Page 308 - Fundamentals of Geomorphology

PERIGLACIAL LANDSCAPES 291

Plate 11.7 Ploughing boulder with furrow, levee, and Plate 11.8 Active rock glacier, Swiss
frontal lobe, Rock and Pillar Range, New Zealand. Alps.
(Photograph by Stefan Grab) (Photograph by Stefan Grab)

development (e.g. Bliss 1990). Permafrost degradation the forests being converted to minerotrophic floating mat
occurswherethethermalbalanceofthepermafrostisbro- fens (Osterkamp et al. 2000). A hundred years ago or
ken, either by climatic changes or by changing conditions more at this site, some 83 per cent of 260,000 ha was
at the ground surface. underlain by permafrost. About 42 per cent of this per-
In the Low Arctic, mineral exploration has led to mafrost has been affected by thermokarst development
the melting of permafrost. Under natural conditions, within the last 100 to 200 years. The thaw depths are
peat, which is a good insulator, tends to prevent per- typically 1–2 m, with some values as high as 6 m. On
mafrost from melting. Where the peat layer is dis- the Yamal Peninsula of north-west Siberia, land-use and
turbed or removed, as by the use of tracked vehicles climatic changes since the 1960s, when supergiant nat-
along summer roads, permafrost melt is encouraged. ural gas fields were discovered, have led to changes in
Ground-ice melting and subsequent subsidence pro- the tundra landscape (Forbes 1999). Extensive explo-
duce thermokarst, which resembles karst landscapes ration meant that large areas were given over to the
(cf. p. 284). In the Tanana Flats, Alaska, USA, ice-rich construction of roads and buildings. Disturbance asso-
permafrost that supports birch forest is thawing rapidly, ciated with this development has affected thousands of