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Unit 48


Unit Overview

This unit examines environments adjacent to glacial regions. The main sections are as follows:

  • Permafrost
  • Geomorphic processes in periglacial environments
  • Landforms of periglacial regions
  • Resource development in periglacial environments

Periglacial refers to being on the periphery of glaciation, and periglacial zones at the present time exist at high latitudes. Permafrost tends to delimit the extent of periglacial conditions. Frost action and solifluction are two dominant geomorphic processes that modify periglacial landscapes.

Associated with frost action are frost wedging, frost heaving, frost thrusting, and frost creep. Mass movements can be achieved through solifluction, which is the slow flowage of saturated soil. Periglacial landscapes exhibit a variety of landforms resulting from frost action and mass movements. These landforms include ice wedges, patterned ground, pingos, and boulder fields.

Unit Objectives

  • To discuss the unique landscapes that develop when near-glacial conditions prevail at high latitudes and high altitudes
  • To highlight the important weathering and mass-movement processes that shape periglacial landscapes

Glossary of Key Terms

Active layer The soil above the permafrost table that is subject to annual freezing and thawing.
Boulder field The area of blocky rock fragments formed when weathered rocks particularly from frost wedging - remain near their original location; also known as a felsenmeer.
Frost creep The movement of particles within the active layer above the permafrost under the influence of gravity; on the surface, rocks will move downslope during the thawing phase.
Frost heaving The upward displacement of rocks and rock fragments within the active layer above the permafrost after frost wedging has loosened them; triggered by the formation of ice in the ground that expands the total mass of rock materials.
Frost thrusting The horizontal movement of rocks and rock fragments within the active layer above the permafrost.
Frost wedging The forcing apart of a rock when the expansion stress created by freezing its internal water into ice exceeds the cohesive strength of that rock body.
Ice-wedge polygon Polygonal features formed by the freezing and thawing of sediments that fill surface cracks caused by very cold winter temperatures in periglacial zones.
Patterned ground Periglacial rock and soil debris shaped or sorted in such a manner that it forms designs on the surface resembling rings, polygons, lines, and the like.
Periglacial A high-latitude or high-altitude environment on the perimeter of a glaciated area.
Permafrost The permanently frozen layer of subsoil that is characteristic of the colder portions of the D-climate zone as well as the entire E-climate zone; can exceed 300m in depth.
Pingo A mound-like, elliptical hill in a periglacial zone whose core consists of ice rather than rock or soil.
Solifluction A special kind of soil creep in which soil and rock debris are saturated with water and flow in bulk as a single mass; most common in periglacial zones.

Unit Outline

  • Permafrost
    • Periglacial areas are on the periphery of glaciation
      • Dfc, Dfd, E climates
      • Gravity is the main mover of loosened materials in these areas
    • Permafrost is permanently frozen ground (Fig. 48.2)
      • usually extends from 15 cm (6 in.) to 5m (16.5 ft) below surface
      • upper surface called permafrost table
      • active layer is the soil layer above the permafrost table, freezes and thaws annually
      • continuous permafrost in more northerly latitudes, discontinuous (alpine) permafrost further south
  • Geomorphic processes in periglacial environments
    • Frost action
      • frost wedging (shattering) occurs when stress of freezing and thawing breaks down the strength of rock
        • produces patterned ground
      • after frost wedging, frost heaving causes vertical rock displacement when ice expands
      • frost thrusting moves material horizontally
      • frost creep occurs when particles in the active layer move due to gravity
    • Solifluction (Fig. 48.5)
      • actually a form of soil creep, slow flowing of saturated soil when water cannot infiltrate soil due to permafrost
      • vegetation in more moderate areas stabilizes solifluction
  • Landforms of periglacial regions
    • Ice wedges
      • created when ground becomes cold and cracks, filling with mud, ice, etc. and then expands each time it refreezes, widening the cracks
        • can form ice-wedge polygons over large areas
    • Patterned ground
      • rock and other solid debris sorted to form patterns such as rings or polygons
    • Pingos
      • mounds consisting of ice that form under permafrost conditions, possibly from drained lakes where the permafrost level rises to the surface
    • Boulder fields (felsenmeers)
      • slopes covered by blocks of rock that have been removed from rock faces by frost wedging
      • some believe that ice may have filled the openings between the boulders, creating a moving rock glacier
  • Resource development in periglacial environments
    • Search for resources has lead to further investigation of remote periglacial areas

Review Questions

  1. Name and elaborate on three of the major landforms associated with periglacial regions.
  2. Define frost wedging, frost heaving, and frost thrusting.
  3. Draw the layers of permafrost, using Fig. 48.2 as a guide.