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7.1 Isostatic subsidence 195
Figure 7.1. A plot of the Earth’s crustal thickness made by USGS (1999). The continental crust is
normally thick at the center of the continents and under high mountains, like the Andes and Tibet,
and thin at the margins.
w
w 1 2
s
l
l
a 1
a 2
Figure 7.2. The pressure at the same depth in the ductile asthenosphere remains the same after load-
ing the rigid lithosphere with sediments. The sediments replace some of the water (a=asthenosphere,
l=lithosphere, s=sediments and w=water).
same at the same depth in the ductile asthenosphere after loading, because of its fluid-like
behavior. See Figure 7.2. The same depth before and after deposition of sediments is
w 1 + l + a 1 = w 2 + s + l + a 2 (7.1)
where w 1 is the water depth before loading, w 2 is the water depth after loading, s is the
sediment thickness and l is the thickness of the lithosphere. The thickness of a part of the
