152
Climatic Geomorphology
The study of the freezing process may be relatively complex because it involves
knowing the magnitude and duration of the temperatures below 0~ Besides, it may affect
materials with different moisture contents and variable heat conductivity (Williams
and Smith, 1989). The understanding of the frost-related processes requires knowing the
evolution of the freeze-thaw cycles in the ground surface throughout the year. These
physical changes may correspond to seasonal variations or may have a daily regime as in
the alpine periglacial environments of the low latitudes. The daily variations in these zones
are due to insolation changes. In some parts of the Andes more than 300 freeze-thaw
cycles have been recorded (Troll, 1944). In the Ransol meteorological station of the
Pyrenees in Andorra, at an altitude of 1640 m, an annual average of 80 freeze-thaw cycles
have been recognized (Raso and Garcfa Loureiro, 1998) and 117 in the University Hostel
(2510 m) of the Sierra Nevada in the Betic Cordillera of Spain (G6mez Ortiz and Salvador
Franch, 1997). The different processes associated with frost action and some of the most
relevant effects are analysed below.
3.1.1. Congelifraction
This is the most important weathering process in the periglacial zones. It is also called
gelifraction or frost-shattering. Liquid water undergoes a volume increase of approximately 9% with the freezing process. The rocks break when the stresses exerted by the
interstitial water surpass their tensile strength. If the water freezes in a confined space, it
may transmit a large pressure to the host rock. However, the freezing of the water in cracks
does not take place in a totally confined condition because it may extrude and only part of
the expansion contributes to the disintegration of the rock (Ollier, 1984). The frost
expansion may cause the fracturing of the rock producing angular clasts (also called
cryoclasts) or its granular disintegration. The mechanical fragmentation of the rocks is
an important geomorphological agent as is demonstrated by the block fields developed
on planated surfaces and the talus accumulations on the slopes of mountain areas.
Gelifraction has a larger effect during the spring when the water derived from snow
melting penetrates into the cracks and subsequently freezes. In the southern slopes of
Sierra Nevada in the Betic Cordiller of Spain, frost shattering reaches a maximum
intensity during the pre- and post-nival periods (G6mez Ortiz and Salvador Franch, 1998).
The continuous production of cryoclasts produces the back-wearing of the rock slopes.
According to field measurements, the retreat rates may range from 0.3 mm/yr in
Spitzbergen (Jahn, 1976) to 2.5 mm/yr in the Swiss Alps (Barsch, 1977a).
The effectiveness of the congelifraction depends on the rock type and the water content.
Generally, slates disintegrate to a larger extent than igneous crystalline rocks. This process
is largely controlled by the structural characteristics of the rock, mainly by the density
of discontinuity planes in the rock mass (Figure 7.6). The role of porosity is particularly
important in sedimentary rocks (French, 1996). Numerous experiments about the
cryogenic weathering of different rock types under variable conditions have been carried
out. Those performed with schists indicate that fragmentation increases with the frost
intensity due to a fatigue effect (Lautridou, 1988) (Figure 7.7). An increase in the
disintegration of the schists with the number of freeze-thaw cycles and a decrease in the
median of the cryoclasts with time are also observed.
Climatic Geomorphology
The study of the freezing process may be relatively complex because it involves
knowing the magnitude and duration of the temperatures below 0~ Besides, it may affect
materials with different moisture contents and variable heat conductivity (Williams
and Smith, 1989). The understanding of the frost-related processes requires knowing the
evolution of the freeze-thaw cycles in the ground surface throughout the year. These
physical changes may correspond to seasonal variations or may have a daily regime as in
the alpine periglacial environments of the low latitudes. The daily variations in these zones
are due to insolation changes. In some parts of the Andes more than 300 freeze-thaw
cycles have been recorded (Troll, 1944). In the Ransol meteorological station of the
Pyrenees in Andorra, at an altitude of 1640 m, an annual average of 80 freeze-thaw cycles
have been recognized (Raso and Garcfa Loureiro, 1998) and 117 in the University Hostel
(2510 m) of the Sierra Nevada in the Betic Cordillera of Spain (G6mez Ortiz and Salvador
Franch, 1997). The different processes associated with frost action and some of the most
relevant effects are analysed below.
3.1.1. Congelifraction
This is the most important weathering process in the periglacial zones. It is also called
gelifraction or frost-shattering. Liquid water undergoes a volume increase of approximately 9% with the freezing process. The rocks break when the stresses exerted by the
interstitial water surpass their tensile strength. If the water freezes in a confined space, it
may transmit a large pressure to the host rock. However, the freezing of the water in cracks
does not take place in a totally confined condition because it may extrude and only part of
the expansion contributes to the disintegration of the rock (Ollier, 1984). The frost
expansion may cause the fracturing of the rock producing angular clasts (also called
cryoclasts) or its granular disintegration. The mechanical fragmentation of the rocks is
an important geomorphological agent as is demonstrated by the block fields developed
on planated surfaces and the talus accumulations on the slopes of mountain areas.
Gelifraction has a larger effect during the spring when the water derived from snow
melting penetrates into the cracks and subsequently freezes. In the southern slopes of
Sierra Nevada in the Betic Cordiller of Spain, frost shattering reaches a maximum
intensity during the pre- and post-nival periods (G6mez Ortiz and Salvador Franch, 1998).
The continuous production of cryoclasts produces the back-wearing of the rock slopes.
According to field measurements, the retreat rates may range from 0.3 mm/yr in
Spitzbergen (Jahn, 1976) to 2.5 mm/yr in the Swiss Alps (Barsch, 1977a).
The effectiveness of the congelifraction depends on the rock type and the water content.
Generally, slates disintegrate to a larger extent than igneous crystalline rocks. This process
is largely controlled by the structural characteristics of the rock, mainly by the density
of discontinuity planes in the rock mass (Figure 7.6). The role of porosity is particularly
important in sedimentary rocks (French, 1996). Numerous experiments about the
cryogenic weathering of different rock types under variable conditions have been carried
out. Those performed with schists indicate that fragmentation increases with the frost
intensity due to a fatigue effect (Lautridou, 1988) (Figure 7.7). An increase in the
disintegration of the schists with the number of freeze-thaw cycles and a decrease in the
median of the cryoclasts with time are also observed.