Geology Reference
In-Depth Information
experience diurnal rather than seasonal conditions. Sub-arctic locations which experience
seasonal freezing and thawing but which are not underlain by permafrost are a second.
In both environments, the absence of an impermeable frozen layer means that meltwater
percolates to the groundwater table. There is also an absence of up-freezing in the autumn
freeze-up period. The net effect of both is to reduce the importance of the zero
curtain.
4.4.2. Short-Term Fluctuations
Short-term temperature fl uctuations are frequently imposed upon the long-term tempera-
ture cycle just described. These fl uctuations may be classifi ed as being either diurnal or
cyclonic in nature, although it is often diffi cult to distinguish between the two.
Diurnal variations usually relate to changes in solar insolation and surface heating
brought about by variations in angle and azimuth of the sun. In high latitudes, these
diurnal infl uences are relatively weak because the sun is above the horizon for much of
the Arctic day. In middle latitudes, diurnal ranges are better developed. For example,
the range between mean daily maximum and minimum air temperatures, at 3050 m in the
Colorado Front Range, can reach 15 °C in July (Fahey, 1973). It is also in middle latitudes
that the effects of orientation become well developed, with south- and west-facing
slopes in the northern hemisphere experiencing signifi cantly higher near-surface soil
temperatures.
Local climatic events of a cyclonic nature may also produce marked fl uctuations in soil
temperature. This is especially the case in High Arctic regions, where periods of direct
solar radiation may heat the ground surface to give a 20-30 °C temperature difference
between air and ground surface. In Antarctica, for example, thawing of snow when air
temperature is
20 °C may be caused by the localized heating of dark rock surfaces. The
passing of a cloud, or the onset of precipitation, will also affect soil temperatures. For
example, on Banks Island, the author documented the rapid changes in soil temperatures
which followed a summer snowfall of 2.5 cm (French, 1970); within 3-4 hours, the entire
active layer had attained a near-isothermal state due to downward percolation of the
snowmelt. Even on the Tibet Plateau, at altitudes in excess of 4800 m a.s.l., air tempera-
tures fl uctuate signifi cantly during the day in accordance with periods of direct sunshine
(Wang and French, 1994, 1995a). A fi nal example is provided by data obtained over a 31-
day period in early 1992 on Livingston Island, in the South Shetlands, maritime Antarctic
(Serrano et al., 1996); a total of 14 daily fl uctuations above and below freezing were
recorded.
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4.5. ROCK (FROST?) SHATTERING
The disintegration, or mechanical breakdown, of bedrock in periglacial regions is well
known. The most dramatic features that result are the extensive surfaces of angular
rock fragments, commonly referred to as “blockfi elds” in the English-language literature
(Washburn, 1969), or “kurums” in the Russian literature (Romanovskii and Tyurin, 1983,
1986; Romanovskii et al., 1989). Other features include tors and near-vertical rockwalls
with extensive debris slopes beneath (see Chapter 9), and brecciated bedrock (see Chap-
ters 7 and 13). Traditionally, this mechanical disintegration has been attributed to intense
frost action, usually combined with the presence of moisture and ice segregation. In recent
