Unit 45

GLACIAL DEGRADATION AND AGGRADATION

Unit Overview

This unit examines glaciers and their actions on the landscapes of high-latitude and high-altitude regions. The major sections are as follows:

  • Glaciers of the past
  • The formation of glaciers
  • The glacier as a system
  • Glacial movement and erosion

A glacier is a body of ice┬┐which exists on land and is in motion┬┐that is formed through the compaction and subsequent recrystallization of snow. The two broad types of glaciers are mountain glaciers and continental glaciers (ice sheets). A glacier has a zone of accumulation and a zone of ablation. If the glacier is in equilibrium, then gains through precipitation equal losses through melting, evaporation, and sublimation.

Glaciers are powerful erosional agents, but it must be noted that mountain glaciers in temperate areas erode more slowly than similar glaciers in very cold polar areas. Much of the erosional action of glaciers involves abrasion, which is the scraping process produced by the impact of rock debris carried in the ice upon the bedrock below. The amount of global glaciation has varied throughout Earth's history; long periods of reduced atmospheric temperatures are known as ice ages.

Unit Objectives

  • To discuss the different categories of glaciers
  • To give a brief history of how glaciation has influenced the Earth's surface
  • To outline how glaciers form, move, and erode the landscape


Glossary of Key Terms

Abrasion A glacial erosion process of scraping, produced by the impact of rock debris carried in the ice upon the bedrock surface below.
Basal ice The bottom ice layer of a glacier.
Continental glacier Huge masses of ice that bury whole countrysides beneath them.
Crevasse One of the huge vertical cracks that frequently cut the rigid, brittle upper layer of a mountain glacier.
Cryosphere The collective name for the ice system of the Earth.
Deglaciation The melting and receding of glaciers that accompanies the climatic warmup after the peak of a glaciation has been reached.
Firn Granular, compacted snow.
Glacial creep One of the two mechanisms by which glaciers flow; involves the internal deformation of the ice, with crystals slipping over one another as a result of downslope movement.
Glacial sliding One of the two mechanisms by which glaciers flow; involves the movement of the entire glacier over the rocks below it, lubricated by a thin film of water between the basal ice and the bedrock floor.
Glacial surge Episode of rapid movement in a mountain glacier, as much as one meter per hour, that can last for a year or more.
Glaciation A period of global cooling during which continental icesheets and mountain glaciers expand.
Glacier A body of ice - formed on land - that exhibits motion.
Ice age A stretch of geologic time during which the Earth's average atmospheric temperature is lowered; causes the expansion of glacial ice in the high latitudes and the growth of mountain glaciers in lower latitudes.
Ice sheet See continental glacier.
Interglacial A period of warmer global temperatures between the most recent deglaciation and the onset of the next glaciation.
Late Cenozoic Ice Age The last great ice age that ended 10,000 years ago; spanned the entire Pleistocene Epoch (2,000,000 to 10,000 years ago) plus the latter portion of the preceding Pliocene Epoch, possibly beginning as far back as 3 million years ago.
Mountain (alpine) glacier River of ice that forms in mountainous regions; confined in valleys that usually have steep slopes.
Plucking A glacial erosion process in which fragments of bedrock beneath the glacier are extracted from the surface as the ice advances.
Rouche moutonnee The most common landform associated with glacial plucking; an asymmetrical mound.
Striation Scratches made on underlying rock layers by boulders or pebbles dragged by a glacier.
Surge Rapid movement by a mountain glacier, as fast as one meter per hour, sustained for a month or more; produces overall movement of several kilometers in a season.
Zone of ablation A glacier's lower zone of loss; ablation refers to all forms of loss at the lower end, including melting and evaporation.
Zone of accumulation A glacier's upper zone of growth, where new snow is added.


Unit Outline

  • Glacier definitions
    • A glacier is a moving body of ice, formed on land
    • Glaciers of Switzerland and Alaska are mountain ( alpine) glaciers
    • Glaciers of Antarctica and Greenland are continental (sheet) glaciers
  • Glaciers of the past
    • An ice age is a long period of geologic time with lower average temperature, resulting in expansion and growth of glaciers
    • A glaciation is a cooling period when ice sheets advance, deglaciation then follows
    • Interglacials are periods of time in-between deglaciation and the next glaciation
    • Earth at present is in an interglacial
    • The most recent ice age is the Late Cenozoic
    • Glaciation associated with falling sea levels, deglaciation with rising sea levels
  • The formation of glaciers
    • Glaciers consist of ice, which is recrystallized snow
    • When summer snow loss is less than winter snow gain, conditions favorable for glacier formation
    • Snow is converted to ice in stages
      • newly fallen snow flakes
      • melting occurs or later snowfall compacts earlier snowfall
      • firn(compacted snow) undergoes further compaction
  • The glacier as a system
    • An open system, gaining snow in its zone of accumulation, losing ice in its zone of ablation
      • matter enters in solid state as snow, undergoes two changes to granular and crystalline states, and exits as liquid or vapour
    • When mass balance is positive, glacier advances with steep icy edge
    • When mass balance is negative, glacier is slushy and edge is less pronounced
  • Glacial movement and erosion
    • The temperature of ice does not decrease steadily with depth
    • Basal ice is affected by several factors, including:
      • pressure exerted by weight of overlying ice
      • temperature of underlying bedrock
    • When basal ice is at the melting temperature, glaciers move faster and erode more effectively
      • mountain glaciers in temperate areas are therefore more effective erosional agents than ice sheets
    • The movement of ice (Fig. 45.6)
      • glaciers move very slowly
      • continental glaciers move slower than alpine glaciers
      • a surge is an occasional period of rapid movement (up to 1 m/hr)
      • crevasses are long cracks in the brittle upper layer of a glacier
      • ice is plastic-like below upper layers of a glacier, with crystals slipping over each other due to downslope movement; results in glacial creep
      • glacial sliding is the movement of the glacier as a whole over rock
        • enhanced by thin film of water between glacier and rock
    • Glacial erosion
      • plucking (or quarrying) is glacial erosion in which blocks of bedrock are pulled up as the glacier slides (Fig. 45.8)
      • abrasion is the process by which a glacier erodes by scraping the bedrock below
        • when glacial debris fine but hard and bedrock hard, a polished result
        • when glacial rocks larger, striations (grooved scratches) produced in eroded bedrock


Review Questions

  1. Describe the cycle of glaciations and interglacials, and list general climatic conditions associated with each.
  2. How are glaciers formed from snow? What are the steps involved in this process? See Fig. 45.2.
  3. What are glacial surges? Which type of glacier are they associated with?