Structure of Rock - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 6

Igneous rocks, also referred to as “magmatic rocks”, are formed from the solidification
of the magma. They are divided into plutonic, dyke and volcanic rocks, depending on
the depth and rate of their cooling (Fig. 2.1 and 2.2). Volcanic and dyke rocks are formed
by the cooling of extruded magma either in voids and cracks of other rocks in various
depths (dyke rocks) or at the earth's surface (volcanic rocks). Plutonic rocks are formed
at greater depth. It is also convenient to subdivide igneous rocks into acid, intermediate
and basic rocks depending on the amount of silica in their composition (Farmer 1968).
Sedimentary rocks are largely formed by the sedimentation and subsequent cementing
of mechanically or chemically degraded rocks from all three main groups. They are di-
vided into three subgroups according to their formation: clastic, chemical and organic
sedimentary rocks. Clastic sediments may be further subdivided into pyroclastic sedi-
ments of volcanic origin and detrital sediments, and they are classified according to the
grain size of their components as in the usual classification of granular soil. Important
subgroups of the detrital sediments are arenaceous rocks such as sandstone, and argil-
laceous rocks such as mudstone and shale. Chemical sediments are deposits from wa-
ter-based solutions in the sea or inland waters. Important representatives are salt rocks
and calcareous rocks such as dolomite and limestone (Fig. 2.2).
Figure 2.2 Main rock groups, examples