Geoscience Reference
In-Depth Information
Plate 15.18
Earth hummocks 50 cm high in the Mackenzie valley, Canada. Note the bare, granular surface of the mound, and
the denser vegetation in the inter-hummock troughs. The permafrost table is at the base of the measure.
Photo: Ken Atkinson
wedge ice grows in the cracks to give ice-wedge polygons.
Whilst desiccation polygons are about two metres only in
diameter, ice-wedge polygons can reach 100 m diameter.
Black (1952) was the first to propose a cycle for ice-wedge
polygons that progress from a flat surface with polygonal
cracks (
youthful stage
) through low-centre polygons
(
mature stage
) to high-centre polygons (
old age stage
).
Initially the development of ridges and vegetation above
ice wedges inhibit surface drainage and cause flooding of
central areas. Lateral growth of ice wedges, mass wasting,
thaw and vegetation growth eventually convert low-centre
polygons into high-centre ice-wedge polygons (
Plate
Lachenbruch (1966), in classic studies of ice-wedge
polygons on the North Slope of Alaska, agreed that high-
centre ice-wedge polygons can form where polygonal
troughs are deepened by erosion and where peripheral
ridges are destroyed, but he suggested a further mecha-
nism in which soil texture is critical. Low-centre polygons
form where the material has finite shear strength when
thawed, for example sand, peat and coarse silt. This
material is extruded into the active layer and accumulates
as peripheral ridges. High-centre ice-wedge polygons form
where the permafrost is fluid when thawed, for example
where considerable ice, clay and fine silt are present. When
this material flows and disperses into the active layer, it
leaves either no surface ridge around the polygon or only
a trough over the ice wedges.
Large-scale landforms due to ground ice:
palsas and pingos
Palsas are prominent features formed by ground ice in
wetland and boggy terrain. Water is attracted to the many
thin ice lenses, from 5 cm to 20 cm in thickness, that are
present in palsas. Palsas are usually mounds but can be
plateau-like, with steep edges, or esker-like, in winding
ridges. They are 10-30 m in width, 15-500 m in length,
and 1-10 m high, with surfaces that are usually domed
and cracked. The characteristic structure of palsas is
