P2 Flashcards
Periglacial environments
- climate conditions and landscape that characterised the areas near the margins of glacier ice during the Pleistocene, or temporally before the onset of glacier conditions.
- However, the term is now more widely used to include all non-glacial cold climate areas with a high range of different high latitude and high altitude environments which may or may not contain glaciers.
Periglacial climates typically have many of the following characteristics:
- intense frosts during winter and on any snow-free ground in summer
- highest average annual temperatures usually range from 1 °C to -4 °C
- daily temperature below 0°C for at least nine months, and below -10°C for at least six months per year
- temperatures rarely rise above 18°C, even in summer
- low precipitation: typically under 600 mm per year (<100 mm in winter and <500 mm in summer)
- temperatures fluctuating through frequent cycles of freezing and thawing to cause interstitial ice (ice within cracks) to melt (Figure 4.9).
These climate conditions give rise to a variety of processes,
collectively known as periglacial. These processes combine to produce distinctive landscapes, containing landforms that are unique to periglacial areas. Some of these processes, such as frost action, occur elsewhere quite widely, although with less intensity than in periglacial areas.
Distribution past and present
- Around 20 per cent of the Earth’s land surface experiences periglacial conditions (Figure 4.8, largely in the northern hemisphere, with extensive areas in Siberia, northern Scandinavia, northern Canada and Alaska.
- The distribution of periglacial environments in the Pleistocene glacial periods was very widespread, with an estimated 33 per cent of the world experiencing these conditions, at much lower latitudes than today.
- For example, periglacial conditions extended across Europe as far south as southern France, northern Italy and the Balkans.
- Interestingly, relict (surviving) landform evidence shows that southern Britain was not covered by ice but experienced periglacial conditions.
Active layer:
The top layer of soil in permafrost environments that thaws during summer and freezes during winter.
Permafrost
- Permafrost is often loosely defined as ‘permanently frozen ground’, but technically the term refers to soil and rock that remains frozen as long as temperatures do not exceed 0 °C in the summer months for at least two consecutive years.
- Continuous permafrost forms in the coldest areas of the world where mean annual air temperatures are below -6 °C. It can extend downwards for hundreds of metres.
- Discontinuous permafrost is more fragmented and thinner.
- Sporadic permafrost occurs at the margins of periglacial environments and is usually very fragmented and only a few metres thick; it often occurs on shady hillsides or beneath peat.
- In summer, the energy balance is positive, which causes overlying snow and ice to melt away to produce a seasonally unfrozen zone above the permafrost called the active layer, which varies from a few centimetres to as deep as 3.0 m.
up to….
- 25 per cent of the Earth’s surface is currently experiencing permafrost conditions (almost 50 per cent of Canada and 80 per cent of Alaska).
- While periglacial environments usually contain permafrost, sometimes areas of periglacial activity involving intense frost action do exist outside the permafrost zone.
A number of factors influence the distribution and character of permafrost:
- Climate is the main control, as temperature and the amount of moisture available determine the presence or absence, depth and extent of permafrost.
- On a local scale, the depth and extent of permafrost is influenced by a number of interrelated factors:
- Proximity to water bodies is important; lakes are relatively warm so remain unfrozen throughout the year with a deep active layer.
- Slope angle and orientation influence the amount of solar radiation, and therefore melting, freeze-thaw and wind.
- Character of ground surface (different rock and soil types) can determine the degree and depth of permafrost; for example, dark compact rocks absorb a greater amount of solar radiation.
- Vegetation cover can insulate the ground from temperature extremes.
- Snow cover can slow the freezing process in winter and, in spring, delay the thaw and development of the active layer.
Periglacial processes and landforms
Cold climate environments develop distinctive geomorphology because of four basic processes.
• The nine per cent expansion of water on freezing; this causes frost shattering, which forms block fields and screes.
• The contraction and cracking of rapidly freezing soils in which ice wedges form, as well as frost heaving, which creates patterned ground.
• The migration of sub surface water to the ‘freezing front’ by suction, which causes the formation of segregated ice leading to the formation of ice lens, palsas and pingos.
• The mass movement of the active layer downslope largely by solifluction, which leads to lobes and terraces.
Of these processes only frost shattering occurs outside periglacial areas; the other three processes are associated with permafrost, and melting and movements within the active layer.
Ground ice features
Ice wedge polygons
Patterned ground
Pingos
Ice wedge polygons
- Major ground ice features include networks of ice wedge polygons which are unique to periglacial areas.
- The process of frost cracking creates areas of irregular polygons, 5 to 30 m across, usually found on valley floors.
- When the active layer thaws, ice wedges can begin to form as water flows down into the cracks; it subsequently freezes and contracts, which means the ice wedge can build up over time.
- Larger ice wedges are usually a tapering shape 1 to 2 m wide and up to 10 m deep, extending down into the permafrost, taking over 100 years to form.
- e.g banks of the River Till, near Etal Northumberland
Patterned ground
- Patterned ground is the general term for a range of features including circles, nets, polygons, steps and stripes.
- These features are unique to periglacial areas and are formed by a series of movements resulting from frost action.
- Frost push propels the stones upwards, while frost heave causes the stones to migrate outwards to form circles, which provides the basis for each of the patterns.
- The up doming of the circle created by heave means that larger stones roll outwards as a result of gravity, while finer sediments remain central
- As a result of mass movement, stone polygons are elongated into stone nets and stripes, with a clear relationship between slope angle and the type of patterned ground
- Once slope gradients go beyond 30° patterned ground features no longer form and rock avalanches may occur.
- e.g on the Glyders, North Wales
Pingos
- Pingos are another unique periglacial feature.
- They are ice core mounds 30 to 70 m in height and 100 to 500m in diameter.
- The mounds can be either conical or elongated.
- The growth of an ice core forces up the overlying sediments, causing dilation cracks.
- Once the ice core is exposed at the surface it melts, causing the top of the pingo to collapse forming a crater, which may be filled with meltwater and sediments.
- e.g the north york moors
Two types of pingo occur:
- Open system (hydraulic pingos, or East Greenland type)
- are found in the discontinuous zone of permafrost or valley floors.
- Freely available groundwater is drawn towards the expanding ice core, so the pingo grows from below the ground.
- Closed system (hydrostatic pingos, or Mackenzie Delta type),
- which are associated with low-lying flat areas and only form in zones of continuous permafrost.
- They form from the downward growth of permafrost, often after a small lake is gradually enclosed with sediments.
- The loss of the insulating influence of the lake allows permafrost to advance, trapping the body of water and putting it under hydrostatic pressure and, ultimately, freezing it to push up the earth above it.
- Smaller features similar to pingos, known as palsas, occur within peat beds.
The role of frost shattering
Freeze-thaw weathering puts pressure on any cracks in rocks and shatters them. While the process is not unique to periglacial environments, it occurs with greater severity within them. The features created by freeze-thaw include:
Block fields
Tors,
Scree or talus slopes
Pro-talus ramparts
Rock glaciers
Possible relict periglacial landforms found in Great Britain
