Geology Reference
In-Depth Information
profiles or as contoured (isogal) maps. Interpretation of
the latter may be facilitated by utilizing digital image
processing techniques similar to those used in the display
of remotely sensed data. In particular, colour and shaded
relief images may reveal structural features that may not
be readily discernible on unprocessed maps (Plate 5.1a).
This type of processing is equally appropriate to mag-
netic anomalies (Plate 5.1b; see for example Lee
et al.
1990).
Table 6.2
Approximate density ranges (Mg m
-3
) of some
common rock types and ores.
Alluvium (wet)
1.96-2.00
Clay
1.63-2.60
Shale
2.06-2.66
Sandstone
Cretaceous
2.05-2.35
Triassic
2.25-2.30
Carboniferous
2.35-2.55
Limestone
2.60-2.80
Chalk
1.94-2.23
Dolomite
2.28-2.90
Halite
2.10-2.40
6.9 Rock densities
Gravity anomalies result from the difference in density,
or
density contrast
, between a body of rock and its
surroundings. For a body of density
r
1
embedded in
material of density
r
2
, the density contrast
Dr
is given by
Granite
2.52-2.75
Granodiorite
2.67-2.79
Anorthosite
2.61-2.75
Basalt
2.70-3.20
Gabbro
2.85-3.12
Gneiss
2.61-2.99
Quartzite
2.60-2.70
D
rr r
=-
1
2
Amphibolite
2.79-3.14
The sign of the density contrast determines the sign of
the gravity anomaly.
Rock densities are among the least variable of all geo-
physical parameters. Most common rock types have
densities in the range between 1.60 and 3.20 Mg m
-3
.
The density of a rock is dependent on both its mineral
composition and porosity.
Variation in porosity is the main cause of density
variation in sedimentary rocks. Thus, in sedimentary
rock sequences, density tends to increase with depth,
due to compaction, and with age, due to progressive
cementation.
Most igneous and metamorphic rocks have negligible
porosity, and composition is the main cause of density
variation. Density generally increases as acidity decreas-
es; thus there is a progression of density increase from
acid through basic to ultrabasic igneous rock types. Den-
sity ranges for common rock types and ores are present-
ed in Table 6.2.
A knowledge of rock density is necessary both for ap-
plication of the Bouguer and terrain corrections and for
the interpretation of gravity anomalies.
Density is commonly determined by direct measure-
ments on rock samples. A sample is weighed in air and in
water.The difference in weights provides the volume of
the sample and so the dry density can be obtained. If the
rock is porous the saturated density may be calculated by
following the above procedure after saturating the rock
with water. The density value employed in interpreta-
tion then depends upon the location of the rock above or
below the water table.
Chromite
4.30-4.60
Pyrrhotite
4.50-4.80
Magnetite
4.90-5.20
Pyrite
4.90-5.20
Cassiterite
6.80-7.10
Galena
7.40-7.60
NB. The lower end of the density range quoted in many texts is
often unreasonably extended by measurements made on samples
affected by physical or chemical weathering.
It should be stressed that the density of any particu-
lar rock type can be quite variable. Consequently, it is
usually necessary to measure several tens of samples of
each particular rock type in order to obtain a reliable
mean density and variance.
As well as these direct methods of density determina-
tion, there are several indirect (or
in situ
) methods.These
usually provide a mean density of a particular rock unit
which may be internally quite variable.
In situ
methods
do, however, yield valuable information where sampling
is hampered by lack of exposure or made impossible
because the rocks concerned occur only at depth.
The measurement of gravity at different depths be-
neath the surface using a special borehole gravimeter
(see Section 11.11) or, more commonly, a standard
gravimeter in a mineshaft, provides a measure of the
mean density of the material between the observation
levels. In Fig. 6.14 gravity has been measured at the
surface and at a point underground at a depth
h
im-
mediately below. If
g
1
and
g
2
are the values of gravity
