Page 95 - Fundamentals of Geomorphology

78 INTRODUCING LANDFORMS AND LANDSCAPES

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