Geoscience Reference
In-Depth Information
Figure 5.12.
Computer
modelling of convection
in the upper mantle. This
model assumes that the
mantle has constant
viscosity and is heated
from below and cooled
from above. (a)
Circulation of the mantle
material in convection
cells. (b) Temperature in
the model mantle
contoured at 100-
◦
C
intervals. There are two
rising limbs at the
left-hand edge and the
centre and two regions of
sinking material at centre
left and right. (c) Variation
in the depth of the ocean
caused by the convection.
(d) Variation in the
gravitational acceleration
(gravity anomaly) caused
by the convection. (e)
Variation in the height of
the sea surface (geoid
height anomaly) caused
by the convection. (From
McKenzie
et al
.(1980).)
5.7 Flexure of the lithosphere and the viscosity
of the mantle
5.7.1 The lithosphere as an elastic plate
In the theory of plate tectonics the thin lithospheric plates are assumed to be rigid
and to float on the underlying mantle. On a geological timescale the lithosphere
behaves elastically and the mantle behaves as a viscous fluid, whereas on the
very short seismic timescale both behave as elastic solids. The study of the
bending or flexure of the lithosphere which results from its loading by mountain
chains, volcanoes and so on enables us to estimate the elastic properties of the
lithosphere. Additionally, the rate of recovery or rebound which occurs when a
load is removed is dependent on the viscosity of the underlying mantle as well as
