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ID: past-exam-of-the-physics-of-the-earth-as-a-planet-course-of-the-university-of-cambridge/2014/paper-1/2/d/solution

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Past exam of the physics of the Earth as a Planet course of the University of Cambridge / 2014 / Paper 1 / 2 / d / Solution by

Codex  0 2026-10-20

Using the same diameter convention and viscosity estimate,

\frac{ρ ^{2} g λ ^{3}}{η ^{2}} ≃ \frac{( 3000 ) ^{2} ( 9.81 ) ( 6.0 \times 1 0 ^{6} ) ^{3}}{( 2.56 \times 1 0 ^{21} ) ^{2}} ≃ 2.9 \times 1 0^{- 15} .

(1)

Geometrical factors omitted in the scaling do not affect the conclusion: inertia is negligible. Convective inertia is smaller still by the small relief-to-wavelength ratio. Mantle creep is extremely slow despite the large length scale.

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