Geology Reference
In-Depth Information
pore ice and the migration of unfrozen pore water. Thus, permafrost creep will be most
effective in ice-rich soils on steep slopes in areas of warm permafrost. This is because of
the large amount of unfrozen water existing at these temperatures and the increasingly
plastic nature of the ice. It follows that, ground temperature and ground-ice amount are
the two obvious controlling variables; the warmer the permafrost and the greater the
amount of ground ice, the greater will be the deformation.
Several fi eld studies have documented the nature and magnitude of permafrost
creep on gentle slopes (Table 6.2). In relatively warm permafrost, such as in the Mac-
kenzie Valley and in Tibet, annual rates of deformation of 0.1-0.4 cm/yr appear typical,
while in colder permafrost of the High Arctic, movement is one order of magnitude
less, in the vicinity of 0.03-0.05cm/year. Permafrost creep is discussed further in
Chapter 9.
6.4.2. Types and Distribution
There are two types of periglacial rock glaciers: (a) “talus-rock glaciers” occur below talus
slopes, and (b) “debris-rock glaciers” occur below glaciers (Figure 6.10). Active rock gla-
ciers are probably best developed in continental and semi-arid climates since, under these
conditions, the extent of the mountain periglacial zone is greatest (i.e. the snowline is
highest).
Rock glaciers form where permafrost is present and where there is an adequate supply
of debris. Two supply mechanisms are common: fi rst, from talus slopes and their associ-
ated mechanical weathering, and second, from adjacent moraines, where the moraine
forms the debris supply. Given an adequate accumulation of coarse clastic debris, percolat-
ing snowmelt infi ltrates and freezes to form an ice-debris matrix which then deforms.
Typical movement rates vary from several centimeters to several meters per year, and a
typical surface relief consists of arcuate ridges and furrows aligned, in general, perpen-
dicular to the fl ow direction.
Rock glaciers are reported from most of the major mountain regions of the world,
including the European Alps (Baroni et al., 2004; Guglielmin and Smiraglio, 1997;
Haeberli, 1985), the central Asian mountains (Cui Zhijui, 1983; Gorbunov, 1988b; Gor-
bunov and Tytkov, 1989), and the Cordillera of both North and South America (Corte,
1978; Brenning 2005; White, 1971). In high latitudes, talus rock glaciers have been
described from Svalbard (André, 1994; Berthling, et al., 1998, 2000; Sollid and Sörbel,
1992), Greenland (Humlum, 1996, 1997, 1998a, 2000), the Faeroe Islands (Humlum,
1998b), and the Canadian Arctic (Evans, 1993). Relict rock glaciers have been described
from numerous locations, including the northern Yukon (Harris et al. 1983, p. 69; Vernon
and Hughes 1966), Gaspésie, Québec (Hétu et al., 2003), the Italian Alps and Apennines
(Carton et al., 1988; Dramis and Kotarba, 1992), the Pyrenees (Chueca, 1992), and the
southern Carpathian Mountains (Urdea, 1992).
6.4.3. Origin
The key to their origin lies in their internal structure. This is often diffi cult to ascertain
without costly and sometimes impossible drilling. As a result, geophysical (seismic)
methods are frequently used. Referring specifi cally to the Galena Creek rock glacier
(Potter, 1972), D. Barsch (1988) comments that this rock glacier is probably composed of
a mixture of debris and ice since P-wave velocities of 2400-4000 m/sec are typical, whereas
