Tundra Polygons and Ice Wedges

Tundra polygons consist of a polygonal network of cracks that penetrate the surface of permafrost. (Photo) Each polygon ranges in size from a few metres to 50m wide. They form in response to repeated annual cycles of thermal contraction and cracking of the ground during winter. When ground temperatures fall below -20^oC contraction stresses develop and these cause the ground to rupture into an interconnecting series of surface cracks.

The infilling of these open cracks with snow and ice leads to the formation of ice wedges, which are wedge shaped bodies of ice that penetrate down into the permafrost from the base of the active layer. (Photo) As this process repeats itself over hundreds of years ice wedges can reach widths of up to 3m and vertical heights of 10m. Since tundra polygons and ice wedges only form under very cold climate conditions they are restricted to areas of continuous permafrost.

 

Pingos

Pingos are dome-shaped hills that are formed by the bulging of the surface in response to the growth of a large body of ground ice (Photo). They are generally oval or circular in plan and may attain diameters of up to 600m and heights up to 50m. There are two types of pingos

Open System: form in discontinuous permafrost where a gap within the frozen ground enables pressurised water to flow upwards through rock and soil. As this water rises towards the surface, it encounters progressively colder ground temperatures and begins to freeze to form a large body of ice. Fuelled by the continuous freezing of groundwater from beneath, the growing ice mass eventually deforms the ground surface into a hill. (Fig)

Closed System: these pingos form following the draining of a lake above an unfrozen talik in continuous permafrost. With the removal of the insulating cover of water, permafrost begins to advances into the saturated talik from all direction. This causes the talik water to be squeezed upwards where it eventually freezes to form a massive body of ice, which in turn deforms the ground surface into a pingo. Closed system pingos are particularly common on the Mackensie Delta, northern Canada, where the drainage of lakes is caused by the erosion of the soft delta sediment by rivers. (Fig)