Introduction

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 negative gravity anomaly.