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Glacial Environments

Glacial Environments. EG1103 March 2003 Dawn Nicholson. Introduction. c. 10% Earth’s surface covered by ice today – but declining due to global warming In the past, % ice cover has been much higher (eg 30% c. 20,000BP) c. 96% glacier ice in Antarctic and Greenland ice caps.

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Glacial Environments

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  1. Glacial Environments EG1103 March 2003 Dawn Nicholson

  2. Introduction • c. 10% Earth’s surface covered by ice today – but declining due to global warming • In the past, % ice cover has been much higher (eg 30% c. 20,000BP) • c. 96% glacier ice in Antarctic and Greenland ice caps

  3. The Growth of Glaciers • Ice originally falls as snow: Density 50 to 300 kg/m3 (compare to density of water 1000 kg/m3) • Snow crystals reduced by melting and sublimation (ablation – prevails in summer) • Physical compaction by melting and re-freezing • Snow transformed into small round crystals called névé (density >500 kg/m3) • When névé survives summer ablation it becomes firn – firn layers accumulate each year • Further density increase produces glacier ice (density 850kg/m3) – takes 25 to 100 years

  4. The Growth of Glaciers

  5. Glacier Movement • Ice + motion = glacier • Movement occurs when ice mass too heavy to maintain its rigid shape and flows by plastic deformation • Normally, ice flow begins where snow/ice depth >20m • Flow rates not uniform • Middle of glacier flows fastest • At the margins of a glacier, surface movement reduced due to friction with valley wall • Bottom of glacier also moves slowly due to friction with base

  6. Glacier Movement

  7. Ice Flow Patterns • Up-glacier accumulation • Mid-glacier converging flow lines (compression) • Down-glacier ablation and extensional flow (accelerating movement) with icefalls, crevasses

  8. Ice Flow Patterns

  9. Velocity of Glacier Movement • Velocity influenced by valley gradient, temperature, ice thickness, friction with valley walls and underlying bed • Glaciers flow along path of least resistance • Valley glaciers often have lobe at snout • Adjacent lobes may coalesce

  10. Velocity of Glacier Movement

  11. Mechanisms of Glacier Movement: Basal Sliding • Pressure caused by weight of glacier produces melting at the bedrock – ice interface • Pressure Melting Point (PMP) is the temperature at which ice will melt under a given pressure • At Earth’s surface PMP = 0oC • At depth beneath glacier PMP << than 0oC • Melted ice layer reduces friction  movement • Some glaciers move 50m/day (ave. 1m/day)

  12. Mechanisms of Glacier Movement: Internal Deformation • Basal sliding only occurs in temperate glaciers • In colder environments (cold-based glaciers) temperature << PMP and so no basal sliding • Instead, glacier movement occurs by internal deformation • Internal deformation occurs by deformation of the lattice of ice crystals • Also occurs by sliding along the film of water in between ice crystals (internal slippage)

  13. Glacier Mass Balance • Glaciers can be regarded as systems, influenced by inputs and outputs • Inputs = snow and sediment • Outputs = water (water or vapour) and sediment • Analysis of the relative balance of inputs and outputs can help us to understand why glaciers expand (advance) and shrink (retreat) • This form of modelling is known and glacier mass balance

  14. Glacier Mass Balance: Accumulation Zone • Two main components of glacier mass balance: Accumulation (snow) and ablation (ice) • Accumulation zone: Occurs in the upper reaches. • Additions of snow exceed annual losses due to melting, evaporation and sublimation. • Surface is snow-covered throughout the year

  15. Glacier Mass Balance: Ablation Zone • Ablation zone: Below accumulation zone • Losses from melting, evaporation and sublimation exceed additions • Line separating accumulation and ablation zones is known as the Equilibrium Line (or firn line)

  16. Glacier Mass Balance and Movement • A glacier moves forwards under control of gravity • However, for terminus to advance, accumulation must also exceed ablation • May be some time delay (lag) between net accumulation and ablation and corresponding movement – can get surges (10-20m/day) • Ablation processes include melting, evaporation, sublimation and calving • Calving occurs where terminus is in water • Calving produces icebergs

  17. Glacier Mass Balance and Movement

  18. Glacier Mass Balance and Movement • Today, most glaciers are retreating because of general warming of global temperatures since the early 1900’s • Therefore mass balance of glaciers is negative (less snow accumulating at higher levels) • Many European glaciers made strong advances during the Little Ice Age • Anomalously, some Norwegian glaciers are advancing and some Icelandic glaciers are prone to surging

  19. Glacier Mass Balance and Movement

  20. Glacier Mass Balance and Movement

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