Periglacial Processes (Edexcel A Level Geography)

Past & Present Periglacial Landscapes

• A periglacial landscape is characterised by permanently frozen ground or permafrost and is defined as:

• Approximately 25% of the Earth's surface is considered periglacial, but during the Pleistocene epoch it covered as much as 45% of the surface, extending into the lower latitudes of southern England and Europe

• Permafrost is not controlled by soil moisture and ice does not need to be present

• Permafrost is prevalent in Siberia (up to 1500m deep) and Northern Canada (depths of 700m) 

• Most permafrost water stays frozen as ground ice

Comparison of Glacial and Periglacial Areas

• Areas of unfrozen ground within the permafrost are known as talik

• Talik can be:

  • Open - a small area of unfrozen ground exposed to the surface

  • Through - a large mass of unfrozen ground beneath a small open area

  • Closed - unfrozen ground surrounded by permafrost

• Permafrost can be subdivided into:

  • Continuous - large, unbroken stretches of permafrost, that reach depths of up to 1,500 meters – largest areas are located in Canada, Alaska and Siberia

  • Discontinuous - mostly permafrost, with some small, localised unfrozen ground (talik)

  • Sporadic - where small patches of frozen ground occur in talik (unfrozen ground)

  • Isolated  - there are random pockets of permafrost within the talik 

  • Subsea - or offshore permafrost is frozen ground within seabed sediments. Originally formed on land and was subsequently submerged as sea levels rose after the last Ice Age

• Seasonal melting at the surface produces the active layer  of between 2cm to 5m in depth 

• However, any meltwater cannot drain through the impermeable permafrost below and sits on the surface as thaw lakes

• Thaw lakes are common in these poorly drained areas and as the water continues to absorb solar radiation, the depth and size of these lakes increase in size

• In the past, the area of permafrost was more extensive and permafrost covered most of southern England, giving it a tundra landscape 

• The periglacial landscape is littered with angular rocks across its surface

• Quickly weathered through freeze-thaw processes, these areas are called felsenmeer, meaning 'field of rocks' in German

Periglacial Processes

• Processes within periglacial environments are due to frost action and include:

  • Ground ice

  • Solifluction

  • Nivation

  • Frost contraction and ice wedges

  • Frost heave

  • Freeze-thaw

• Ground ice

  • The most common form of ground ice is pore ice

  • It develops in the pore spaces between soil and rock particles, where meltwater has accumulated and frozen

  • Needle ice are thin slivers of ice and can be several cm long

  • Found mostly in moist soil where temperatures drop below freezing at night

  • Needle ice helps with loosening material for erosion and moving soil particles in soil creep

• Frost contraction and ice wedging

  • As temperatures drop, the active layer freezes and contracts

  • Crack begin to form in the permafrost as a result

  • When the active layer thaws, meltwater will fill the cracks

  • The cold of the permafrost freezes the water in the crack, forming ice-wedges

  • Continued melting and thawing can enlarge the crack to sizes of 3m wide and 10m deep

• Nivation

  • A blanket term for active processes that occur at the edges of snow patches 

  • The processes include physical and chemical weathering that occur underneath patches of snow

  • Fluctuating temperatures and meltwater promote chemical weathering and freeze-thaw action 

  • Weathered material is transported with the summer meltwater

  • Repeated cycles of melting, freezing and transportation forms nivation hollows

  • Saturated debris (due to summer meltwater), destabilises the slope and slumping may occur 

• Frost heave

  • Frost heave occurs when soil particles or small stones are forced to the surface by ground ice

  • Freezing occurs from the surface downwards, which helps ice crystals to form either within the soil pores or as ice needles

  • As the ground ice expands, these crystals force soil and stones to the surface

• Freeze-thaw

  • The action of freeze-thaw weathering (frost shattering) results in rough, angular broken rocks 

  • At the foot of a slope, these rocks are known as scree

  • Periglacial regions are also characterised by blockfields or felsenmeer, caused through quick freeze-thaw action of temperatures of between -5° to -14°C

• Solifluction is a form of mass movement

• There are two types of solifluction:

  • Fast 

  • Slow

• Fast 

  • When an active layer on a slope becomes heavily waterlogged due to melting, gravity acts to pull it downhill

  • This flow of material is called solifluction and produces characteristic lobes on the sides of the slopes

  • The steeper the slope, the faster and further the material will travel, and the larger the lobe will be

  • Movement can be up to 10 cm/yr

• Slow 
  

  • Also known as solifluction or soil/frost creep occurs when water in the soil expands as it freezes

  • This expansion forces soil particles to rise perpendicular to the ground (frost heave)

  • As the ground thaws, the particles are dropped vertically downwards (due to gravity)

  • With each repeated cycle, soil particles gradually 'creep' downslope at a rate of a few cm/yr

• Wind erosion 

  • Periglacial regions often have strong winds and these continue to erode rock debris produced by glaciers and ice caps

• Meltwater erosion

  • Meltwater is seasonal, plentiful and freely flows from a tunnel at the base of the glacier's snout or across the surface of the glacier and into moulins or crevasses