National Ecological Framework (14 of 23)

Permafrost

These data are presented as a set of components within each polygon.  The following attributes are included:  

The extent and nature of permafrost, including estimated ice content and typical ground ice forms are derived from the map "Canada - Permafrost" (Natural Resources Canada, 1995).

"The map depicts current knowledge of the distribution, characteristics and boundaries of permafrost and ground ice in Canada, using a physiographic approach for the delineation of mapping units. For the first time, information on the distribution and extent of ground ice is presented in a consistent manner for the entire country." …

"Permafrost is defined as a state of the ground, whether soil or rock, that remains at or below a temperature of 0° C for long periods (NRC, Permafrost Subcommittee, 1988). The minimum period is from one winter, through the following summer, and into the next winter; however, most permafrost has existed for much longer. This formal definition considers only the temperature of the ground, and thus permafrost is a strictly thermal phenomenon, and not a material. At temperatures below 0° C , almost all of the soil moisture occurs in the form of ground ice. Ground ice usually exists at temperature close to its melting point and so is liable to melt if the ground warms."

"Ground ice occurs in three main forms: as coatings on soil particles and crystals within the pores of sedimentary rocks and unconsolidated deposits (pore ice); as thin, lamellar lenses and veins of ice (segregated ice, intrusive ice and reticulate ice); and as larger bodies of more-or-less pure ice, in the form of ice wedges, extensive sheets of massive ice, and pingo ice. The quantity of ice in the ground varies widely. At one extreme, it can comprise 50-70% by volume of the upper 2-3 m of permafrost over broad areas; locally, it can exceed 90% of the volume of the ground. In other areas, permafrost contains essentially no ground ice and it is termed "dry". (Natural Resources Canada, 1995)"

References in the database to low, medium and high ice contents correspond to volume percentages of < 10%, 10-20% and > 20% respectively as presented in the map legend. The extent of permafrost is given as a percentage of land area underlain by permafrost. The five classes of extent are: continuous (90-100%); extensive discontinuous (50-90%); sporadic discontinuous (10-50%); isolated patches (< 10%); and no permafrost (0%).

Original research by J.A. Heginbottom, Terrain Sciences Division, Geological Survey of Canada, Natural Resources Canada. Additional research and adaptation for the National Atlas of Canada by M.A. Dubreuil and P.T. Harker, National Atlas Information Service, Geomatics Canada, Natural Resources Canada.