Karst features may also develop, though rarely, on very weakly soluble rocks, such as basalt, granite, or quartzite.. Rock solubility and water are the primary factors in karst developme
Trang 1with extensive gypsum karsts known from Russia and
the Ukraine, but their greater solubility renders such
landforms more dynamic and, for rock salt,
ephem-eral in all but the most arid climates Karst features
may also develop, though rarely, on very weakly
soluble rocks, such as basalt, granite, or quartzite
Rock solubility and water are the primary factors
in karst development Arid climates, whether hot
or cold, support little karst Physical rock properties
also are important Highly porous rocks seldom
sup-port well-defined karst features, which instead are
favoured by low porosity and good secondary
per-meability, in the form of fractures, focusing the
drain-age into specific conduits through the karst rock The
removal of rock in solution allows the development
of drainage through the rock, rather than just across
its surface as happens largely with rocks removed
by mechanical erosion Consequently, karst
land-scapes generally lack well-developed surface drainage
but have underground drainage conduits, or caves
Hence a significant component of karst terrains
typ-ically lies beneath the surface, sometimes extending to
depths of hundreds, or even thousands, of metres
Intimately associated with the dissolutional aspects
of karst are depositional ones The latter include
clas-tic sediments within the caves and, parclas-ticularly,
min-erals deposited by precipitation from karst waters
both above and below ground
Many subdivisions of karst have been proposed
Relict karst is used to denote landforms inherited
from earlier climatic or drainage regimes but still
subject to modification by the current conditions
Palaeokarst refers to karst features buried by younger
rocks and so largely isolated from current karst
modi-fication; where uncovered by later denudation, this
isolated karst is called exhumed karst Biokarst
encompasses small-scale sculpting of limestone by
animals and plants, although the distinction between
dissolutional sculpting (true biokarst) and mechanical
excavation (bioerosion) is seldom made Pseudokarst
is, as its name implies, ‘false karst’ Such features
superficially resemble karst but form by quite different
processes, such as lava tubes, soil piping, and
thermo-karst, or cryothermo-karst, formed by localized melting of
permafrost
Often karst geomorphology is regarded as a
spe-cialist discipline that is of limited general application
to geology or geomorphology However, 12% of
Earth’s terrestrial, ice-free surface is composed of
lime-stone, with 7–10% supporting some form of karst
landscape Furthermore, as much as 25% of the
world’s population may depend to some extent on
karst water supplies Consequently, the study of karst
is crucial to understanding landscape and drainage
development over a significant area of Earth’s surface
Karst Processes The basic process of karst dissolution involves ion dissociation For rock salt (NaCl), gypsum (CaSO42H2O), and quartz (SiO2), this requires only the presence of water but, on a global scale, the out-crop area of evaporites and the low solubility of quartz render them of only minor and localized sig-nificance for karst development Limestone (CaCO3) and dolomite (CaMg[CO3]2) are by far the dominant karst rocks but experience very low rates of dissoci-ation in pure water The addition of free Hþ ions greatly increases the rate of carbonate dissociation and hence even weak acids become effective solvents
In most karst environments, carbonic acid, derived from atmospheric or soil CO2, is the main source
of Hþions, although other organic or inorganic acids may be significant locally The solubility of CO2 in-creases with decreasing temperature, in common with other gases The same is true also for limestone, in marked contrast to most solids for which solu-bility increases with temperature Nonetheless, the availability of liquid water and biogenic CO2 is far more significant for karst development than are low temperatures
Water with an excess of Hþ ions is commonly referred to as ‘aggressive’, and continues to take up HCO3 ions until an Hþ/ HCO3 equilibrium is reached at saturation, as in the following equation:
CO23 þ 2Hþ! HCO3 þ Hþ! H2CO3
Different karst waters may reach saturation at differ-ent concdiffer-entrations, determined by the initial CO2
concentration, but this is not a simple straight-line relationship and mixing of different karst waters may increase aggressivity This phenomenon, called mixing corrosion, may be significant in certain karst environments Carbonate solubility also is increased
by the foreign ion effect, the addition of ions such as
Naþ, Kþ, and Cl Seawater is saturated and cannot directly dissolve limestone, but mixing with fresh-water can considerably increase carbonate solubility and is of major significance in certain environments
In the same way that an increase in CO2 concentra-tion increases carbonate uptake, so degassing of CO2
from a saturated solution causes reprecipitation of calcite
Dissolution can and does occur in static or laminar flow conditions, though constrained by diffusion rates through the boundary layer Permeable soil or sediment cover, even when vegetated, may offer only limited resistance to downward percolation of water
to the limestone beneath Although this subsoil water movement may be slow, its dissolutional efficacy is enhanced by higher CO2concentrations generated by
SEDIMENTARY PROCESSES/Karst and Palaeokarst 679