Frost boil

Mud boils near Lapporten, Sweden

Frost boils (also known as mud boils, frost scars and mud circles) are upwellings of mud that occur through frost heave and cryoturbation in permafrost areas, such as arctic and alpine regions. They are typically 1 to 3 metres in diameter with a bare soil surface, and dominantly circular, lacking "a border of stones".

Frost boils often occur in swarms and may form terraces if a series of them surface in a slope. On slopes, frost boils are sometimes protected from erosion by a thin layer of mosses and lichens which retain surface tension as sediments flow downslope, to form a lobe. These landforms eventually settle like a caterpillar track (Van Everdingen, 1998).

Common characteristic include a bowl-shaped boil, an elevated center, a formation of an organic layer on the outer edge, and resistance of the soil surface to vegetation colonization.

Drainage on frost boils differs as a result of micro relief across the frost boil surface. In warm seasons (summer), the elevated center of the frost boil is moderately well drained compared to the depressed inter boil. The permafrost table surface is also affected by differing activity across the boil. The inner boil is more active and generally has more than twice the active depth than the inter boil, which causes the permafrost table surface to be in a nearly perfect bowl shape.