Periglacial Processes
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Periglacial Processes

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    Periglacial Processes Periglacial Processes Presentation Transcript

    • Periglacial processes and landforms Pgs 145-151
      • ‘ 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
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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
    • Europe’s Loess Belt C Loess
    • 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
    • The End!