The cores of mountain belts formed by continental collisions often contain metamorphic rocks, formed when sediments were buried in the collision and transformed by heat and pressure.
But the heat and pressure don’t happen simultaneously – rocks can be buried (and increase in pressure) much faster than they can heat up. When the rocks are not allowed to heat up significantly, this process can create blueschists, the high pressure/low temperature metamorphic rocks formed in subduction zones. In continental collisions, subduction stops, and the metamorphic rocks sit around at depth, heating up until a new geothermal gradient forms.
When I first read Philip England and Alan Thompson’s 1984 paper that modeled the temperature-pressure evolution of rocks in a continental collision, metamorphic rocks suddenly made sense. (And I understood why my interpretation of my senior thesis rocks was simplistic and wrong. Oh, well.) England and Thompson showed how individual metamorphic rocks could potentially record histories of burial, heating, exhumation, and cooling, and inspired lots of metamorphic petrologists to publish papers with diagrams that look like this:
But there was one part of the paper that stuck with me. There were some metamorphic rocks that they just couldn’t seem to model. Rocks that seemed associated with continental collisions, but which just got too hot for the model to explain. Maybe the crust had more radioactive elements, or there was some other, unexplained heat source.
It appears now that the problem was with one of their modeling parameters.
The ability of heat to flow through rock is determined by its thermal diffusivity (and a related parameter, its thermal conductivity). Heat flows easily through materials with a high thermal conductivity, like a metal cooking pot. Heat flows slowly through materials with a low thermal conductivity (like, oh, maybe a really thick steak in a cooking pot). Heat one side of a conductive material, and the other side also heats quickly. (Touch a hot pan. Ouch!) Heat one side of a material with a low conductivity, and you end up with a steak that’s burned on one side and still rare on the other. (Yuck.)
It’s hard to measure the thermal diffusivity (and conductivity) in rocks, and to tell what kinds of changes may occur as temperature or pressure change. So modelers typically make their math simple, and use one value to represent the thermal conductivity of the entire crust.
It turns out that the modelers might be really, really wrong.
Last month, there was a paper in Nature that described the measurement of thermal diffusivity with increasing temperature. It changes. It changes a lot. At temperatures around 575 °C, the thermal conductivity is only half of its value at Earth’s surface.
And that means that hotter rocks are better at trapping heat than colder rocks are. Which means that there’s a positive feedback that should occur.
For instance: imagine that some magma intrudes the crust. Heat immediately begins flowing from the magma into the surrounding rock, heating the crust and cooling the magma. The surrounding rock is metamorphosed, the magma crystallizes, and all is well. If the magma is basalt (which is hotter than granite), and there’s enough of it, then maybe the surrounding rock will melt to form a granitic magma. (And maybe if there’s a lot of heat, that granitic magma will make its way to the surface and erupt like Yellowstone or Chaiten. Boom!)
If hotter rocks trap heat, that means that the magma will take longer to cool, and the surrounding rock will get hotter (at least close to the magma). More of the surrounding rock might reach its melting point, so more granitic magma might form, and take longer to cool, and work its way to the surface.
Magmas aren’t the only thing that could heat rock and set off the thermal feedback. Brittle faults also melt rock during earthquakes, forming little veins of a rock called pseudotachylite. There’s also the possibility of other types of shear heating – the authors model the effect of shear heating during a continental collision, and argue that it could be responsible for the melting that created Himalayan granites. Radioactive decay also heats the crust, and the resulting temperature distribution should be different if hot rock traps heat better than cold rock does. And even if there isn’t a source of heat other than that coming from the mantle, the insulation of the hot lower crust should make it hotter than it’s normally thought to be. It’s like there’s a giant comforter down there, except the comforter gets warmer at higher temperatures. (Not the way I want my comforter to behave. Very uncomfortable.)
The implications of this are huge. The structural behavior of rocks changes with temperature: hot rocks are easier to deform than cold rocks are. If hot rocks stay hot, that means that rocks should be even weaker in the lower crust and in places where magmas intrude. It could make it possible for magmas to work their way through the crust more easily, whether in cracks or shear zones or in other models (some of which had seemed unlikely because of the amount of flow required). It also means that the rock below thick mountain belts should be hotter and weaker, even without extra heat flow from the mantle (due to lithosphere delaminating or otherwise blobbing off into the deeper mantle). I don’t know much about frictional heating of faults, but I wonder if thermal conductivity that changes with temperature could change rock behavior from strengthening during slip to weakening during slip – and therefore have implications for earthquake initiation (and for how big an earthquake becomes).
It also really screws up existing thermal models. Which means I need to do some thinking about what all this means for this paper I’m trying to write…
Whittington, A., Hofmeister, A., & Nabelek, P. (2009). Temperature-dependent thermal diffusivity of the Earth’s crust and implications for magmatism Nature, 458 (7236), 319-321 DOI: 10.1038/nature07818
Braun, J. (2009) Hot blanket in Earth’s deep crust Nature 258, 292-293.
England, P.C., & Thompson, A.B. (1984). Pressure–Temperature–Time Paths of Regional Metamorphism I. Heat Transfer during the Evolution of Regions of Thickened Continental Crust Journal of Petrology, 25 (4), 894-928.
Thompson, A.B., & England, P.C. (1984). Pressure–Temperature–Time Paths of Regional Metamorphism II. Their Inference and Interpretation using Mineral Assemblages in Metamorphic Rocks Journal of Petrology, 25 (4), 929-955.