Page 119 - Fundamentals of Geomorphology

102 STRUCTURE

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