Wednesday, September 29, 2010

Guess this means it's columnar jointing week

Well, since everyone else is getting in on the columnar jointing meme/festival/whatever, I suppose I could throw a few photos out there...

Columns in an ignimbrite on the east side of Santa Maria volcano in Guatemala. (A drive-by, unfortunately. I would have loved a close-up look at these!)

Some of the oldest volcanics in the Vulsini volcanic district, Italy: a jointed trachyte from the Bolsena caldera complex. (The tree at the top of the cliff is about 3 m tall.) This is one of my favorite photos, because you can see multiple cooling sets divided by fractures - and it really emphasizes how columns form from inward-directed cooling.

Willow Mountain, Terlingua, Texas (near Big Bend). Beautiful columnar jointing throughout. (No scale on this one, but I'd say it's about 200 meters from top to base.)

And, finally, an actual example of columnar jointing in basalt, which is probably much more common than jointing in an any of the other stuff I've mentioned: a lava flow near St. George, Utah. (Wish I could figure out where, but Google Earth isn't helping today. Has anyone seen this abandoned water tank?)

Perhaps we should submit a request for a Columnar Jointing Week to someone in Washington. As far as I can tell, we're sadly lacking in geological observances anyway...

Tuesday, September 28, 2010

Upcoming changes

Magma Cum Laude will be moving to a new location soon - more details on that when it happens! In the meantime, posting will go on here (and I'll try and cross-post if I can). Stand by for new developments...

Monday, September 27, 2010

"If we knew what we were doing, it wouldn't be called research"

I talk a lot about my experiences with volcanoes and molten lava and such, but if I want to talk about my most important geological experience overall, I have to skip my volcanological career entirely and go back to the very first field course I ever took. (I mention it a lot; it was a 3 1/2 week trip to the Colorado Plateau, which is an excellent place for a crash course in field work.) We were in Utah, just outside of Capitol Reef and a few miles away from what was later to become my undergraduate thesis field area, in a little campground outside of Bicknell. Sunglow Campground was aptly named; at the end of the day the red rocks surrounding us seemed to light up as the sun was setting.

If I'm remembering correctly, this was our first real mapping assignment. I, having had no classes in mineralogy, petrology, structure, or mapping, was working with some of the older students. At this point, I'd managed to pick up basic Brunton skills and was reasonably certain I could tell sandstone from limestone. I was still pretty nervous about the assignment, since at that point I was still a bit of a perfectionist (left over from high school, I guess). 

We spent the entire day climbing over rocks, gazing at outcrops from high spots, breaking off hand samples, squinting through our hand lenses. And by the end of the day, we still hadn't managed to cover the whole area. I was excited when we found what I guessed was volcanic ash right next to a pretty obvious fault, but mainly because I could actually stand on the thing and say "this is a fault". Beyond that, I was definitely having trouble putting lines and color on our map. Looking back at my field notes, I know I didn't get as detailed as I could have, mostly because I didn't really know what was important to write down. I felt like I should have been able to figure out field relations and rock types by that point on the trip.

I was completely frustrated with myself and the assignment by the end of the day, especially when it became apparent that we'd neglected to investigate a few of the units properly and left one completely off the map. Once we'd turned in our assignments, however, and were sitting around the campfire toasting food and joking, I realized something important: that I had expected myself to come up with all the answers even when I was still developing the tools I needed to find them. There was no way I was going to complete the assignment perfectly. And that was okay, because I was still learning. And that's something I've had to remind myself of all the time, especially when I get frustrated or upset over something: I'm still learning. I'll still be learning when it's time to retire - and I'll still be learning after that.

As a freshman, I may have been bright enough that my advisor invited me to come on a trip that was normally restricted to upperclassmen, but I should also have been bright enough to realize my own limitations. That was one of the best things I learned on that trip, and one that's helped me immensely since. If I can take a step back from whatever frustrating problem I'm dealing with at the moment, I can figure out what I tools and knowledge I need to acquire to solve it. That process may be slow and frustrating itself, but it's much better than thinking I know everything (and then finding out, usually in embarrassing ways, that I'm wrong). Come to think of it, this is a great way to look at life in general, not just geology. 

Monday, September 20, 2010

Distinguishing deposits from andesitic eruptions

Telling apart different kinds of deposits associated with volcanic eruptions isn't always easy. There are a lot of factors that can affect their appearance: the location and type of eruption, the magma/lava type, where they're emplaced, etc. On Montserrat, volcanologists are lucky to have both ancient and modern deposits; they can look at what's currently being erupted and compare it to the older volcanics on the island. We did quite a bit of this on our field trip, and one of our assignments was to summarize the characteristics of andesitic eruption deposits on Montserrat. (I emphasized that because the characteristics we saw are not necessarily going to be the same for all volcanic eruptions, or even for all andesitic eruptions. I've tried to generalize a bit, but apply these cautiously if you're going into the field to look at other volcanic deposits; things may look very different in your field area.)

Block and ash flows are a kind of pyroclastic density current. "Pyroclastic flow" is a kind of catchall term, but there are more specific ones that better describe the makeup of one of these currents. "Block and ash flow" implies that the flow is composed of blocks (either of denser lava or pumice or both) and ash; "pumice flow" means that the contents are mainly pumice and ash, "ash flow" that there are few blocks and mostly ash in the current.

Faint reverse grading in a block and ash flow deposit at Old Road Bay
Block and ash flows contain a range of clast sizes from mm-sized ash to m-scale boulders. The clasts are usually somewhat angular and tend to be a combination of lithics (lava) and pumice. The matrix of these deposits (what larger clasts are embedded in) is generally ashy and may be crystal-rich. The deposit may be clast or matrix supported (which describes whether there is enough matrix that the clasts are not touching) and poorly sorted (clasts are not separated by size). Oxidation and alteration of clasts as well as fractured blocks (including radially jointed clasts) may occur if the flow is deposited into water and cools quickly. Sometimes you can see grading in the deposits (i.e., a change from small to large clasts, or the reverse), which has to do with the energy of the pyroclastic current and the conditions under which clasts are being deposited from it.

Pointing out matrix in a deposit on the side of the Belham River Valley
Debris avalanches form from older volcanic material that's unstable enough to collapse, either because of erosion or alteration or both. They form "hummocky" deposits that are very obvious in aerial photos, but in cross section they're also fairly easy to identify.

Panorama of a debris avalanche deposit near Jack Boy Hill. Different colors mark chunks of preserved stratigraphy.
Debris avalanche deposits contain mm to m sized clasts (or larger), as well as chunks of material from older deposits that retain their original stratigraphy. These clasts and chunks are poorly sorted, and rounded to angular in shape. Clast compositions are almost always mixed, and the matrix is often clayey and highly altered. Jigsaw jointing is apparent in individual clasts, with clast fragments separated by matrix material but orientation with respect to neighboring clasts preserved. Clasts or deposit blocks may be "smeared" out into trains or show internal faulting.

A "jigsaw" fractured block in a debris avalanche deposit
Lahars form when water mixes with volcanic material and flows downslope. These can have the consistency of soup to concrete, and they're a major concern even when a volcano isn't erupting (especially if the volcano is located in a tropical area that gets lots of rain.)

House buried in lahar deposits in the Belham River Valley
Clasts in a lahar deposit range from mm to multi-meter size. They are matrix supported, poorly sorted, and often contain clay or silt in the matrix. The clast shapes vary from rounded to angular and may be either monolithologic (all one rock type) or mixed rock types, depending on the source material. Sometimes the deposits show bedding features such as cross-bedding. There is one absolutely diagnostic feature for lahars: preserved voids where bubbles of air were trapped in the matrix mud. The voids are spherical and usually pretty tiny, but they don't form in pyroclastic currents. (The lack of these voids does not, however, mean that a deposit is not a lahar, so this feature is only useful if it's present.)

Cross-section of a fairly recent lahar deposit. There's not much in the way of sorting or grading here.

A fourth type of pyroclastic current, called a surge, also shows up on Montserrat, but they're not often preserved because they consist of a blast of ash and hot gases (sometimes derived from block and ash flows), and they leave very thin deposits. I don't have any good photos of them, but they tend to be fine-grained (mm to cm sized clasts at the most), and sometimes show cross-stratification. It's rare that they're preserved, especially in a tropical environment, because the fine material washes away very easily.

Tuesday, September 14, 2010

A question of time

*Note: Having been temporarily flattened by my yearly fall cold, I'm putting up a non-geology post that I was working on earlier this month and have just enough energy to finish now. I'll make it back to talking about andesitic eruption deposits just as soon as I emerge from the haze of cold drugs.

'Tis the season for the arrival of new grad students (geology and otherwise), and 'tis also time to talk about time. As in, time management - possibly one of the most important skills a grad can have (or develop).

You've heard it a lot of other places, but I'll repeat it here as well: Graduate school isn't like undergraduate, unless you were part of a crazy tough undergrad department. You're much more responsible for yourself, including how you portion out your time between classes, research, work (if you happen to be a TA), and downtime. I treat it as a job, because that's what it should be. And it's really, really important to know how much time to devote to the different parts of your job - and when to take a break.

Classes will be important, but unlike undergrad, they're not the biggest part of your life. Most grad programs will expect you to maintain a certain GPA of to remain in the program, but you shouldn't be working yourself to death over your classes. Eventually, you won't be taking as many (or any at all), and if you're not used to working on things other than classwork, it will be harder to adjust. Pace yourself if you can - sometimes you can put off a class that's not essential to your research. 

Likewise, if you're TAing, remember that the teaching experience is useful - and if you plan to be a professor someday, it may be some of your only training - but that research is your primary mission. Put in the work that you need to, but don't get sucked into agonizing over grading or class preparation. Ask for help if you need it, from your professor or the other TAs. Don't get too emotionally invested in it; not every student is going to like you or your teaching style (or their grades).

Research should be the main focus of your time in grad school. Even if you don't have a solid idea of what you want to work on when you come in, start by doing as much reading as possible (without frying your brain). Use your class projects to help develop (and answer!) research questions. Set daily, weekly and monthly goals for yourself, and keep track of things like conference deadlines and committee meeting dates (they're good things to schedule your work around). If you have an idea for a research proposal, start writing whatever you can, a little bit at a time - it will save you a lot of work come crunch time. Keep a calendar (or two) to remind you of your appointments; being able to look ahead at a deadline will help you plan out your time better. 

And most importantly, remember to take time for your own well-being. It's really, really easy to get overstressed in grad school. Between teaching, research, writing, taking classes, and dealing with everyday life, you're going to be really busy. You may well be putting in longer hours than a regular job, just to get things done - and that's okay. But make sure that you take time off for yourself, because your mental health is just as important as your degree. I feel like I came from a rigorous academic background, and I still had rough patches getting adjusted to grad school; everyone does. But I learned that I had to step away from it and do fun things for myself - hiking, shopping, going out with friends, watching cheesy horror movies, blogging, etc. It's made the whole experience a lot more enjoyable, and I think that even though grad school is difficult, if you're not enjoying it on some level, there's something that needs to change. 

Wednesday, September 8, 2010

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!