Geology Reference
In-Depth Information
Figure 1.5.
Tectonic regions of the continents: granitic rocks occur overwhelmingly in the shields and orogens.
Plutons are classified partly on their size and shape, but mainly according to whether they are pre-
dominantly discordant or concordant in respect of structures in the host mass. Batholiths are mas-
sive intrusive bodies, and are oval or shield-shaped in plan. They appear either to maintain or
increase in diameter in depth, and they eventually may cut out in depth, suggesting a globular or
lenticular overall form. Most batholiths are complex for they consist of several individual plutons.
Thus, the Sierra Nevada batholith, in California, is about 60,000 km
2
in area and comprises per-
haps 200 individual bodies emplaced over some 100 Ma. Stocks are small batholiths, convention-
ally less than 100 km
2
. Diapirs are discordant globular bodies terminating below in the pipe or
conduit through which the magma was emplaced. Laccoliths are intrusive masses located at shal-
low depths that have caused doming of the host rocks, phacoliths sheet-like bodies located in the
crests of anticlines, lopoliths lenticular masses with sunken centres, gneiss domes, structural
domes in granitic rocks, and so on. Sills are tabular bodies emplaced parallel or concordantly with
such structures as bedding or foliation in the country rock, whereas dikes (or dykes) are of similar
geometry but are discordant with structures in the host rock. Ring complexes are oval, circular or
arcuate groups of sills or dikes related to an intrusive centre. Though they vary in shape and size,
all these bodies originated deep in the crust and are therefore plutonic.
1.4
GRANITE - DEFINITION AND COMPOSITION
The word granite is derived from the Italian granulo, meaning a grain or particle, and was first
used by Caesalpus in 1596 (in Twidale, 1982). From the Renaissance onwards the term was used
of all crystalline rocks. Nowadays, the word granite is widely recognised in a general context, so
much so that laymen commonly refer to any crystalline rock as granite. The rock itself is also famil-
iar to the general public, for granite is a beautiful ornamental stone and, polished, is widely used as
a facing on major buildings and monuments. Many headstones of graves are also of granite.
Various definitions have been suggested (Read, 1957), but granites can be taken as being plu-
tonic rocks, in which individual crystals are a few millimetres diameter, are clearly visible to the
naked eye, and are described as macrocrystalline. Granite contains at least 10% and up to 40% free
