Geology Reference
In-Depth Information
(A)
(B)
-2.0
+24
-3.0
see (B)
+12
-4.0
-5.0
0
-6.0
-12
9.11
9.12
9.13
9.14
9.15
24 January
2001
Noon
23 January
2001
P
.M.
January 23, 2001
Time
Figure
4.4.
High-frequency temperature measurements observed on a south-facing vertical brick
surface, January 23-24, 2001, Prince George, northern British Columbia, Canada. Modifi ed from
Hall (2004). Adapted with permission from John Wiley & Sons Ltd.
Collectively, the type of data presented in Table 4.1. led K. Hall to explicitly question
the effectiveness of freeze-thaw weathering (Hall, 1995, 1997a, 1998). More recently, in
a series of studies that involved high-frequency surface and near-surface rock temperature
measurements, attempts have been made to further quantify the magnitude and nature of
freeze-thaw (Hall, 2004; Hall and André, 2001, 2003). One study involved temperature
measurements made at 1-minute intervals upon a set of paving bricks exposed to freezing
and thawing in northern British Columbia, Canada (Hall, 2004). The high frequency of
measurement enabled the recognition of so-called “exotherms” (cf. exothermic (adj.),
chemical change that generates heat) (Figure 4.4). The temperature at which freezing
occurred varied through the year, with some inferred exotherms indicating that freezing
occurred when air temperatures were approaching
30 °C. On one occasion, the 1 cm
depth froze before the surface of the brick. These changes in freeze temperature may be
related to the chemical composition of the paving brick coupled with salt from the melt
of snowfall. More details concerning the possible interactions between chemical and
mechanical weathering at cryogenic temperatures are discussed later in this chapter.
−
4.4. THE GROUND-TEMPERATURE REGIME
The majority of periglacial environments experience some form of seasonal change
through the year. It is possible, therefore, to recognize a summer thaw period of limited
duration and a longer winter period during which temperatures remain below 0 °C. The
exception is the alpine periglacial environment of low latitudes, which experiences a
diurnal rather than a seasonal cycle.
4.4.1. The Seasonal Regime
Numerous data sets record seasonal ground temperature changes. The typical regime is
illustrated by data from Fenghuo Shan on the Tibet Plateau, China (Figure 4.5). The most
obvious characteristic is that, at a depth of 1.2 m, the ground remains frozen throughout
