Before reading this week’s Published Research Synopsis, hop on over to DamnInteresting.com and check out my latest entry A Big, Big Hole in the Ground. Now that you have the basics down about the Yellowstone Supervolcano, we can dig a little bit deeper into what geologists are doing to monitor it.
Astonishingly, modern satellite remote sensing techniques are capable of actually “seeing” the magma flow deep beneath the surface. The magma slightly deforms the crust above it, and by measuring that deformation and comparing it to computer models, the authors of today’s paper convincingly estimate the volumes and timing of magma flow beneath the surface. For the first time, then, we are given a window into the hidden world deep beneath Yellowstone’s feet. And a very important window it is.
Citation
Wicks, CW, Thatcher, W, Dzursin, D, and Svarc, J (2006). “Uplift, Thermal Unrest and Magma Intrusion at Yellowstone Caldera.” Nature 440(7080), pp. 72-75. [online at CiteULike.org]
Synopsis
Before proceeding into the synopsis this week, if you aren’t familiar with Yellowstone’s geologic activity you may want to check out General Explanations, below. The maps on the right show the interferograms from several years of satellite radar measurements of uplift of the surface around Yellowstone. Hotter colors (reds, oranges) indicate a large amount of uplift (or subsidence, the opposite of uplift) while cooler colors indicate no change of altitude. See the General Explanations below for how to interpret this image in greater detail. Basically, part a in the image shows that the feature labeled NGB in the map below experienced uplift from 1996-2000. Then, in 2000-2001, NGB continued to rise while the region of the caldera floor between the South Creek and Mallard Lake domes experienced subsidence. In 2001-2002, NGB continued to rise, but in 2002-2003 the entire system seemed to pause and no significant elevation change was measured.
After “unwrapping” the interferograms and summing them between 1996 and 2003, the total surface elevation change is plotted in part a of the map on the left. In this map, blue colors indicate uplift and red subsidence. Notice that the caldera floor appears to have subsided approximately 60 mm while the NGB area rose by about 120 mm. The authors then created a variety of computer models of the magma chamber in order to try and reproduce these changes. Plotted on the part a map are the rectangular outlines of the magma sills that they assert are deflating and inflating. Part b of this figure shows their model result in comparison to the observed elevation changes. In general the model fit appears excellent.
As the authors have modeled them, sills are relatively flat areas of magma intruding between layers of rock beneath the surface. In this case, their best fit model indicates that a sill of magma oriented as shown in the left-hand part of the figure on the right is expanding at about 15 km of depth. The amount of magma required to produce the change in elevation at the surface is between 0.06 and 0.1 km3, or about 1/10th of the volume erupted from Mt. St Helens. The right-hand part of this figure shows the cross-section indicated by the line X-X’ on the left. The red and blue circles are the locations of earthquakes, with the magnitudes given by the diameter of the circle. The red circles are earthquakes between 1998 and 2003, while the blue circles are from 1992-1997. The colored contours in this images are the modeled strain in the Earth’s crust. Blue colors are areas of compression, and green-red show areas of extension. Notice that the earthquakes have occurred mostly in areas that lie between compressive and extensive portions of the crust, according to the authors’ model. This is what is expected because these regions are experiencing the highest gradient of strain, and that is what leads to tearing of the crust and earthquakes.
Context
Whew! If you’ve made it this far and are still with me, congratulations! But, if you just skipped the words in the last section and looked at the pretty pictures, never fear, for a summary is here. Basically, the authors present a convincing model of the movement of magma beneath Yellowstone. They describe this system as Yellowstone “breathing.” I prefer to think of it as a menacing pulsation instead. After all, the Yellowstone Supervolcano is powered by a mantle plume where hot magma is coming directly from the mantle itself, rather than from melted portions of the crust as in subduction zone volcanoes (like the Cascade Range, for instance). Hawaii was and is being produced by a mantle plume as well.
As the North American plate slides over that blow-torch of a mantle plume, a chain of enormous calderas serves to remind us of the destructive power of the Earth. For at least the last 15 million years, that mantle plume has been creating some of the largest volcanic eruptions in the world. Mantle plumes beneath continental crust can be particularly eruptive because they are made up of relatively thick magmas. In contrast, the Hawaii mantle plume erupts much more liquid magmas that do not erupt explosively. Yellowstone can be particularly dangerous because the mantle gases within it only remain dissolved in the magma because of the enormous pressures at depth. If fractures in the crust open up even slightly, those gases can come out of solution just like the CO2 from a can of soda. Thus, the magma chamber beneath Yellowstone is extremely unstable, as was very nicely shown in the Discovery Channel movie: Supervolcano.
Alright, so perhaps I’m being a bit gloomy here. Or perhaps I’m not. Short only of meteorite impacts, a supervolcano explosion has the capability of reducing our grand civilization to a whimpering mass of starving creatures. Lying in the path of a massive eruption is the breadbasket of America, that in a very few years will also be sustaining a large fraction of the expanding world population. Cutting off that food supply will create a famine that the world has never seen, especially since most of the crops around the world would have a hard time surviving due to the extreme global cooling caused by the eruption. Wars would break out, hundreds of millions would likely die (if not more), and the world order will be forever shifted. The United States would be crippled, its vast wealth nothing without the sustenance provided by the land. Its neighbors, Canada and Mexico will be facing a crisis as waves of American emigrants struggle to find a new home. We talk about a post-9/11 world as if we’ve experienced a real catastrophe. Just imagine a post-Yellowstone world.
So the next time that a geologist talks about the lungs of Yellowstone slowly filling and emptying, and the peaceful breathing of Mother Nature. Just keep in mind that it is the peaceful breathing of a terrible slumbering giant that is very likely going to wake up in the next few hundred thousand years.
General Explanations
Volcanism and Geothermal Activity at Yellowstone
The map on the right gives quite a bit of details about geothermal and volcanic activity at Yellowstone. The caldera itself is shaded salmon. The Two lakes labeled “YL” and “HL” are Yellowstone and Hebgen Lakes (Hebgen Lake was the epicenter of a magnitude 7.3 earthquake in 1959). “SC” and “ML” are the Sour Creek and Mallard lake Domes. These domes are large protrusions of rock that have formed gradually as the magma chamber beneath Yellowstone has refilled. “NGB” is the Norris Geyser Basin, a particularly active geothermal area of the park, and “M” is the Mammoth Hot Springs. The yellow blobs on the map are areas of hydrothermal activity: vents, geysers, hot springs, mud pots, etc. The black lines are known fault locations active in the last 2 million years (during the Quaternary period). The red asterisks show areas where eruptions have occurred since the caldera formed 640,000 years ago. Finally, the white arrows give the authors’ interpreted paths of magma movement beneath the surface.
The caldera formed when approximately 1,000 km3 of material erupted from a magma chamber whose top is 5 km beneath the surface. With so much material evacuated from the chamber, the crust above caved in, creating what is probably one of the world’s largest sinkholes: the Yellowstone Caldera. The present-day caldera has been mostly filled in due to volcanic activity since that eruption, but it is still visible along many parts of its rim.
This sudden collapse of material left behind an enormously complicated set of rock fractures that has only grown more complicated as the South Creek and Mallard Lake domes have risen. Because the entire subsurface of the crust is filled with water (unless it’s filled with something else), these fractures have become the piping of Yellowstone’s hydrothermal activity. The results are simply spectacular; and I say that having seen the pictures only. The picture on the left shows Yellowstone’s Grand Prismatic Spring. The colors are produced not by precipitating minerals or any such inorganic chemistry. Instead, they are created by communities of extremophile bacteria that exist in very specific regions of the spring that are optimum in terms of temperature and chemistry for each bacterium variety. Yellowstone was one of the places where extremophiles were first discovered.
Interferometry (not for the faint of heart!)
Since I’ve been talking about interferometry for the last few weeks (here and here) at Anthonares.net, I thought I would discuss the basic concept very briefly. Waves (like light waves, sounds waves, or ocean waves) travel right through each other, but when they are in the same location at the same time, they interfere. Interference results in the addition of the amplitude of the two waves. So, if two waves are in phase, they interfere constructively and the combined result has a greater amplitude than either wave did to start with. If they are out of phase, meaning the peak of one is aligned with the trough of another, then they interfere destructively and the result has a smaller amplitude than both did originally.
This is scientifically very useful, as in the case of the Terrestrial planet finder interference can cancel out the light of the very bright central star and allow us to perhaps see the much much dimmer planets circling it. There are a whole host of other uses of interference in science, but the one one used in this paper uses relatively long wavelengths of light, radar waves. Satellites bounce radar waves off of the surface and record the phase and amplitude of the returned signal. Over successive passes, if the height of the Earth’s surface changes the phase and amplitude of the wave will change. Thus, by adding the two radar images together, even very small changes in altitude of the surface are visible.
Hotter colors (reds, oranges) indicate a large amount of uplift (or subsidence, the opposite of uplift) while cooler colors indicate no change of altitude. In the interferogram above (first figure in the Synopsis section), the NGB feature is experiencing positive uplift from 1996-2000. Concentric rings should not be interpreted as some strange wavelike pattern of elevation change, but instead indicate regions where the change in elevation is greater than the radar wavelength used to take the measurements, in this case 28.3 mm. To calculate the total amount of uplift, you have to “unwrap” the interferogram by summing the number of concentric red rings between the background and the peak. For example, an unwrapped interferogram is shown in the next figure, part a. You can see that the total change in elevation of the NGB feature is about 120 mm between 1996 and 2003, or approximately 5 red rings. There are three in 1996, and two more in each 2000-2001 and 2001-2002.
