Of Ancient Plate Tectonics and Unknown Carbon Reservoirs
By Stephanie Osborn
A couple of articles were recently brought to my attention by my particle physicist friend. One of them is a patently alarmist article in a UK newspaper. (http://www.dailymail.co.uk/sciencetech/article-4226566/Scientists-discover-massive-reservoir-greenhouse-gases.html) The other is the scientific article it purports to reference. (http://www.sciencedirect.com/science/article/pii/S0012821X16307543/#ec0040) [NOTE: this one is behind a purchase wall, but the abstract is available to read.] I won’t go into the whole contents of the articles; you can read those for yourself, or at least the abstract of one. The gist of it is, the UK newspaper is claiming that a recently-discovered deep-Earth structure and a possible ‘volatiles’ reservoir associated with it could create a catastrophically huge release of CO2 into the atmosphere.
This brought on some brief email discussion, as not everyone has a strong background in some of these sciences, and I thought it might be good to explain some of the things being discussed in the articles.
Now, I don’t know how much the average person knows about seismology, but it’s one of the reasons I picked up an undergrad minor in geology and did some graduate subspecialty work in geology, as well. I grew up in an area of the country that regularly felt quakes from the New Madrid Fault Zone, which is a whole ‘nother article in itself — several, actually. I can do those at some point, if there is enough interest. Anyway.
Seismology is really a form of optics; the very same rules apply, since you are looking at wave propagation, reflection, and refraction. (Having just finished the Optics sequence in the Physics dept., when I got to seismology in my Geology studies, I was better and faster than the Geology majors, because the concepts were already very familiar to me. For that matter, I made use of the concepts in my racquetball course, too, and the coach couldn’t believe I’d never played the sport before. Hee!)
So there are various ‘types’ of seismic waves, which is really just another way of saying they are polarized differently. (The only kind of wave that seismology has that optics doesn’t is the longitudinally-polarized wave — aka the acoustic wave. And so if you’ve studied acoustics, you even already have THAT.) Now the interesting thing is that certain of these wave polarizations create differing effects on the ground surface, and the budding science of seismology therefore named them accordingly. (A ‘shear wave,’ which the article references, is a ‘transverse body wave’ — this means that the wave motion is perpendicular to the direction of motion, and it moves through the body of an object like the Earth via elasticity within the object. It was named ‘shear’ by geologists because it had a shearing effect upon structures when it arrived.)
However, just like in optics, when the medium changes, so does the refractive index. And just like in optics, the boundary between media creates a reflective surface, which in turn also generates additional polarized reflected waves. And this is what complicates the thing so much. But certain polarizations are easier to ‘read’ than others, and they can tell us a lot about the various strata, including what state of matter they are in — liquids tend not to transmit some of those waves at all because, once inside the melt, the waves usually experience total internal reflection, and thus you get a blank zone.
So we know when there is a blob of actual melt down there, because we get all reflections from it, and no refraction through it to speak of. If it’s partly molten, you can get some refraction, but it tends to generate ‘mushy’ surfaces, is maybe a way to put it.
Now, the Farallon Plate referenced by the articles is an ancient oceanic-floor tectonic plate under what has become the Pacific Ocean. There are a few remnants of it left that have not yet been subducted under the North and South American plates; they’re most notable in the Cascadia Subduction Zone, where there are some triangular bits, now known as the Juan de Fuca Plate, and the adjacent Gorda Plate. Another notable remnant is the tongue-shaped plate (Cocos Plate) that forms the west coast of Central America, and the better-known Nazca Plate just off the western South American coast. In all cases, the principal direction of motion takes them east and under the continental plates in intensive subduction zones. (It’s worth noting that these are serious quake zones, capable of generating monster quakes and tsunamis, in some cases equivalent to the Boxing Day quake/tsunami combo in 2004.) You can find out more about it by plugging in ‘Farallon Plate’ to Wikipedia.
Now, it is also worth noting that there are volcanic and regular mountain ranges that run parallel to, and just inland of, the west coast from Alaska/Canada all the way down to the tip of South America, and it is this subduction of the Farallon Plate that is responsible for both types of ranges. Obviously the whole ‘big crunch’ thing is responsible for the standard mountain ranges, in various forms — when two plates slam together, buckling occurs, and mountains result. But what about the volcanic chains, such as the heavily volcanic Cascade Range?
Well, since most crustal plates are a mixture of mineral types, and various families of minerals melt at different temperatures, as the plate is subducted, low-melting-point minerals melt out of the solid plate. Being liquid, they’re more buoyant and rise upward through whatever cracks and crevices and imperfections they can find in the overlying plate, or force such cracks and crevices to open by dint of increasing pressure (which, I might note, tends to form magma chambers, either way). When they reach the surface, blooie, volcano.
Note also that the type of volcano tends to change as you move from the coast inland; this is because, as you go farther inland, the plate being subducted is being shoved deeper and deeper into the mantle, encountering hotter and hotter temperatures, and thus melting out minerals with increasingly higher melting points. This results in a separation of the minerals, and a corresponding chemical difference in the melts, in a smooth transition moving from coastal volcanoes and progressing inland. It’s been theorized that this is the reason why certain areas have more explosive volcanoes — the chemistry resulting from the melt leads to a more viscous lava, trapping the dissolved gases inside and allowing for pressure buildup.
There is also increasing evidence that the heat resulting from subduction was insufficient to fully melt the Farallon Plate, and the continental plates overrode the Farallon, which may have fragmented/faulted and ‘stacked up’ in slabs under the continents. According to a NASA research group, a significant portion of the Farallon sank to the bottom of the mantle, and is much farther east, most likely under the eastern USA. (http://svs.gsfc.nasa.gov/vis/a000000/a002400/a002410/) The footprint area is quite considerable. And no, it wasn’t that the Farallon drove so far eastward, as much as it was that the North American Plate just moved over it. Still and all, dang big plate, when you think about it.
That said, it strikes me that the Daily Mail has once again gotten its science mixed up. (Yes, I’ve dealt with anxieties produced by articles from this and other similar UK newspapers several times.) It references the same area that the Science Direct article does. And it does link to the Science Direct article. But that doesn’t mean they interpreted it correctly.
They, of course, immediately focus on the fact that the Yellowstone supercaldera is supposedly in the middle of it. I say ‘supposedly’ because the NASA research plainly indicates the Farallon Plate remants in a VERY different location from that depicted in the Daily Mail article. I’d really love to know where they got their graphic, and how accurate it really is, relative to what they think they’re talking about.
HOWEVER, all that said:
1) The Daily Mail article immediately assumes that virtually the entire volume of ‘volatiles’ referenced by the scientific paper is carbon, when the first volatile mentioned by the science paper is hydrogen. And even that is speculative, as denoted by the phrase, ‘such as.’
1a) Typically the constituents of volcanic gases are: water vapor, carbon dioxide, sulfur dioxide, hydrogen sulfide, nitrogen, methane, carbon monoxide, hydrogen, and several of the noble gases such as neon, helium, and argon. (Other gases may be found in trace quantities as well.) According to Wikipedia (and this matches my training), “The abundance of gases varies considerably from volcano to volcano. Water vapor is consistently the most common volcanic gas, normally comprising more than 60% of total emissions. Carbon dioxide typically accounts for 10 to 40% of emissions.”
2) The Yellowstone hotspot is separate from the Farallon Plate structures, and goes far down, into the mantle. Its upper regions have been 3-D mapped, and are not a part of the Farallon structures. More, it has a tracked geologic history of eruptions, with fossil calderas that can be traced back from its current location, regressing southwest almost all the way to the northeast corner of California. There is no indication of catastrophic gaseous emissions of which I am aware; the volume of ejecta ultimately came from much deeper. The danger from a Yellowstone eruption is in the massive blast which would devastate the area for at least 1-2 hundred miles in every direction, followed by the truly titanic volume of ash which would be pumped high into the atmosphere. There have been discoveries of fossilized, fully-articulated herd animals in mass deaths from acute silicosis as far east as, if memory serves, at least the vicinity of the Mississippi River. (I distinctly recall reading this, but cannot now find the article online. However, not so very far away, near the Missouri River somewhat westward of my recollection, is also found the Ashfall Fossil Beds State Historical Park in NE Nebraska. A huge die-off of animals, from turtles all the way up to herds of horses, camels, and rhinos, are found intact, embedded in what is evidently the Mesa Falls Tuff — tuff being a type of loose, porous rock composed of ‘welded’ ash. The Mesa Falls Tuff was the ash layer deposited by a Yellowstone eruption some 1.3 million years ago. Evidence indicates they died of acute silicosis and were then entombed in the very ash that caused their deaths. You can find lots of information here http://museum.unl.edu/research/vertpaleo/ashfall.html and here http://www.rhinoresourcecenter.com/index.php?s=1&act=pdfviewer&id=1378684580&folder=137 )
Is it possible for some of these volatile gases to ‘leak’ into the Yellowstone magma chamber? Sure it is. That’s how natural gas and petroleum get around, after all, not to mention groundwater. But there are limits; impermeable strata effectively block such migrations (which is how artesian wells occur).
And frankly, if we have a Yellowstone eruption, we got way bigger, and much more immediate, problems than trying to figure out how much carbon dioxide the thing is belching.