Glacial Environments

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Glacial Environments

• EG1103 March 2003
• Dawn Nicholson

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Introduction

• c. 10 Earths 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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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 gt500 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

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The Growth of Glaciers
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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
  gt20m
• 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

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Glacier Movement
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Ice Flow Patterns

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

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Ice Flow Patterns
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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

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Velocity of Glacier Movement
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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 Earths surface PMP 0oC
• At depth beneath glacier PMP ltlt than 0oC
• Melted ice layer reduces friction ? movement
• Some glaciers move 50m/day (ave. 1m/day)

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Mechanisms of Glacier Movement Internal
Deformation

• Basal sliding only occurs in temperate glaciers
• In colder environments (cold-based glaciers)
  temperature ltlt 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)

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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

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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

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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)

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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

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Glacier Mass Balance and Movement
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Glacier Mass Balance and Movement

• Today, most glaciers are retreating because of
  general warming of global temperatures since the
  early 1900s
• 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

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Glacier Mass Balance and Movement
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Glacier Mass Balance and Movement