Geoscience Reference
In-Depth Information
RHYOLITE-ANDESITE
STRATO-VOLCANOES
ISLAND ARCS
CONTINENTAL
CRUST
Metasomatism
Accretion
BASALT OCEANIC CRUST
AND SHIELD VOLCANOES
GRANITE PLUTONS
Terrestrial
Retention
Weathering, Mass Wasting
and Erosion
Metamorphism
and
Deformation
PRIMARY
IGNEOUS
LOOP
SECONDARY
SEDIMENTARY
LOOP
SEDIMENTARY
ROCK
DEBRIS
Migmatization
B-Subduction
and Resorption
Sediment Entrainment,
Transport and Deposition
Magmatization
Lithification
Syngenesis
and
Diagenesis
MANTLE
ASTHENOSPHERE PERIDOTITE
Figure 12.1
The rock cycle, following Primary (igneous) and Secondary (sedimentary) loops. Rock material assemblages are
highlighted in boxes between the operational processes of the cycle. Metamorphism and deformation can occur in any part of
the loop.
The rock cycle
SYSTEMS
Rock-forming and recycling processes are tied closely to tectonic supercontinental cycles and constitute several
interconnected sub-cycles in their own right. Tectonic processes and morphotectonic landforms determine the general
location of each stage of the cycle, which operates through two interconnected loops. The primaryor
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 (
Table 12.1
).
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 lithologicalstyles 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.
attraction in
ionic bonds
or the sharing of electrons by two
atoms by
covalent bonds
. Precise patterns depend largely
on the size and internal packing of the anions and the
presence of suitably sized cations in the spaces between.
A snug, compact fit restores electrical equilibrium at the
points of contact. Electron sharing provides a symmetrical
framework which provides minerals with distinct,
crystalline structures. The geometric arrangement of
