The document discusses isostasy and models of crustal compensation. It describes Airy and Pratt models of isostatic equilibrium, where topography is supported by either lateral density variations within the crust (Pratt model) or variations in crustal thickness (Airy model). Continental collision leads to the highest elevations on Earth because the thick, buoyant continental crust uplifts vast areas when plates converge.
6. The actual deflection for the Himalayas was less than expected , due to a deficiency of mass beneath the mountains. Introduction to Geophysics-KFUPM Expected deflection of a plumb bob ( highly exaggerated ), due to the attraction of the mass of a mountain range .
8. Models of Isostasy Isostatic Equilibrium - Compensation Introduction to Geophysics-KFUPM
9. Airy Model Airy proposed that crust is thicker beneath mountains and thinner beneath the oceans. Excess mass under the oceans from a shallower, high density mantle. Mass deficiency beneath mountains due to crustal root. Mass Excess Mass Deficit Introduction to Geophysics-KFUPM
10. Pratt Pratt proposed that observation could be explained by lateral changes in density within a uniform thickness crust. Introduction to Geophysics-KFUPM
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14. P= ρ g z Where: P =pressure at the point within the earth ρ = average density of the material above the point G = acceleration due to gravity (9.8 m/s 2 ) Z =depth Hydrostatic pressure is the pressure exerted on a point within a body of water. Similarly, pressure at a given depth within the Earth can be viewed as a Lithostatic pressure Local Isostasy Introduction to Geophysics-KFUPM Fig 8.18 of Lillie
15. P= ρ g h Where: P= pressure exerted by crustal block ρ = density of the crustal block h = thickness of the crustal block The pressure exerted by a crustal block for the models (Pratt/Airy) can be expressed as: Introduction to Geophysics-KFUPM Local Isostasy
17. P/g = ρ 2 h 2 = ρ 3 h 3 = ρ 4 h 4 = ρ 5 h 5 Dividing out a constant gravitational acceleration (g): For Pratt model shown above, ρ 5 < ρ 4 < ρ 3 < ρ 2 < ρ 1 Where ρ 1 is the density of Earth’s mantle. Introduction to Geophysics-KFUPM Local Isostasy : Pratt Model