Introduction

The phenomenon of isostasy concerns the response of the outer shell of the Earth to the imposition and removal of large loads. This layer, although relatively strong, is unable to support the large stresses generated by, for example, the positive weight of a mountain range or the relative lack of weight of an ocean basin. For such features to exist on the Earth's surface, some form of compensating mechanism is required to avoid the large stresses that would otherwise be generated.

Isostasy was first recognized in the 18th century when a party of French geodesists were measuring the length of a degree of latitude in Ecuador in an attempt to determine if the shape of the Earth corresponds to an oblate or a prolate ellipsoid. Plumb lines were used as a vertical reference in the surveying and it was recognized that a correction would have to be applied for the horizontal deflection caused by the gravitational attraction of the Andes. When this correction, based on the mass of the Andes above sea level, was applied, however, it was found that the actual vertical deflection was less than predicted (Fig. 2.27). This phenomenon was attributed to the existence of a negative mass anomaly beneath the Andes that compensates, that is to say, supports, the positive mass of the mountains. In the 19th century similar observations were made in the vicinity of the Himalaya and it was recognized that the compensation of surface loading at depth is a widespread phenomenon.

The presence of subsurface compensation is confirmed by the variation in the Earth's gravitational field over broad regions. Bouguer anomalies (Kearey et al., 2002) are generally negative over elevated continental areas and positive over ocean basins (Fig. 2.28). These observations confirm that the positive topography of

Figure 2.27 Horizontal gravitational attraction of the mass of the Andes above sea level would cause the deflection (c) of a plumb bob from the vertical (a). The observed deflection (b) is smaller, indicating the presence of a compensating mass deficiency beneath the Andes (angles of deflection and mass distribution are schematic only).

Figure 2.27 Horizontal gravitational attraction of the mass of the Andes above sea level would cause the deflection (c) of a plumb bob from the vertical (a). The observed deflection (b) is smaller, indicating the presence of a compensating mass deficiency beneath the Andes (angles of deflection and mass distribution are schematic only).

Figure 2.28 Inverse correlation of Bouguer anomalies with topography indicating its isostatic compensation.

continents and negative topography of oceans is compensated by regions at depth with density contrasts which are, respectively, negative and positive and whose mass anomaly approximates that of the surface features.

The principle of isostasy is that beneath a certain depth, known as the depth of compensation, the pressures generated by all overlying materials are every-

where equal; that is, the weights of vertical columns of unit cross-section, although internally variable, are identical at the depth of compensation if the region is in isostatic equilibrium.

Two hypotheses regarding the geometric form of local isostatic compensation were proposed in 1855 by Airy and Pratt.

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