Environmental Engineering Reference
In-Depth Information
cliffs. The basalts were extruded onto a land surface near the
North Atlantic mid-ocean ridge, as sea-floor spreading
separated Greenland and Europe 55-58 Ma ago.
Photo: Ken Addison.
known individuals such as Mount Kenya, Kilimanjaro and Ruwenzori over 5 km high. In
Europe the Eifel volcanoes of Germany are associated with the Rhine graben.
Sea-floor spreading also leaves
passive margins
on the trailing edge of continents
which often form major escarpments, upwarped either by the initial crustal elevation
producing the rift or a subsequent isostatic or thermal (epeirogenetic) response. Passive
margin escarpments, reaching elevations of 1·8-3·5 km, are best developed on the eastern
seaboards of southern Brazil (Serra da Mantiqueira), southern Africa (Drakensburg
range) and Australia (the Great Escarpment), both sides of the Red Sea and the west coast
of India (the Western Ghats). The Piedmont Fall Line marks a lower, persistent passive
margin on the south-east coast of the United States. In all cases, escarpments source
continental-margin sedimentation on their seaward side.
THE GEOLOGICAL EVOLUTION OF BRITAIN
Few modern continental areas illustrate crustal evolution better than the British Isles. Into
their diminutive 150,000 km
2
are crammed rocks representative of half Earth's history
and structures of all three
Phanerozoic
orogens. Origins are traced through rocks which
tell a story of fragments lost, gained and surviving as terranes were assembled and
dismantled in the long drift across Earth's surface. Britain's familiar coastline is less than
10 ka old and dependent on global sea level. The story is elaborated by three vital strands
of the science of
stratigraphy
.
Litho-
stratigraphy and
bio
-stratigraphy reveal the physical
and biological character of past environments and
chrono
-stratigraphy provides a time
scale, based on the decay of constituent radioactive minerals. The early history is very
obscure but we have a clearer view of the past 0·5 Ba, in which fragments originating 60°
south
of the equator were joined by others as 'Britain' drifted to its modern position at
50°-60° north.
En route
, subtropical Silurian coral reefs were joined by Devonian and
Permo-Triassic desert sands 'sandwiching' Carboniferous equatorial swamp forests, and
the whole was subjected most recently to Quaternary glaciation (Figure 10.13).
CONCLUSION
Earth is of almost unimaginable age and yet its modern character and geological
processes can be traced directly to its astronomic origins. Many geological processes are
imperceptibly slow, measured against our own life spans, but earthquakes and volcanic
eruptions are sharp reminders of Earth's relentless evolution. The accretion of a cocktail
of planetary matter over 4·6 Ba ago set in train enduring processes of chemical
fractionation, heat generation and cooling which progressively refine our planet's
constituent parts. Some processes are necessarily familiar to us - the weathering of
