The principle of hydrostatic equilibrium applied to
the Earth, referring to the position of the lithosphere essentially
floating on the asthenosphere, much like the level at which
ice floats on water. Isostatic forces are of major importance in
controlling the topography of the Earth’s surface. There are
several different models for how topography is supported,
referred to as isostatic models. The simplest models have
blocks of crust that are essentially floating as isolated blocks
in a fluid substrate (the asthenosphere) and are free to migrate
vertically and do not interact with neighboring blocks. There
are two main variations of these simple isostatic models. In
the Pratt model, crustal blocks of different density are
assumed to extend to a constant depth known as the depth of
compensation, and the height of the topography varies
inversely with the density of each block. Thus, high-density
oceanic crust resides at a lower level than lower-density continental
crust. In the Airy model, the level of isostatic compensation
varies for each block, but the crustal layer is assumed
to have a constant density. This way, thick blocks have high
topography and a thick root to compensate the topography,
whereas thin crustal blocks have subdued topography. Both of
these models are simplistic descriptions of a complex lithosphere,
and they were both derived in the 1700s before an
appreciation of plate tectonics. The Airy model is generally
more applicable than the Pratt model, but the Airy model
does not accommodate variations in crustal density that are
known to exist, such as that between continents and oceans.
Isostatic anomalies are variations in measured gravity
values from those expected using an assumed isostatic model
and depth of compensation. The anomaly indicates that the
model used or the compensation depth assumed needs to be
adjusted in the model.
See also GRAVITY ANOMALY.
isostatic anomaly See ISOSTACY.
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