Environmental Engineering Reference
In-Depth Information
oceanic crust which acts as the conveyor belt in plate motion. The persistence of plate
tectonics over geological time accounts for the extreme youth of ocean crust, with a mean
age of only 55 Ma and none older than 200 Ma; and the relative youth
Table 10.1 The nature, time scales and relevance
of geological and related processes.
Processes
Time scale
(years)
10 6-8
Macro-scale geological processes drive the ever-changing global configuration of
ocean basins, continents and mountain ranges, and in turn:
10 1-9
• Created, and continue to modify, the composition of atmosphere and oceans
through volcanic outgassing and rock weathering
10 1-8
• Cycle rock material from its formation, through degradation to reformation
10 6-7
• Create global seismic belts which locate most earthquake and volcanic activity
10 5-7
• Create random variations in the pattern of surface materials and relief, influencing
solar radiation exchange and, hence, global climate
10 1-7
• Channel global ocean currents and disturb meridional and zonal atmospheric
circulation, with major impacts on climate systems and weather events
10 1-7
• Drive geomorphological processes through vertical displacement of continental
crust and sea levels
10 1-7
Meso-scale geomorphological processes etch and shape the continental crust into
distinct landforms and landform assemblages or landsystems
10 1-4
Micro-scale pedological processes drive the formation of soils and, through them:
• Physical support and attachment sites for flora and fauna
• Principal inorganic source and cycling components of the biosphere nutrient cycle
Geological, geomorphological and pedological processes collectively also create:
• All our nuclear and fossil fuels, metal and non-metalliferous ores, building stones
and aggregates
• The variety of surface materials and landsystems which provide the foundation of
cultural landscapes
• The substrate on which we construct our buildings, urban and industrial regions,
farmland and other economic, cultural and recreation systems
of Earth's crust as a whole, with 98 per cent less than 2·5 Ba and 90 per cent less than 0·6
Ba old. Continental crust is, on average, fifteen to twenty times older than oceanic crust
because it is recycled more slowly. Fragments of Archaean Earth, 3·7-4·3 Ba old, survive
in parts of Canada, Greenland, Australia and South Africa. Modern continents are a
collage of quite different crustal terranes , or fragments of widely dispersed origin and
form, which reflect the repeated accretion and break-up of older crust (Figure 10.1).
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