Geoscience Reference
In-Depth Information
Sea-floor spreading
KEY PROCESSES
The Earth science revolution after 1960 confirmed that
sea-floor spreading
is the mechanism driving plate tectonics,
through the convection of new crust from the asthenosphere. Palaeomagnetic signatures reveal changes in Earth's
magnetic field, involving polar wandering and total reversal, and allow us to reassemble the former global location
of crustal rocks. Deep-sea drilling into ocean sediments and lithosphere provides evidence of past environments,
age-correlated by isotopic dating. Seismology (see box, pp. 209) confirms that narrow belts of intense earthquake
activity, girdling the earth for over 60,000 km, are located at intraplate boundaries. Bathymetry demonstrates that
their mid-ocean segments form submarine ridges. Satellite geo-positioning now provides accurate measurements
of the rates and directions of plate motion.
How, then, do convection and gravity forces enable these huge plates to move over distances of 10
3-4
km? Mantle
convection, stirred by local thermogenesis and heat conduction from the core, appears to be the dominant process.
Convection in fluids occurs as material is heated, becomes less dense and therefore more buoyant. Rising to the
surface, it spreads, cools and eventually sinks as it becomes denser than the continuing warm plumes. High-
temperature rock flows rapidly as molten
lava
only when free of confining pressures at Earth's surface. Pressure
increases with temperature as depth increases, raising the melting point of any particular mineral assemblage. The
'solid' nature of the mantle thus reduces normal fluid motion to an extremely slow crystalline creep. However, the
pressure-temperature balance in the adjacent asthenosphere permits a partial melt of up to 10 per cent of its mass,
giving it the texture of a stiff, granular slush whose lower viscosity and ductility accelerate convection.
Crucially, it also provides the basis for decoupling at the lithosphere-asthenosphere boundary at the depth of the
1,400C isotherm, which represents the minimum melting point of upper mantle rock at the pressures found there.
Since continental lithosphere is lighter and cooler than oceanic lithosphere, the boundary is found at mean depths
of 100-150 km and 70-80 km below each respectively. It was thought originally that plates were moved by
viscous
drag
, coupling the base of the lithosphere to the asthenosphere, as convection cells in the latter spread out over a
rising
mantle plume
and rafted crust around the Earth. The cooling limb of the cell would eventually return into the
mantle. This convective drive does not account for enough of the thermal energy dissipated at the surface, however,
and is augmented by gravitational effects at plate margins. In essence, the ascending convection current develops
a
thermal bulge
in lithospheric slab, which then slides away under gravity. Cooling as it moves and ages, it eventually
subsides into the subjacent warmer asthenosphere and develops a gravitational pull. This mechanism decouples the
lithosphere from the asthenosphere, as melting reduces friction at their common boundary and
ridge push
or
slab
pull
gravity forces are applied at opposite ends of the lithospheric slab (
Figure 10.5
).
This combination of thermally direct and gravity-induced forces accounts for the motion of plates and imparts direction
and velocity to them. Simultaneously, ascending currents transport the raw material of new crust with them and
descending currents recycle older crust back into the mantle. These processes, acting at opposing ends of the 'crustal
conveyor belt', broadly maintain a continuity of mass, since it appears that Earth is neither expanding nor contracting.
If anything, global continental crust is growing extremely slowly at the expense of ocean crust. An estimated 12-15
km
3
of crust is recycled annually in this way.
MID-OCEAN RIDGE
e
THERMAL
BULGE
MANTLE PLUME
Lithosphere
Asthenophere
Slab pull
triggered by mantle convection currents.
CONVECTION CURRENTS
