We’ve often heard it said that warmists will link any disaster with global warming with earthquakes being the quintessential disaster that has no connection with the weather. But personally, I’m not sure that we should be so dismissive. Although what follows is pure speculation. It has about the same scientific basis as: “CO2 must cause warming”, in that it may be very simplistic simply lacks empirical evidence to support it but it is worth more investigation.
If the surface of the earth warms, then, we’d expect that warming to penetrate beneath the surface of the earth and for the earth’s crust to warm as well. We can calculate how fast this might happen based on basic principle. Rock has a thermal conductivity of about 5 W/m.K (range is 2-7). Basalt rock has a heat capacity of 0.84 Kj/KG K and a density of 3011 kg/m3. If we assume a 10km thick crust and a 1C rise in temperature spread evenly through the crust, the heat conduction through this is:
5 x10-4 W/m2.
How long would this heat flow take to heat the crust? The weight of 10km of basalt is:
3011 x 10,000 = 3.011 x 107 KG/m2
Heat capacity per square meter of 10km of basalt rock is therefore
3.011 x 107 x 840 j/K = 2.5 x 1010
So a rough estimate for this mass of rock to heat by an average of 0.5C (1C at surface dropping to 0C change at 10km) with a typical value of heat conduction is :
(2.5 x 1010 j/K x 0.5K)/5 x10-4 W/m2. =2.5 x 1013 seconds. = 800,000 yrs
Whilst this is a very rough and ready calculation of the heating and falsely assumes a linear heat profile, it’s near enough for our purposes to show if there is an appreciable affect. But, even given the hand waving argument, there’s no way that a century of global warming is going to significantly warm the crust. But what about a much longer period change, like the last iceage. An iceage tyically has around 10C variation in temperature. That increased temperature difference brings down the time to see appreciable warming (an average 0.5C change) to perhaps 80,000 years and this is very comparible with the time between ice ages as there have been 4 in the last 400,000 years.
The Creaking earth
As everyone knows, warming causes expansion, so how much will the earth’s crust warm for a 0.5C change in crustal temperature? Expansion coefficient of basalt is about 5×10-6 per degree change. So the expansion of the crust at the equator of approximately 40,000km due to a 0.5C change in temperature is approximately:
0.5 x 40,000 x 1000 x 5×10-6 = 100m
For a 100,000 year heating cycle that amounts to:
100/100,000 = 1mm/year
Which is comparable with plate tectonic movements which are in the range of 0-100mm annually. At the very least, the heating and cooling of the crust as a result of the ice-age cycle, must significantly increase or decrease the rate of movement of plates.
But, there is a much bigger problem. Whilst we might have assumed an average temperature change of the order of 0.5C, the surface will heat disproportionately more by around 10C, which is 20x the “average” heating, and so 20x the expansion forcing a massive 20mm of plate expansion if this happened over the full 100k time period, but much greater if rock is closer to the surface.
If we repeat the calculations with a 1km slice, the rate of heat flow increases 10x, the thermal mass reduces to 1/10 and so instead of 80,000 years, we are talking around 800 years and for a 3.1km slice, the time to warm by 0.5 is of the order of 8000 years, the time since the last ice age.
And, here lies the real crux of the issue. Different layers of the crust will be heating at different rates. The result is that changing temperature on the surface will create stresses in the earth that build up with greater surface temperature change and with longer periods of cooler/hotter climate.
These stresses will sooner or later result in movement in the earth. We see very similar effects in all structures, and a notable example is a house. As the house heats (or cools), the various materials expand (or contract) at different rates. The result is that sooner or later the stress is enough to cause movement to start, and once the movement starts, it continues until the force is insufficient to make it move, then there is a pause as the temperature continues to rise (fall) until the stress is again enough to initiate movement. The result is a series of punctuated movements which we hear as creaking. Because of the low mass of the materials and the rapid heating/cooling, the sound is audible. But in the case of the earth, these punctuated movements are literally movements in the form of earthquakes.
Moreover, this thermal stress will move through the crust. Those layers nearest the surface temperature change will heat most rapidly tending to expand the surface layers and tending to exert a buckling force trying to force up the surface layers. But later, after the surface tensions have been “resolved” by earth quakes, those further down will begin to expand, creating an opposite buckling stress on the crust.
Worse still, the different composite rocks of the crust have different rates of expansion. For example, sandstone has twice the rate of expansion of basalt, so a 1km section of basalt will expand 5mm less than a 1km section of sandstone for each degree centigrade change. That might no sound much, but over tens of kilometres, and with a 10 centigrade change, the total difference is around a meter, more than enough to cause a major earthquake.
An interplay between plate tectonics &climate?
So far this has been a purely passive response of the earth to the changes in surface temperature, but is it possible that plate tectonics could act the other way to affect affect climate?
If the crust of the earth expands due to temperature increase, then that crust can’t just get bigger, part of the crust will be forced down under adjacent plates so that the rocks of the crust expand but the total size of the crust remains the same.These rocks will be forced down into the mantel and then subject to huge temperatures and pressures. As many of these rocks will be carbon rich, the result will be to liberate many carbon containing chemicals including huge quantities of CO2 which will eventually make its way into the atmosphere (it may also create oil … but that’s another grand theory) … obviously CO2 causes more warming (wink) so, the rate of warming will increase resulting in run-away global warming and further expansion and subduction of crust releasing more CO2 – leading to the end of the world – which as it hasn’t happened suggests it will never happen. However, it is possible that subduction and release of CO2 could magnify the effect of a small but sustained period of warming as caused by solar cycle variation.
However, sooner or later the expansion will stop, the temperature will start to decline (possibly by external forcing such as the earth’s tilt) and then the planet crust will start to shrink. A shrinking crust will cause “gaps” in the crust through which the mantle will come resulting in the expansion of areas like the mid-Atlantic ridge, this will continually fill in the gaps as the crust shrinks and the rate of subduction will stop, reducing the level of CO2 given out by volcanic activity until the earth goes back through another warming cycle.
There have been many theories postulated to explain tectonic plate movement. I’ve never been entirely convinced by these, but just assumed that they were the best guess of the experts. But if you do the math(s) it turns out that regular heating/cooling cycle like the ice-age cycle could be a significant driver of plate movement. Indeed, the expansion of the near surface layers may be more important than deeper layers, because expansion of the near surface layers causes a downward buckling that tends to assist subduction and/or because cooler surface layers could be stiffer and so exert more force compared to deeper layers which are hotter under extreme pressure so more likely to behave like a plastic than a rigid solid.
So is it possible that the main reason for plate movement is due to a cycle of expansion/contraction of plate due to cyclic temperature changes. Moreover, if it were possible that the main impact of CO2 were as cooling gas (which it is in part!!)- or some other similar feedback mechanism were found, then the release of CO2 as the crust expands could result in a delayed cooling (due to the time the surface change takes to reach significant depths) which then drives the cycle into shrinking, allows the reduction in CO2, which then leads to warming, until you have a regular CO2 climate cycle of about 100,000 years, causing the bulk of plate movement. It’s contrary to all perceived notions of CO2 as a warming gas, but as CO2 is also an excellent emitter of IR and higher concentrations in the upper atmosphere would help radiate heat. it is just possible – but testing such a theory is well above my pay grade.
Why the theory is unlikely
Unfortunately, under such a model, you would expect to see periods of subduction/volcanic release of CO2 interspersed with periods of little volcanic activity and ridge building at expansion zones. And whilst, I’ve not done any research, there’s nothing to suggest it is happening.
The earth’s crust must expand due to changes in surface temperature, however the rate of change is extremely slow and of the order of 10,000 to 1,000,000 years depending on the depth of penetration. This makes it very unlikely that even after a century of significant warming that there will be any perceivable impact. However, the inter-glacial period is long enough and there is a sufficiently large temperature change so that this will cause significant and measureable stresses on the earth’s crust. One would expect tectonic plate subduction zones to be most active in the warming phase, and expansion zones in the cooling phase.