thermokarst

thumb|Permafrost thaw ponds in Hudson Bay, Canada, in 2008 Thermokarst is a type of terrain characterised by very irregular surfaces of marshy hollows and small hummocks formed when ice-rich permafrost thaws. The land surface type occurs in Arctic areas, and on a smaller scale in mountainous areas such as the Himalayas and the Swiss Alps.

These pitted surfaces resemble clusters of small lakes formed by dissolution of limestone in some karst areas, which is how they came to have "karst" attached to their name, even though no limestone is actually present. Small domes that form on the surface due to frost heaving with the onset of winter are only temporary features. They collapse during the following summer thaw, leaving a small surface depression. Some ice lenses grow and form larger surface hummocks ("pingos") which can last for many years, and sometimes become covered with grasses and sedges, until they begin to thaw. These domed surfaces eventually collapse – either annually or after longer periods – and form depressions which become part of the uneven terrains included under the general category of thermokarst.