Geology Reference
In-Depth Information
Figure 1.4.
Composition and classification of common granitic rocks, according to Streckeisen (1967).
Wilhelmy (1958) published a monograph concerned specifically with granite forms, but with the
emphasis on climatic influences, and Godard (1977) has provided a delightfully concise account of
granitic terrains considered both from the geomorphological and geographical points of view, the
last contribution is the monographicbook of Ikeda (1998). Some years ago Twidale (1982) and
later Vidal Romaní and Twidale (1998) published an account of granite geomorphology which in
some measure forms the basis of the present review, though the scope of the present monograph is
broader and the analysis is both extended and updated.
1.3
OCCURRENCES OF GRANITE
Granitic rocks, i.e. granite
sensu stricto
or its close petrological relatives (see Fig. 1.4 and
Streckeisen, 1967) form the rafts of sial (a mnemonic derived from its two major components, silica
and alumina) materials that underlie the continents, though, enigmatically, granite occurs in such
oceanic islands as the Seychelles. Such occurrences can be explained in terms of plate tectonics,
the Seychelles having splintered from the host crustal block (what is now the Indian Peninsula) and
been left behind during the migration of the latter. Granites form extensive outcrops in the shield
lands that form the ancient nuclei in each of the continents (
Fig. 1.5),
and also of orogens. They
also protrude through the sediments of platform areas in small, but frequently notable, exposures.
The continental land masses occupy almost one third of the Earth's surface, though both on the
continents and on the continental shelves and slopes, veneers of other materials reduce exposures of
granite to about 15% of the continental areas, or about 4.5% of the Earth's surface.
Many granites originate in subduction zones where plates are melted and the resulting magma is
fractionated. Where continental plates collide and obduct pre-existing acid rocks are remelted cre-
ating migmatites or hybrid magmas. Thus, many granite masses result from the repeated emplace-
ment, magmatic assimilation and fractionation of crustal materials. They are acidic magmas which
did not reach the surface and cooled comparatively slowly, allowing time for relatively large crys-
tals to form. The larger masses of granite, and especially migmatites, however, are metamorphic.
They formed as a result of the transformation of other rocks, including argillites, by hot fluids and
gases spreading through the Earth's lithosphere. These processes have gone on throughout geolog-
ical time, and they continue to operate.
Regardless of their origins and petrological characteristics, these magmatic bodies are referred
to as plutons, after Pluto, the god of the underworld and of the dead in Greek and Roman mythology.
