Glaciers by NvDEAf4

VIEWS: 8 PAGES: 11

									                                       Glaciers
                         Why YOU Should Care About Glaciers
•   Important agents of erosion which redistribute sediment
•   Store within them information on climate change
•   A major storehouse of water
•   … and they are just so darn ‘cool’!

                              Glaciers Are Made Out of Rock
•   Recall, ice is a mineral and minerals are rocks!
•   IGNEOUS – liquid water freezes
•   SEDIMENTARY – snow crystals accumulate, compact and stick together
•   METAMORPHIC – existing crystals recrystallize without melting … this is glacier ice!

                            Glaciers Are Born in the Clouds

                          Where Do Glaciers Come From?
• Glaciers are made of ice
• The ice develops from snow

                                      What is Snow?
•   There are two fundamentally different types of snow:
     – Precipitated snow
        • That which falls from the sky

    – Metamorphosed snow
        • That produced by later changes within deposited snow

                                   Precipitated Snow

                           The Formation of Precipitated Snow
Ice nuclei > Ice crystal > Snow crystal > >Snowflake

                                   In the Atmosphere
•   Aerosol particles
        • Dust, volcanic ash, ocean salt, smoke particulate, etc.
        • Act as condensation nuclei or ice nuclei
•   Water vapour
        • Condenses on condensation nuclei to form cloud droplets
        • Freezes onto ice nuclei to form ice crystal

                                   In the Atmosphere




                                                                                           1
                               Growth of an Ice Crystal
• Snow crystal
   – Grown by sublimation (freezing directly from gaseous state)
   – Large enough to see with naked eye
   – Often intricate shape

                               Growth of an Ice Crystal

                                Conditions of Formation
• Different conditions = different crystal shapes
• Examples:
   – at -15º C the primary axis of a snow crystal grows very slowly, while the basal axis’
     grow very rapidly = plates
   – at -5º C the basal surfaces grow more slowly than the primary axis = columns
• The physics behind this:
   – complex
   – not well understood
   – the subject of considerable research

                                        Dendrite

                                         Stellar

                         Plate - note the double sheet structure

                                  Column and Bullet

                                         Needle

             Hexagonal snow crystal composed of 2 offset 3 branches

                        Capped Column and Attached Bullets

                                        Grauple

                                   Irregular Crystals

                                     Rimed Plate
        Rime on: Plates (1), Dendrites (2), Capped Column (3) and Needles (4).

                               A Twelve Sided Crystal ?

                          What’s Wrong With This Picture?

                          Formation of a Glacier From Snow




                                                                                        2
                                  Will a Glacier Form?

For a glacier to form the following requirements must be met:
•   Precipitation in the form of SNOW;
•   Must fall on a surface where it can accumulate in large volumes;
•   It must accumulate in an area where the temperature regime allows the snow to
    remain through at least one melt season; and
•   The snow must undergo metamorphism which changes it to glacier ice.

                                 Metamorphosed Snow

                              Instability Leads to Change
• Snow crystals are unstable by nature:
      • Differences in vapour pressure – center to tips
      • Large surface area to volume
• A sphere (ice pellet) is more stable:
      • Equal vapour pressure
      • Smallest surface area to volume ratio

                       Equi-Temperature (E.T.) Metamorphism
• Destroys the original shape of the snow; also known as destructive metamorphism

• Most rapid close to freezing point and diminishes as the temperature drops. It virtually
  halts at –40º C

• Changes density of snow:
   –
     Newly fallen snow has a density from 0.004-0.3 gm/cm3
   –
     It is complete when density reaches about 0.5 gm/cm3
   – Lowering of the snow surface through settlement (but not melt) gives the external
     evidence that E.T. metamorphism is taking place.

• Proceeds in bodies of snow which are not far from uniform in temperature
• Transfer of water molecules from one part of the crystal to another (high vapour
  pressure to low vapour pressure)

                            Early to Late E.T. Metamorphism

                                         Sintering
•   The development of inter-crystalline bonds - binding grains together

•   Again, occurs to reduce the surface area to volume ration between grains

                                 Early to Late Sintering

                                      Firnification
• Further changes will lead to further density increases and the formation of firn


                                                                                         3
• Metamorphosed ‘snow’ which:
  – has survived a summer melt season;
                                    3
  – has a density of 0.4 - 0.8 gm/cm ; and
  – is still permeable.

•    Firnification takes place in two ways:
    – Refreezing of meltwater and rain among the grains
        • Trapped in between grains of snow (surface tension)
        • Refreezes when temperatures drop (diurnal; seasonal)
        • Dominant mechanism in the early stages of firnification.
    – Compaction under pressure
        • As more snow accumulates firn subjected to more pressure as it ‘moves
            deeper into the overall accumulation
        • Becomes dominant during the later stages of firnification

                               Melt Freeze Grain Clusters
• Melt freeze grain clusters from the surface layer of a melting snow pack
• Free water is evident
                                     Glacier Ice
•                              3
  Density of 0.8 – 0.85 gm/cm ; 0.9 gm/cm3 is considered ‘pure’ glacier ice;
• Impermeable - most air spaces have been filled in or squeezed out
                                   Snow – Firn – Ice

                                How Long Does it Take?
• Operates most quickly at conditions (temperature/pressure) close to the melting point
  of the crystals
   – Cold conditions:
       • Firn/glacier ice boundary will be deep; WHY?
       • Rate of increase of crystal size and density will be slow; WHY?
   – Warm conditions:
       • Firn/glacier ice boundary will be closer to the surface
       • Rate of increase of crystal size and density will be faster

                               How Long Does it Take?
It takes MUCH longer for glaciers to form in cold climates

                                 How Thick Is a Glacier?
10’s of meters (glacierette) to 4 km (ice sheet)
                g                     i




                                                                                      4
                        Clearly, Not All Glaciers Are Created Equal
•   There are different types of glaciers and different ways of classifying them
•   For now, we’ll just consider shape and size (morphological classification)

                                   Continental Glaciers

                        Glaciers Unconstrained by Topography
• Ice dome
  – Situated approximately symmetrically over the land area involved
• Outlet glacier
  – Radiates from the periphery of an ice dome

                                Mountain/Alpine Glaciers
• Valley glacier
   – Flows in a rock valley overlooked by rock cliffs
   – May originate in an icefield or a cirque
• Cirque glacier
   – Small ice mass occupying and restricted to an amphitheatre-shaped depression in
     bedrock

                            Glaciers - Up Close and Personal

                                   Glaciers Aren’t Clean

                                 Debris-Covered Glaciers

                                       Rock Glaciers

                                        Is It a Glacier?
•   Just because glacier ice has formed doesn’t mean we have a glacier
•   In order for the ice to be considered a glacier there must be movement of the ice

                              How and Why Glaciers Move

                                Movement Is Affected By:
•   Gravity
     – Compaction under weight of ice
     – Downslope movement
•   Temperature of ice at base of glacier

                                 The Influence of Gravity




                                                                                        5
                            Stress Leads To Deformation
• A glacier flows because the ice deforms in response to stress set up in the ice mass
  by the force of gravity
      • Downward compression
      • Downslope

                                      How Cold is the Ice?
• Warm (temperate) glacier
        •   Glacial ice is at or near it’s pressure melting point.

      Pressure melting point: the temperature at which melting occurs at a given
        pressure. At sea level pressure ice melts at 0ºC; at higher pressures (like
        below a glacier) melting point is reduced; e.g. 2,000 m thick glacier (Antarctica)
        melting point = -1.6ºC; increased pressure has the effect of lowering all change
        of state temperatures.
      • This is the principle that allows you to skate on ice!
• Cold (polar) glacier
      • Glacial ice is below the pressure melting point.

* It is now recognized that an individual glacier can possess both warm and cold
    ice.

                                     Thermal Classification

                             Warm Basal Ice = Water to Slide On

                                  Slippage Over a Water Layer
•   Water, from melting (pressure or surface), rain or groundwater, finds itself at the base
    of the glacier (only warm-based ice)
•   Unique properties of water:
     – It can’t be compressed
     – Ice is less dense than liquid water
•   The ice ‘floats’ on the layer of water
•   As the water ‘lifts’ the ice it decreases contact with the bed = less friction between
    rock and ice – ice moves more easily
•   A water layer only a few mm thick can increase velocity by 40-100%
•   Can account for up to 90% of total movement

•   Given all this information on glacier movement, describe the flow velocity pattern
    through a glacier:
     – Where is the ice moving fastest/slowest?
        • Top surface of ice?
        • Ice at base of glacier?

        • Along valley walls?
        • Away from valley walls, towards center of ice?


                                                                                           6
                          Internal Flow Velocity Distribution

                                      Ice Velocity

                                       Crevasses
• Millimeters to several meters in width
• Maximum depths:
       • Temperate glaciers to 25-35 m
       • Polar glaciers to 60 m

                                         Ogives
• Commonly originate at ice falls:
     • Transverse crevasses
     • Input of sediment and dirty ice in summer
     • Input of clean snow and ice in winter
     • Movement reconstitutes crevasse trapping sediment
     • Formation of alternating light / dark bands


                                     Surging Glaciers
•   Sometimes a glacier will display a sudden, brief, and large-scale displacement
    (downslope movement)
•   Ice velocity may be 10, 100, up to 1000 times faster than normal (the flow rate
    between surges)!
•   Sometimes the ice moves so fast that you can actually hear and feel the glacier move
    (e.g. 350 m per day!)
•   E.g. Iceland:
         • 1963-4
         • Advanced 8 km
         • Speeds up to 5 m PER HOUR!

Some interesting facts about surging glaciers:
     • Not all glaciers surge
     • Some researchers believe that they are a specific type of glacier
     • Are not evenly distributed around the world – clusters: northwestern N.
       America, Iceland, western Asia
     • Surging glaciers don’t surge all the time; they have a periodicity of 10 – 100
       years

Some interesting facts about surging glaciers:
     • Temperate and subpolar glaciers can surge; polar glaciers frozen to their beds
       are not known to surge
     • Surge-type glaciers may tend to be longer, wider and less steep than normal
       glaciers

•   Reasons for surges … unknown


                                                                                        7
•   Some ideas:
       • Part of glacier acts as reservoir which fills – when critical state is reached the
         ice, or part of it, is discharged downglacier at high velocity;
       • Amounts of basal water – greater than average amounts of meltwater flow from
         a surging glacier, buildup of which might be the critical state which is reached

                              Glacier Advance and Retreat

                                    Advance or Retreat?
•   Advancing glacier = position of the toe is moving forward
•   Retreating glacier = position of the toe is moving back

•   In BOTH cases the ice is still moving forward

                                 Like a Bank Account
•   Same money in as out = zero balance
•   More money in than out = positive balance
•   More money out than in = negative balance

                          The Effects of Glacial Ice Movement

                                When a Glacier Moves …
•   A moving glacier moves:
     – Ice
     – Sediment
Erosion and Deposition:
• Sediment is dumped on the ice and gets transported by it;
• Ice picks up sediment and transports it;
• Sediment carried by the ice abrades the surface over which it moves; and
• Eventually the sediment is deposited.
                                      Glacial Erosion

                                         Abrasion
• Debris, caught in glacial ice, is transported and may erode the surface it moves over

                                 Microscopic to Massive




                                                                                          8
                                          Polish
• Size of abrasive material in a glacier varies
• The finest abrasives produce such fine grooves that the surface appears polished
                                       Striations
• Larger abrasive debris forms larger, deeper grooves
• These scratches can be used to determine the localized direction of glacier
  movement

                                    Deep Striations

                                        Grooves

                                     Chatter Marks

                                  Crescentic Gouges

                                  Roche Moutonnees
•   Asymmetrical hill
•   One sided moulded
•   Other side steepened and often craggy
•   Stoss = abrasion
•   Lee = blocks loosened and removed (freeze/thaw, rock fracture, regelation,
    entrainment processes)

                                     Troughs
• The Finger Lakes of New York state are glacial troughs filled with fresh water
                                         Fjord
• A glacial trough which extends into sea and fills with water
                                    U-Shaped Valley

                                         Cirque

                                       Arete
• The ridge which forms when two cirques cut back to back
• Steep, straight slope
                                           Col
• A deep cut in an arete
                                      Horn
• A peak formed when several cirques have been cut towards each other
                     Can You See The Erosional Features?



                                                                                     9
                                     Glacial Deposition

                                       2 Types of Drift
•   Unstratified
        • No layering
        • Deposited by glacier ice
•   Stratified
        • Layered
        • Sediment transported by moving water before it’s final deposition (fluvioglacial,
           glaciolacustrine, marine)

                               Till = Unstratified Glacial Drift

                                       Moraine
•   A depositional landform composed of unstratified drift (till)
•   2 main forms
        • Ridges
        • Areal extent

                                       Medial Moraine

                                       Lateral Moraine

                                           Erratics

                                           Drumlin

                                     Drumlin Formation

                                        Stratified Drift

                                            Esker

                                       Outwash Plain

                                  Varves = Stratified Drift

                                            Varves

                                  Loess = Windblown Silt

                             Ice Age – Not Just a Movie Title!




                                                                                         10
                         Note: these are not Plummer diagrams
                                      Fig. 22.40 a
                                      Fig. 22.41 a
                                      Fig. 22.41 b
                                      Fig. 22.41 c
                                      Fig. 22.35 a
                                       Fig. 22.39

                             Effects of Continental Glaciation
•   Ice loading and glacial rebound
•   Sea level changes
•   Ice dams, drainage pattern changes, lake development
•   Erosion and deposition

                                      Fig. 22.28 a
                                     Fig. 22.28 b, c
                                     Fig. 22.28 d, e
                                      Fig. 22.29 c
                                      Fig. 22.29 a
                                      Fig. 22.29 b
                                     Fig. 22.30 a1
                                     Fig. 22.30 a2
                                     Fig. 22.30 a3
                                     Fig. 22.30 b1
                                     Fig. 22.30 b2
                                      Fig. 22.32 a
                                       Fig. 22.31
                                      Fig. 22.35 b
                                       Fig. 22.38


                                       pp.686-687




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