Periglacial Processes
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Periglacial Processes

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Periglacial Processes Presentation Transcript

  • 1. Periglacial processes and landforms Pgs 145-151
  • 2.
    • ‘ Peri’ comes from the Greek meaning round or about
    • Therefore,
      • periglacial is applied to areas at or around the fringes of glaciers
    • Occur in areas where there is frost action such as freeze-thaw and frozen ground
    • Periglacial zone has no fixed location because of frequent and major ice advances and retreats over the last 2 million years – constant change
    • S. and E. England fossil landscape features formed by periglacial processes can be recognized as there was no later ice advances to destroy them
    • Today… periglacial processes within British Isle’s are restricted to tops of higher mountains
    • Worlds main periglacial zone covers Alaska and Artic lands of Canada and Russia
    E
  • 3. Permafrost
    • It means permanently frozen.
    • Permafrost itself is not a process, it’s a ground condition.
    • Bedrock, regolith, and soil are frozen, between these layers are filled with ice.
    • Porous rock (chalk) is turned into impermeable rock/mass by permafrost.
    • Weathered rock and soil are cemented together and become solid.
    • In winter months permafrost can be up to 600m deep.
    • Permafrost has greater presence on mountains and ridges compared to river valleys.
    • Active layer in shallow surface zone defrosts in summer.
    • Water released is unable to drain away as ground beneath is frozen.
    • Upper soil becomes saturated and can move on slopes of as little as 2°-Solifluction.
    • In Autumn refreezing begins on surface where heat is lost first.
    • As active layer narrows, expansion of water above and below causes surface heaving and associated landforms layer, eg patterned ground.
    D
  • 4. Mass movement
    • There are several factors affecting mass movement:
    • - The ground freezes and re-freezes throughout a year.
    • This leads to water being trapped above the permafrost.
    • - The temperature isn’t high enough for the water to evaporate.
    • - There is little vegetation to hold the soil together
    • The ground is often on slopes
    L
  • 5. Mass Movement Cont…
    • Solifluction = important periglacial process as widespread occurrence
    • Definition = when the active later thaws in summer, excessive lubrication reduces the friction between soil particles – leads to solifluction or lobes
    • Appearance = these are rounded, tongue-like features often forming terraces on the sides of valleys
    • Effect = to reduce and smooth out the relief as weathered and loose deposited materials are transferred from upper slope to downslope
    • Therefore, there are more features at the foot of the slope:
    E
  • 6. Mass Movement Cont
    • The formation of lobes and terraces gives it a greater form
    • by making the lower slopes rounder and flatter areas:
    • There are two types of lobes
    • These are both controlled by the amount of vegetation
    • present:
    • Lack of vegetation cover, stone-banked lobes/terrace formed:
    • Small-lobes – rising up to 5m then extending up slope to form terrace between 10-30 metres long
    • Distinctive feature – each lobe is the stone wall behind the finer material
    • Stones on surface travel quicker downslope that finer materials until something changes the movement, e.g. vegetation
    • Finer material accumulates behind the wall to form a terrace
    D
  • 7. Mass Movement Cont
    • Forms under continuous vegetation cover where frost heaving is less effective at raising stones to surface
    • Turf-banked lobes and terraces form instead, as it is the surface layers of turf and topsoil which are moved by solifluction
    • Terraces are created where minor relief obstructs movement, rolling up the turn flow backwards under continued surface movement
    • = forms as the riser below the lobe
    • When held together by vegetation and roots, lobes usually and cover larger areas on lower slopes then stone-banked areas
    L
  • 8. Mass Movement Finally!!!
    • Fossil solifluction deposits are the most widespread periglacial relict feature in the British Isles
    • General term for this = Head deposits
    • Widespread below outcrop of granite on Dartmoor and elsewhere in S.W England
    • Coombe Rock =
    • Name given to fans of chalk below escarpments and in dry valleys in down land regions in Southern England
    • Lobes/terraces identified in uplands of Scotland and Wales are exposed to actions of solifluction for the longest time in the British Isles
    E
  • 9. Frost Action and Resulting Landforms
    • Freeze-thaw is the most common weathering process in periglacial areas.
    • It brakes down the majority of rocks on the mountainsides and many of the landforms lower down.
    L
  • 10. Thermokarst
    • Name given to very irregular surfaces of mostly hollows and small hummocks.
    • These pitted surfaces resemble those formed by solution in some karst areas of limestone.
    • Small domes that form on surface due to frost heaving with the onset of winter are only features.
    • They then collapse in summer thaw leaving small surface depressions.
    • Some ice lenses grow and form surface hummocks whish may last many years before they thaw.
    D
  • 11. Pingos
    • Small circular hills up to 50 or 60 m high
    • Occur in groups
    • Add to variety of thermokarst scenery
    • Pond in central crater makes even more distinctive
    • Greatest number found on flat lands in and around the Mackenzie Delta on Artic coast of N.W. Canada
    • Within each pingo is a large ice core, responsible for the frost heaving which forms the conical hill
    • After the lake forms, its ice core is no longer insulated against summer warmth – the gradient thaws away and the pingo form is lost
    E
  • 12. Patterned Ground
    • Stones in periglacial areas seemed to sort themselves out in to patterns. This is due to frost heaving.
    • The ice freezes underneath the stones which expand and pushes the stones upwards.
    • When they reach the surface they roll outwards.
    L
  • 13. Other Periglacial Processes and Landforms •    Mechanical weathering by freeze-thaw is widespread. •    Low temperatures, sparse vegetation cover and poorly developed soils reduce the likelihood of chemical and biological weathering taking place. •    Lack of vegetation increases wind (aeolian) action. •    Further erosion occurs as the wind-blown sand grains abrade rock outcrops. •    Wind action produces semi-circular dunes of sandy outwash material. •    It also produces wind-blown loess. •    Loess is a fertile, easy to work soil. •    A loess belt extends across Europe C
  • 14. Europe’s Loess Belt C Loess
  • 15. Artic Streams •    The amount of transport and erosion in Arctic streams is also huge. •    The high load comes from flowing over areas liberally covered by loose debris from glaciers. •    The high discharge comes from melting ice. •    80% of the discharge comes in one or two months of the year. C
  • 16. The End!