Engineering Geology

E n g i n e e r i n g G e o l o g y
Metamorphic reactions are influenced by the presence of fluids or gases in the pores of the
rocks concerned. For instance, due to the low conductivity of rocks, pore fluids may act as a
medium of heat transfer. Not only does water act as an agent of transfer in metamorphism,
but it also acts as a catalyst in many chemical reactions. It is a constituent in many minerals
in metamorphic rocks of low and medium grade. Grade refers to the range of temperature
under which metamorphism occurred.
Two major types of metamorphism may be distinguished on the basis of geological setting.
One type is of local extent, whereas the other extends over a large area. The first type refers
to thermal or contact metamorphism, and the latter refers to regional metamorphism. Another
type of metamorphism is dynamic metamorphism, which is brought about by increasing
stress. However, some geologists have argued that this is not a metamorphic process since
it brings about deformation rather than transformation.
Metamorphic Textures and Structures
Most deformed metamorphic rocks possess some kind of preferred orientation. Preferred orientations
may be exhibited as mesoscopic linear or planar structures that allow the rocks to
split more easily in one direction than in others. One of the most familiar examples is cleavage
in slate; a similar type of structure in metamorphic rocks of higher grade is schistosity.
Foliation comprises a segregation of particular minerals into inconstant bands or contiguous
lenticles that exhibit a common parallel orientation.
Slaty cleavage is probably the most familiar type of preferred orientation and occurs in rocks
of low metamorphic grade (see also Chapter 2). It is characteristic of slates and phyllites
(Fig. 1.11). It is independent of bedding, which it commonly intersects at high angles; and it
reflects a highly developed preferred orientation of minerals, particularly of those belonging
to the mica family.
Strain-slip cleavage occurs in fine-grained metamorphic rocks, where it may maintain a regular,
though not necessarily constant, orientation. This regularity suggests some simple relationship
between the cleavage and movement under regionally homogeneous stress in the
final phase of deformation.
Harker (1939) maintained that schistosity develops in a rock when it is subjected to increased
temperatures and stress that involves its reconstitution, which is brought about by localized
solution of mineral material and recrystallization. In all types of metamorphisms, the growth
of new crystals takes place in an attempt to minimize stress. When recrystallization occurs
under conditions that include shearing stress, a directional element is imparted to the newly
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