Geology Reference
In-Depth Information
Fig. 14.7
Mechanism of isostatic compensation. Starting
from a hypothetical initial situation where the elevation
is zero everywhere, a lateral increase of crustal thickness
in the continental crust (
green regions
, density
¡
c
) will
determine
upward
motion to compensate the decreased
weight of the column. Similarly, the denser oceanic crust
(
¡
o
>
¡
c
) will subside to compensate the excess weight
with water (
blue regions
, density
¡
w
). In this example,
the lithospheric mantle has constant thickness and density
¡
l
>
¡
c
,
¡
o
.Theair(
yellow regions
) has negligible density.
Finally, it is assumed that the asthenosphere has density
¡
a
>¡
l
Fig. 14.8
Geoid anomalies,
N
, and dynamic topography
of a fluid mantle layer that incorporates small density
anomalies. The geoid undulations result (
thick solid line
)
from the combined effect of static density contrasts (
thin
solid line
) and topography variations (
dashed lines
)at
the
upper
and
lower
boundaries of the layer induced by
mantle flows (
dotted lines
)
However, in some regions the elevated or
depressed topography cannot be explained in
terms of isostasy. A classic example is given by
the East African and Ethiopian plateaux (Moucha
and Forte
2011
). Hager (
1984
) and Richards
and Hager (
1984
) were the first to propose
a mechanism that today is known as
dynamic
topography
.
The basic idea behind the concept of dynamic
topography is illustrated in Fig.
14.8
.Itstarts
from the simple observation that mantle con-
vection should cause deformation and topogra-
phy variations at the Earth's surface. In fact,
sinking density anomalies pull downwards man-
tle material located above them. At the same
time, they push the mantle downwards at their
front. Similarly, a rising density contrast pulls
or pushes mantle material upwards. Both these
situations determine flexure of the upper man-
tle boundaries and surface topography changes.
The same mechanism would cause variations
in the topography of the CMB associated with
