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variation of igneous and metamorphic activity, and sed-
imentary facies. In fact, it explains all major aspects of
the Earth's long-term tectonic evolution (e.g. Kearey and
Vine 1990). The plate tectonic model comprises two
tectonic 'styles'. The first involves the oceanic plates and
the second involves the continental plates.
would probably not develop fractures (transform faults).
But, although convection is perhaps not the master driver
of plate motions, it does occur. There is some disagree-
ment about the depth of the convective cell. It could
be confined to the asthenosphere, the upper mantle, or
the entire mantle (upper and lower). Whole mantle con-
vection (Davies 1977, 1992) has gained much support,
although it now seems that whole mantle convection and
a shallower circulation may both operate.
The lithosphere may be regarded as the cool surface
layer of the Earth's convective system (Park 1988, 5).
As part of a convective system, it cannot be consid-
ered in isolation (Figure 4.3). It gains material from the
asthenosphere, which in turn is fed by uprising material
from the underlying mesosphere, at constructive plate
boundaries. It migrates laterally from mid-ocean ridge
axes as cool, relatively rigid, rock. Then, at destructive
plate boundaries, it loses material to the asthenosphere
and mesosphere. The fate of the subducted material is
not clear. It meets with resistance in penetrating the
lower mantle, but is driven on by its thermal inertia
and continues to sink, though more slowly than in the
upper mantle, causing accumulations of slab material
(Fukao et al. 1994). Some slab material may eventu-
ally be recycled to create new lithosphere. However, the
basalt erupted at mid-ocean ridges shows a few signs of
being new material that has not passed through a rock
cycle before (Francis 1993, 49). First, it has a remark-
ably consistent composition, which is difficult to account
for by recycling. Second, it emits gases, such as helium,
that seem to be arriving at the surface for the first time.
Equally, it is not 'primitive' and formed in a single step
by melting of mantle materials - its manufacture requires
several stages. It is worth noting that the transformation
of rock from mesosphere, through the asthenosphere, to
the lithosphere chiefly entails temperature and viscos-
ity (rheidity) changes. Material changes do occur: partial
melting in the asthenosphere generates magmas that rise
into the lithosphere, and volatiles enter and leave the
system.
Oceanic plate tectonics
The oceanic plates are linked into the cooling and recy-
cling system comprising the mesosphere, asthenosphere,
and lithosphere beneath the ocean floors. The chief cool-
ing mechanism is subduction. New oceanic lithosphere
is formed by volcanic eruptions along mid-ocean ridges.
The newly formed material moves away from the ridges.
In doing so, it cools, contracts, and thickens. Eventu-
ally, the oceanic lithosphere becomes denser than the
underlying mantle and sinks. The sinking takes place
along subduction zones . These are associated with earth-
quakes and volcanicity. Cold oceanic slabs may sink well
into the mesosphere, perhaps as much as 670 km or
below the surface. Indeed, subducted material may accu-
mulate to form 'lithospheric graveyards' (Engebretson
et al . 1992).
It is uncertain why plates should move. Several driv-
ing mechanisms are plausible. Basaltic lava upwelling at
a mid-ocean ridge may push adjacent lithospheric plates
to either side. Or, as elevation tends to decrease and slab
thickness to increase away from construction sites, the
plate may move by gravity sliding. Another possibility,
currently thought to be the primary driving mechanism,
is that the cold, sinking slab at subduction sites pulls
the rest of the plate behind it. In this scenario, mid-
ocean ridges stem from passive spreading - the oceanic
lithosphere is stretched and thinned by the tectonic pull
of older and denser lithosphere sinking into the mantle
at a subduction site; this would explain why sea-floor
tends to spread more rapidly in plates attached to long
subduction zones. As well as these three mechanisms,
or perhaps instead of them, mantle convection may be
the number one motive force, though this now seems
unlikely as many spreading sites do not sit over upwelling
mantle convection cells. If the mantle-convection model
were correct, mid-ocean ridges should display a consis-
tent pattern of gravity anomalies, which they do not, and
Continental plate tectonics
The continental lithosphere does not take part in
the mantle-convection process. It is 150 km thick and
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