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of the Earth and the temperatures at the base
of the mantle. On average the rotational pole
and the magnetic pole are close together, but
the instantaneous poles can be quite far apart.
The hotspot frame is no longer considered
valid. The fixed-hotspot hypothesis led to the view
that hotspots are anchored deep in the mantle
and may reflect a different kind of convection
than that which is responsible for large-scale con-
vection in the mantle. The apparent motions of
hotspots has been used to argue for
true polar
wander
(TPW) but this is not a well-founded
argument.
Density inhomogeneities in the mantle grow
and subside, depending on the locations of conti-
nents and subduction zones. The resulting geoid
highs reorient the mantle relative to the spin
axis. Whenever there was a major continental
assemblage in the polar region surrounded by
subduction, as was the case during the Devonian
through the Carboniferous, the stage was set for
a major episode of true polar wandering.
The outer layers of the mantle, including
the brittle lithosphere, do not fit properly on a
reoriented Earth. Membrane stresses generated as
plates move around the surface, or as the rota-
tional bulge shifts, may be partly responsible for
the breakup and dispersal of Pangea. In this sce-
nario, true polar wandering and continental drift
are intimately related. A long period of continen-
tal stability allows thermal and geoid anomalies
to develop. A shift of the axis of rotation can
cause plates to split. Horizontal temperature gra-
dients, along with slab pull, force continental
fragments to drift away from the thermal anoma-
lies that they caused. The continents drift toward
cold parts of the mantle and, in fact, make the
mantle cold as they override oceanic lithosphere.
Polar wandering can occur on two distinct
time scales. In a slowly evolving mantle the rota-
tion axis continuously adjusts to changes in the
moments of inertia. This will continue to be
thecaseaslongasthemajoraxisofinertia
remains close to the rotation axis. If one of the
other axes becomes larger, the rotation vector
swings quickly to the new major axis (Figure 6.6).
This is called
inertial interchange true
polar wander
(IITPW). The generation and
decay of thermal perturbations in the mantle
S. p.
w.
Catastrophic
jump
Pole position
x
2
x
1
Fig. 6.6
The principal moments of inertia shown on a cusp
catastrophe diagram. As the moments of inertia vary, due to
convective processes in the interior, the pole will slowly
wander unless the ratios of the moments
x
1
, and
x
2
, pass
through unity, at which point a catastrophe will occur, leading
to a rapid change in the rotation axis.
are relatively gradual, and continuous small-
scale polar wandering can be expected. The inter-
change of moments of inertia, however, occurs
more quickly, and a large-scale 90-degree shift
can occur on a timescale limited only by the
relaxation time of the rotational bulge. The rate
of polar wandering at present is much greater
than the average rate of relative plate motion,
and it would have been faster still during an
interchange event. The relative stability of the
rotation axis for the past 200 million years sug-
gests that the geoid highs related to hotspots
have existed for at least this long. On the other
hand, the rapid polar wandering that started
500 Ma may indicate that the Atlantic--African
geoid high was forming under Gondwana at the
time and had become the principal axis of iner-
tia. With this mechanism a polar continental
assemblage can be physically rotated to the equa-
tor as the Earth tumbles.
The southern continents all underwent a
large northward displacement beginning some-
time in the Permian or Carboniferous (280 Ma)
and continuing to the Triassic (190 Ma). During
this time the southern periphery of Gondwana
was a convergence zone, and a spreading cen-
ter is inferred along the northern boundary. One
