Geoscience Reference
In-Depth Information
fromthesinkeritself.Foranincreaseinviscos-
ity with depth, the deformation of the upper
boundaryislessandthenetgeoidanomalyis
positive.
Basalt chemistry exhibits lateral variations on
length scales of 150 and 400 km that may be
related to intrinsic heterogeneity of the man-
tle. Large variations in magma output along vol-
canic chains occur over distances of hundreds
to thousands of km; most chains -- often called
hotspot tracks
-- are less than 1000 km long. These
dimensions may be the characteristic scales of
mantle chemical and fertility variations. This
provides a straightforward explanation of the
order of magnitude variations in volcanic output
along long volcanic chains and along spreading
ridges.
Shorter wavelength features
There is a broad range of dominant wavelengths --
or peaks in the spectrum -- in the geoid and
bathymetry, ranging in wavelength from 160 km
to 1400 km. Although these have been inter-
preted as the scales of convection and thermal
variations they could also be caused by density
variations due to chemistry and, perhaps, par-
tial melt content, in the upper mantle. Several
of the spectral peaks are similar in wavelength to
chemical variations along the ridges. The shorter
wavelengths may be related to thermal contrac-
tion and bending of the lithosphere. The longer
wavelengths probably correspond to lithologic
(major element) variations in the asthenosphere
and, possibly, fertility and melting point vari-
ations. Intermediate-wavelength (400--600 km)
geoid undulations are continuous across fracture
zones and some have linear volcanic seamount
chains at their crests.
Profiles of gravity and topography along the
zero age contour of oceanic crust are perhaps
the best indicators of mantle heterogeneity.
These show some very long wavelength varia-
tions,
Interpreting the geoid
Quantitative interpretations of the geoid are
often based on relations such as
wavelength
vs. spherical harmonic degree;
the geoid
bears little relation to
global tectonic maps
or to present tectonic features of the Earth other
than trenches.
The Earth's largest pos-
itive geoid anomalies have no simple
relationship to continents and ridges
.
The Mesozoic supercontinent of Pangea, however,
apparently occupied a central position in the
Atlantic--African geoid high. This and the equato-
rial Pacific geoid high contain most of the world's
hotspots although there is little evidence that the
mantle in these regions is particularly hot. The
plateaus and rises in the western Pacific formed
in the Pacific geoid high, and this may have been
the early Mesozoic position of a subduction com-
plex, the fragments of which are now the Pacific
rim portions of the continents. Geoid highs that
are unrelated to present subduction zones may
be the former sites of continental aggregations,
the centers of large long-lived plates -- which
cause mantle insulation and, therefore, hotter
than normal mantle. The pent-up heat causes
uplift, magmatism, fragmentation, and the sub-
sequent formation of plateaus, aseismic ridges
and seamount chains. However, the effect must
be deep in order to also affect the long wave-
length geoid.
When the subduction-related geoid highs are
removed from the observed field, the residual
geoid shows broad highs over the central Pacific
and the eastern Atlantic--African regions. Like the
total geoid, the residual geoid does not reflect
1000 km, and also abrupt
changes. Ridges are not uniform in depth, grav-
ity or chemical properties. Complex ridge-plume
interactions have been proposed, the assumption
being that normal ridges should have uniform
properties. The basalts along midocean ridges are
fairly uniform in composition but nevertheless
show variations in major oxide and isotopic com-
positions.
Major and minor element chemistry shows
spectral peaks with wavelengths of 225 and 575 km
.In
general, one cannot pick out the ridge-centered
and near-ridge hotspots from profiles of grav-
ity, geoid, chemistry and seismic velocity. This
suggests that short-wavelength elevation anoma-
lies, e.g.
hotspots
, do not have deep roots or deep
causes. Some hotspots have low seismic velocities
at shallow depths, shallower than 200 km, consis-
tent with low-melting-point constituents in the
asthenosphere.
∼
5000 km and
∼
