Geoscience Reference
In-Depth Information
only mark plate boundaries but they antici-
pate new plate boundaries, changes in boundary
conditions and dying plate boundaries. If the
shallow mantle is close to the melting point or
partially molten, volcanoes have a simple cause,
stress.
The mode of convection in the Earth depends
on the distribution of radioactive elements and
physical properties and how these properties
depend on temperature and pressure and melt-
ing point. An Earth with most of the radioac-
tive elements in the crust and upper mantle,
and with strongly pressure-dependent thermal
properties will not behave as a uniform fluid
beingheatedonastove.Theseeffects,plus
continents and sphericity, break the symmetry
between the top and bottom thermal boundary
layers.
The main plate tectonic cycle is the ridge-
trench-slab system, primarily playing out in the
ocean basins. There is a secondary cycle involving
underplating, freezing at depth, delamination
and asthenospheric upwelling. A mafic lower
crust, if it thickens and cools sufficiently, will
convert to a high density mineral assemblage,
leading to a gravitationally unstable configura-
tion in which the lower crust can sink into the
underlying lower-density mantle, cooling it and
fertilizing it.
The solid Earth can rotate rapidly underneath
its spin axis through a process known as
true polar
wander
(
TPW
). The spinning Earth continuously
aligns its maximum moment of inertia with the
spin axis. Melting ice caps, plate motions, con-
tinental uplift and drift and ridge-trench anni-
hilations can all cause TPW. The magnetic and
rotational poles are good terrestrial reference
systems; the hotspot frame is not. But fertile
patches in the asthenosphere move more slowly
than the plates and plate boundaries and there-
fore
melting anomalies appear to be
relatively fixed.
