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icesheets, and as far back as the Pliocene. While coastal
erosion and flooding can result from this regional sub-
sidence, it is possible for humans to adjust to these
Natural mechanisms causing land subsidence fall
into three groupings: chemical, mechanical and
tectonic. Chemical agencies of land subsidence mainly
involve solution by groundwater of limestone, halite,
gypsum, potash, or other soluble minerals. Because
limestone is very extensive, the development of karst
topography can lead to sudden ground subsidence over
large areas. This is a particular problem in the south-
eastern and mid-western parts of the United States,
areas that are underlain by extensive limestone
deposits. In Australia, subsidence features are wide-
spread across the Nullarbor Plain; however, the region
is neither densely settled nor intensely farmed, so that
karst subsidence has little effect on people. A rarely
recognized chemical cause of subsidence is the natural
ignition (or, more recently, industrial vandalism) of coal
seams such as has occurred at Burning Mountain,
north of Scone, New South Wales, Australia.
Mechanical factors involve the removal, by extrusion,
of material at depth in the soil. This can include the
ejection of quick clays. However, by far the most effec-
tive process is the melting of ice lenses in permafrost
areas. Because living and decayed vegetation provides
an insulating blanket against the penetration of heat
into permanently frozen ground, its removal will often
result in catastrophic and irreversible melting of ground
ice and subsequent surface collapse. The emplacement
of heat conduits, such as foundations and telephone
poles, will also permit the transfer of heat from the
surface to depth where melting will occur. Even heat
from a building can conduct into permafrost to cause
melting and subsidence unless it is prevented from
doing so by an intervening layer of insulation. The
effect of subsidence in permafrost regions has major
implications for landform development and the survival
of fragile, cold-climate ecosystems.
Tectonic factors involve warping of the Earth's crust.
The Earth is an elastic body that can be deformed by
the weight of glaciation, and that will return to its
original shape after the ice load is removed. Glaciation
causes forebulging some distance in front of the ice
front. In these regions following glacial retreat, the
crust settles instead of rebounding. Large sections of
the southern coastline of England, including London
and northern Europe, are affected by this process, and
are presently undergoing subsidence at rates in excess
of 2-3 mm yr -1 . This is being exacerbated by the long-
term downwarping of the southern North Sea Basin. If
there is a threat of sea level rise in the next century,
then it is possible for this rise to be exacerbated by
tectonic subsidence in these regions. Subsidence can
also be caused by ground loading. This is an obvious
factor where major rivers are presently depositing large
volumes of sediment on shallow continental shelves.
Much of the coastline surrounding the Mississippi
Delta is subsiding because of continual, long-term
deformation of the crust due to the deposition of
sediment debouched from the Mississippi River.
Land instability hazards encompass the most diverse
range of processes and phenomena of any hazard dis-
cussed in this text. They also occur over the greatest
time span, from soil creeping downslope at rates
of millimetres per year, to the various types of
avalanches reaching speeds in excess of 300 km hr -1 .
In many respects, land instability is the most
commonly occurring hazard. For instance, solifluc-
tion affects 20 per cent of the Earth's landmass, while
expansive soils and landslides impact on 33 per cent
and 14 per cent, respectively, of the conterminous
United States. However, the topic of land instability
hazards is not an easy one to cover. The fact that
these hazards have attracted equal attention from
engineering- and geology-related disciplines has led
to a confusion of terms and a discipline-specific
emphasis. This has meant that some important
phenomena such as expansive soils are relegated in
treatment to disciplines, in this case soil science, that
do not always attract the attention of natural hazard
researchers. It has also made exchange between
disciplines difficult, not only because of the clash
over terminology, but also because engineers tend
to centre discussion around mathematics while
geologists and geomorphologists are more interested
in processes and morphology. Despite attracting
the attention of a wide range of researchers, many
aspects of land instability remain unclear. For
instance, little is known about the mechanics of
debris avalanche movement. This has meant that our
ability to predict the occurrence of many land
instability hazards is far from effective. For example,
research in the Wollongong region of Australia
would indicate that landslides should be prevalent
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