Environmental Engineering Reference
In-Depth Information
of radioactive elements, so 95 per cent of its heat flux must come from greater depth.
Gradients are steepest in oceanic lithosphere, which conducts heat twice as efficiently,
and continental lithosphere is cooler than ocean lithosphere. Overall, oceanic crust
accounts for 75 per cent of global geothermal heat flux by virtue of its larger area and
superior rate. Volcanoes and hot spots, not surprisingly, experience the highest fluxes of
200-250 mW m
−2
. All of this indicates considerable thermal activity in the shallow
lithosphere. In particular, persistent contrasts between 'hot' oceanic and 'cool'
continental lithosphere show that the sea floor holds the key to crustal evolution via plate
tectonics.
CRUSTAL EVOLUTION: PLATE TECTONICS
From the great voyages of exploration in the Age of Discovery after AD 1450 it was
noted that many continental coastlines appeared to fit together, particularly those
bordering the Atlantic Ocean, and seemed to have become separated like the dispersed
parts of a jigsaw puzzle - perhaps by Noah's Flood! In 1912 Alfred Wegener
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. Deepsea drilling into ocean sediments and lithosphere
provides evidence of past environments, age-correlated by isotopic dating. Seismology
(see box, pp. 197-99) 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 1400° C isotherm which represents the minimum melting
