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Principles of Geology. Glaciers. Mian Liu. Outline of Chapter 18. What are glaciers? Formation and movement Glacial landforms The ice age . Outline of Chapter 18. What are glaciers? Formation and movement Glacial landforms The ice age. Glacier:.

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Principles of geology l.jpg

Principles of Geology

Glaciers

Mian Liu


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Outline of Chapter 18

  • What are glaciers?

  • Formation and movement

  • Glacial landforms

  • The ice age


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Outline of Chapter 18

  • What are glaciers?

  • Formation and movement

  • Glacial landforms

  • The ice age


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Glacier:

  • A mass of ice (with included rocks and air) on land formed by compaction and recrystallization of snow, and moves under its own weight.


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Why are they important?

  • < 10% of continental area, but occurred at various times over the Earth’s history at 3/4 of continental surface (last major iceage:Pleistocene (1.6-0.01 Ma));

  • Store ~75% of the fresh water on earth;

  • Glacier meltwater: major source for summer stream flow for many regions.


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Outline of Chapter 18

  • What are glaciers?

  • Formation and movement

  • Glacial landforms

  • The ice age


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Basic conditions for glacier formation

  • Snow to survive summer melting

  • Enough precipitation

    • Winter snowfall - summer melting = accumulation

  • Metamorphism of snow:

    • Snow -> firn -> ice



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Glaciers

  • Types of glaciers

    • Valley (alpine) glaciers

      • Exist in mountainous areas

      • Flows down a valley from an accumulation center at its head

    • Ice sheets

      • Exist on a larger scale than valley glaciers (also called continental glaciers)

      • Two major ice sheets on Earth are over Greenland and Antarctica


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Movement of glacial ice

Two basic types

  • Plastic flow (creep)

    • Occurs within the ice

    • Under pressure, ice behaves as a plastic material

  • Basal slip

    • Entire ice mass slipping along the ground

    • Most glaciers are thought to move this way by this process


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Movement of glacial ice

  • Rates of glacial movement

    • Rates of up to several meters per day

    • Some glaciers exhibit extremely rapid movements called surges



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Glacial structures

  • 1st order: stratification (discernible layers or bands resulted from annual cycles of snow accumulation and ablation above the snowline)

  • 2nd order:

    • Foliation: layering due to shear in the ice

    • Crevasses: cracks formed in the top layer, ranging from miniature fractures to gaps several meters wide.


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Outline of Chapter 18

  • What are glaciers?

  • Formation and movement

  • Glacial landforms

  • The ice age


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Erosional

Glacial Landforms


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Glacial erosion

  • Glaciers are capable of great erosion and sediment transport

  • Glaciers erode the land primarily in two ways

    • Plucking – lifting of rocks

    • Abrasion

      • Rocks within the ice acting like sandpaper to smooth and polish the surface below


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Glacial erosion

  • Glacial erosion

    • Glacial abrasion produces

      • Rock flour (pulverized rock)

      • Glacial striations (grooves in the bedrock)

  • Landforms created by glacial erosion

    • Erosional features of glaciated valleys

      • Glacial trough

      • Truncated spurs

      • Hanging valleys


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    Glacial erosion

    • Landforms created by glacial erosion

      • Pater noster lakes

      • Cirques

      • Tarns

      • Fiords

      • Arêtes

      • Horns


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    Glacial deposits

    • Glacial drift – refers to all sediments of glacial origin

      • Types of glacial drift

        • Till – material that is deposited directly by the ice – poor sorting

        • Stratified drift – sediments laid down by glacial meltwater – relatively good sorting


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    Glacial deposits

    • Landforms made of till

      • Moraines

        • Layers or ridges of till

      • Moraines produced by alpine glaciers

        • Lateral moraine

        • Medial moraine

      • Other types of moraines

        • End moraine – terminal or recessional

        • Ground moraine


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    Glacial deposits

    • Landforms made of till

      • Drumlins

        • Smooth, elongated, parallel hills

        • Steep side faces the direction from which the ice advanced

        • Occur in clusters called drumlin fields

        • Formation not fully understood


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    Glacial deposits

    • Landforms made of stratified drift

      • Outwash plains (with ice sheets) and valley trains (when in a valley)

        • Broad ramp-like surface composed of stratified drift deposited by meltwater leaving a glacier

        • Located adjacent to the downstream edge of most end moraines

        • Often pockmarked with depressions called kettles


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    Glacial deposits

    • Landforms made of stratified drift

      • Ice-contact deposits

        • Deposited by meltwater flowing over, within, and at the base of motionless ice

        • Features include

          • Kames

          • Kame terraces

          • Eskers


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    Ice ages and the Ice Age


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    Glaciers of the past

    • Ice Age

      • The Ice Age began between two million and three million years ago

      • Most of the major glacial stages occurred during a division of geologic time called the Pleistocene epoch


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    Causes of glaciation

    • Any successful theory must account for

      • What causes the onset of glacial conditions

      • What caused the alteration of glacial and interglacial stages that have been documented for the Pleistocene epoch


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    Causes of glaciation

    • Some possible causes of glaciation

      • Plate tectonics

        • Continents were arranged differently in the past

        • Changes in oceanic circulation

      • Variations in Earth’s orbit

        • The Milankovitch hypothesis


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    Causes of glaciation

    • Some possible causes of glaciation

      • Milankovitch hypothesis

        • Shape (eccentricity) of Earth’s orbit varies

        • Angle of Earth’s axis (obliquity) changes

        • Earth’s axis wobbles (precession)

        • Changes in climate over the past several hundred thousand years are closely associated with variations in the geometry of Earth’s orbit

      • Other factors are probably also involved