Glacial Cycles: Earth's Ice Evolution

1. Earth Science, 13e Tarbuck & Lutgens

2. Glaciers, Deserts, and WindEarth Science, 13eChapter 6 Stanley C. Hatfield Southwestern Illinois College

3. Glaciers: a part of two basic cycles in the Earth system • Glaciers are a part of both the hydrologic cycle and rock cycle • Glacier – a thick mass of ice that forms over land from the compaction and recrystallization of snow and shows evidence of past or present flow

4. Glaciers: a part of two basic cycles in the Earth system • Types of glaciers • Valley, or alpine glaciers – form in mountainous areas • Ice sheets, or continental glaciers • Large scale • e.g., Over Greenland and Antarctica • Other types • Ice caps and piedmont glaciers

5. Currently ice sheets cover Greenland and Antarctica

6. Glaciers: a part of two basic cycles in the Earth system • Movement of glacial ice • Types of glacial movements • Plastic flow • Slipping along the ground • Zone of fracture • Uppermost 50 meters • Crevasses form in brittle ice

7. Glaciers: a part of two basic cycles in the Earth system • Movement of glacial ice • Zone of accumulation – the area where a glacier forms • Zone of wastage – the area where there is a net loss due to melting • Calving – the breaking off of pieces of ice, usually dropping into the ocean

8. The glacial budget

9. Glaciers: a part of two basic cycles in the Earth system • Glaciers erode by • Plucking – lifting of rock blocks • Abrasion • Rock flour (pulverized rock) • Striations (grooves in the bedrock)

10. Glaciers: a part of two basic cycles in the Earth system • Landforms created by glacial erosion • Glacial trough - a wide U-shaped valley • Hanging valley - a smaller U-shaped valley leading into the trough • Cirque - round valley at top of a glacier • Arête – sharp crowned ridge between troughs • Horn - sharp peak at top of glaciers • Fiord - glacial trough in sea, inlet to land

11. Erosional landforms created by alpine glaciers

12. The Matterhorn in the Swiss Alps

13. Glaciers: a part of two basic cycles in the Earth system • Glacial deposits • Glacial drift • All sediments of glacial origin • Types of glacial drift • Till – material that is deposited directly by ice • Stratified drift – sediment deposited by meltwater

14. Glacial till is typically unstratified and unsorted

15. Glacial deposits • Depositional features • Moraines – layers or ridges of till • Types of moraines • Lateral - gravel deposits on side of valleys • Medial - gravel ridge in between valleys • End - large gravel ridge crosswise to valley • Terminal end – the furthest the glacier advanced • Recessional end – the glacier sometimes melted then advanced leaving smaller End moraines • Ground - filling in depressions with glacial till

16. Glacial deposits • Outwash plain, or valley train - sloping plains consisting of deposits from melting edge • Kettles - depressions created when a block of ice stays in a hole while the glacier around it melts and leaves deposits • Drumlins - streamlined, asymmetrical hills composed of glacial dirt • Eskers - ridges composed largely of sand and gravel deposited by a stream flowing beneath a glacier near its terminus • Kames - large round hills of glacial till

17. Glacial depositional features

18. Glaciers: a part of two basic cycles in the Earth system • Glaciers of the past • Ice Age • Began 2 to 3 million years ago • Division of geological time is called the Pleistocene epoch • Ice covered 30 percent of Earth’s land area

19. Maximum extent of ice during the Ice Age

20. Glaciers: a part of two basic cycles in the Earth system • Glaciers of the past • Indirect effects of Ice Age glaciers • Migration of animals and plants • Rebounding upward of the crust • Worldwide change in sea level • Climatic changes

21. Glaciers: a part of two basic cycles in the Earth system • Causes of glaciation • Successful theory must account for • Cooling of Earth, as well as • Short-term climatic changes • Proposed possible causes • Plate tectonics • Continents were arranged differently • Changes in oceanic circulation

22. Glaciers: a part of two basic cycles in the Earth system • Causes of glaciation • Proposed possible causes • Variations in Earth’s orbit • Milankovitch hypothesis • Shape (eccentricity) of Earth’s orbit varies • Angle of Earth’s axis (obliquity) changes • Axis wobbles (precession) • Changes in climate over the past several hundred thousand years are closely associated with variations in Earth’s orbit

23. Deserts • Geologic processes in arid climates • Weathering • Not as effective as in humid regions • Mechanical weathering forms unaltered rock and mineral fragments • Some chemical weathering does occur • Clay forms • Thin soil forms

24. Deserts • Geologic processes in arid climates • Role of water in arid climates • Streams are dry most of the time • Desert streams are said to be ephemeral • Flow only during periods of rainfall • Different names are used for desert stream valleys including • Wash – American • Arroyo – Mexican Spanish • Wadi – Middle Eastern, biblical

25. Deserts • Geologic processes in arid climates • Role of water in arid climates • Desert rainfall • Rain often occurs as heavy showers • Causes flash floods • Poorly integrated drainage • Most erosional work in a desert is done by running water

26. A dry stream channel in the desert

27. The same stream channel following heavy rainfall

28. Deserts • Basin and Range: the evolution of a desert landscape • Uplifted crustal blocks • Interior drainage into basins produces • Alluvial fans and bajadas – piles of silt spread in a fan shape as the Wash becomes a valley • Playas and playa lakes – usually dry lake beds with silt bottoms

29. Deserts • Basin and Range: the evolution of a desert landscape • Erosion of mountain mass causes local relief to continually diminish • Eventually mountains are reduced to a few large bedrock knobs called inselbergs projecting above a sediment-filled basin

30. Landscape evolution in a mountainous desert – early

31. Landscape evolution in a mountainous desert – middle

32. Landscape evolution in a mountainous desert – late

33. Wind erosion • Deflation • Lifting of loose material • Produces • Blowouts – rocky ground where topsoil has blown away • Desert pavement – rocky ground over bedrock with no soil • Abrasion – eroding rock worn away by blowing sand

34. Formation of desert pavement

35. Types of wind deposits • Loess • Deposits of windblown silt • Extensive blanket deposits • Primary sources are deserts and glacial stratified drift

36. Types of wind deposits • Sand dunes • Mounds and ridges of sand formed from the wind’s bed load • Characteristic features • Slip face – the leeward slope of the dune • Cross beds – sloping layers of sand in the dune

37. Formation of sand dunes

38. Types of wind deposits • Sand dunes • Types of sand dunes • Barchan dunes – parabolic dunes pointing away from the prevailing wind • Transverse dunes – long waves of sand cross-ways to the prevailing wind • Longitudinal dunes – long waves of sand parallel to the prevailing wind • Parabolic dunes – water formed dunes pointing toward the water and prevailing wind • Star dunes – dunes with pointed tops where there is little prevailing wind

39. Sand dune types

40. End of Chapter 6