Glaciers and Glaciation

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Glaciers and Glaciation

• The Earths climate has varied greatly through
  geologic time
• During the Mesozoic (Age of Reptiles), global
  climate was warm and humid and the globe is
  thought to have been ice free
• The Cenozoic has shown a general cooling trend
  and includes the Pleistocene Epoch, also known as
  the Ice Ages (1.8 ma-10 ka)
• Currently the Earths climate is in a warming
  trend and glaciers are in global recession

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Glaciers and Glaciation

• Glacier a mass of ice on land consisting of
  compacted and re-crystallized snow that flows

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Glaciers and Glaciation

• Glaciers cover about 1/10 of the Earths surface
  there is enough glacial ice on the Earth to cover
  all of the U.S. and Canada in 1.5 km of ice!
• Found on all continents except Australia
• Valley or alpine glaciers are found in the
  mountains, but the majority (95) of glacial ice
  on Earth is found in the continental glaciers of
  Antarctica and Greenland

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Glaciers

• Glaciers provide important data that is enhancing
  our understanding of climate change and global
  warming

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Glacial Ablation

• Glaciers can lose ice by
• melting
• calving of icebergs
• sublimation (evaporation without a liquid phase)
• The general term for glacial ice loss is
  ablation

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Glacial Ice

• Ice is a mineral, which is an inorganic,
  naturally occurring solid, with an orderly
  internal arrangement (crystalline structure), and
  a definite chemical composition
• Ice under stress reacts with strain similar to
  other minerals and Earth materials
• Firn Glacial snow that has re-crystallized into
  compact, granular glacial ice (90 solid fresh
  snow is 20 solid )

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Glacial Movement

• Firn reacts to the stress of mass and gravity by
  deforming plastic flow occurs when glacial
  depths reach 40 m this allows glaciers to move
• Subglacial meltwater may also produce basal slip

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Glacial Movement

• Flow rates vary within an individual glacier
  friction slows the flow of ice along the base and
  margins, thus flow is greater in the upper middle
• Glacial ice above 40m depths behaves as a
  brittle solid, thus glacial flow produces
  crevasses in this region of the glacier

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Glacial Movement
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Types of Glaciers

• Alpine glaciers (valley or mountain glaciers)-
  occur in areas of high elevation, are confined to
  valleys, and flow toward lower areas
• May be fed by smaller tributary glaciers
• Can be several kilometers across, hundreds of
  meters thick, and as in the case of glaciers such
  as the Bering Glacier in Alaska, as much as 200
  km long
• Valley glaciers may coalesce (join together) to
  form a piedmont glacier

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Types of Glaciers

• Continental glaciers-
• Also known as ice sheets
• Cover large areas (at least 50,000 km2) and are
  unconfined by topography
• Flow can be outward from the zone of accumulation
  in all downslope directions
• Currently exist only in Antarctica and Greenland,
  where thicknesses exceed 3000m and only the
  higher mountains are not covered

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Glacial Budget

• Expansion and contraction of a glacier, known as
  the glacial budget, is controlled by climate
  fluctuations
• Zone of accumulation the upper part of the
  glacier where precipitation adds snow and ice at
  a rate exceeding losses
• Zone of wastage the lower section of a glacier,
  where melting, sublimation and calving losses
  exceed accumulation

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Glacial Budget
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Glacial Budget

• Firn limit marks the division of the zones of
  accumulation and wastage
• If the firn limit is stationary the glacier has a
  balanced budget and the terminus will remain
  stationary
• If accumulation exceeds losses the glacier has a
  positive budget and the terminus will move
  downslope
• If losses exceed accumulation, the glacier is in
  a negative budget and will move upward into its
  valley

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Glacial Erosion and Transport

• Areas exposed to glaciation exhibit distinctive
  erosive and depositional structures
• Plucking (or quarrying) occurs as glaciers pull
  loose blocks of rock formed by ice wedge
  fracturing
• Glacial polish formed by abrasion on bedrock
• Striations grooves created as particles imbedded
  in glacial ice scratch rock surfaces that are in
  contact with the glacier
• Rock flour glacial abrasion of bedrock creates
  this accumulation of clay and silt sized
  particles, often clouds glacial steams making
  them appear milky

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Glacial Erosion

• Alpine glaciers create a variety of distinctive
  structures as they flow through and erode
  mountainous terrain, including
• U-shaped valleys glacial valleys have
  distinctive U-shaped cross sections with steep,
  almost vertical walls
• Fiords alpine glacial valleys flooded by rising
  sea levels
• Hanging valleys created by tributary glacier
  mouths often leave spectacular hanging
  waterfalls

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Glacial Erosion

• Cirques unique bowl-shaped eroded valleys are
  created in the zones of accumulation at the head
  of glaciers
• Aretes steep, sharp ridges found between cirques

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Glacial Erosion

• Mountain peaks surrounded by cirques form
  steep-walled, pyramidal pinnacles called horns
• Lakes in cirques are called tarns

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Glacial Deposition

• Structures formed by glacial deposition include
• Glacial till general term for all glacial
  deposits, always poorly sorted and unstratified
• Erratics rocks transported by glaciers far from
  their source area
• Moraines ridges of till deposited at the ends
  and margins of glaciers include lateral, and
  medial, and end moraines

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An erratic from Scotland. Analysis of its
composition can be used to identify its source
and allow determination of glacial movements
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Glacial Depositional Forms

• Drumlins streamlined hills formed from till
  deposited by continental glaciers
• Kettle lakes form in depressions created by ice
  blocks dropped by a retreating glacier
• Kames are conical hills formed from sediments on
  a glacier that are left behind as the glacier
  retreats
• Stratified, sinuous elongate hills called eskers
  are deposited by meltwater steams running beneath
  continental glaciers

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The Ice Age

• Glacial periods controlled by climate change have
  occurred several times throughout geologic time,
  most recently in the Pleistocene Epoch, also
  called the Ice Age
• Periods of global climate change and glaciation
  are attributed to Milankovitch cycles, variations
  in the orbit and rotation of the Earth that
  affect the amount of solar energy that reaches
  the Earths surface
• These variations include rotational tilt, orbital
  eccentricity, and orbital precession

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The Ice Age

• When the Milankovitch cycles coincide, a major
  reduction in insolation occurs, depressing global
  temperatures
• These depressed temperatures can produce global
  glaciation
• We are able to calculate when the Milankovitch
  cycles have occurred and will occur
• Geologic records confirm the validity of the
  Milankovitch cycle effects
• We are currently entering a cooling period,
  however human influences have apparently
  mitigated the effects