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.