CSIR NET Dec 2015 Q96: Geophysics

The surface heat flow contributed by a crustal layer with uniform radiogenic heat production per unit mass (A, in W/kg), density (rho), and thickness (t) is given by the simple heat-balance relation Q = A * rho * t (heat generated throughout the layer's mass column, per unit surface area, assumed to flow entirely to the surface in this idealized steady-state problem). Substituting A = 2.6x10^-11 W/kg, rho = 2900 kg/m^3, and t = 6000 m: Q = 2.6x10^-11 x 2900 x 6000 = 2.6x10^-11 x 1.74x10^7 = 4.524x10^-4 W/m^2 = 0.4524 mW/m^2, which rounds to approximately 0.45 mW/m^2, matching option 2.

Option 1 (0.8 mW/m^2) overestimates the heat flow, inconsistent with the direct multiplication of the given heat production rate, density, and thickness. Option 2 is correct: Q = A*rho*t = 2.6x10^-11 x 2900 x 6000 ≈ 0.45 mW/m^2. Option 3 (0.6 mW/m^2) and option 4 (0.55 mW/m^2) are both close but do not match the precise arithmetic product of the given values, likely arising from minor rounding or unit-conversion errors rather than the correct direct computation.