What Is Permafrost?

Permafrost landscape with a pingo hilltop in the mid-ground and mountains in the distance. Yukon Territories, Canada. Photo © Ron Erwin / Getty.

Permafrost is any soil or rock that remains frozen—below 32°F—throughout the year. For a soil to be considered permafrost, it must be frozen for at least two consecutive years or longer. Permafrost can be found in cold climates where the mean annual temperature is less than the freezing point of water. Such climates are found near the North and South poles and in some alpine regions.

Some soils in areas that experience warmer temperatures thaw for a short time during warmer months.

The thawing is restricted to the top layer of soil and a permafrost layer remains frozen several inches below the surface. In such areas, the top layer of soil—known as the active layer—warms up enough to enable plants to grow during the summer. The permafrost that lies below the active layer traps water close to the soil's surface, making it quite soggy. The permafrost ensures a cool soil temperature, slow plant growth, and slow decomposition.

Several soil formations are associated with permafrost habitats. These include polygons, pintos, solifluction and thermokarst slumping. Polygon soil formations are tundra soils that form geometric shapes (or polygons) and are most noticeable from the air. The polygons form as the soil contracts, cracks, and collects water trapped by the permafrost layer.

Pingo soil formations form when the permafrost layer traps a large amount of water in the soil.

When the water freezes, it expands and pushes the saturated earth upward into a large mound or pingo.

Solifluction is a soil formation process that occurs when thawed soils slide down a slope over the permafrost layer. When this happens, the soils form rippled, wave patterns.

Thermokarst slumping occurs in areas that have been cleared of vegetation, usually due to human disturbance and land use.

Such disturbance leads to the melting of the permafrost layer and as a result the ground collapses or slumps.