Geology Reference
In-Depth Information
6.6.5. Patterned Ground
Most patterned-ground phenomena form within the active layer. Circles, earth hummocks,
and mudboils are undoubtedly the most common. Some patterned ground may also form
in seasonally-frozen ground.
Patterned ground attracted much attention from early explorers and scientists working
in the tundra and Arctic regions, who often traveled on foot or by dog team. The relative
lack of vegetation makes the various patterns distinctive, and early botanical explorers
quickly identifi ed the intimate relationships that exist between patterned ground and
vegetation. A voluminous literature describes patterned ground phenomena, and A. L.
Washburn (1956) lists no fewer that 19 hypotheses for its origin. Today, more recent studies
suggest more universal explanations (Gleason et al., 1986; Hallet, 1990; Hallet et al., 1988;
Krantz et al., 1988; Mackay, 1980b; Werner and Hallet, 1993).
The main geometric forms that are recognized include circles, polygons, and stripes.
Where circles interact with each other, the resulting pattern is sometimes called a net.
Patterned ground features are also described in terms of whether they are sorted or non-
sorted as regards the material in which they are formed. Circles, nets, and polygons usually
occur on fl at or nearly-fl at surfaces. Then, as slope angle increases, these features become
elongate and irregular and, depending upon local conditions, may change to stripes further
down slope.
Undoubtedly, the most common form of patterned ground is the non-sorted circle
(Figure 6.20A) sometimes termed a “plug circle” (Washburn, 1997). If the feature assumes
a raised topography, it is termed a “hummock” (Mackay, 1979b, 1980b). Hummocks grade
from those which are completely bare (mud hummocks) to those which are vegetated
(earth hummocks) (Figure 6.20B). Non-sorted circles, so defi ned, occur singly or in
groups, and commonly vary from 0.5 m to 3.0 m in diameter and up to 0.5 m in height.
Beneath the circle or hummock, the late-summer frost table is gently bowl-shaped.
These circular forms of patterned ground are likely the result of cryoturbation (see
above). Typically, they are composed of fi ne-grained, frost-susceptible, soil. The upward
displacement of material is caused by the freeze and thaw of ice lenses at the top and
bottom of the active layer, with a gravity-induced, cell-like movement (Mackay, 1979b,
1980b). Evidence of cell-like circulation is deduced from the grain-size distribution of the
hummocky soil, radiocarbon dating of organic materials intruded into the hummock
center from the sides, and from upturning tongues of saturated soil observable in late
summer (Figure 6.20B) (Tarnocai et al., 1973; Zoltai and Tarnocai, 1974; Zoltai et al.,
1978). An equilibrium model is inferred in which cell-like movement occurs because the
top and bottom of the freeze-thaw zones have opposite curvatures (Figure 6.20C).
Support for some sort of circulatory movement within the active layer is provided by
measurements of soil movement within sorted circles (Hallet and Prestrud, 1986; Hallet
et al., 1988) (Figure 6.21). These indicate periods of activity in the summer separated by
long quiescent periods. One possibility is that the density difference that exists between
water at
3-4 °C (at the near surface) may be
suffi cient to induce free convection of water in the active layer (Gleason et al., 1986; Ray
et al., 1983). This may lead to spatially non-uniform thawing at the top of frozen ground
caused by convective heat transfer. Such a process would explain the concave upward
curvature of the frost table beneath circles, a phenomenon not adequately explained in
the equilibrium model (see Figure 6.20C).
Even though soil circulation can be demonstrated, it still does not completely explain
the origin of all circular forms. For example, circulation may evolve subsequent to a pro-
toform, such as a diapir (Washburn, 1989, pp. 953-954). It has also been suggested that
∼
0 °C (at the base of the active layer) and
∼
