Australia is well known as a dry continent; this reputation can be best
demonstrated by the fact that two thirds of the continent has no runoff at
all. One third has less than 250 mm of rainfall and the 750 mm isohyet is
rarely more than 250 kilometres inland.
Much of the remainder is only seasonally or episodically moist and
therefore karst processes can only operate for relatively short periods.
The fact that many of the larger bodies of soluble rocks are found in the
drier areas further reduces the opportunity for karst development. Even
the well watered fringe is subject to extreme variability and the
precipitation/evaporation ratio generally does not favour the ready
availability of water.
The distribution of Australian karsts is shown this is about 4% of
the continent and is an underestimate due to incomplete geological
mapping. . It should be noted that a much larger area (around 15%) is
underlain by limestone groundwater basins with a thick overburden of
Climatic control of karst development can be seen dramatically in the
difference between the dry Nullarbor Plain and the moist southeast of
South Australia. Although the lithologies of the horizontally bedded,
Tertiary limestones are very similar, the latter has extensive
karstification in stark contrast to the Nullarbor.
This may in part be due to watertable change through tectonics as well as
sea level fluctuations. Sea level changes have had an influence on the
development of the Nullarbor caves but it may well be that the karst
features of the Nullarbor are relics of former, more effective,
precipitation regimes. There is, however, little evidence for this over
the fifteen million years that the limestones have been above sea level.
In common with the other Gondwanaland continents Australia is poor in
carbonate rocks. This is because much of the continent developed before
carbonate rocks became an important proportion of the geological makeup of
the earth. However, if the Precambrian shield area is ignored limestones
and dolomite are present in relative abundance, but there is an extreme
paucity of evaporite bedrocks in which karst features occur elsewhere in
the globe. Australia is the only continent lacking Tertiary orogenic belts
(excepting the Cape Range of W.A.); these usually contain extensive
limestones and Australia misses out on this count as well. Illustration by
Contact:- David Gillieson,
Dept. of Geography & Oceanography,
University College, University of New South Wales, Canberra, ACT 2601,
Phone 61 6 2688305 Fax 61 6 2688313
Karst is a distinctive topography in which the landscape is largely shaped
by the dissolving action of water on carbonate bedrock (usually limestone,
dolomite, or marble). This geological process, occurring over many
thousands of years, results in unusual surface and subsurface features
ranging from sinkholes, vertical shafts, disappearing streams, and
springs, to complex underground drainage systems and caves. (Photo by J.
The process of karst formation involves what is referred to as "the carbon
dioxide (CO2) cascade." As rain falls through the atmosphere, it picks up
CO2 which dissolves in the droplets. Once the rain hits the ground, it
percolates through the soil and picks up more CO2 to form a weak solution
of carbonic acid:
H2O + CO2 = H2CO3
The infiltrating water naturally exploits any cracks or crevices in the
rock. Over long periods, with a continuous supply of CO2-enriched water,
carbonate bedrock begins to dissolve. Openings in the bedrock increase in
size and an underground drainage system begins to develop, allowing more
water to pass, further accelerating the formation of karst. Eventually
this process leads to the development of subsurface caves.