Random thoughts on Volcanology 2

Yesterday while trying to circumvent DRM, I accidentally my whole computer. So I spent all day yesterday backing up data and reinstalling Windows XP (I have XP on my PC and 7 on my netbook). I wanted to spend my time this weekend studying for my Volcanology and Structure exams, but as I’ll be out of town with those classes the next two weekends, this was my only chance to fix it. Given that my Volcanology exam is tomorrow morning, I think it’s time for another edition of random thoughts on Volcanology!

This section of the class was a little on the boring side I must say. We spent alot of time talking about pyroclastic flow deposits, which sound cool, but at the end of  the day, aren’t the most interesting thing one studies in a class about volcanoes.

Firstly pyroclastics, or ‘fire fragments’ are volcanic rock fragments. So pyroclastic flows therefore, are flows that contain a large amount of those pyroclastics.

I’ve said flows so far, but we talked about several types of pyroclastic ‘events’ in a more general pyroclastic density currents. These density currents are gravity controlled laterally moving mixes of pyroclastics and air.  From that umbrella term, you can separate them into two categories, flows and surges. These are separated by their pyroclastic content. Pyroclastic flows are more dense than surges, meaning that the ratio of pyroclastics to air is much higher. Surges are much more dilute than flows and contain more air than pyroclastics.

Furthermore, pyroclastic flows can be broken down into block and ash flows and ash flows. Block and ash flows are just what their name suggest (convenient, huh?), flows consisting of ash and blocks (pyroclastic material larger than about 64mm). These flow types are favored in Vulcanian eruptions and lava dome collapses. Ash flows travel further than block and ash flows and are generally produced in Plinian eruptions.

(I’m sorry if this ends up being hard to follow, I’m just rewording my notes and my teacher is not one for cohesive order)

Those relationships should be obvious, because Plinian eruptions aren’t going to be producing tons of blocks, but tons of ash, and the opposite with Vulcanian eruptions. Lava dome, likewise, result in the formation of many blocks and bombs leading to those block and ash flows.

Lava dome collapse is influenced by two main factors: gas pressure inside the dome and the tensile strain of the dome itself. When pressure exceeds the tensile strength of the dome, you get an explosive collapse, if a collapse occurs when the tensile strength exceeds the gas pressure, you end up with a gravitational collapse. Either way, you get a lot of large pyroclastic material released.

Pyroclastic surges are generated in sometimes similar ways, but distinctly different ways as well. Surges, if you recall, are composed more of hot air than pyroclastic material. So they can be formed by lava dome collapses, but they’re also produced by boiling over eruptions, eruption plume column collapses, directed blasts and from the base of an explosion column in the form of base surges.  They can also occur at the end of pyroclastic flows, and from the upper, less dense part of pyroclastic flows. That all makes sense when you think about it, all of those events are going to causes much less pyroclastic material than an actual eruption would as they’re almost all secondary features of eruptions.

So what are you left to find when one of these pyroclastic density currents occurs? Ignimberites, of course. Ignimberties are thick, widespread, often pumice-rich pyroclastic flow deposits that are poorly sorted and generally unstratified with a wide variety of characteristics that give us clues as to where they came from. That means it’s a lot like sedimentology and…. Zzzzzzz….

I’m tired of writing. I’m sure this was generally useless to anyone who read it. Apologies.

Random thoughts on Volcanology

I am currently studying for a Volcanology exam I have in the morning. I’m going to muse about random things we’ve covered in an attempt to know them better.

Magma resevoirs

Magma reservoirs are continuous regions of magma , and serve as the magmatic source for volcanism. Resevoirs are made up of two key components: the magma chamber, and the magma mush. Typically magma reservoirs are described or depicted simply as a magma chamber, but within what people commonly view as the chamber, there is a zone called the mush.

The magma chamber is the zone where eruptable magma is stored, eruptable meaning a non-rigid supply of magma. Generally, if the composition of the magma contains less than 50% crystals, it is considered part of the magma chamber.

The magma mush zone exists where the magma is rigid with greater than 50% crystal content. In the image below, the magma mush is represented by where the crystals are settling.

Volume and size of magma reservoirs can be estimated by measuring the volume of eruptions in the form of lava and pyroclastic deposits. Both lava and pyroclastics were once magma within the chamber. Lava measurements are 1:1 when inferring volume of magma, converting a measurement of pyroclastic however, is more complicated.

Taking measurements of pyroclastic deposits and turning them into value of magma volume requires using the dense rock equation; (volume of pyroclastic deposit) x [(density of pyroclastics)/(density of magma)].

Indirect measurements of magma reservoirs can be made by studying seismic waves in a few ways. One way is  through the observation seismic wave attenuation. Attenuation meaning the weakening of a force, this basically means observing when seismic waves slow down as they pass through the earth. Waves will become attenuated when they pass through a magma reservoir because the magma is a liquid. Seismic waves travel faster through country rock than molten rock.

This also means that magma reservoirs can be detected by locating anomalies in seismic reflections, again because of the properties of solid vs. liquid.

The more interesting way to infer size of magma reservoirs is by analysis of earthquake foci (aka the hypocenter or the spot within the earth, at depth, below the epicenter). Earthquakes only occur where brittle deformation can take place, meaning no liquids. As volcanoes typically occur in seismically active places and can causes earthquakes themselves, you can use that information to map a magma reservoir.

The beginning of the class was focused a lot of magma compositions and tectonic settings while this part of the class has focused on eruption types. So the above may seem a little out of place but it fits.

Here’s some eruption classification junk:

Volcanic eruptions can be broadly split into effusive and explosive eruptions. Effusive meaning non-explosive while explosive eruptions are rather self-explanatory. Within explosive eruptions there are Phreatomagmatic eruptions which are explosive eruptions caused by the violent interaction between magma and some external water source.

To get more descriptive there is also the Lacroix classification system that classifies eruptions by comparing them against iconic eruption types.

Hawaiian Eruptions and predominantly effusive, with lava fountaining common and primarily basaltic lavas/magma.

Strombolian Eruptions are moderate, discrete eruptions with lavas/magmas ranging from basaltic to andesitic in composition.

Violent Strombolian Eruptions are similar to Strombolian except that they have moderate sustained explosions and have a sustained ash plume.

Surtseye Eruptions are basaltic, phreatomagmatic eruptions with a large “rooster tail” ash plume.

Vulcanian Eruptions are eruptions with moderate-strong, sustained explosions with intermediate composition magmas that are sometimes phreatomagmatic.

Plinian Eruptions are eruptions with very continuous blasts and a very high eruption plume. When interacting with water, they are the largest of the phreatomagmatic eruptions.

Outside of those classification schemes, there are also those of George Walker, which measures pyroclastic fall desposits, and the more common Volcanic Explosivity Index which is a semi-quantitative scale of eruption magnitudes using a variety of measurable quantities associated with an eruption.

I am tired now.

*crickets*

I left home at around 8am this morning to make it to my 9am class. 9 hours later, I left my physics lab and got in my car to head to work. Several hours later, I arrived home.

Here is me and my lady love.

That is kind of my set up for this semester, and being that I have homework that is due in the morning, I don’t have much to post about. I guess that is the danger of challenging myself to write something daily.

I think that Volcanology is quickly becoming my favorite class this semester. I knew I would enjoy it, but I’m finding myself more and more engaged with it. Currently, I am reading a paper on intraplate volcanism (as in, volcanic activity that doesn’t occur at plate margins) and it’s really interesting. If you have access to Nature, check it out here. I really recommend reading it if you’re into volcanism.

Lemuria came up on shuffle today and I’ve been listening to them all day as a result.

Fish Canyon Tuff

Exploring another volcanology topic today. The Fish Canyon Tuff.

This, like yesterday’s post, is about a very specific volcanic occurrence, not a broad topic. Tuff, for those non-geologically inclined, is a type of rock that is formed by volcanic ash as it compacts and welds itself together as it is very hot (you know, having come from a volcano).

The significance of this in Fish Canyon is that there is a lot of it. So much of it, in fact, that if you measure it, look at it’s composition and infer what kind of eruption produced it, you end up with one of the largest, most confidently estimated, eruptions in the history of the Earth.

The Fish Canyon Tuff is the result of a supervolcano that formed a massive caldera near La Garita, Colorado around 28 million years ago. A caldera forms after certain types of eruption occur. The ground rises because of the eruption and then collapses upon itself after the eruption is finished leaving a large depression. Here is a cool animation I found on Wikipedia showing off how it works.

To deposit the amount of tuff it did, the eruption would’ve had to been a supermassive, explosive eruption… and it was. The energy released for this eruption is estimated to be the most energetic event on Earth since the asteroid that struck the Earth leading to the K/T extinction occurred 65 million years ago.

Another cool topic, but I’m not sure it’s for me. I WAS thinking supervolcanos when I first learned of the paper, but some of the other topics look a little more promising. Either way, it would be a cool looking place to visit.

Ol Doinyo Lengai

I have a Volcanology class this semester. Thus far it’s been pretty cool. We’ll go to the St. Francois Mountains later this semester, and it doesn’t seem to be that heavy of a course load for the class. The only thing of real importance is the 10-15 page term paper due at the end. We were given a list of topics to choose from and I haven’t decided what I wish to do yet so I’m just reading up on different ones.

One of the topics is Ol Doinyo Lengai in Tanzania, Africa. Most of the topics on the list are on there because of some sort of interesting property about it. There are a handful of volcano’s on the list, but most of the rest of them are more general topics of interest or extra-terrestrial things.

What is interesting about this volcano is that it’s magma is of a composition that isn’t very common to most volcanoes. It’s lava is natrocarbonitic in nature, meaning it is a volcanic source for the mineral carbonatite. As most lavas are very silica rich, this is kind of cool and unique. Not having much experience with carbonate minerals, this might be an interesting topic to tackle.

This makeup of these lava flows result in unique properties for the lava, including how it flows and looks. It seems pretty neat.