Geology Reference
In-Depth Information
Table 3.1 Size grades of sedimentary particles
Particle names
Particle diameter
Deposits
(phi) units
a
φ
mm
Unconsolidated
examples
Consolidated examples
Gravel
b
Boulders
<
−
8
>
256
Rudaceous deposits
Cobbles
−
6to
−
8
64-256
Till
Conglomerate, breccia,
gritstone
Pebbles
−
2to
−
6
4-64
Granules
−
1to
−
2
2-4
Sand
Very coarse sand
0 to
−
1
1-2
Arenaceous deposits
Coarse sand
1 to 0
0
.
5-1
Sand
Sandstone, arkose,
greywacke, flags
Medium sand
2 to 1
0
.
25-0
.
5
Fine sand
3 to 2
0
.
125-0
.
25
Very fine sand
4 to 3
0
.
0625-0
.
125
Silt
8 to 4
0
.
002-0
.
0625
Argillaceous deposits
Clay
>
8
<
0
.
002
Clay, mud, silt
Siltstone, claystone,
mudstone, shale, marl
Notes:
a
The phi scale expresses the particle diameter,
d
, as the negative logarithm to the base 2:
φ
=−
log
2
d
b
The subdivisions of coarse particles vary according to authorities
rocks, such as granite, it may produce convex hills known
as
exfoliation domes
.
vapour circulating through the still open pores and fis-
sures that comes into contact with the ice condenses
and freezes. The result is that ice lenses grow that push
up the overlying layers of soil. This process is called
frost heaving
and is common in glacial and periglacial
environments (cf. p. 66).
Frost action
Water occupying the pores and interstices within a soil
or rock body expands upon freezing by 9 per cent.
This expansion builds up stress in the pores and fis-
sures, causing the physical disintegration of rocks.
Frost
weathering
or
frost shattering
breaks off small grains
and large boulders, the boulders then being fragmented
into smaller pieces. It is an important process in cold
environments, where
freeze-thaw cycles
are common.
Furthermore, if water-filled fissures and pores freeze
rapidly at the surface, the expanding ice induces a hydro-
static or cryostatic pressure that is transmitted with equal
intensity through all the interconnected hollow spaces to
the still unfrozen water below. The force produced is
large enough to shatter rocks, and the process is called
hydrofracturing
(Selby 1982, 16). It means that frost
shattering can occur below the depth of frozen ground.
In unsaturated soils, once the water is frozen, the water
Heating and cooling
Rocks have low thermal conductivities, which means
that they are not good at conducting heat away from
their surfaces. When they are heated, the outer few
millimetres become much hotter than the inner por-
tion and the outsides expand more than the insides.
In addition, in rocks composed of crystals of different
colours, the darker crystals warm up faster and cool down
more slowly than the lighter crystals. All these thermal
stresses may cause rock disintegration and the formation
of rock flakes, shells, and huge sheets. Repeated heating
and cooling produces a fatigue effect, which enhances
the
thermal weathering
. The production of sheets by
thermal stress was once called exfoliation, but today
