P4 Flashcards
Periglacial Processes:
• Periglacial areas are those that experience a cold climate, with intense frost action and the development of permafrost.
• Today up to 25% of the Earth’s surface may be described as periglacial (both mountainous areas and areas within the Arctic circle
Permafrost:
• Is permanently frozen ground.
• Subsoil temperatures much remain below zero for two years or more for it to develop
• The extent, depth and continuity of the layer varies through time according to fluctuations in climate.
• During the summer, when air temperatures rise above freezing, the surface layer thaws to form an active layer up to 4m deep.
• There are three main types of permafrost:
Continuous permafrost
found in the coldest regions such as the Arctic, where there is little summer thawing. It affects the soil and rock to a depth of up to 700m in parts of Canada and up to 1500m in some regions of Siberia.
Discontinuous permafrost
this is found in slightly warmer regions where freezing conditions do not penetrate to such great depths (av. 20-30m). It is called discontinuous as there are breaks in the permafrost around rivers, lakes and the sea.
Sporadic permafrost
here mean annual temperatures are around or barely below freezing point and permafrost occurs only in isolated areas
Frost Heave:
- This process results from the direct formation of ice crystals or lenses in the soil.
- On freezing, fine-grained soils expand unevenly upwards to form small domes.
- As stones cool down faster that the surrounding soil, small amounts of moisture in the soil beneath the stones freeze and turn to ice, expanding by 9% as they do so.
- By repeated freezing and thawing over time, these crystals and lenses heave stones upwards in the soil profile.
- In areas where temperatures fluctuate between 0 and -4 degrees Celsius the frost heaving and thawing is able to sort material into patterned ground.
- The larger stones move outwards down the very low slopes of smaller domes because of their weights.
- On gentler slopes stone polygons are created, but where the ground is steeper than 6 degrees, the stones are dragged by gravity into more linear arrangements - stone stripes.
Groundwater Freezing:
• Freezing of water in the upper layer of soil where permafrost is thin or discontinuous leads to the expansion of ice within the soil
• This causes the overlying sediments to heave upwards into a dome-shaped feature known as a pingo:
• Found on flat tundra plains
• Dome shaped and isolated hills
• Top may be indented or ruptured to expose an icy core or water filled hollow
Up to 50m high and 500m in diameter
East Greenland type Pingo Formation (open type):
• As temperatures fall, there is progressive downward freezing of water-saturated sediments
• Then, as subsurface pressure increases, it forces the ground to bulge upwards
Mackenzie type Pingo Formation (closed type):
- Found in the Mackenzie delta, Canada
- Develop beneath lakes that are surrounded by permafrost
- The soil beneath the central part of the lake is unfrozen as it is insulated by the water
- Sediments are washed into the lake, which slowly infills, continuing the insulation
- Over time, the water in the sediments freezes, but some trapped unfrozen material (talik) remains beneath this.
- As temperatures continue to decrease, the permafrost encroaches on this unfrozen material; as it does, the pressure is increased due to water expanding on freezing.
- To relieve the pressure, the surface bulges upwards
- Eventually all the water is converted to ice, forming a core of clear ice under the bulge.
For both types of Pingo:
• There may come a time when the intrusion of ice and the stretching of the overlying soil causes fracturing
• This leads to the collapse of the pingo as the cracks allow warmer air to penetrate the ice core of the mound, and so it begins to melt
• Thus, ruptured pingos (ognips) are characterised by a collapsed centre as the ice has disappeared.
• The longest known pingo lifespan is 1000 years
Ice Wedges:
- Widespread in periglacial environments
- Develop in areas of continuous permafrost where the soils are poorly drained
- During the winter, soil temperatures drop below -15 degrees, causing the soil to contract and form cracks in the frozen ground
- When temperatures rise during spring, the soil expands and moisture collects in the cracks and freezes
- Freezing causes the ice to expand and prevents the crack from closing
- The average dimension of a wedge ranges from 15m to 40m
- Example: Long Hanborough Carrot near Oxford
Nivation:
- Occurs mainly between north- and east-facing slopes beneath patches of snow in hollows of bare rock
- It is essentially frost action affecting the land beneath a blanket of snow and may involve freeze-thaw weathering as well as solifluction and meltwater.
- Freeze-thaw action causes the underlying rock to disintegrate.
- During the spring thaw, the weathered particles are moved downslope by the meltwater and by solifluction.
- Over a period of time this leads to corrie formation.
Solifluction:
- The effect of the summer thaw on the active layer is to release a great deal of meltwater
- As the water is unable to percolate downwards (as ground is frozen), it saturates the soil, reducing the internal friction between particles, thus making it highly mobile.
- The lack of substantial vegetation to fix the saturated soil means that it begins to flow even on slopes of very few degrees
- The deposits it leaves behind are characterised by rounded, tongue-like features often forming terraces on the sides of valleys - solifluction lobes/terraces.
- These stepped lobes may be formed beneath a turf of vegetation which is pushed forwards and rolled under (like a caterpillar truck)
- Where vegetation is sparse stones heaved to the surface are pushed to the front of the advancing lobe and form a small stone bank at the front
