Pingos and ice-wedge polygons are large scale landforms found in periglacial regions and are responsible for some of the most striking features in periglacial environments today.
Pingos
From the Inuit word ‘hill’, pingos are dome-shaped hills found in periglacial environments.
Pingos can be up to 500m in diameter and 50m high (see Figure 1 for an example) and are generally found in permafrost regions (where ground temperatures have remained below 0°C for at least 2 years) of Arctic North America and northern Asia 1.
FUN FACT: The highest concentration of pingos is located in the Tuktoyaktuk Peninsular, on the Mackenzie Delta, Canada, home to 1350 identified examples! 2.
Formation of pingos
Generally, two types of pingo exist. Firstly we have ‘closed- system’ (hydrostatic) pingos and second we have ‘open-system’ (hydraulic) pingos 4 (Figure 2).
Both types of pingo can rupture through the surface and collapse, to leave behind relic pingos, identifiable by circular ramparts generally filled with water (Figure 1b).
| Open system pingos | Closed system pingos |
| Open system pingos occur in areas with discontinuous permafrost. Here, islands of frozen ground are separated by small pockets of unfrozen ground. As a result, open system pingos are generally found in valley bottoms. Open system pingos develop as groundwater flowing down through permeable soils is forced to the surface by artesian pressure and freezes, forming an ice lens 5. | Closed system pingos occur in areas of continuous permafrost. Here the ground only melts superficially (on the surface). As a result, closed system pingos are generally found in lowland regions. In closed systems, water confined in unfrozen material within the permafrost – known as talik – freezes to form an ice lens. This ice lens is forced to the surface through upheave activity of the permafrost beneath2. |
Ice wedge polygons
In short, ice wedge polygons are the result of repeat frost cracking and ice-vein growth occurring over hundreds to thousands of years. The process of ice wedging happens exclusively in the upper horizons of permafrost, resulting in striking landforms.
Firstly, water held within surface cracks freezes in the winter to form an ice vein. In doing so the crack expands. Then in the following spring, these veins thaw, before refreezing the following winter.
Over time, this cycle of freeze and thaw expands the cracks significantly to form ice-wedges (Figure 4).
Irregular Polygons
As a result of the repeat freeze-thaw activity described above networks of interconnected ice wedges develop beneath the ground. However on the surface, they appear as irregular polygons (Figure 4). Generally, irregular polygons are between 5 and 40 m in diameter 6 and are one of the most recognisable landforms found in periglacial landscapes today.
