Geoscience Reference
In-Depth Information
12
Rock formation
As tectonic processes drive plates around Earth's surface
they are constantly gaining and shedding material, so that
the rough form of continental land masses may be
recognized beyond one or more supercontinental cycles
but the specific materials may have changed. Ancient
cratons resist this most of all, whereas oceanic plate
is much more transient, as we have seen. Each plate in
this global mosaic is rather like a ship which picks up
commodities and crew at one port and leaves them at
another, processing raw and waste materials en route and
undergoing repairs and structural refits as job require-
ments change. A ship's plates, rivets and fittings as its
working life ends are not all those of its maiden voyage.
In a similar way, individual
lithologies
or rock types
accreted to continental plates are subjected to alteration
or erosion and whole terranes may be added, removed or
relocated. These changes may appear to occur randomly.
In practice, rock type, process and resultant rock condition
and landforms are intimately linked through a rock cycle
whose components also share specific plate and global
locations (
Figure 12.1
).
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 composi-
tion, and a crystalline structure. The existence of well over
2,000 known minerals makes their study a formidable
proposition until it is appreciated that just two ele-
ments, 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) magne-
sium (Mg) and titanium (Ti); oxygen alone forms 95
per cent by
volume
. Lithospheric minerals reflect these
concentrations.
Mineral structure
We need to consider atomic structures in order to
understand minerals further. Atoms comprise a nucleus
of protons and neutrons inside an electron shell. Protons
and electrons have positive and negative electrical charges
respectively and, when balanced, give atoms electrical
neutrality. However, atoms exchanging electrons create an
electrical imbalance and become known as
ions
. Net loss
of negatively charged electrons leaves a smaller, positive
cation
and net gain leaves a larger, negative
anion
. Mineral
structure is created by three-dimensional arrangements of
suitable anions and cations in a repeated geometric
pattern. They are held together primarily by
electrostatic
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
