Geology Reference
In-Depth Information
10.3.1 Fracture formation
Veins and fractures form by several mechanisms and are controlled by the
local and regional stress fields acting on the rock mass. We have discussed
the formation of cooling joints in plutons in Chapter 7. In brittle rock, fracture
propagation is influenced by grain boundaries, smaller cracks and other pre-
existing discontinuities. Increased stress at the fracture tip may activate new vein
formation within a 'fracture process zone' surrounding the propagating crack.
Fractures in rock can be classified into two main kinds and it is important to be
able to identify each kind in the field as they contain information about the style
of deformation responsible for their formation.
Mode I
fractures are formed by
tensile stresses and are symmetrical in that little or no displacement occurs either
side of the crack opening.
Mode II
fractures form under shear and are asymmet-
rical (Figure 10.7). In plutonic rocks both kinds can occur as primary fractures
during cooling, or afterwards during regional tectonic deformation. Identification
of primary Mode II shear fractures formed during cooling can yield important
information about the local stress field in operation during pluton emplacement.
10.3.2 Permeability and porosity
The most highly altered igneous rocks occur in active geothermal areas where
fracture development has led to elevated permeability and porosity, allowing large
quantities of hot fluids to circulate through the rock mass. It is thus not depth
beneath the surface but high fracture density and interconnectivity that allow
hydrothermal fluids to permeate and alter the composition of igneous rocks.
Hydrothermal alteration of individual minerals can result in the formation
of small voids that form a secondary porosity in some altered igneous rocks.
While more commonly associated with sediments undergoing digenesis, sec-
ondary porosity can nonetheless lead to a significant increase in the non-fractured
permeability of hydrothermally altered igneous rocks. Figure 10.8 shows the
range of total porosity in selected igneous rocks where the total is a function
of both fracture and void (diffusive) porosity. Porosity and permeability have
been measured in volcanic rocks and is a measure of their fracturing, primary
gas content and vesicularity (for example, Chapters 4 and 5). Not surprisingly
scoria is the most porous and permeable volcanic material (Figure 10.9).
10.4 Rock Mass Classification
At its most basic level, a volcanic edifice is made up from discrete layers of
different rock types cut through on a range of scales by pervasive and pen-
etrative discontinuities including faults, fractures and older contact surfaces
(Figure 10.10). In addition, different scales of heterogeneity mean that assign-
ing an overall strength is problematic. Characterisation of rock mass properties
can be further hindered by long time gaps in volcanic activity that result in
