Geoscience Reference
In-Depth Information
KEY POINTS
1
Over 2,000 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
(1,000-1,200
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.
C) are replaced by increasing proportions of silicate minerals as magma cools below 1,000
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 with moving plates. Crustal shortening,
extension, uplift and subsidence are accompanied by folding, faulting and thrusting and large-scale
displacement of original terranes. Tectonic activity strongly influences all other Earth surface systems.
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.
Leeder, M. (1999) Sedimentology and Sedimentary Basins: from turbulence to tectonics, Oxford: Blackwell. This text
commences with a useful review of sedimentary properties and processes as a prelude to comprehensive cover of a
full range of the sedimentary environments. Good tectonic and climate contexts are provided and the whole is well
illustrated.
Park, R. B. (1997) Foundations of Structural Geology, third edition, Cheltenham: Thornes. This extensively revised edition
focuses on deformation and geological structures with good, if rather technical, explanations of folding, faulting, etc.
It concludes with modern interpretations of plate tectonic roles in deformation processes and zones.
Sigurdsson, H. (ed.) (2000) Encyclopedia of Volcanoes, San Diego, CA: Academic Press. Encyclopaedic in size and extent,
although with a conventional article-based structure, the 1,400 pages of this splendid text provide state-of-the-art and
extensively illustrated coverage of volcanic processes, landforms, hazards and applications.
