Thermal expansion can release quite some force as is seen in bridges and railroad tracks that are deformed due to hot weather. I am wondering whether thermal expansion of the Earth's crust due to global warming can similarly lead to seismic activity, especially at subduction zones where one crustal plate slides under another one.
I have searched the scientific literature, but was unable to find any articles on the topic, while it sounds quite straight forward to me. Did really no one have this idea before, or was it already debunked in 1960?
|Dear journalists, This is not a press release; I do not like science by press release. While I am a scientist, I have no special expertise in geology. Please only write about this if the standards of your publisher are so low that you would also write about it if you next-door neighbour would have this idea. In other words, please wait until there is a scientific publication on this. Even better would be to wait until there are several papers, but I realise that that is illusory. |
Dear "skeptics", I did not write that your SUV is responsible for the seismic atomic genocide of the the Japanese population. This post is only written to bring the idea under the attention of the right scientists. At this stage, it is very likely that the idea is wrong and that nothing will happen.
Dear colleagues, the reason to write this post is to attend someone with the skills to investigate it. It is thus highly appreciated if you could forward it to someone who may have the right skills. I guess two skills are needed: 1) modelling of the warming of the crust and 2) dynamical modelling of the crust expansion. If you have one of these skills, please contact me, maybe someone with the second skill also responded. If you have both skills or know someone, feel free to act as if it is your own idea. Ideas are cheap and this back-of-the-envelope computation was quickly made. The real innovative step is deciding which idea is worth spending a year of your time on to make it into science. It would be nice if after publication you could act as if a colleague attended you to this post and send me a copy of the article. I am curious.
Back-of-the-envelope computationThe length of the Equator is roughly 40x106 metres. And my high-school tables book gives thermal expansion coefficients for various stone types between 4 and 12x10-6 K-1. So for a one degree temperature change one would expect that the Earth's crust would expand by 160 to 480 metre. (More accurately, one would use the size of a plate not the circumference of the Earth. On the other hand, the projected temperature change is larger than one degree.) For the sake of the argument, let's say that it is about 100m. One thing I do not know is whether the crust is sufficiently brittle/solid (like a railway track or bridge that deforms by heat) or whether there are air and liquid pockets and cracks which could absorb the expansion. Furthermore, maybe the additional pressure would mainly increase the width of the crust not its length or the crust would just crumple a little more.
The next question would be how fast the Crust would expand. I would expect that whether you get additional earthquakes depends on how fast the thermal expansion is relative to tectonic plate movements already happening. Only then could the thermal expansion be a significant source of additional stress at the plate boundaries. This speed depends on how fast the heat is taken up by the surface and on how deep the brittle top layer is (does the full crust need to be warmed?). If the speed of heating of the "full" crust is in the order of 10,000 years, the contribution of expansion to tectonic plate movements would be in the order of 100m/10,000a = 0.01 m/a = 10 mm/a, which is 5 to 10% of current movements at faster plates.
The continental crust is about 10 km thick. Thus the temperature signal should penetrate the crust with a speed of around 1 meter per year. One meter is about the depth at which you no longer see an annual cycle in soil temperatures. Thus it seems to be possible, but at the margin of the possible.
An important limiting factor for the thermal conductivity is likely the fluffy humus layer, which isolates well as it contains many air pockets. The thermal conductivity of solid rock is much larger. At the end of an interglacial, just after the retreat of the ice sheets, the humus layer would still be thin, as it is just being build up. Such a situation would make the mechanism stronger. Thus, this could be an interesting period to study. Also nowadays the thickness of the humus layer is decreasing due to erosion. If the spatial variations in effective thermal conductivity are sufficiently large, it may be needed to also take lateral heat fluxes into account, for instance to model heat flows from mountains (with little or no isolating humus) to its surroundings.
Scientific literatureI have a hard time believing no one thought of thermal expansion of the Earth's crust before. Do you know of any pointers to the literature? I did find some articles on the relationship between volcanoes and climate. It may be easier to study volcanoes as earthquakes as volcanoes leave an ash layers, which can be dated. Naturally, volcanic eruptions can cool the climate up to several years. In the other direction, there are publications that rapid warming can cause volcanic activity (Rampino et al, 1979. A recent interesting paper by Kutterolf et al. (2012) studied the influence of temperature fluctuation within a ice age on volcanoes in the Pacific “Ring of Fire” based on volcanic data from deep sea drilling cores. Their assumed mechanism is that fast melting glaciers and rising sea levels change the gravitational forces on the crust and produce regional stresses in it.
Help wantedAre able to compute how fast the crust would heat up or the reaction of the Earth's crust to warming? I would be very grateful for any help. Do you maybe know a colleague that may be interested in looking into this mechanism? Please forward this post.
ReferencesKutterolf, S., M. Jegen, J. X. Mitrovica, T. Kwasnitschka, A. Freundt, P. J. Huybers. A detection of Milankovitch frequencies in global volcanic activity. Geology, DOI: 10.1130/G33419.1, 2012.
Rampino, M.R., S. Self, R.W. Fairbridge. Can rapid climatic change cause volcanic eruptions? Science, 206, pp. 826-829, 1979.