active layer

The active layer is the surface layer of ground that freezes in the winter and thaws in the summer. This layer is found in permafrost regions, where the ground is frozen for two or more consecutive years. The thickness of the active layer varies depending on moisture content, ranging from less than a foot in wet sediments to several feet in dry gravels.1

Biological activity, such as root growth and decomposition, is primarily confined to the active layer. In Arctic tundra soils, the summer thaw typically penetrates 6 to 12 inches, while in areas with full plant cover and well-drained soils, it can reach 1.5 to 10 feet. The freeze-thaw cycle within the active layer is responsible for phenomena like patterned ground and solifluction.2