Geology Reference
In-Depth Information
In fresh igneous and metamorphic rocks, the interlocking mineral
grains are linked by strong chemical bonds. As illustrated in
Figure 5.5,
there is almost no void space, although there may be
some tiny
fluid inclusions trapped within mineral grains. As weath-
ering takes place close to the Earth
'
fluids pass through the
rock, it develops more voids as minerals decompose chemically and
weathering products such as clay are washed out. The bonds between
and within individual grains are weakened.
Figure 5.6
illustrates how
rock that starts off with a dry density of about 2.7Mg/m
3
(typical of
granite) becomes more and more porous so that by the completely
s surface and
Figure 5.5 Thin
section through
granite, illustrating
tightly interlocking
fabric. Width of
20mm.
grade V
Completely decomposed
but with original rock fabric
1.2
grade
IV
soil-like
grade VI
Residual soil
(collapsed)
2.0
III
II
2.7
Fresh
grade I
Degree of weathering
decomposed material, at which stage the density can be as low as 1.2 despite still having the
appearance of granite (fresh state 2.7). At that stage, the material is prone to collapse to a denser,
reworked, grade VI state. Based on Lumb (1962).
