Geoscience Reference
In-Depth Information
Figure 5.7.
Left, profiles across the U.K. continental margin including Hatton
Bank, (a) bathymetry and (b) free-air gravity anomaly (D. McKenzie, personal
communication 2004); and right, profiles across the Mid-Atlantic Ridge, (c)
bathymetry and (d) free-air gravity anomaly. Since the large changes in bathymetry
are not accompanied by large changes in the gravity anomaly, both structures must
be compensated. (After McKenzie, D. and Bowin, C., The relationship between
bathymetry and gravity in the Atlantic Ocean,
J. Geophys. Res.
,
81
, 1903-15,
published 1976 American Geophysical Union. Reproduced by permission of
American Geophysical Union.)
5.5.4 Use of gravity anomalies to investigate isostasy
Gravity measurements are used to determine whether an area is in isostatic equi-
librium. If a region is in isostatic equilibrium, there should be no excess or lack
of mass above the compensation depth and hence almost no gravity anomaly.
To a first approximation this can be illustrated by using Eq. (5.35), the
Bouguer correction, of 42 mgal for a layer 1 km thick with a density of 1000
kg m
−
3
.Figures 5.7(a) and (b) show the bathymetry and free-air gravity anomaly
across part of the U.K. continental margin. The water depth decreases by
2km
across the margin with water (density 1000 kg m
−
3
) 'replaced' by crust (density
∼
∼
2700 kg m
−
3
). If this structure were uncompensated then gravity should increase
from ocean towards the continent by
2700
−
1000
1000
42
×
2
×
≈
140 mgal
It is clear from Fig. 5.7(b) that the change in gravity anomaly is much smaller
than this, indicating that the structure must be compensated. Figures 5.7(c) and
(d) show the Mid-Atlantic Ridge over which there is a change in water depth
