Thursday, June 4, 2015

Man Made Maar experiments (the science)

-Alison



Experimental volcanology is a pretty fun sounding job description, but it is also one that isn’t as obvious in terms of what that entails. There are a lot of different specialties in volcanology, and the day to day activities for volcanologists can be pretty diverse. You can describe what I do as making deposits from simplified versions of volcanic processes using experiments to understand what evidence is left behind in the rocks. The simplifications mean that I can study the complex phenomena of an explosive eruption in parts, one or two at a time. Then I relate isolated processes to the deposits they form, which I compare to natural deposits that are the result of anywhere between 2 and 10 different processes. Every volcanic rock you see is the result of whatever process gets it out of the ground, some form of transport and then deposition. After that the deposits can be altered through physical processes like erosion by water and wind, chemical breakdown, collapse if the deposit is on a steep slope, and so on. To get into the nitty gritty of the experiments that I’ve been working on the last few years we need to consider those first three steps of making a deposit. Then, I promise I'll post some photos of the experiments we did this past Monday.

I study volcanic explosions that happen underground when rising magma interacts with some sort of liquid water, typically ground water. The resulting volcano is called a maar, and it forms a crater that cuts down into the surrounding rock, rather than building upward like a classic stratovolcano. Much of the action takes place underground and makes a downward tapering cone of debris called a diatreme. These maar-diatremes share a lot of similarities with kimberlite pipes where diamonds are found. The volcanic explosions that brings diamonds to the near surface so miners can find them did not form the diamonds, but anyone who loves diamonds should consequently love volcanoes. That's another story.


So how do we make our own maar volcanoes? And what do they tell us?


Step 1) Get that stuff out of the ground: ejection! The fact that these magma-water explosions happen underground and not in an open vent like at Stromboli or Kilauea is really important to how material is moved up and out of the volcano to form deposits. To model this in our Man Made Maars (well, woman made maars, but I do love the alliteration) we bury a chemical explosive in a pre-made gravel ‘substrate.’ So here are two big simplifications: 1) we have an explosion and are not studying what makes the magma and water go boom, but instead we are studying the effects of this explosion. 2) The second assumption is that our test material, layers of gravel are a good ‘host’ for our explosions and that we can use the same material over and over. The reason we can use gravel and not hard rock is maar volcanoes are the products of 100s to thousands of individual explosions. So really, once you have one or two explosions the later ones will happen under a bunch of debris, much like our gravel pits. We have done some explosions where we changed the strength of the host material, but you will have to wait to hear about those results as they are being written up by a M
asters student for publication.
Example of the 'substrate' that we make for our man made maars. The explosives are buried at various depths beneath the surface. The different material types help us track the mixing underground and the ejecta at the surface.

What have we learned so far? Our homemade maars have shown us that the size of the explosion and its depth below the surface are very important factors controlling how big the crater will be, how far material will be throw upward into the air, and how far away the material will travel. It is not a one size fits all type scenario at a maar volcano. The size, number and depth of explosions likely changes over time during an eruption. But most importantly, we have learned that only the shallow explosions get material from underground to the surface. Deeper explosions move material around underground and mix up material in the diatreme, taking nice flat layers and making a jumble. When we look at natural maar tephra ring deposits we see that there is material from great depths in the deposits and what our models have shown us is that it take a combination of deep and shallow explosions to make this happen. If you want to read the papers about this here are some links about our experiment results: Ross et al. 2013 and Graettinger et al. 2014*. And the real rocks that help show evidence of this underground mixing by explosions: Lafebvre et al. 2013.


Step 2) How that stuff gets somewhere else: transport. So volcanic deposits are proof that material from the volcano made it out, but the big question is how did it get where it did? The experiments give us a great chance to watch how debris, what I call ejecta, travels from inside the crater to wherever it lands. We get to use lots of high speed video and watch the explosions go forward, and my personal favorite, backward.  This transport happens in a jet of upward propelled gravel and gas. What our experiments have helped us understand is that the nature of the material above the explosion influences the shape that jet will have and the shape influences where the gravel ejecta ends up. If there is nice flat ground above the explosion the jet is really wide and produces a very dramatic jet. However, if it happens under a well formed crater the jet is narrow and focused. This means lots of the ejecta falls back down into the crater. This is an important process because it means that the crater is full of debris throughout the eruption and that future explosions have to interact with the material on top of it.
Transport: material is thrown into the air by the explosion in a jet. The shape of the jet is controlled by depth of the explosion under a surface (flat, cratered, hummocky) and the energy of the explosion.
The more detailed papers on this subject are Taddeucci et al. 2013 and Graettinger et al. 2014.  The angle of the jet also influences the distribution of the deposits. You can read about those inclined jets in Valentine et al. 2015, and another paper by myself soon.

Step 3) Finally the flying debris must come to rest: deposition. If material goes up in a jet, it must come down. A lot of the gravel in the jets falls back down, we call this ballistic emplacement. Where it falls is a result of the shape of the jet, as I described above, but these gravel, dust and gas mixtures can actually do much more before the come to rest. When the jet collapses it is a mix of big particles, small particles and gas. As the big particles land they displace the gas. This gas escapes between the larger particles and can take the smaller particles along for the ride. This expulsion of dusty gas travels outward along the ground as a density current. A density current is a liquid or gas that moves as a result of gravity acting along differences in density. The dusty gas mixture is more dense than the atmosphere, but less dense than the ground. The density difference prevents the dusty gas from mixing with the atmosphere and moves outward. The smaller particles drop out of the current along the bottom and when the current runs low on particles it stops. The deposit formed by the ballistics in the jet is full of a range of particle sizes, poorly sorted, and is controlled by the shape of the jet. The density current deposits are fine grained and are controlled not by jet shape, but by the amount of material expelled during the collapse. These are very different deposits from the same explosion.
Deposition from the jet occurs in two forms. Falling down in a classic ballistic trajectory, or being pushed outward as fine particles and gas escape from between bigger particles during collapse (seen at the end of the loop).
After establishing the influence of such simple variables as the depth and size of the explosion we started to vary other elements of the explosion set up to better understand these existing patterns and look for others.  This past week we wanted to test the influence of the number of explosions in a crater. We did 10 explosions in the same crater. This meant a lot of work collecting ejecta and crater data. So it will take some time to get all the data sorted. The videos take a lot of time to cut down to a size suitable for viewing (or posting). So please enjoy these photos of the deposits and hard work instead!




Ejecta gets on EVERYTHING, including our sensors.

As the crater got deeper, the graduate student helper had to get personal with the crater and crater bridge.

The last explosion of the day! We've now had over 63 individual 'eruptions' of man made maars.

This camera was on a flag pole. There may be some interesting views of the jets from above. Or they may be dirty. We'll have to wait and see.




* Papers mentioned in text can be found via the following links.

Valentine, G.A.; Graettinger, A.H.; Macorps, E.; Ross, P.-S.; White, J.D.L.; Sonder, I.; Döhring, E. 2015 Experiments with vertically- and laterally- migrating explosions with applications to the geology of phreatic and phreatomagmatic eruptive centers and diatremes. Bulletin of Volcanology.  doi: 10.1007/s00445-015-0901-7
Valentine, G.; Graettinger, A.; Sonder, I., 2014. Explosion depths for phreatomagmatic eruptions. Geophysical Research Letters. DOI: 10.1002/2014GL060096.
Graettinger, A.H.; Valentine, G.A.; Sonder, I.; White, J.D.; Ross, P.-S. 2014. Maar-diatreme geometry and deposits: subsurface blast experiments with variable explosion depth. Geochemistry, Geophysics, and Geosystems.  DOI: 10.1002/2013GC005198.
Taddeucci, J., G. A. Valentine, I. Sonder, J. D. L. White, P.-S. Ross, and P. Scarlato (2013), The effect of pre-existing craters on the initial development of explosive volcanic eruptions: An experimental investigation, Geophys. Res. Lett., 40, 507–510, doi:10.1002/grl.50176.
N. S. Lefebvre, J. D. L. White, B. A. Kjarsgaard, Unbedded diatreme deposits reveal maar-diatreme-forming eruptive processes: Standing Rocks West, Hopi Buttes, Navajo Nation, USA, Bulletin of Volcanology, 2013, 75, 8
P.-S. Ross, J.D.L. White, G.A. Valentine, J. Taddeucci, I. Sonder, R.G. Andrews, Experimental birth of a maar–diatreme volcano, Journal of Volcanology and Geothermal Research, 2013, 260, 1    

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