Consider a magma chamber in the Earth's crust ... models show that 1-2 km surrounding this chamber are heated above the brittle-ductile transition zone. Let's say that you know the yield strength of the crust in general (not heated) is 15 MPa ... and after that, rocks under heavy confining pressure behave plastically anyway. Let's say they behave as an ideal plastic. If you want to create a plastic model of the area, how would you define the extra plasticity of that hot zone? A lower yield strength? How much lower? Is there a function of temperature you know of as a standard?