Geoscience Reference
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would expect that this configuration would be
consistent with a stationary or a southern migra-
tion of Gondwana, unless a geoid high centered
on or near Africa was rotating the whole assem-
blage toward the equator. The areas of very low
upper-mantle velocities in northeast Africa and
the western Indian Ocean may be the former site
of the center of Gondwana.
Thus, expanding the paradigm of continental
drift and plate tectonics to include continental
insulation and true-polar wandering may explain
the paradoxes of synchronous global tectonic and
magmatic activity, rapid breakup and dispersal
of continents following long periods of conti-
nental stability, periods of static pole positions
separated by periods of rapid polar wandering,
sudden changes in the paths of the wandering
poles, the migration of rifting and subduction,
initiation of melting, the symmetry of ridges
and fracture zones with respect to the rotation
axis, and correlation of tectonic activity and
polar wandering with magnetic reversals. Tum-
bling of the mantle presumably affects convec-
tion in the core and orientation of the inner core
and offers a link between tectonic and magnetic
field variations. Global plate reorganizations are
a necessary part of plate tectonics on a sphere.
New plate boundaries are often accompanied by
extensive volcanism and enriched magmas, pre-
sumably from the shallow mantle.
The largest known positive gravity anomaly
on any planet is associated with the Tharsis vol-
canic province on Mars. Both geologic and grav-
ity data suggest that the positive mass anomaly
associated with the Tharsis volcanoes reoriented
the planet with respect to the spin axis, placing
the Tharsis region on the equator. There is also
evidence that magmatism associated with large
impacts reoriented the Moon. The largest mass
anomaly on Earth is centered over New Guinea,
and it is also almost precisely on the equator.
The long-wavelength part of the geoid correlates
well with subduction zones, and these appear to
control the orientation of the mantle relative to
the spin axis. Thus, we have the possibility of a
feedback relation between geologic processes and
the rotational dynamics of a planet. Volcanism
and continental collisions cause mass excesses to
be placed near the surface. These reorient the
planet, causing large stresses that initiate rifting
and faulting, which in turn affect volcanism and
subduction. Curiously, Earth scientists have been
more reluctant to accept the inevitability of true-
polar wandering than to accept continental drift,
even though the physics of the former is better
understood.
