| Term: | Active layer |
| Definition: | The Active Layer is a layer of ground that is subject to annual thawing and freezing in areas underlain by Permafrost. In the zone of Continuous Permafrost the Active Layer generally reaches the Permafrost Table; in the zone of Discontinuous Permafrost it often does not. The Active Layer includes the uppermost part of the Permafrost wherever either the salinity or clay content of the Permafrost allows it to thaw and refreeze annually, even though the material remains cryotic (T < 0C). The Active Layer depth may vary considerably from one year to another depending on a number of variables (cf. Active Layer Thickness). If the ground thermal regime is no longer in long-term climatic equilibrium, Active Layers tend to show trends of increasing thickness. |