Advances in Periglacial Geomorphology
E<li,ed by :V1. J. Clark
191988 John Wiley & Sons Ltd
./
10
Earth Hummocks (Thufur)
E. SCHUNKE
Geographisches lnstitut, Universitiit G6ttingen
and
S. C. ZOLTAI
Canadian Forestry Service, Edmonton, Canada
ABSTRACT
Dome-shaped hummocks (thufur) occur in arctic, alpine and subarctic
environments where climate allows characteristic snow distribution and seasonal
ground frost patterns. They occur in level or IJ.ear-level areas that have imperfect
drainage. Their size varies within narrow limits, averaging 50 cm in height and
100 cm in diameter. The soil material is fine textured, stone-free or sparingly
stony. Their internal structure shows displaced or distorted layers. Mounds are
formed by the permanent displacement of local surface material in frost-sensitive
soil in the presence of plentiful moisture under climatic conditions that generate
seasonal frost penetration. Such conditions are conducive of frost-generated
soil movements-cryoturbation. Internal structure indicates that cryoturbation
is playing an active role in generating and preserving the shape of the hummocks.
The exact mechanism of the initiation of hummock formation is not known.
Speculation
and circumstantial
evidence
indicate
that
uneven
surface
(microrelief, texture) and related differences in vegetation may create thermal
variations in the soil that can initiate hummock formation. Once formed,
differences in moisture content, insulative vegetation cover and soil texture act
to create cryoturbations that will maintain the hummocks.
RESUME
Les buttes gazonnees (thufurs) en forme de domes sont tres frequentes dans
les milieux arctiques, subarctiques et alpins, caracterises par une forte
differentiation de la couverture de neige et des structures de gel saisonnier.
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Advances in Periglacial Geomorphology
Les buttes gazonnees se trouvent sur des surfaces horizon tales aussi bien que
sur des surfaces legerement indinees a drainage incomplet. Les dimensions des
buttes ne varient que dans des Iimites assez etroites: dIes atteignent en moyenne
50 cm de haut et 100 cm de diametre. Les thufurs se forment dans des materiaux
mineraux fins, dans des limons et des limons sablonneux tres pauvres en
pierrailles. La structure interne des thufurs montre souvent une perturbation
de la stratification originelle et un deplacement des fines. Ce deplacement
superficiel permanent des fines est responsable de la morphogenese des thufurs.
Les facteurs les plus importants sont la gelivite des sols en condition d'humidite
et l'existence d'un gel saisonnier qui favourisent les cryoturbations. L'observation
des coupes montre clairement que ces cryoturbations jouent un role actif dans
la formation et dans la conservation de la forme de ces buttes. Les stades initiaux
de la formation des buttes restent encore problematiques: probablement ils
dependent du microrelief, des irregularites de la granulometrie et de la couverture
vegetale. Le developpement de la forme est plus evident: la tendence au
soulevement s'accentue grace aux differentiations dans l'humidite, la texture
du sol et l'isolation thermique par la vegetation, qui s'accentuent de plus
en plus.
ZLISAMMENFASSLING
Kuppelformige Erdbulten (Thufur) mit vollsUindiger Vegetationsbedeckung
haben im arktischen, subarktischen und alpinen Milieu, dessen klimatische
Ausstattung zu einer charakteristischen Differenzierung der Schneeverteilung
und des saisonalen Bodenftostes fuhrt, weite Verbreitung. Die Erdbulten treten
auf ebenen wie auf schwach geneigten Arealen auf, denen es an einer guten
Drainage mangelt. Die GroBe der Formen variiert in engen Grenzen: Die Hohe
betragt zumeist um 50 CIn und der Durehmesser um 100 em. Das Lockermaterial
der Erdbulten weist sehIuffige bis feinsandige Kornung auf und ist in aller Regel
weitgehend steinfrei. Die innere Struktur der Erdbulten laBt haufig eine
Deformation oder Umlagerung des Feinmaterials erkennen. Die Bildllng der
Erdbulten geht auf langsame und langfristige Umlagerllngen des
oberfIaehennahen Loekersubstrates zuruck. Voraussetzungen hierflir sind
frostempfindliehe Boden mit ausreichender Durehfeuehtung sowie die
Ausbildung von saisonalem Bodenfrost. Diese klimatisehen und edaphisehen
Bedingungen bewirken frostbedingte Bodenbewegungen von der Art der
Kryoturbation. Die innere Struktur der Erdbulten dokumentiert, daB fi.ir die
Genese und den weiteren Bestand der Erdbulten diese kryoturbaten Prozesse
eine aussehlaggebende Rolle spielen. Die Einzelheiten jener Mechanismen. die
zur Einleitung der Thufur-Bildung fiihren, sind nicht genau bekannt.
Wahrscheinlicb sind es die Differenzierungen des Oberbodens (Mikrorelief,
Bodentextur u.a.) und der Vegetation, die thermisehe Differenzierungen im
Earth Hummocks (Thujur)
233
Substrat bewirken, die schliesslich durch subkutane Feinmaterialumlagerung
die Biiltenbildung initiieren. Nach dem Initialstadium fiihren die mit ihm
gegebenen Differenzierungen der Bodenfeuchte, der Bodentextur und der
Isolationswirkung der Vegetation zu jenen kryoturbaten Prozessen im Boden,
die die Bildung ausgereifter Erdbiilten hervorrufen.
10.1
INTRODUCTION
This paper summarizes recent cryopedological field studies on earth hummocks
(thufur) in the American and European arctic and subarctic. These periglacial
forms have for many years attracted the interest of scientists in many countries.
The earliest descriptions of Icelandic thufur (singular: thufa) appear in Gruner
(1912) and Thoroddsen (1913). In Sweden, earth hummocks (jordtuva) were
studied by Bergstrom (1912) and G. Lundqvist (1944), and are described by
J. Lundqvist (1962). Sod-covered hummocks, found under permafrost
conditions, have been reported from various parts of the USSR ( Kachurin, 1959).
In North America, Sharp (1942) describes earth hummocks from the Yukon
Territory, but he includes several unrelated forms in his discussion. 'Turf
hummocks' are described from Greenland by Raup (1965), but it is not clear
whether these are earth hummocks.
In fact, the casual and non-specific use of the term 'earth hummock' and
the eagerness of some authors to liken various periglacial forms to the Icelandic
thufur, have created a great deal of confusion. In many cases, forms that have
entirely different origins have all been called earth hummocks. In most of these
instances the internal structure dT'lhe mounds was not investigated, and they
were grouped as earth hummocks on the basis of their superficial appearance
alone. Recent studies, however, have shown that one group of non-sorted circles,
occurring in large numbers in similar environments under arctic and subarctic
conditions, share both internal and external characteristics and have a common
possible genesis. This paper therefore describes and defines earth hummocks
(thufur) on the basis of their external and internal characteristics, indicates
factors important to their development, and establishes their significance in terms
of climatic and non-climatic factors.
The practical importance of earth hummocks lies in the fact that they are
formed by frost heaving. In non-permafrost areas, such as Iceland, thufur can
develop on previously tilled land within a few decades. Such fields are unsuitable
for mechanized agriculture unless the thufur are destroyed by levelling. In
permafrost areas, earth hummocks indicate actively heaving surfaces, as shown
by the tilting of trees. In addition, they are associated with ice accumulations
just below' the permafrost table. Any disruption of the insulating surface
vegetation causes this ice to thaw, resulting in very unstable conditions where
erosion and mass movement will damage the land surface. An understanding
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Advances in Periglacial Geomorphology
FIGURE 10.1
FIGURE 10.2
Closely spaced thufur, Iceland
Earth hummock development on gentle slopes in fine-grained soils with
imperfect surface drainage
Earth Hummocks (Thujur)
235
of the mechanisms that lead to the establishment and maintenance of
earth hummocks may well lead to the avoidance of the problems associated with
them.
10.2
10.2.1
THE CHARACTERISTICS OF EARTH HUMMOCKS
External morphology
The terms 'thufur' (Thoroddsen, 1913) and 'earth hummocks' ( Sharp, 1942)
describe a type of ground patterning which can be categorized as hemispherical
and domed non-sorted circles or nets (Washburn, 1956, 1980). Unless otherwise
qualified, the term earth hummock as used in this paper can be taken as the
equivalent of thufur. The paper thus implies that these two terms are
interchangeable, but that they are both restricted to forms which meet the strict
morphological and genetic criteria discussed below.
In ground view the hummocks are generally circular or oval, but elongated
hummocks occur on gentle slopes if their angle is less than 6°. Their height
varies from 20 cm to 100 cm, and averages around 50 cm. Their basal diameter
is between 50 cm and 150 cm, with the majority close to 100 cm. Earth
hummocks usually occur closely spaced in distinct fields (Figure 10.1), although
they may be found on occasion as scattered individuals. When closely spaced,
they are separated from each other by narrow grooves or somewhat wider
troughs. In general, the distance between hummocks is less than the diameter
of the mounds (Tarnocai and Zoltai, 1978).
Earth hummocks occur on flat or gently sloping areas of fine-grained soils
where the internal drainage is im rfect but there is no excess surface water
(Figure 10.2). They invariably develop in fine-grained, stone-free or sparingly
stony soils of volcanic-aeolian, lacustrine or glacial origin. Their distribution
is widespread in the arctic and subarctic regions of the northern hemisphere,
though infrequently they are also found in alpine areas outside the polar regions.
The associated vegetation is tundra or meadow, or open subarctic woodland
in North America.
The individual mounds are usually covered with vegetation, but in the
high arctic their apices may be bare. In permafrost areas, the insulating
vegetation is usually thicker in the interhummock troughs, hence the permafrost
table is higher under the trough than under the mounds. This results in the
development of small 'basins' in the permafrost table under the mounds-a
mirror image of the ground surface (Figure 10.3). In non-permafrost areas,
different vegetation develops on the earth hummocks compared \vith that in
the intermound troughs due to different drainage conditions caused by
microrelief (Lotschert, 1974). Such vegetation differences undoubtedly cause
contrasts in the thermal regIme of various parts of the hummock, as is
discussed later.
pe
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Advances in Periglacial GeomorphologJ'
FIGURE 10,3
Section through earth hummock showing the development of a 'basin'
in the underlying permafrost table
FIGURE
10.4
Section through earth hummock showing disrupted and displaced
sedimentary layers of volcanic ash, Iceland. (Scale in centimetres and decimetres)
Earth Hummocks (Thujifr)
10.2.2
237
Internal morphology
The internal morphology of earth hummocks is characterized by disrupted and
displaced horizons and strata. The intruded material may be parts of soil
horizons, organic layers or sedimented layers such as volcanic ash (Figure 10.4).
Tongues of such materials may extend downwards, especially at the perimeter
of the mound, thence tending to\vards its centre and then often turning upwards.
The material displaced by these intrusions often shows flow patterns that
normally develop in a viscous liquid.
Earth hummocks consist of fine-grained mineral soil. The combined clay­
and silt-sized particles dominate in the soil (58-99 per cent in Canada; average
of 60 per cent in Iceland), and the remainder is mostly fine sand (Tarnocai and
Zoltai, 1978; Schunke, 1977a). The texture of the unfrozen hummock material
is usually homogeneous, but there may be a slight increase in coarser-textured
particles on the sides and tops of the mounds. Mackay (1980) found that the
sand content was higher at the top of the permafrost table and at the centre
than on the sides, indicating a cell-like circular pattern. Finely disseminated
organic carbon is common within the unfrozen earth hummock material; the
concentration may reach 2-5 per cent ( Zoltai and Tarnocai, 1974)
Hollow spaces are not observed inside the hummocks, although the soil is
usually loosely structured within the rooting zone of plants. The bulk densities of
the mineral soil in hummocks have a fairly wide range (1.00-2.18 g/cm3), such
as is characteristic of soil materials affected by cryoturbation. In areas of high
rainfall the earth hummocks show homogeneous soil moisture content. In areas
of continental climate, however. the soil moisture content is low at the hummock
apex, increases towards the centre, ��d reaches a maximum above the permafrost
table. The ice content of the permafrost in hummocky areas is high, being well
in excess of 100 per cent by weight. The ice-rich layer, however, is only about
1 m thick (Zoltai and Tarnocai, 1974). The presence of such concentration of
segregated ice is not necessarily related to earth hummock formation, but it
does reflect the availability of plentiful water in the imperfectly drained
hummocky areas.
10.2.3
Dynamic features
The dynamic nature of earth hummocks is indicated by their external and internal
characteristics. Externally, some hummocks may be split across the apex, or
the organic mat may be ruptured near the base, indicating internal pressures.
Trees grO\ving on hummocks are invariably tilted by the heaving of hummocks
(Figure 10.5) (Zoltai, 1975). Internally, the presence of disrupted and displaced
materials points to cryoturbation activity.
The age of earth hummocks is difficult to determine. In North America,
radiocarbon dates indicate that most earth hummocks originated 2500-5000 BP
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Advances in Periglacial Geomorphology
FIGURE 10.5
Tilting of trees caused by heaving of the earth hummocks upon which
they are growing
FIGURE 10.6
Relationship between thufur and snow retention, Iceland. Deep snow has
an insulating effect and therefore decreases frost penetration
Earth Hummocks (Thufur)
239
( Zoltai et al., 1978), although they have remained active since then. In Iceland,
thufur formation is taking place at the present time, as is shown by the
development of thufur on cultivated land ( Schunke, 1977a). Other thufur,
however, are of greater antiquity, as shown by the deposition of volcanic ash
layers of kno\vn age that conform to the shape of the mounds.
10.3
CLIMATIC CONDITIONS FOR
EARTH HUMMOCK FORMATION
Virtually all earth hummocks in North America are underlain by permafrost
where the seasonally-thawed layer freezes every winter. The exceptions are those
occurring in alpine areas ( Scotter and Zoltai, 1982) or those that are inactive
fossil forms (Tarnocai and Zoltai, 1978). In the USSR earth hummock-like
structures were identified from permafrost regions ( Kachurin, 1959), but
hummocks in non-permafrost areas were reported from eastern Siberia
(Naumov, 1963). In Europe, earth hummocks are again not restricted to
permafrost terrain. The climate is severe enough to allow relatively deep seasonal
frost penetration, although in the lowlands of Iceland the seasonally frozen layer
is only 30-40 em thick ( Schunke, 1977a). Local climatic conditions, especially
the distribution of snow, become important (Figure 10.6). Deep and late-thawing
snow has an insulating effect on the soil and therefore tends to decrease frost
penetration. Thorarinsson (1951) notes that thufur are absent from regions where
snow cover is particularly thick and melts late. The occurrence of earth
hummocks in Scandinavia is virtually restricted to areas above the tree line
(J. Lundqvist, 1962). This may be J;�lated to snow redistribution, which is more
effective in treeless areas than in those with trees. In Greenland, too, earth
hummocks are not limited to permafrost terrain (Schunke, 1977b).
10.4
THE GENESIS OF EARTH HUMMOCKS
The mechanism of earth hummock formation is not known with certainty. It
is known that the mounds are produced by the permanent displacement of local
surface soil materials. The process that can produce such displacement under
the given circumstances is cryoturbation. All the necessary ingredients are
present: heave-sensitive soil, plentiful soil moisture and seasonal frost
penetration. Should any of these conditions be lacking, hummock formation
does not take place.
As is the case with several periglacial features, the initiation of the formative
process is more difficult to explain than its subsequent maintenance or
development. The initiation of mound development may be connected \'lith the
movement of moisture toward a freezing front. The moving moisture may carry
some fine-grained soil particles. Cellular centres often develop in relatively
homogeneous fine-textured soil, and clay-sized soil particles may be concentrated
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Advances in Periglacial GeomOlphology
20-150
em
A
I
120-80
I em
L
FIGURE 10.7
A model of cellular moisture and fine sediment movement in an earth
hummock (after Schunke,1981)
in such centres by water movement associated with freezing (Figure 10.7)
(Schunke, 1981). Another mechanism for the initiation of earth hummocks may
be the random development of frost-heaved spots ( Mackay, 1980), which may
remain without insulating vegetation for several years. Such spots have different
thermal regimes from the surrounding vegetated areas and may serve as the focus
for mound development. Fields of hummocks can develop by the infilling of
spaces between hummocks.
Once an embryonic earJh hummock has been initiated, the vegetation will
develop differentially on the mounds compared with the intervening areas. The
small mounds are somewhat better drained, and an insulating, more mesic
vegetation develops on them than in the moister troughs. This results in
differential thermal regimes: the drier mound will lose heat more slowly, than
the moist surroundings. In the more moist spots the freezing front penetrates
faster and deeper than under the mound. This sets up lateral pressures toward
the centre of the mound, displacing more materials and eventually forming the
earth hummocks (Schunke, 1977b).
An interesting difference between earth hummocks with and without a
permafrost substrate is the depth of cryoturbation. In non-permafrost soils the
maximum deformation takes place at 30-60 cm (Schunke, 1977b), and layers
at a greater depth are completely undisturbed (Figure 10.8). In permafrost areas
the entire hummock above the permafrost table shows severe mixing. These
differences show that in non-permafrost areas the seasonal frost (depth and
rate of penetration) is responsible for the maintenance of the earth hummocks.
In permafrost areas the greater depth of mixing may be accomplished by a
cellular motion, maintained by basin-shaped depressions in the permafrost table
under the hummocks (Mackay, 1980).
Earth Hummocks (Thufur)
FIGURE 10.8
241
Section through an earth hummock in an area without permafrost,
Iceland. l\1aximum deformation occurs at depths of 30-60 cm,while deeper layers remain
undisturbed
The presence of a permafrost taGle constitutes another difference between the
permafrost and non-permafrost environments for hummock formation. The
permafrost table acts as a layer virtually impervious to water and as a base that
can firmly withstand pressures associated with cryoturbation. In non-permafrost
areas where such a firm impervious base is absent, earth hummocks develop
in layered deposits, such as volcanie ash, that have a uniformly fine texture but
are underlain by different materials. If underlain by coarser material, moisture
tends to accumulate at the textural boundary. If underlain by finer materials,
internal drainage may be impeded, thus giving moister conditions. If underlain
by bedrock or dense till, these materials will (like permafrost) resist pressures
and direct the resulting forces upward.
to.5
DEFINITION AND TERMINOLOGY
The equivalent terms 'earth hummocks' and 'thufur' should be restricted to
those mounds that have the internal (texture, structure, moisture, cryoturbation)
and external (size, shape) characteristics described in this paper, and that occur
within arctic, alpine or subarctic regions of the northern or southern
hemispheres. To conform to traditional usage, those forms which were produced
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Advances in Periglacial Geomorphology
FIGURE 10.9
Small, very closely spaced hummock-like mounds in arctic Canada,
possibly formed by water deepening of desiccation cracks rather than by true earth
hummock processes
FIGURE 10.10
Classic thufur development in an agricultural field in Iceland
Earth Hummocks (Thufurj
243
under permafrost conditions may be called earth hummocks, and those occurring
in non-permafrost soils should continue to be called thufur. According to the
current state of knowledge, these forms are produced by the same process, and
they differ only in the degree of cryoturbation: earth hummocks show the effects
of frost churning throughout the mound above the permafrost table, while in
thufur the disruption occurs only in the upper 60 cm. When in doubt, a non­
genetic term such an non-sorted circle (Washburn, 1956) can be used.
Superficially similar, but basically different, forms should not be confused
with earth hummocks or thufur. To clarify these distinctions, it is worth
considering some of the major categories of features which cannot be equated
with true earth hummocks:
(1) Small hummock-like mounds that occur in the high arctic of Canada on
moderate to steep (5-20°) slopes may be mistaken for earth hummocks.
However, when examined and measured at twelve locations throughout
the Canadian Arctic Archipelago ( Zoltai, unpublished data), they were
found to be considerably smaller than true earth hummocks: their average
height is 20 cm (range lO-52 cm), and their average diameter is 31 cm
(range 12-68 cm). The mounds are closely spaced (Figure lO.9); a 30 m
long straight line touches an average of 76 mounds. The ball-like mounds
are completely covered with dwarf shrubs (1-2 cm high) of Dryas or
Cassiope. Internally, they are composed mainly of fine sand
(0.08-0.4 mm), and show no evidence of cryoturbation. The Bernard soil
on Banks Island (Tedrow and Douglas, 1964) has developed on areas of
such small mounds. The PaLent material is stone-free or very sparsely stony
colluvial, aeolian, lacustrine or fluvial deposits. At six locations in the
Canadian study, desiccation polygons (average diameter 28 cm) were
found on level ground above the mound-covered slopes. It is, therefore,
suggested here that the small mounds are formed by water eroding and
overdeepening the desiccation cracks that originated in a uniform readily
erodible fine sand material. These desiccation mounds are not earth
hummocks as here defined.
(2) Other mounds, not associated with periglacial environments, are the
'gilgai' that are formed by the s\velling of clay ( Hallsworth et al., ]955).
(3) 'Pimple mounds' are believed to be erosional pedestals that were protected
by the roots of single trees ( Cain, ]974).
(4) 'Mirna mounds' appear to originate by the excavations and earth-moving
activities of pocket gophers (Geomys spp., and Thomomys spp.) (Cox,
1984).
(5) The hummocky microrelief affecting over 90 per cent of the forested land
in the Appalachian region of North America is caused by the uprooting
of trees by wind (Beke and McKeague, 1984). There is a characteristic
mound and pit relief, and the soil horizons are disrupted (arbotur bated).
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Advances in Periglacial Geomorphology
None of these numbered forms are earth hummocks. On the other hand, mounds
that lack cryoturbation features but are otherwise comparable to earth
hummocks have been noted in northern Canada (Tarnocai and Zoltai, 1 978),
and these are believed to be old inactive forms of earth hummocks. The
dimensions of the mounds are similar to those of the active forms, but soil
horizons are well developed under both the hummocks and the troughs, and
the soils are not disrupted. As there are no detailed descriptions of fossil earth
hummocks, it is not known in what form they might be preserved. Studies of
earth hummocks in terms of human influences are equally lacking; however,
the development of particularly regular and striking forms on man-made
hayfields in Iceland (Figure 1 2.10) introduced the term 'thufur' to the
international periglacial terminology.
10.6
PERSPECTIVES
When dealing with a little-known periglacial phenomenon, it is prudent to apply
a generalized, non-specific nomenclature and classification. Thus the term 'non­
sorted circle' (Washburn, 1956) served for many years to describe various forms
that were somewhat similar. How'ever, as knowledge increased through careful
observations, analyses and measurements, it became apparent that certain groups
of non-sorted circles were distinctly different from others. Earth hummocks
and thufur are different from most other non-sorted circles in their internal
and external morphology and perceived genesis, hence these terms are proposed
for the specific forms to which they have been applied in this paper. An
agreement on common terminology would greatly facilitate communications
between various workers.
In the last few years there has been an explosion of information on soil
conditions under freezing conditions. However, little of this has been applied
to the study of periglacial phenomena. Knowing the conditions under which
earth hummocks and thufur are formed, it should be possible experimentally
to initiate and monitor the development of such forms, with full instrumentation.
This vl'Ould supply substantive confirmation of models hitherto based on
speculative indirect evidence.
10.7
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__
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