Heat flow, thermal regime, energy budget of the Earth

1. Heat flow, thermal regime, energy budget of the Earth • Definitions • Measuring heat flow • Kelvin and the age of the Earth • Radioactivity • Continental heat flow (1) • Oceanic heat flow (1) • Global budget (1) • Continental vs oceanic heat flow • Plate model for the oceans • Continental heat flow (2) • Global budget (2) • Constraints on the temperature regime

2. Thermodynamics • 1st law: heat is a form of energy dU = dQ -PdV • ΔQ = C ρΔT • 2nd law (simple form): heat goes from hot to cold (Entropy can only increase unless work is done. Work can be extracted from system only if there are a cold and a hot source). • Conduction of heat (Fourier): q = -k grad T • Other mechanisms of heat transport (radiation, convection).

4. Some numbers • k (thermal conductivity) 3 W/m/K (average for most rocks) • Temperature gradient 20-30 K/km • Heat flow • If gradient does not decrease with depth, temperature at 100km >2000K and temperature at CMB > 60,000K

7. Kelvin and the age of the Earth

9. Radioactivity

12. Oceanic heat flow

13. Surprise !!! • Continental crust is radioactive and thick • Oceanic crust is thin with almost no heat generation • Oceanic heat flow < Continental heat flow ? • NO difference?

15. Energy Budget of the earth (1) • Birch (1951) • Total flux = 30 TW • Heat production in chondrites = 5 pW/kg • Mass of earth = 6 1024 kg • Coincidence?

32. Thermal model for continents