Geoscience Reference
In-Depth Information
Chapter 6
The shape of the Earth
When Galileo let his balls run down
an inclined plane with a gravity which
he had chosen himself . . . then a light
dawned upon all natural philosophers.
I. Kant
it precisely a sphere or even an ellipsoid of rev-
olution. Although mountains, ocean basins and
variations in crustal thickness contribute to the
observed irregular shape and gravity field of the
Earth, they cannot explain the long-wavelength
departures from a hydrostatic figure.
The distribution of elevations on the Earth is
distinctly bimodal, with a peak near
Terrestrial planets are almost spherical because
ofgravityandtheweaknessofrockinlarge
masses. The largest departures from sphericity
are due to rotation and variations in buoyancy
of the surface and interior shells. Otherwise,
the overall shape of the Earth and its heat flow
are manifestations of convection in the interior
and conductive cooling of the outer layers. The
style of convection is uncertain. There are var-
ious hypotheses in this field that parallel those
in petrology and geochemistry. The end-members
are whole-mantle convection in a chemically
uniform mantle, layered convection with inter-
change and overturns, and irreversible chemi-
cal stratification with little or no interchange of
material between layers. Layered schemes have
several variants involving a primitive lower man-
tle or a depleted (in U and Th) lower mantle.
In a convecting Earth we lose all of our refer-
ence systems. The mantle is heated from within,
cooled from above and experiences secular cool-
ing.
0.1 km
representing the mean elevation of continents
and a peak near --4.7 km corresponding to the
mean depth of the oceans [see Google
Images
hypsometry
]. This bimodal character contrasts
with that of the other terrestrial planets. The
spherical
+
harmonic
spectrum
of
the
Earth's
topography shows a strong peak for
l
1, cor-
responding to the distribution of continents in
one hemisphere, and a regular decrease with
increasing n. The topography spectrum is simi-
lar to that of the other terrestrial planets. There
are small peaks in the spectrum at
l
=
=
3and
l
10, the latter corresponding to the dis-
tribution of subduction zones and large oceanic
swells.
The wavelength, in kilometers, is related to
the spherical harmonic degree l and the circum-
ference of the Earth (in km) approximately by
=
9
−
Wavelength
=
40 040
/
(
l
+
0
.
5)
Global
topography
and
gravity
provide
Thus, a wavelength of 10 degrees or 1100 km
corresponds to a spherical harmonic degree of
about 40.
Active orogenic belts such as the Alpine and
Himalayan are associated with thick crust, and
high relief, up to 5 km. Older orogenic belts such
as the Appalachian and Caledonian, because of
constraints on mantle dynamics.
Topography
Although the Earth is not flat or egg-shaped, as
previously believed at various times, neither is
