Environmental Engineering Reference
In-Depth Information
Archaean crust. Its outer terrestrial fractionates formed an upper layer of volcanic and
intrusive granitic, low-melting-temperature products on a metamorphosed granitic base.
Only 15 per cent survives, which emphasizes the role of erosional, sedimentary and
metamorphic processes in reworking primary continental crust without removal from the
continental system; and offscrape, accretion and B-subduction recycling at destructive
plate boundaries and remelt of continental lithosphere over hot spots.
Continental crust is more extensive than is suggested by the ratio of land to sea area. It
accounts for 39 per cent of all crust, or 0·6 per cent of Earth's volume, whereas
continents cover only 29 per cent of the surface. The difference is explained by the
presence of
epicontinental seas
, such as the North Sea, the Black Sea, parts of the
Mediterranean and Hudson Bay, on continental crust and a portion of the
continental
shelf
(see Chapter 11). Continental architecture consists primarily of stable ancient
cratons
swathed in the remnants of six to eight orogens of various, younger ages (Figure
10.12). Cratons are stable cores around which continents form and reform with only
minor 'bruising', contrasting with the high geothermal flux, land surface elevation and
geomorphic activity of the Cenozoic orogens. Despite their stability and location, usually
over cool spots, the cratons have endured sustained erosion since their Archaean or
Proterozoic formation and are now areas of modest overall and relative relief. Between
stable cratons and unstable orogens, epeirogenesis and isostatic adjustments create lesser,
long-term disturbances of continental lithosphere. Their principal effect is to adjust
gravitational energy inputs and hence erosion rates, with elevated plateaux forming
sediment sources and subsidence zones creating basins which act as terrestrial sediment
traps.
What are the consequences of rifting, which initiates the Wilson cycle and eventually
forms new oceans, for the continents themselves? Emerging mid-ocean rifts may
propagate across ocean-continent boundaries. The East African rift, for example, is the
landward extension of the triple junction formed with the Red Sea and Gulf of Aden. If it
fails to propagate further and develop into a full midocean ridge system it will join a long
list of
aulacogens
. Although classed as failed rifts, they still play a significant role in
continental architecture, often forming major topographical depressions which channel
world-scale rivers and their sediment fluxes. The North Sea basin and Rhine graben
extension is another example. Continental rifting in which significant crustal extension or
thinning occurs has other consequences. Asymmetrical rifting may gener-
