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Gravity anomalies

So far only the broad scale features of the variatoion of g have been discussed. The topographic features of the earth's surface together with variations in density within the earth's crust and mantle give rise to geographic anomalies of gravity from the mean value for a given latitude. The quantitative evaluation of such anomalies derives from the pendulum measurements. However, certain qualitative effects can be predicted for the broader scale topographic features represented by the continents.

It should certainly be expected that the plumb line will be deflected (relative to a radial line to the earth's center) towards to continental masses for those positions at or near the continental coastlines. Presumably, the continental land masses are of lesser density than the rock underlying the ocean sediment in order that an isostatic balance be maintained. However, this density difference is not sufficiently great to offet the effect of the difference in density of the water and adjacent land masses in the outer crust of the earth.

A deflection of the plumb line towards to continental masses at the coastline infers an upward warping of level surfaces along the coastlines. The ocean surface under static equilibrium is consequently drawn upward along the continental coastlines and is somewhat analogous to the meniscus effect of water in a glass vessel. (However, the forces involved in the latter situation are quite different.)


next up previous contents
Next: Tidal forces Up: Geostatics and Dynamics: The Previous: Gravity

Steve Baum
Mon Dec 1 08:50:29 CST 1997