Page 55 - Global Tectonics

42 CHAPTER 2

the top of the lower mantle or mesosphere (Karato,
1998). However, this latter interpretation is highly
controversial and awaits testing by continued investi-

gation. If such a zone of horizontal flow does exist
then convection in the mantle probably occurs in
layers and does not involve the whole mantle (Section
12.5.3).

2.11 ISOSTASY

2.11.1 Introduction 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 phenomenon of isostasy concerns the response The observed deflection (b) is smaller, indicating the

of the outer shell of the Earth to the imposition and presence of a compensating mass deficiency beneath

removal of large loads. This layer, although relatively the Andes (angles of deflection and mass distribution

strong, is unable to support the large stresses gener- are schematic only).
ated by, for example, the positive weight of a moun-
tain 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 rec-
ognized 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 defl ection
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 Figure 2.28 Inverse correlation of Bouguer anomalies
to say, supports, the positive mass of the mountains. In with topography indicating its isostatic compensation.
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 wide- continents and negative topography of oceans is com-
spread phenomenon. pensated by regions at depth with density contrasts
The presence of subsurface compensation is con- which are, respectively, negative and positive and whose
firmed by the variation in the Earth’s gravitational fi eld mass anomaly approximates that of the surface

over broad regions. Bouguer anomalies (Kearey et al., features.
2002) are generally negative over elevated continental The principle of isostasy is that beneath a certain
areas and positive over ocean basins (Fig. 2.28). These depth, known as the depth of compensation, the pres-

observations confirm that the positive topography of sures generated by all overlying materials are every-