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Principles of Geology

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

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  1. Principles of Geology Glaciers Mian Liu

  2. Outline of Chapter 18 • What are glaciers? • Formation and movement • Glacial landforms • The ice age

  3. Outline of Chapter 18 • What are glaciers? • Formation and movement • Glacial landforms • The ice age

  4. 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.

  5. 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.

  6. Outline of Chapter 18 • What are glaciers? • Formation and movement • Glacial landforms • The ice age

  7. Basic conditions for glacier formation • Snow to survive summer melting • Enough precipitation • Winter snowfall - summer melting = accumulation • Metamorphism of snow: • Snow -> firn -> ice

  8. Formation of glacier ice

  9. 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

  10. 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

  11. Movement of glacial ice • Rates of glacial movement • Rates of up to several meters per day • Some glaciers exhibit extremely rapid movements called surges

  12. Glacial budget

  13. 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.

  14. Outline of Chapter 18 • What are glaciers? • Formation and movement • Glacial landforms • The ice age

  15. Erosional Glacial Landforms

  16. 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

  17. 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

  18. Glacial erosion • Landforms created by glacial erosion • Pater noster lakes • Cirques • Tarns • Fiords • Arêtes • Horns

  19. 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

  20. 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

  21. 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

  22. 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

  23. 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

  24. Ice ages and the Ice Age

  25. 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

  26. 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

  27. 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

  28. 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

  29. End

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