Geology Reference
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initiated, an absence of snow from the surface of the palsa allows deeper frost penetration
and further growth. Implicit in the defi nition is the fact that permafrost extends beneath
the organic surface layer into underlying fi ne-grained mineral soil.
The initial growth (heave) of a palsa or peat plateau is highly problematic. Random
conditions, such as the growth of a spruce tree on a sedge tussock in a bog, may be suffi -
cient to reduce the annual snow cover at a locality, thereby initiating deeper frost penetra-
tion. Another hypothesis is that palsa formation is triggered when wind turbulence causes
a thinning of the snow cover on certain parts of a bog, also enabling frost to penetrate
more deeply. This hypothesis was tested over a 3-year period in Finnish Lapland (Seppälä,
1982, 1995a) when snow was systematically removed from an experimental plot each
winter. The result was the formation of permafrost and the growth of a small artifi cial
palsa some 30 cm high.
The somewhat contradictory term “mineral palsa” (Pissart and Gangloff, 1984) has
now been replaced by the less ambiguous term “lithalsa” (Pissart, 2000, 2002) to describe
a shallow ice-segregation mound not covered with peat. The process of lithalsa growth is
similar to that of a palsa but ice segregation is favored not by organic material but by the
relatively high thermal conductivity of bare mineral soil, typically silty in nature.
Palsas are sometimes confused with seasonal-frost mounds. They differ in two impor-
tant ways: (1) growth is primarily by ice segregation, with water moving towards the palsa
from the surrounding wetland by cryosuction (see Chapter 4), and (2) palsas grow gradu-
ally over time, and may then persist for many decades. Also, although palsas are a type
of frost mound, they are quite distinct from pingos. Likewise, various earth- or vegeta-
tion-covered mounds, locally referred to as “thufurs” or “pounus,” and other small per-
mafrost-related mounds, exist in permafrost terrain. All these forms are described later
in this chapter. A. L. Washburn (1983a, b) attempted to bring order to the literature on
palsas but it is now clear that a number of different mechanisms result in similar mound-
like forms of varying sizes.
6.3.2. Peat Plateaus
Peat plateaus are fl at-topped expanses of frozen peat, elevated above the general surface
of a wetland. They are the simple result of freezing of peat with formation of segregated
ice lenses and consequent uplift of the surface (Zoltai, 1972; Zoltai and Tarnocai, 1975).
Segregated ice lenses may, or may not, extend downwards into underlying mineral soil.
This is probably the main difference, genetically, between palsas and peat plateaus. Buoy-
ancy effects may also initiate “fl oating” palsa-like elevations within water-saturated bogs
(Outcalt et al., 1986).
Peat plateaus may be several square kilometers in extent. Towards the northern limit
of the discontinuous permafrost zone, peat plateaus become increasingly more common
as surrounding unfrozen zones become increasingly rare.
In summary, the recognition of palsas, lithalsas, and peat plateaus rests not only upon
the nature of the surface material, which promotes deeper frost penetration than else-
where, but also upon the presence of segregated ice, which causes the uplift.
6.4. ROCK GLACIERS
A rock glacier is a lobate or tongue-shaped body of frozen debris, with interstitial ice and
ice lenses, which moves down-slope or down-valley by deformation of the ice contained
within it (Figure 6.9A). Although some investigators regard rock glaciers as simply
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