Periglacial / permafrosts

Permafrost

Ground that remains frozen (below 0 degrees C) for at least two consecutive years.

Pingos

Periglacial landforms, ice-cored hills, conical in shape and vary between 3-70m in height.

Talik

A layer of year-round unfrozen ground that lies in permafrost areas.

Ice lens

Group of frozen moisture in the soil.

Two different types of Pingos

1. Open-system Pingo
2. Closed-system Pingo

Open system Pingo formation: Areas with discontinuous permafrost

When temperatures drop, hydrostatic pressure increases and moisture in the ground freezes and rises.
Subsequently, creating a mound in the ground that rises upwards
If pressure in mound becomes too great, ground above ice lens will become too great and expose ice, later creating a lake (lake pingo).

Closed system Pingo formation: Areas with permafrost

When temperatures drop, hydrostatic pressure increases and moistures in soil freezes and rises becoming an ice lens, or a lake can freeze over.
As permafrost advances, it pushes up ice lens / frozen lake, forming a mound.

Patterned ground

terrain that exhibits regular or irregular surface patterning, most commonly in the form of circles, polygons.

Formation of patterned ground

As hydrostatic pressure increases in ground, moisture in the ground begins to freeze and rise, forming an ice lens as they group together.
Capillary action then leads to more moisture joining the ice lens.
As ice lens grows, it pushes stones (above) up, then smaller sediment in soil filters into gap left by rising soil (stopping stones from falling down).
Stones now poke out of the ground

Capillary action

When moisture moves through the soil to join ice lens

Ice wedge polygons (unique to periglacial areas and found on valley floor)

During summer - when active layer thaws - water flows down into cracks which become filled with meltwater.
During subsequent winter, any water-filled cracks expand as ice forms. Over time cracks widen to form ice wedges.
Repeated freezing and thawing leads to formation of wedges through frost cracking (1to 2m wide at top and 8 to 10m deep) (features take ~100 years to form)

How do ice wedge polygons and patterned ground differ?

Ice wedges polygons differ from patterned ground in that they are larger, and edges are slightly higher than their centres, whereas frost-heave polygons are domed.