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must overcome the local strength; this reason-
ing spawned the plume hypothesis . With the second
definition the global stress field, dictated by plate
boundary and subplate conditions, and cooling
plates, controls the locations of stress conditions
appropriate for the formation of dikes and vol-
canic chains, and incipient plate boundaries; the
underlying mantle is already at or near the melt-
ing point. This is the plate hypothesis .
Plates are not permanent; they are tempo-
rary alliances of subplates. Global plate reorga-
nization processes episodically change the orien-
tations of spreading centers, the directions and
speeds of plates, and redefine the plates. Plates
annex and lose territory to adjacent plates and
they break up or coalesce. New plate bound-
aries do not form all at once but evolve as age-
progressive chains of volcanoes. Volcanic chains
can also be extinguished if lateral compression
takes over from local extension. Volcanism can
be turned on and off by changing stress but it
is not so easy to turn off plume volcanism, or to
suddenly reduce the temperature of the mantle.
Important aspects of plate tectonics are the
necessity for ridges and trenches to migrate,
for triple junctions and boundary conditions to
evolve, and for plates to interact and to reconfig-
ure when boundary conditions change. Second
order features of plates and plate boundaries (e.g.
fracture zones, accreted terranes ,trans-
form faults, broad diffuse zones, swells, sutures,
lithospheric architecture and microplates) and
boundary reorganizations are actually intrinsic
and provide the key for a more general view of
plate tectonics than contained in the rigid plate-
fixedhotspotscheme .
high temperatures and flow more rapidly at high
stress, the lithosphere appears to be thicker at
low stress levels and short times than it does for
high stress levels and long times. Thus, the elastic
lithosphere is thick when measured by seismic or
postglacial-rebound techniques. At longer times
the lower part of the instantaneous elastic litho-
sphere relaxes and the effective elastic
thickness decreases. Thus, the elastic litho-
sphere is relatively thin for long-lived loads such
as seamounts and topography. Estimates of the
flexural thickness of the lithosphere range from
10--35 km for loads having durations of millions
of years. A more complete definition of the litho-
sphere is that part of the crust and upper mantle
that deforms elastically for the load and time scale in
question .
The viscosity and strength of the man-
tle depend on composition -- including water
content -- mineralogy and crystal orientation
as well as on temperature and stress. If the
upper mantle is compositionally layered, then
the lithosphere--asthenosphere boundary may
be controlled by factors other than tempera-
ture.Forexample,ifthesubcrustallayerisdry
olivine-rich harzburgite, it may be stronger at a
given temperature than a damp peridotite, or
a clinopyroxene--garnet-rich layer. If the latter
is weak enough, the lithosphere--asthenosphere
boundary may represent a chemical boundary
rather than an isotherm. Likewise, a change
in the preferred orientation of the dominant
crystalline species may also markedly affect the
creep resistance. The boundary may represent a
dehydration boundary -- wet minerals are weak.
The effective elastic thickness of the lithosphere
depends on many parameters but these do not
necessarily include the parameters that define
plates,
The many lithospheres
thermal
boundary
layers
and
cratonic
keels.
The layer that translates coherently, the plate
of plate tectonics , is often taken to be identical
with the elastic lithosphere . This is probably a valid
approximation if the stresses and time scales of
the experiment that is used to define the flexu-
ral thickness are similar to the stresses and time
scales of plate tectonics. It must be kept in mind,
however, that mantle silicates are anisotropic in
their flow and thermal characteristics, and that
The lithosphere is that part of the cold outer
shell of the Earth that can support stresses elas-
tically. The lithosphere is defined by its rheolog-
ical behavior. There are other elements of the
outer shell that involve lateral motions, buoy-
ancy, chemistry, mineralogy or conductivity and
these may or may not be part of the lithosphere.
Lithosphere is not the same as thermal boundary
layer or plate . Since mantle silicates flow readily at
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