Environmental Engineering Reference
In-Depth Information
THE ROCK CYCLE
systems
Rock-forming and recycling processes are tied closely to supercontinental/Wilson
tectonic cycles and constitute a cycle in their own right. Individual plate tectonic
processes and morphotectonic landforms determine the general location of each stage of
the cycle, which operates through two interconnected loops. The
primary
or
igneous
loop
is, at its simplest, concerned only with cycling oceanic lithosphere between magma
extrusion and resorption. However, resorption accompanied by ocean water, mineral and
organic debris triggers alternative igneous processes which also penetrate adjacent
continental crust. The
secondary
, or
sedimentary
, loop exposes magmas accreted to and
retained by continental crust to exogenetic
weathering
and
erosion
.
Transportation
and
deposition
of the debris forms sediments and sedimentary landforms in terrestrial, coastal
and marine environments.
Either sequence may be interrupted at any time and material can be relocated to any
other point in the cycle, before eventually re-entering the primary loop via subduction
and remagmatization. At any point in either loop, rock material may also be subjected to
irreversible change or metamorphism by significant increases in temperature or pressure,
usually through volcano-tectonic activity. These processes represent, simultaneously, a
geochemical cycle
of continued fractionation and a rock cycle of particular
lithological
styles and masses of rock materials. Both cycles are important to geographers and
environmental scientists and are integrated here. A concern for rocks and their
geochemistry underpins our understanding of not only rock formation but also soils,
nutrient cycles and lithospheric material exchanges with the biosphere, atmosphere and
oceans - all of which are dynamic, evolving material systems sourced from Earth's rocks.
ROCK
-
FORMING MINERALS AND PROCESSES
ROCK-FORMING MINERALS
Rock-forming minerals are the crucial link between broadly
homo
geneous magmas
derived from the upper mantle and particular
hetero
geneous assemblages which form a
distinct
lithology
or rock type. Lithology is measured by geochemical and textural
character - rather like a particular dish derived from the general stock of ingredients in a
restaurant. We start with upper mantle geochemistry and the formation of a suite of
igneous fractionates, determined largely by temperature/pressure environments. Each
mineral species
has unique chemical and physical properties, related to its elemental
composition, and a crystalline structure. The existence of well over 2000 known minerals
makes their study a formidable proposition until it is appreciated that just two elements,
oxygen (O) and silicon (Si) form 75 per cent of the lithosphere by
mass
, with a further 24
per cent formed (in declining abundance) by aluminium (Al), iron (Fe), calcium (Ca),
sodium (Na), potassium (K) magnesium (Mg) and titanium (Ti); oxygen alone forms 95
per cent by
volume
. Lithospheric minerals reflect these concentrations.
