Freezing progresses down from the surface. Meanwhile, the ground expands upwards, lifting a stone by first freezing onto the upper surface. The freezing front progresses downwards and space is created below the stone as it is lifted. Ice crystals grow into this space and push up the stone. Thawing progresses down from the surface causing contraction and lowering of the surface level but the stone is still supported by the ice crystals which have collected beneath it. Thawed sediment collapses around the stone supporting it. The stone is held in thawed sediment while ice crystals melt and space is filled by collapsing sediment.
Patterned ground is formed as stones collect, or stone polygons when ice wedges (see thermal contraction) are present. These stones remain in place when the gradient is lower than 6. Shapes elongate on steeper slopes to form stone stripes or garlands.
Pingos are huge landforms, roughly 500m in diameter and 50m in height. Pingos are often located in Canada and Greenland. There are two types of pingos:
- Closed system pingos are known as Canada or Mackenzie pingos due to their location. Canada pingos occur in areas of continuous permafrost. On the site of a lake, groundwater is trapper by freezing from above and permafrost below expanding inwards. The water freezes and expands, pushing overlying sediment upwards. If the pressure is too great, the top of the sediment heap can rupture and collapse. It may then infill with water.
- Open system pingos are known as Greenland pingos and occur in areas of discontinuous permafrost. Water infiltrates from the surface to the layer below during the summer. Freezing and expansion cycles force sediment up into a dome shape which can rupture if the pressure becomes too great.
When a pingo ruptures, the collapsed structure is known as an ognip.
