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Heat flow, thermal regime, energy
budget of the Earth
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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
Thermodynamics
 1st law: heat is a form of energy dU = dQ -PdV
 ΔQ = C ρ ΔT
 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).
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
Radioactivity
Oceanic heat flow
Surprise !!!
 Continental crust is radioactive and thick
 Oceanic crust is thin with almost no heat generation
 Oceanic heat flow < Continental heat flow ?
 NO difference?
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?
Thermal model for continents