Page 589 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

(a) A crack forms in the tundra.
Frozen active layer
Open crack (thermal contraction)
Permafrost Winter
Frozen active layer
Open crack
Permafrost
500th winter
Thawed active layer
Ice
Permafrost
Fall
Mounded sediments in response to subsurface volume of ice
Thawed active layer
Developed ice wedge
Permafrost
500th fall
Humans and
Periglacial Landscapes
In areas of permafrost, people face certain problems related to periglacial landforms and phenomena. Because thawed ground above the permafrost zone frequently shifts, highways and rail lines may warp or twist, and utility lines may be dis- rupted. In addition, any building placed directly on frozen ground will “melt” (subside) into the de- frosting soil (Figure 17.23).
In periglacial regions, structures must be suspended slightly above the ground to allow air circulation beneath. The airflow permits the ground to cycle through its normal annual temperature pattern. Utilities such as water and sewer lines must be enclosed aboveground in “utilidors” to protect them from freez- ing and thawing ground. The trans-Alaska oil pipeline was constructed aboveground on racks for 675 km of its 1285-km length to avoid thawing the fro- zen ground and causing shift- ing that could rupture the line (Figure 17.24). Where it runs un-
Chapter 17 glacial and Periglacial Landscapes 553
(c) An example of an ice wedge and ground ice in northern Canada.
(b) The crack evolves into an ice wedge over a time span of hundreds of years.
▲Figure 17.21 Evolution of an ice wedge. Sequential illustration of ice-wedge formation. [(a) Bobbé Christopherson. (b) illustration adapted from a. H. Lachenbruch, “Mechanics of thermal contraction and ice-wedge polygons in permafrost,” Geological Society of America Bulletin Special Paper 70 (1962). (c) H. M. French.]
by cave-ins, bogs, small depressions, pits, standing water, and thaw lakes.
Hillslope Processes: Gelifluction and Solifluc- tion Soil drainage is poor in areas of permafrost and ground ice. The active layer of soil and regolith is saturated with soil moisture during the thaw cycle (summer), and the whole layer commences to flow from higher to lower elevation if the landscape is even slightly inclined. This flow of soil is generally called solifluction. In the presence of ground ice or permafrost, the more specific term gelifluction is ap- plied. In this ice-bound type of soil flow, movement up to 5 cm per year can occur on slopes as gentle as a degree or two.
The cumulative effect of this flow can be an overall flattening of a rolling landscape, combined with identifi- able sagging surfaces and scalloped and lobed patterns in the downslope soil movements. Other types of periglacial mass movement include failure in the active layer, pro- ducing translational and rotational slides and rapid flows associated with melting ground ice. Periglacial mass- movement processes are related to slope dynamics and processes discussed in Chapter 14.
derground, the pipeline uses a cooling system to keep the permafrost around the pipeline stable.
The Pleistocene Epoch
Imagine almost a third of Earth’s land surface buried be- neath ice sheets and glaciers—most of Canada, the north- ern Midwest, England, and northern Europe, with many mountain ranges beneath thousands of metres of ice. This occurred at the height of the Pleistocene Epoch of the late Cenozoic Era (see Chapter 12, Figure 12.1). Dur- ing this last ice age, periglacial regions along the margins of the ice covered about twice the areal extent of perigla- cial regions today.
The Pleistocene Epoch, thought to have begun about 2.5 million years ago, was one of the more prolonged cold periods in Earth’s history. As discussed in Chapter 11, the term ice age, or glacial age, is applied to any extended pe- riod of cold (not a single brief cold spell), in some cases last- ing several million years. An ice age includes one or more glacials, characterised by glacial advance, interrupted by brief warm spells known as interglacials. The Pleistocene