Geology Reference
In-Depth Information
both alpine permafrost and polar mountain permafrost (Cheng and Dramis, 1992), for
simplicity, the term “alpine” permafrost is used here.
Alpine (mountain) permafrost is not to be confused with plateau (montane) perma-
frost, a term reserved exclusively for permafrost on the Tibet Plateau (see below). This is
because many of the controls over the latter are quite different to those over alpine per-
mafrost. Moreover, alpine permafrost implicitly involves consideration of steep slopes,
solar radiation and aspect, and snow. Like latitudinal permafrost, the most important
control over alpine permafrost is climate; this depends on the geographic location of the
mountains in question (i.e. latitude, altitude, and distance from ocean).
Zonation of alpine permafrost is not unlike that of high-latitude (polar) permafrost;
namely, a zone of continuous permafrost that occurs below the snow (fi rn) limit is replaced
by a zone of discontinuous permafrost at lower elevations. This, in turn, is replaced by a
zone of sporadic permafrost.
In the North American Cordillera, the distribution of permafrost varies both with
altitude and latitude (Péwé, 1983a). Field observations suggest that the lower altitudinal
limit of permafrost rises progressively from approximately 1000 m a.s.l. at 60° N to over
3000 m a.s.l. in Colorado and New Mexico at latitudes 39° N-33° N. In Central Mexico,
isolated occurrences of frozen ground have been reported from the summits of volcanic
peaks at elevations of 4500-4800 m a.s.l. (Figure 5.7A). In the European mountains, a
number of recent mapping and monitoring studies have improved our understanding of
alpine permafrost (Haeberli and Burn, 2002; Harris et al., 2001; Isaksen et al., 2001; King,
2000). The nature of permafrost in the Japanese Alps has also been subject to detailed
investigation (Matsuoka and Ikeda, 1998).
In general, the relationship between the lower altitudinal limit of alpine permafrost
and latitude can be expressed by a Gaussian curve (Figure 5.7B) (Cheng, 1983; Cheng
and Dramis, 1992; Corte, 1988). This resembles, in broad outline, the snowline, the alpine
cold-desert zone, and the timberline plotted against latitude. Regional controls over alpine
permafrost are usually related to an increase in continental conditions away (inland) from
nearby oceans. For example, in Scandinavia, as the snowline or glacial limit rises, the
lower limit of permafrost decreases (King, 1986). Similar changes occur in the Cordillera
of North America, with greater snowfall in the coastal ranges causing the lower limit of
permafrost to decrease eastwards (Harris and Brown, 1978, 1982).
The extent and lower altitudinal limit of alpine permafrost is complicated by the
relationships that exist between glaciers, timberline, and permafrost in the mid-latitude
alpine environment (Figure 5.8). In humid-maritime environments, the lower parts of
temperate glaciers often extend into forested valleys where summer warmth and winter
snow accumulation prevent permafrost formation. As a result, no tundra or periglacial
zone exists. By contrast, in dry-continental environments, as in many central Asian
mountain chains, the glacier equilibrium line may be far above timberline. The occur-
rence of cold-air drainage, or temperature inversions and ice-caves (Harris, 1979, 1983),
further complicates the situation. Surface features, such as rock glaciers, are sometimes
used to infer the lower limit of alpine permafrost, but this can be controversial because
some rock glaciers may be inactive while others may be glacial in origin (Barsch,
1988).
One method used to predict the occurrence of permafrost in the humid mountains of
Europe is the bottom temperature of the winter snow cover. This is called the BTS
method. W. Haeberli (1973, 1978) and others (Ikeda and Matsuoka, 1999; King, 1983)
have demonstrated that permafrost is usually present in the Alps and in Scandinavia
if the BTS beneath at least 80-100 cm snow cover during February and March is less than
−
2 °C. According to M. Hoelzle (1992), the following “rules of thumb” apply: values of
