Geology Reference
In-Depth Information
7.2.4. Vein Ice
Vein ice is formed by the penetration of water into open fi ssures developed at the ground
surface. The origin of the water prior to freezing is atmospheric, usually meltwater from
snow and summer rain. Vein ice can be distinguished from segregation ice on account of
its vertical foliation and structures.
There are two types of vein ice. First, single-vein ice develops in small cracks, usually
less than 0.2 cm thick and less than 70 cm deep, that form by thermal-contraction cracking
at the ground surface. Second, repeated-vein ice forms in a thermal-contraction crack that
opens in approximately the same place for a number of years. At depth the result is a
complex of ice veins that mirror the different cracks; nearer the surface, the result is a
vertical, or near-vertical, sheet of foliated ice that is termed an ice wedge. The latter are
discussed later in this chapter (Section 7.5, pp. 176-181).
7.2.5. Other Types of Ice
Other types of ground ice occur in response to specifi c situations. These often refl ect the
complexities of thermokarst processes (see Chapter 8). For example, the term “thermokarst-
cave ice” was initially used by Russian scientists to describe bodies of near-pure ice that
lack foliation, may contain organic fragments dispersed throughout the ice, and that posses
ice fabrics and bubble patterns that indicate inward freezing (Gasanov, 1969; Shumskii,
1959; see the English translation by Leuchtenberg, 1964, pp. 45-46). In North America,
the more colloquial term “pool ice” is used to describe this type of ice (Mackay, 1988b,
p. 87, 1997). Typically, pool ice forms in gullies or tunnels that are preferentially eroded
along ice wedges. In ice-rich unconsolidated sediments, the rapidity of gully erosion along
tundra polygons is well known, and can lead to lake drainage (see Chapter 8). As thaw
progresses during the summer, fallen masses of soil and slumped material plug the tunnel
or allow standing water to accumulate in the channel fl oor. These water bodies subse-
quently freeze during the following winter. On poorly-drained tundra, pool ice may be
more common than generally thought (Shur et al., 2004). For example, pool ice was
present in the near-surface of at least three of the seven ice wedges that were exposed
in the gully that was eroded when Lake Illisarvik was artifi cially drained (Mackay, 1997,
p. 22). It is exposed also in the walls of the CRREL permafrost tunnel in Alaska (Shur
et al., 2004).
Ice within caves and tunnels in bedrock must also be considered a type of ground ice.
Karst geomorphologists report ice forming when either groundwater seeps into a cave
system and freezes (Marshall and Brown, 1974) or when warm and humid air circulates
within a cave system (Lauriol et al., 1988). On Svalbard, the ice present within expanded
bedrock joints beneath warm-based glaciers (see Figure 7.6C) refl ects the refreezing of
downward percolating glacial meltwater (Christiansen et al., 2005).
7.3. ICE DISTRIBUTION
7.3.1. Amounts
The total volume of ground ice present within permafrost varies from negligible, as in
certain igneous and metamorphic rocks, to considerable, as in unconsolidated, fi ne-grained
Quaternary-age rocks and unconsolidated sediments. For example, Table 7.2 summarizes
the typically high ground-ice volumes which exist in the upper 5.0 m of permafrost at three
