Heat flow from Earth's interior — conductive vs advective transport, borehole profiles
Typical gradient: 25–30°C/km. Heat sources: radioactive decay (U, Th, K in crust ~40%), mantle heat flux (~40 mW/m²). Surface heat flow: ~65 mW/m². Moho (~35 km): ~400–600°C. Precambrian cratons: cool, 10–15°C/km.
Gradient steeper near ridge (~100°C/km), flattens with age as crust cools. Heat flow: ridge 200–400 mW/m² → old ocean 40–50 mW/m². Hydrothermal circulation removes ~30% of oceanic heat. Moho (~7 km): ~200–300°C.
Conductive: Fourier's law q = −k∇T. Thermal conductivity k ≈ 2–3 W/m·K for crust. Advective: moving fluids/magma transport heat 100–10,000x more efficiently. Convection dominates in mantle (Ra >> Ra_c).
Drilled 12.26 km (1970–1994, USSR). Expected 100°C at depth; found 180°C — gradient higher than predicted. Encountered unexpected geology. Halted by heat (260°C at base). Deepest artificial point on Earth for 20 years.