Geology Reference
In-Depth Information
Myagkov, 1973, both cited in Hall and André, 2001). Even on the relatively cloudy and
damp Antarctic Peninsula, rock temperature measurements on a granodiorite outcrop
indicate rates of change in excess of the critical threshold value of 2 °C/min (Hall and
André, 2001).
Despite several papers that invoke thermal stress as the cause of rock disintegration in
cold regions (Hall, 1999; Hall and André, 2001; Hall et al., 2002, pp. 589-592), there are
few direct measurements of the actual tensile stresses that must be involved. Furthermore,
the thermal stresses that are associated with bedrock at the cold end of the temperature
spectrum are often neglected by physicists and others interested in rock properties. The
few fi eld studies that are of relevance usually deal with either thermal-contraction cracking
in unconsolidated sediments (Mackay, 1974c) or frost jacking (Dyke, 1984). The most
useful information comes from geocryology. Here, E. Yershov (1990, pp. 139-140) states
that the coeffi cients of linear and volumetric thermal expansion are highest in rocks and
minerals that have a low energy of the crystalline lattice. It follows that the greater the
SiO
2
content of a rock the larger the coeffi cient becomes. Figure 4.9 provides summary
data that show the coeffi cient of linear thermal expansion of acidic igneous rocks to be
several times greater than that of ultrabasic rocks. Linked to this is the observation that
quartz-containing igneous rocks appear subject to more micro-cracking than quartz-free
igneous rocks (Winkler, 1977). This suggests that the anomalous behavior of the mineral
quartz during the process of cooling from magma leads to intra-granular and circum-
granular cracking, when the rock is eventually exposed to sub-aerial conditions. It is sig-
nifi cant that many examples of cold-climate rock shattering attributed to thermal stress
are associated with quartzitic sandstones and silica-rich igneous and metamorphic
rocks.
I n su m ma r y, ther ma l ly-i nduced stress is probably a n i mpor ta nt rock weather i ng process
in cold arid environments. However, until more experimental, fi eld, and laboratory data
become available, it is diffi cult to make a more defi nitive statement.
Figure
4.9.
Variations in thermal expansion of different rock types. (A) Dependence of the coef-
fi cient of volumetric thermal expansion (
10 °C.
Legend: 1, granite; 2, diabase; 3, limestone; 4, tuffola; 5, sandstone. (B) Dependence of coeffi cient
of linear thermal expansion (
α
,
v
) on rocks in the temperature range
−
10 °C to
+
) on silica content in rocks. Legend: 1, dunite; 2, gabbro; 3, diorite;
4, granite. From Yershov (1990). Reproduced by permission of Cambridge University Press.
α
