption, and activity at the crater formed in Santa Maria continued for weeks afterward.

This location is about 4 km from the volcano. The 1902 air-fall deposit here is more than 2 meters thick (the photo is only showing the top bit, and Gustavo is standing a few meters behind the outcrop, so the sense of scale is a bit wonky. The big clast in the center is about fist-sized, though.). This fall deposit contains a mix of lithics (old and new lava and country rock) and pumice, and what's really impressive is that many of the lithics are the same size as the pumice (up to 15 cm). Generally, in a fall deposit the material will have had the same terminal velocity, which means the mass of the pumice and lithics should be roughly equal. Because pumice is much less dense than lithics, this usually means that the lithics are much smaller than the pumice. Possibly the lack of difference in clast sizes here has to do with the proximity to the volcano; in other places that we stopped, the pumices were definitely larger than the lithics.

The view from this location was pretty spectacular, and it highlighted the deposits from the November 2, 1929 dome collapse. Here you can just see the summit of Santa Maria, and the El Brujo and other domes in front of it. Brujo is the dark green dome farthest to the left, and the hummocky low hills in front of it are may be the result of that 1929 collapse. Hummocks are a classic sign of a debris avalanche of some sort, and can be seen in places like Mount St. Helens and Mount Shasta. ***Note: After speaking with Rudiger Escobar, a Santiaguito expert and fellow volcanologist (see the comments section), I wanted to note that the hills pictured here probably have more to do with lava flows coming off of Brujo than older collapse deposits, although some portion of them may be collapse remnants. It's also possible that I have faulty notes, as the conversation was mostly being conducted in Spanish at the time, and my Spanish skills are limited. 

After a bit of backroad driving around the fincas, we reached the Rio Tambor, which was the site of many deaths in the 1929 dome collapse. The 1929 event, in which more than 3 million cubic meters of the lava dome collapsed, was mostly described by foreign geologists who interviewed survivors. Their accounts tell of glowing rains of ash and rock; boiling mudflows in the rivers covered with rafts of glowing rock; a blast that rushed back toward the volcano after it blew down the river valleys, scouring the south side of tree trunks; and people suffocating on gases and hot air. More than 3,000 people were killed, mainly because they were on the fincas for a religious holiday rather than in their villages. Sadly enough, this was a very  minor dome collapse by volcanology standards (a large one might be in the hundreds of millions of cubic meters, such as those at Soufriere Hills on Montserrat).

The coffee plantations that were destroyed in 1929 are no longer here, but the new ones are beginning to encroach. Frankly, I'm not sure I'd want to spend my days working in an area where there are outcrops like this:

These photos are looking east across the Tambor at a mudplain created by lahars, and at the 1902 deposit (the white strip at the base of the cliff) overlain by block-and-ash flow and lahar (volcanic mudflow) deposits. Here the 1902 deposit is about 2 meters thick, and overlain by a thinner gray ashy deposit (perhaps from the 1929 collapse, although it hasn't been well studied and it wasn't clear if that guess was right). The upper blocky deposits are probably post-1929, and consist of a mix of lahar and block-and-ash (pyroclastic flow) leavings.

It was a bit difficult to get a scale into this picture, but the cliff is about 10 meters high, and the bigger boulders at the base are roughly the size of me. This is an interesting photo because it shows a very distinct division between two deposits. What we ended up discussing at this site was what kind of deposits these were, and how to tell them apart. Both block-and-ash flow deposits and lahar deposits look similar at first glance: very poorly sorted, with a clast size range from ash (< 2 mm, in the matrix) to boulders (> 25 cm, supported by the matrix). Both deposits are matrix-supported and both have a bit of a mix of angular and rounded clasts.

So how to tell them apart? One clue might be the angularity of the clasts; block-and-ash flows tend to be a bit more violent in terms of knocking rocks around, and they're dry, which means no muddy cushion for the rocks like a lahar might provide. But what happens if the block-and-ash flow goes down a river (like here)? It could become a lahar, which brings up all sorts of messy discussion about naming conventions. A better indicator of a block-and-ash flow would be the lithology of its clasts; lahars will pick up anything in their path and tend to have a big mix of rock types, while block-and-ash flows tend to be monolithological. Again, however, if a block-and-ash flow travels far enough, it may pick up enough clasts from other places to confuse this.

As it turns out, there is one sure indicator that you've got a lahar deposit, and it's not always easy to find (or present). Because a lahar is a wet flow, it will contain bubbles. When the lahar stops, sometimes those bubbles are unable to rise through the muddy liquid and are trapped; when the deposit dries out, the bubbles leave little spherical cavities in the fine-grained matrix. These are really hard to see and it's easy to debate what you're seeing if you're not sure.

So what was the verdict for these deposits? Based on a bit of arguing discussion, we decided that we were fairly sure we saw bubbles in the lower deposit, but that the upper one had more indicators of a block-and-ash flow deposit. So at this outcrop, we have about 4-5 meters of visible lahar deposits overlain by about 5-6 meters of block-and-ash flow deposits. Needless to say, this would not have been a fun place to have been standing for either event. In fact, Gustavo mentioned that very few people will come down to this area after dark, because some of the victims of the 1929 collapse are thought to haunt the riverbed. Having been down there in the evening, I can attest that it does get a little creepy when it's dark, and it would certainly be easy to turn the jungle sounds into the moaning or wailing of ghosts. Better not to stay there too long, ghosts or no ghosts; it is a channel filled with lahar deposits, after all.

Watch your step: Field work on lava domes

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.

Back in los Estados Unidos

Just a quick note to let you all know that I'm finally back from Guatemala with samples in hand and only a bit of surface damage. (Much better than last time.) It was a really successful trip, minus the bits where we had to deal with driving around Guatemala, and I'm really happy that the field work went as well as it did.

I'm still in the process of unpacking and making up classwork, but I'll hopefully have some photos up in a day or two, as well as a bit about the work our field group did. I've also come back with a mission: INSIVUMEH, the geological survey in Guatemala, desperately needs help with equipment for the Santiaguito observatory, and I volunteered to see what I could do from this end, since they've done so much to help me with my research. Again, more on that in the next few days.

I've come back from this trip with a new appreciation for the difficulty of doing research on volcanic domes, a few bruises, and a lot of things to think about. And more blogging material, of course!

Dome collapses

In light of the recent dome collapse at Soufriere Hills, I thought I'd expound a little on the subject, which is a major part of my research. Lava domes, if they last long enough, tend to go through cycles of growth and collapse. These can be relatively short, like the domes at Soufriere Hills or Mt. St. Helens (remember, a few years is short even on a volcanic timescale), or long, like at Casita volcano in Nicaragua or my own study area of Santiaguito. The collapses vary in volume: a smallish collapse might comprise a few million cubic meters of material (the 1929 collapse at Santiaguito was about 3 million cubic meters), but the collapse of an entire dome might be in the 100s of millions of cubic meters. 


There are a number of reasons why domes collapse. One is gravity; domes can grow on steep slopes or overspill summit craters, which means that parts of them can become very unstable and simply collapse because of their own weight. Another trigger might be an earthquake, which can shake loose dome rock; yet another trigger might be an eruption, which could loosen or even blow up significant portions of a dome. (This could be what happened at Soufriere Hills; there have been some pretty spectacular Vulcanian eruptions going on at the same time as dome-building there.)


My research, however, focuses on how water can cause dome collapses over short and long timescales.  In the short term, intense precipitation events (large storms or hurricanes) have been known to cause domes to collapse (especially at Soufriere Hills). This likely has something to do with water saturating the domes to the point where it can penetrate deep into hot dome rock, and either a) sealing in magmatic gases or b) sealing itself in and vaporizing in cracks and fractures. Either way means that pressure builds up in the dome and water can lubricate structural breaks, which reduce the stability of the dome. In the long term, a dome with an active and well-supplied hydrothermal system can form lots of clays, which are very weak and can also trap water. (Some clays even swell when they absorb water, which pushes dome rock around and destabilizes it that way.) This seems to be what happened when some old domes at Casita Volcano in Nicaragua collapsed and formed a devastating lahar, or mudflow of volcanic material.


What I intend to focus on at Santiaguito is why the domes there haven't experienced any major collapses in the 80 years they've been growing, and what part of the complex might be most likely to fail if a collapse did occur. This is going to involve looking at water-dome interaction in both the short and long term; I'm mixing in hydrology and clay mineralogy with my volcanology, and on this trip I hope to collect clay samples from the inactive domes in the complex. (I am not going anywhere near Caliente, the erupting dome, if I can help it; those videos of people standing on the rim during eruptions are just insane. I've met people who were at Galeras when it erupted in 1993, and I've heard enough about what can happen that I have no intention of putting myself in that kind of danger.)


Further Reading:



Barclay, J., Johnstone, J.E. and Matthews, A.J., 2006. Meteorological monitoring of an active volcano: Implications for eruption prediction. Journal of Volcanology and Geothermal Research, 150(4): 339-358.

Calder, E.S., Luckett, R., Sparks, R.S.J. and Voight, B., 2002. Mechanisms of lava dome instability and generation of rockfalls and pyroclastic flows at Soufriere Hills Volcano, Montserrat. Geological Society, London, Memoirs, 21(1): 173-190.

Elsworth, D., Voight, B., Thompson, G. and Young, S.R., 2004. Thermal-hydrologic mechanism for rainfall-triggered collapse of lava domes. Geology, 32(11): 969-972.

Fink, J.H. and Anderson, S.W., 2000. Lava domes and coulees. In: H. Sigurdsson, B.F. Houghton, S.R. McNutt, H. Rymer and J. Stix (Editors), Encyclopedia of Volcanoes. Academic Press, San Diego, California, pp. 307-319.

Harris, A.J.L., Rose, W.I. and Flynn, L.P., 2003. Temporal trends in lava dome extrusion at Santiaguito 1922-2000. Bulletin of Volcanology, 65(2-3): 77-89.

Matthews, A.J. et al., 2002. Rainfall-induced volcanic activity on Montserrat. Geophysical Research Letters, 29(13): 1644-1647.

Sapper, K. and Termer, F., 1930. Der Ausbruch des Vulkans Santa María in Guatemala vom 2-4 November 1929. Zeitschrift für Vulkanologie, 13: 73-100.

Sheridan, M.F. et al., 1999. Report on the October 30 1998 rockfall/debris avalanche and breakout flow of Casita volcano, Nicaragua, triggered by Hurricane Mitch. Landslide News, 12: 2-4.

Taron, J., Elsworth, D., Thompson, G. and Voight, B., 2007. Mechanisms for rainfall-concurrent lava dome collapses at Soufriere Hills Volcano, 2000-2002. Journal of Volcanology and Geothermal Research, 160(1-2): 195-209.

Voight, B. and Elsworth, D., 2000. Instability and collapse of hazardous gas-pressurized lava domes. Geophysical Research Letters, 27(1): 1-4.