Environmental Engineering Reference
In-Depth Information
how dramatically abrupt geological events can be. Every stage of the rock cycle can be
located within the global mosaic of moving plates and morphotectonic landforms,
permitting a better understanding of Earth's dynamic evolution and a better assessment of
geological resources and hazards.
KEY POINTS
1 Over 2000 minerals, occurring as single or compound elements, are the fundamental
rock-forming units. Mineral species, which grow from melts or solutions, are
distinguishable by their composition, cation and anion bonds, crystal structure and
other properties. Silicates are the most common minerals, constructed of silicate
tetrahedral anions in various combinations or with other cations. Mineral assemblages
are formed by - and later separated, refined or reorganized in - a global rock cycle, at
specific sites and times defined by plate tectonics.
2 The cycle commences with the fractional crystallization of solid minerals from rising
partially melted asthenosphere peridotite (magma). Iron-magnesium-rich minerals
forming at high temperatures (1000°-1200° C) are replaced by increasing proportions
of silicate minerals as magma cools below 1000° C. Magma may intrude, cool and
solidify in older rocks to form plutons below the land surface or erupt as lava flows, or
as effusive or explosive volcanoes, to create new surface landforms.
3 Exposure to significantly different hydrothermal and mechanical conditions at the land
surface triggers denudation. This leads to a cascade of weathering and erosion
products which are deposited as sediments, in the short term on the land surface or
continental shelf, but most eventually reach the sea floor before being recycled.
Biogenic sediments form by the accumulation of dead organisms or the precipitation
of their dissolved derivatives.
4 All rocks experience mild diagenesis during and after formation, usually as chemical
and textural properties stabilize. They can also be altered geochemically to
progressively greater extents by metamorphism, metasomatism or migmatization in
higher temperature and pressure conditions. These processes stop short of remelt, but
where prevailing conditions exceed the melting point of rock and other surface
materials in subduction zones, the cycle is complete.
5 Rock is also deformed mechanically under high stresses, especially the stresses
associated with moving plates. Crustal shortening, extension, uplift and subsidence are
accompanied by folding, faulting and thrusting and the large-scale displacement of
original terranes.
FURTHER READING
Bell, F. G. (1998) Environmental Geology: principles and practice , Oxford: Blackwell. Another
fine example of the author's readable blend of pure and applied Earth science, varying our
perspective on geological, geomorphic and pedological processes in twelve chapters in which
human- environment impacts are never far away, before concluding with four more applied
chapters.
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