frozen ground or permafrost


perennial hummocks formed in either the active layer in permafrost areas, or in the seasonally frozen ground in non-permafrost areas, during freezing of the ground.
These big lumps in the ground are called thufur. Thufur can form when the ground freezes and thaws. These thufur are in Iceland. —Credit: Reinhold Richter


a passive heat transfer device installed to remove heat from the ground.

thermokarst terrain

the often irregular topography resulting from the melting of excess ground ice and subsequent thaw settlement.

thermokarst mound

a hummock remaining after melting of the ice wedges surrounding an ice-wedge polygon.

thermokarst lake

a lake occupying a closed depression formed by settlement of the ground following thawing of ice-rich permafrost or the melting of massive ice.
Increased thawing of frozen ground could create more thermokarst features, like this lake. —Credit: Andrew Slater


the process by which characteristic landforms result from the thawing of ice-rich permafrost or the melting of massive ice.

thermo-erosional niche

a recess at the base of a river bank or coastal bluff, produced by thermal erosion of ice-bonded permafrost.

thermo-erosional cirque

the usually steep, horseshoe-shaped headwall of a retrogressive thaw slump.

thermal-contraction-crack ice

ice formed in thermal contraction cracks in the ground.

thermal-contraction crack

a tensile fracture resulting from thermal stresses in frozen ground.


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