Question 15 of 21: Question 5, Part 2: Permafrost Cross-Section

Question 5, Part 2: Permafrost Cross-Section (5 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. A generic high-latitude Canadian ground column subject to seasonal freeze – thaw at the surface and long-term subsurface freezing (permafrost).

Find. (a) A labelled vertical cross-section through permafrost ground; (b) the two permafrost zones recognized across Canada and how they differ.

Approach. From the surface down: an active layer that freezes/thaws every year, then permafrost (ground remaining below 0°C for ≥2 consecutive years) extending down to the depth where geothermal heat flow raises the temperature back above freezing (the permafrost base); Canada's permafrost distribution is zoned by how much of the ground is actually frozen.

(a) Layers, top to bottom. The active layer is the near-surface zone that thaws every summer and re-freezes every winter (its thickness governs frost-heave and foundation design). Below the seasonally stable permafrost table, the permafrost proper remains below 0°C year-round (often containing segregated ground ice, ice wedges, and unfrozen "cryopeg" pockets in saline ground). At the permafrost base, the upward geothermal heat flux finally warms the ground above 0°C, and unfrozen ground continues below.

(b) The two zones of permafrost in Canada. Canada's permafrost map is divided into a continuous permafrost zone (the far north – Arctic islands, northern mainland tundra) where permafrost underlies essentially the ENTIRE landscape (>90%), is typically hundreds of metres thick, and has only a thin active layer; and a discontinuous permafrost zone (a broad transitional belt across the subarctic, e.g. much of the boreal forest) where permafrost is patchy – occupying anywhere from a few percent to most of the ground, interrupted by taliks (unfrozen ground, often under lakes/rivers or south-facing/well-drained sites), generally thinner, and far more sensitive to disturbance and climate warming than the continuous zone.