Periglacial Processes


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Periglacial Processes

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