Page 60 - Global Tectonics

THE INTERIOR OF THE EARTH 47

Figure 2.33 Free air, Bouguer and Airy isostatic anomalies over an idealized mountain range (a) in perfect isostatic
equilibrium, (b) with 70% isostatic compensation, (c) with 30% isostatic compensation, (d) uncompensated. Densities in
−3
Mg m .

tion and indicate which type of mechanism is in opera- A more sophisticated test of isostasy involves the
tion. This is because the compensation occurs at a spectral analysis of the topography and gravity anoma-
relatively deep level and the differences in the anomalies lies of the region being studied (Watts, 2001). The rela-
produced by a root/antiroot (according to the Airy tionship between gravity and topography changes with
hypothesis) or by different density units (according to wavelength. Moreover, the way in which it changes
the Pratt hypothesis) would be very small. Moreover, varies for different isostatic models. Thus by determin-
the gravity anomalies over most regions contain short ing the frequency content of the gravity and topo-
wavelength components resulting from localized, graphic data it is possible to determine the type of
uncompensated geologic structures that obscure compensation pertaining in the area. The technique

the differences in the regional field arising from the also yields an estimate of T e , the elastic thickness of the
different forms of compensation. lithosphere (Sections 2.11.4, 2.12).