Earth’s Two Principal Components

The force exerted on an element of mass at the surface of the Earth has two principal components. One is due to the gravitational attraction of the mass in the Earth, and the other is due to the rotation of the Earth. Gravity refers to the combined effects of both gravitation and rotation. If the Earth were a nonrotating spherically symmetric body, the gravitational acceleration on its surface would be constant. However, because of the Earth’s rotation, topography, and internal lateral density variations, the acceleration of gravity g varies with location on the surface. The Earth’s rotation leads mainly to a latitude dependence of the surface acceleration of gravity. Because rotation distorts the surface by producing an equatorial bulge and a polar flattening, gravity at the equator is about 5 parts in 1000 less than gravity at the poles. The Earth takes the shape of an oblate spheroid. The gravitational field of this spheroid is the reference gravitational field of the Earth. Topography and density inhomogeneities in the Earth lead to local variations in the surface gravity, which are referred to as gravity anomalies.

The mass of the rock associated with topography leads to surface gravity anomalies. However, as we discussed in Chapter 2, large topographic features have low-density crustal roots. Just as the excess mass of the topography produces a positive gravity anomaly, the low-density root produces a nega-

tive gravity anomaly. In the mid-1800s it was observed that the gravitational attraction of the Himalayan Mountains was considerably less than would be expected because of the positive mass of the opography. This was the first evidence that the crust–mantle boundary is depressed under large mountain belts.

A dramatic example of the importance of crustal thickening is the absence of positive gravity anomalies over the continents. The positive mass 5.2 Gravitational Acceleration 355 anomaly associated with the elevation of the continents above the ocean floor is reduced or compensated by the negative mass anomaly associated with the thicker continental crust. We will show that compensation due to the hydrostatic equilibrium of thick crust leads in the first approximation to a zero value for the surface gravity anomaly. There are mechanisms for compensation other than the simple thickening of the crust. An example is the subsidence of the ocean floor due to the thickening of the thermal lithosphere, as discussed in Section 4–23.

Gravity anomalies that are correlated with topography can be used to study the flexure of the elastic lithosphere under loading. Short wavelength loads do not depress the lithosphere, but long wavelength loads result in flexure and a depression of the Moho. Gravity anomalies can also have important economic implications. Ore minerals are usually more dense than the country rock in which they are found. Therefore, economic mineral deposits are usually associated with positive gravity anomalies. Major petroleum occurrences are often found beneath salt domes. Since salt is less dense than other sedimentary rocks, salt domes are usually associated with negative gravity anomalies.