Geology Reference
In-Depth Information
Table 10.4 A summary of the rock mass properties obtained from volcanic
rocks in the Snowdonia volcanic group a and from the island of Tenerife b .
Values of intact laboratory compressive strength and the Hoek Brown variable
m i are also included for reference.
Rhyolite 1
Tuff 1
Basalt 2
Phonolite 2
Rock type
Micro
granite 1
RMR
52 - 74
47 - 74
57 - 79
48 - 60
38 - 60
Laboratory
compressive
strength (MPa) 3
175
150
250
150
175
Hoek Brown
variable m i
25
±
5 3
±
5 2
±
3 5
±
5 5
±
5
Rock mass
compressive
strength (MPa)
12 - 41
7 - 35
23 - 78
8 - 16
6 - 19
Rock mass tensile
strength (MPa)
(
0.29) to
(
0.18) to
(
0.3) to
(
0.2) to
(
0.1) to
(
1.5)
(
1.4)
(
1.55)
(
0.5)
(
0.5)
3 Values taken from Brown (1981).
not require this (or other) variables to be assessed and while relatively untested
may ultimately prove more the reliable rating system in a volcanic context.
10.5 Summary
Igneous rocks are often hydrothermally altered and are on occasion host to eco-
nomically significant mineral deposits including copper, tin, tungsten, platinum
and clays. They can also act as reservoirs for hydrocarbons where fracturing
has resulted in elevated permeability and porosity. In the field alteration zones
around hydrothermal fractures and veins should be recorded along with struc-
tural data (dip and strike), spacing, relationship to larger structures (for example,
faults) vein width and number density. Any associated metaliferous minerals
should be identified (Table 10.1) and their occurrence noted. Where possible
the mode of fracture (tensile or shear) should be discerned along with any
cross-cutting relationships so that a detailed picture can be constructed to help
shed light on the emplacement and post-emplacement deformation history.
Intact rock, measured in a laboratory, is a relatively homogeneous and con-
tinuous material. However, on larger scales in the field, the notion of continuity
does not hold. Instead the structure of a rock mass on the mesoscale is highly
discontinuous and heterogeneous and it is the presence of discontinuities that
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