Periglacial Landforms
Geography: Edexcel A Level, topic 2.2.2
Ice Wedges and Patterned Ground
Periglacial environments, such as northern Canada and northern Russia, have a range of characteristic landforms, including ice wedges and patterned ground.
Diagram
Ice wedges
- Ice-wedge polygons are generally 20–30 m across.
- They are formed in areas of continuous permafrost by ground freezing in the winter and thawing in the summer.
- When temperatures fall in winter, water in fissures in the active layer freezes and so expands, pushing the ground apart.
- The wedge shape is maintained as water further down from the surface, over 3m down, remains frozen.
- The annual repetition of this process causes the gradual formation of marked ice-wedges which develop in polygonal patterns.
Patterned ground
- Stone polygons are smaller than ice-wedge polygons, less than 10 m across and are the result of frost heave.
- During the winter, soils freeze downwards from the surface, reaching and ‘grabbing’ individual stones, which are pulled upwards by the expansion vertically of the frozen soil above them.
- The empty space left beneath is filled with loose unfrozen soil, so the stone is prevented from moving back when the soil below freezes.
Patterned ground cont.
- Stones are also pushed towards the surface because of the pressure of small ice lenses growing beneath them.
- Stones warm up more quickly so, in the summer, the ice in the soil beneath the stone melts, allowing wet sediment to slump to fill the space beneath it.
- This prevents it from sinking back into its original position.
- As the stones collect on the surface of the ground, the larger stones are pushed towards the edge of the pile by ground expansion. Gravity and smaller stones, sands and silt are left in the middle, resulting in a polygonal shape.
Pingos
Pingos are domed mounds of layered sediments with a core of ice and are usually up to 100 m in diameter. They can be as wide as 2 km.
Open pingos
- Pingos can be open or closed.
- Open pingos can be seen in the Canadian Arctic and Siberia and are more likely to be found in clusters.
- Open pingos are formed in areas of discontinuous permafrost when groundwater from small areas of talik moves upwards because of an increase in hydraulic pressure as the active layer re-freezes in winter.
- As the water freezes near the surface, it causes the ground surface to dome upwards, forming the characteristic shape of a pingo.
Closed pingos
- Closed pingos are less common and are found in Alaska and Greenland.
- Closed pingos are formed in areas of continuous permafrost when water pushed down from frozen lake sediments accumulates at depth, freezes and then expands again, causing the ground surface to dome.
- Some pingos have a small depression at their top, resulting from localised melting there.
Open pingo
Closed pingo
Test yourself on Periglacial Landforms
How wide are ice-wedge polygons usually?
Patterned Ground Formation
In what type of permafrost would you find an open pingo?