Figure 2.11. Crust Thermal Model. This diagram shows a model steady-state geotherm (in red) for uniform crust based on crustal thickness and the values of selected parameters. Move the slider to change the thickness of the crust (think continent-continent collision). Use the small sliders to change the thermal conductivity of the crust (1-6 W/mK), radioactive heat production in the crust (0.1-5 μW/m3), and the heat flux into the crust from the mantle (1-50 mW/m2). Initial parameters are based on the review of the thermal state of the lithosphere by Furlong and Chapman (2013). Click on the "Show P Up" button if you prefer pressure to increase upward.
The surface heat flow and the temperature at the crust-mantle boundary are shown for the steady-state geotherm and will change as the input parameters change. Thickening the crust, with it's significant concentration of heat-producing, radioactive isotopes causes the temperature to increase at every crustal depth and leads to metamorphism. Plate tectonics produces significant crustal thickening by the collision of continents.
The time needed to reach a thermal steady-state following crustal thickening increases as the square of the thickness of the crust. Tectonic thickening also leads to erosional thinning and tectonic unroofing, so the steady-state geotherm shown for a specific crustal thickness gives temperatures higher than those likely to be attained by thickening alone. However, erosional thinning and tectonic unroofing during metamorphism can produce near isothermal decompression, effectively raising the temperature at each depth without additional heating.
For reference, temperatures and depth stabilities of the aluminosilicate minerals andalusite (And), kyanite (Ky), and sillimanite (Sil) are shown by black lines. Pressures for the crust are based on an average density of 2830 kg/m3. Check the "met facies" box in the upper right to show T-P ranges for various metamorphic facies. Check the "Geotherms" box to display the temperatures measured at the bottom of drill holes in the Gulf of Mexico as a function of depth with a blue curve labeled GM, an average continental crust geotherm by Pollack and Chapman (1977) with a brown curve labeled PC, and a geotherm by Hasterok and Chapman (2011) with a purple curve labeled HC. Both geotherm models produce a surface heat flow of 70 mW/m2, the current best estimate of the average continental heat flow, and model a non-uniform distribution of radioactive elements (more near the surface). The radioactivity symbol was taken from Wikimedia Commons. If you would like to see the mathematical model used to construct the steady=state geotherm, click on the "Show Math" button.
