Geology Reference
In-Depth Information
mainly determined by air temperature fluctuations, but
they are modulated by the thermal properties of the
ground-surface materials, vegetation cover, and snow
cover.
process arises are debatable. Two groups of hypotheses
have emerged: the frost-pull hypotheses and the frost-
push hypotheses. In essence,
frost-pull
involves all soil
materials rising with ground expansion on freezing, fol-
lowed by the collapse of fine material on thawing while
larger stones are still supported on ice. When the ice
eventually melts, the fine materials support the stones.
Frost-push
consists of flowing water tending to collect
beneath a stone and on freezing lifting it. On melt-
ing, finer particles fall into the void and the stone falls
back on top of them. The frost-push mechanism is
known to work under laboratory conditions but applies
to stones near the surface. The frost-pull mechanism
is in all likelihood the more important under natural
circumstances.
Frost weathering and shattering
Frost weathering was covered in an earlier section
(p. 51). Many periglacial landscapes are carpeted by
angular rock debris, the origin of which is traditionally
attributed to
frost shattering
. However, frost shatter-
ing requires freeze-thaw cycles and a supply of water.
Field investigations, which admittedly are not yet large
in number, indicate that such conditions may not be as
common as one might imagine. Other processes, such
as hydration shattering and salt weathering (in arid and
coastal sites), may play a role in rock disintegration. It
is also possible that, especially in lower-latitude glacial
environments, the pervasive angular rock debris is a relict
of Pleistocene climates, which were more favourable to
frost shattering.
Mass displacement
Frost action may cause local vertical and horizontal move-
ments of material within soils. Such
mass displacement
may arise from cryostatic pressures within pockets of
unfrozen soil caught between the permafrost table and
the freezing front. However, differential heating resulting
from annual freezing and thawing would lead to a sim-
ilar effect. It is possible that, towards the feet of slopes,
positive pore-water pressures would bring about mass dis-
placement to form periglacial
involutions
in the active
layer. Periglacial involutions consist of interpenetrating
layers of sediment that originally lay flat.
Frost heaving and thrusting
Ice formation causes
frost heaving
, which is a vertical
movement of material, and
frost thrusting
, which is a
horizontal movement of material. Heaving and thrusting
normally occur together, though heaving is probably pre-
dominant because the pressure created by volume expan-
sion of ice acts parallel to the direction of the maximum
temperature gradient, which normally lies at right-angles
to the ground surface. Surface stones may be lifted when
needle ice forms. Needle ice or pipkrake forms from ice
crystals that extend upwards to a maximum of about
30 mm (cf. Table 11.1). Frost heaving in the active layer
seems to result from three processes: ice-lens growth as
downward freezing progresses; ice-lens growth near the
bottom of the active layer caused by upward freezing
from the permafrost layer; and the progressive freezing of
pore water as the active layer cools below freezing point.
Frost heaving displaces sediments and appears to occa-
sion the differential vertical movement of sedimentary
particles of different sizes. In particular, the upward pas-
sage of stones in periglacial environments is a widely
observed phenomenon. The mechanisms by which this
Frost cracking
At sub-zero temperatures, the ground may crack by ther-
mal contraction, a process called
frost cracking
. The
polygonal fracture patterns so prevalent in periglacial
environments largely result from this mechanism, though
similar systems of cracks are made by drying out (
des-
iccation cracking
) and by differential heaving (
dilation
cracking
).
Frost creep and gelifluction
Most kinds of mass movement occur in periglacial
environments, but
frost creep
and
solifluction
are of
paramount significance (p. 66).
Solifluction
commonly
