A prehistoric catastrophic rock avalanche at
Holsteinsborg, West Greenland
MICHAEL KELLY
DGF
Kelly, M.: A prehistoric catastrophic rock avalanche at Holsteinsborg, West Greenland. Bull. geol. Soc.
Denmark, vol. 28, pp. 73-79. Copenhagen, February 22nd, 1980.
The deposits of a rock avalanche have been identified near Holsteinsborg, West Greenland, where they
cover about 2 km2 and involve at least 2.8 X 106 m3 of rock debris which has been transported up to 7 km
from its source. The possible transport mechanisms responsible for this deposit are discussed by analogy
with recent rock avalanches described in the literature. This suggests that the avalanche was a high velocity
flow cum slide with a dispersed load of rock debris, snow and ice which was generated by a rock fall from
the side of the mountain of Avqutikitsoq. The date of the avalanche is estimated from the somewhat
uncertain lichenometrical evidence to be 16th or 17th century.
M. Kelly, Department of Environmental Sciences, University of Lancaster, Bailrigg, Lancaster, England,
May 22nd, 1979.
Introduction
A large area of coarse rock debris covers much of
the valley floor at the head of Kangerdluarssuk
ungatdleq fjord near Holsteinsborg in West
Greenland (67°8'N, 53°18'W).
This is portrayed on the Quaternary Map of
Greenland, 1:500 000 Søndre Strømfjord-Nugssuaq Sheet (Greenland Geological Survey), as a
rock fall and rock glacier deposit derived from
the adjacent northern valley wall, but a subsequent preliminary survey of the area suggests
rather that it is the deposit from a highly mobile
rock avalanche which had its source 7 km away to
the south east. Such rock avalanches, fossil or
contemporary have not been described hitherto
from Greenland.
Deposits
The distribution of the supposed avalanche deposits, (Fig. 1) has been mapped from air photographs supplemented by field observations at the
southern terminus and the glacier margin only.
The topographic base for Fig. 1 is derived from
the 1:2000 000 map of the Geodetisk Institut
and the contours can be considered only as approximate.
The biggest area of avalanche deposits lies on
6 D.G.F. 28
the flat valley floor between the fjord and the
lake Taserssuaq, resting on uplifted Holocene
marine sediments. These sediments have been
dissected by the existing drainage and the avalanche deposits drape over this relative relief of
about 30 m, from the terrace surface to the
stream bed. The burial of terrace features,
obvious at the terminus, can also be discerned on
the air photographs.
The deposit itself consists of very poorly sorted
debris of the major rock types of the area: granulite facies, hypersthene and leucocratic gneisses
(Geological Map of Greenland 1:500 000, Søndre Strømfjord-Nugssuaq Sheet, Geological Survey of Greenland), with a predominant block size
on the surface around 1 m3 but including also
many large blocks up to 1000 m3 (Fig. 2). Grain
sizes range down to sand and silt sizes but this
finer fraction does not appear to be abundant
enough to constitute a matrix. Small block and
gravel sized debris frequently lies on the large
block surfaces and blocks of all sizes are often
delicately perched. Both these features are taken
as indications of the settling of the deposit from a
dispersed state, due either to the avalanche
mechanism or to the melting of included snow
and ice.
A second area of deposits of similar lithology
occur in a partly glaciated hanging tributary valley, north of the 1440 m peak of Avqutikitsoq.
Fig. 1. Map of the avalanche deposits. Reproduced by permission (A.292/79) of the Geodætisk Institut, Denmark.
Bulletin of the Geological Society of Denmark, vol. 28 1979
75
'
3 1
» U J K V ' " . ' • ; ' * • *• « .
":".",,;•;;
<£*i-«- J
-.5
; ; « , , > :
Fig. 2. View from the terminus of the avalanche deposits and its track (arrowed), with the main valley deposits in the foreground and
upper valley deposits at A. (1 m scale on foreground boulder).
The major part of these upper deposits occurs as
an anomalously extensive ice cored moraine on
the valley glacier (Fig. 3). Another part lies in
front of the moraine and yet another, of less certain affinity, may be the deposits which lie
beyond a small knoll to the north of the glacier. It
is clear from the relationship of the moraine to
the proglacial deposits that the former are
avalanche deposits which have been partly reworked or transported by the glacier, and it is
assumed that both deposits are from the same
avalanche. Other smaller areas of moraine which
probably include avalanche material occur behind the northern nunatak in the icefall and along
the northern side of the upper firn basin.
On the moderately steep slope between the
Fig. 3. Reworked avalanche deposits forming the Historical moraine.
76
Kelly: Greenlandic rock avalanche
tributary and main valley only scattered blocks
occur. The largest of these, put at > 5 m across,
are visible on the air photographs and their distribution here and in the other areas is shown in
Fig. 1. Also visible on the photographs, in the
areas of continuous deposits, is a lineation which
is generally parallel to the presumed path of the
avalanche. It would seem likely that these are
traces of the longitudinal surface grooves described from modern avalanche deposits (e.g.
McSaveney, 1978). In one area this lineation appears to be distorted, perhaps by secondary
downslope creep which may have been aided by a
high snow or ice content in the deposit.
The total volume of the deposits can only be
very crudely estimated due to the difficulty in assigning a realistic mean depth to it, because of the
nature of the deposit and the lack of detailed
measurements. The area covered by continuous
deposits derived from the air photographs is put
at 1.85 km 2 and a minimum estimate of the mean
thickness is thought to be 1.5 m, giving a volume
of 2.78 x 106 m3.
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Fig. 4. The mountain of Avqutikitsoq.
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Dispersal mechanism
The overall distribution of the deposits indicates
that their source was the vertical rock wall on the
north side of Avqutikitsoq (Fig. 4), where the
mass movement process was presumably initiated
by a major rock fall. This process, which dispersed the material over a long distance, is considered to have been a rock avalanche. By definition these are highly mobile flows or slides in
which water does not play a significant role in the
flow mechanism, unlike debris flows. Although
other names have been suggested for such dry
rock avalanches (e.g. sturzstrom, Hsu (1975)),
this term which is in common usage seems appropriate.
Something of the likely flow conditions of the
Holsteinsborg avalanche can be deduced from
modern examples, of which the closest analogues
will be those generated by rock falls onto glaciers
or snow (Table 1). Although the avalanche is
small in comparison with the modern ones, in
terms of the area and estimated volume of its
77
Bulletin of the Geological Society of Denmark, vol. 28 1979
Table 1. Characteristics of some rock avalanches.
Location
Holsteinsborg
Elm1
Sherman2
Lyell*
Tahoma"
Maximum path (km)
Height
Distance
1.4
0.61
1.16
1.63
1.9
6.75
2.02
5.7
4.0
6.9
Apparent
coefficient
of friction
(tan <p)
0.21
0.31
0.20
0.4
0.28
Deposit
Area (km 2 )
Vol. (10 6 m3)
1.85
0.67
8.25
0.84
5.0
2.8
10
12.5
4.2
11
Velocity
kmh-1
72-180
94-241
60
130-140
Sources: 1 - Hsu, 1975; 2 - McSaveney, 1978; 3 - Gordon et al., 1978; 4 - Fahnestock, 1978 (composite fall).
deposits, it is at least as mobile. This mobility can
be expressed by the ratio of height lost to distance
travelled, which defines the apparent coefficient
of sliding friction according to Coulomb's Law
(Hsii, 1975). Approximate values of this
parameter, derived using maximum values for
height and length, are given in the table. It is
likely therefore that the velocity of the
Holsteinsborg avalanche was comparable to the
velocities observed or deduced for the modern
avalanches, i.e. 17-67 m s"1 (60-240 km h -1 ). It
may be possible to obtain a direct estimate of the
velocity since evidence of the tilt of the avalanche
surface as it negotiated the 90° bend at the head
of the main valley may have been preserved, allowing calculation of the velocity as with the
Tahoma Peak avalanche (Fahnestock, 1978).
General agreement has not been reached about
the mechanics of rock avalanches, in particular
about the relative role of basal sliding and internal flow, and the processes operating in both
these categories, e.g.:
-
-
-
-
basal sliding over trapped compressed air
(Shreve, 1968)
basal sliding over a low friction layer, e.g.
snow (McSaveney, 1978) or weathered clays
etc.
flow, with grain support by fluidisation, i.e.
excess pore pressures generated by trapped air
(Kent, 1966), or by exsolved gases in the case
of pyroclastic flows (Sparks, 1976)
flow, with grain support by grain dispersive
stress (Bagnold, 1956; Hsii, 1975) or
mechanical fluidisation in the sense of
McSaveney (1978)
flow in which a dispersed fine fraction aids
other mechanisms by reducing bouyant weight
(Hsii, 1975), reducing overall viscosity
(McSaveney, 1978), or increasing fluid
medium viscosity (Sparks, 1976).
The most convincing model for a rock avalanche appears to be McSaveney's for the Sherman Glacier avalanche. This envisages a flow
maintained in a dispersed state by grain-grain
collisions deriving their energy ultimately from
the initial rock fall, but with most of the movement accomplished by basal shear over a low
friction snow layer, and only a small proportion
coming from the laminar viscoplastic flow of the
dispersed medium itself. For the latter he
obtained viscosities of 0.4—1.6 x 105N s m-2 and
a low yield strength of 2 kN m~2. Whilst a precise
analogy cannot be drawn between this and the
Holsteinsborg avalanche it does provide some
idea of the mechanism that may have been involved. At least several of the essential features
existed, with an initial rock fall, and movement at
least for 3.6 km over a snow-ice surface.
A content of snow and ice in the deposit is a
common feature with the snow or ice derived by
basal scour by the avalanche (Hsii, 1975;
McSaveney, 1978) or from material included in
the rock fall. Although the final snow-ice content
in the Sherman Glacier deposit was low, (Gordon
et al., 1978) considered that it constituted 90% of
the Lyell Glacier avalanche deposit, judging from
the thickness changes as it melted. It is possible
that the snow, plus any fine rock debris, played a
role in the flow mechanics by providing a dispersed fine fraction matrix. Another common
feature is the tendency for separation of the flow
from its bed in free ballistic flight where sharp
changes in slope angle occur (Hsii, 1975;
Fahnestock, 1978), and the lack of deposits on
the rock step between the two valleys may reflect
such a situation.
78
Age of the avalanche
The moraines in which the avalanche deposits
occur are from the maximum readvance of the
last few hundred years - the Historical Readvance of Weidick (1968), and the avalanche
therefore predates this. Beschel (1956) has
lichenometrically dated the readvance maximum
at similar local mountain glaciers in the coastal
area south of Holsteinsborg variously to A.D.
1740-1780 and A. D. 1600, and in more continental areas to A.D. 1850 or 1870-1890. Lichen
measurements (Rhizocarpon geographicum) on
the avalanche deposits gave maximum thallus
diameters of 101 mm in the main valley, 109 mm
in front of the glacier, and 28 mm on the moraine
itself. The avalanche therefore appears to be about four times the age of the moraine. Beschel
(1956) quotes lichen growth rates of 2—43
mm/100 y for West Greenland, relating the lower
rates to increasing continentality, although some
more recent work discredits these low values
(Ten Brink, 1973). It seems likely that there was
very little direct control on these rates except at
one locality, which gave one of the higher values.
Equating the moraine with an 1890 maximum
gives a minimum age for the avalanche of 335 y,
or alternatively with a 1750 event gives 880 y.
Using a lichen growth factor of 40 mm/100 y
gives 273 y. Since the freshness of the moraines
suggests a 19th century date for them it seems
probable that the avalanche occured in the 16th
or 17th centuries.
Conclusion
A massive rock fall, involving at least 3680 tonnes of rock (assuming a maximum 50% porosity
for the deposit), fell from some point on the vertical north face of Avqutikitsoq, probably in the
16th or 17th centuries A.D. This generated a
rock avalanche, analogous to several modern
ones, which moved at high velocity down a glacier
and into the main valley below, covering a total of
nearly 2 km2 with massive boulder deposits. It is
interesting that the trigger for recent rock avalanches is frequently seismic shocks since
Holsteinsborg has a record of moderate seismic
activity (Watterson, pers. comm.). If this was the
case other rock avalanches or rock falls in the
Kelly: Greenlandic rock avalanche
area may have been caused by the same event. So
far only one other deposit which may be of the
same age is known, from the north side of Pisigsarfik, north of Taserssuaq, where an anomalously large Historical Readvance moraine occurs.
Although this is the first description of a highly
mobile rock avalanche from Greenland it is unlikely to have been unique in the past, or to be so
in the future. Major rock falls not associated with
extensive horizontal transport, such as described
and photographed by Weidick (1968) are likely
to be even more frequent. It is therefore worth
emphasising that the effects of these categories of
mass movement can be catastrophic, with an
extreme example of this being the 21,000 deaths
from the Huascaran rock avalanche and mudflow
(Plafker & Ericksen, 1978), and thus the assessment of their occurrence should be an integral
part of construction planning in Greenland.
Acknowledgements
This work is published by permission of the Director of the
Greenland Geological Survey. I am grateful to Ian Shaw,
Thorbjørn Pedersen and the skipper Eirik Hansen for help in
the field, to Garolyn Amos for secretarial assistance and to
Anker Weidick for his comments on the manuscript.
Dansk sammendrag
Nær Holsteinborg, Vest Grønland, er lokaliseret et større
stenskred. Det dækker et areal på omkring 2 km2 og omfatter
mindst 2,8 x 106 m3 bjergartsmateriale, som er blevet transporteret op til 7 km fra sit oprindelsessted.
Den mulige transportmekanisme diskuteres på baggrund af
sammenligninger med nyere beskrivelser af tilsvarende skred.
Det må antages at skredet skete med stor hastighed og bestod af
en blanding af bjergartsfragmenter, sne og is. Den udløsende
faktor kan vel have været et stenskred fra siden af bjerget Avqutikitsoq. Tidspunktet for stenskredet kan på basis af lichenometri anslås til det 16. eller 17. århundrede.
References
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