Environmental Engineering Reference
In-Depth Information
fluvial
,
glacial
,
periglacial
,
coastal
environments, etc. They are found everywhere and
behave according to universal rules. Mass
KARST LANDSYSTEMS
applications
Limestones are not uniquely susceptible to solution but, given the right conditions,
develop a unique
karst
landsystem dominated by the solution of carbonate rocks and
progressive development of underground drainage. Carbonate rocks of clastic, biogenic
and inorganic precipitate origins cover 15 per cent of continental land surfaces, in all
climates. They are classed as
limestones
when CaCO
3
(carbonate) content exceeds 50 per
cent as
calcite
or
aragonite
, or
dolomitic limestone
when CaMg(CO
3
)
2
(
dolomite
) is the
principal carbonate mineral. Not all limestones develop karst landforms in all climates,
however, and karst geomorphology provides an excellent illustration of the respective
roles of lithology, structure and climate in shaping Earth's land surface.
Essential hydrogeological ingredients are rocks possessing a geochemical
susceptibility to solution, a porous texture (particle framework) and/or permeable
structures (discontinuities) providing good water access, substantial thickness and high
mechanical strength to support residual landforms. Britain's Carboniferous limestone
illustrates a global tendency for older, Palaeozoic limestones to produce prominent
surface landforms and underground cave systems. Its 'massive' structure provides a
fracture network through which solution is concentrated, more than compensating for its
low porosity (2-5 per cent) and less than pure carbonate content (60-80 per cent). By
comparison, solution is diffused throughout the intense microfracture network and high
porosity of Cretaceous chalk, producing subdued solution landforms despite being an
almost pure carbonate (over 90 per cent).
Hydrometerological conditions required to sustain optimal (near equilibrium) solution
rates, and flush out dissolved carbonates, include moderate to high annual
liquid
precipitation amounts (over 1500 mm yr
−1
) and near uniform regimes.
The temperature regime is more complex. Carbonate solution rates obey the general
rule of being proportional to temperature but the initial solution of CO
2
in H
2
O is
inversely related to temperature (see p. 264). CO
2
saturation concentration at 0° C is
twice that at 20° C. This distorts the otherwise expected spectrum from weak karst
development in cold and/or dry climates to full development in hot, moist climates.
Substantial glacial meltwater flow enhances solution in cool temperate karst regions.
At their best, karst landforms develop a three-tiered system (Figure 1).
Aggressive
water
- charged with solution potential - etches grooves (
karren
) or hollows (
lapies
) 1-
10 m deep into limestone surfaces (Plate 1a), before moving underground through the
fracture network. Solution depressions 10
1-2
m deep (
dolines
) develop and permeability is
enhanced as they are widened or collapsed by solution (Plate 1b). Drainage is transferred
progressively underground, forming cave systems, 10
2-3
m deep and 10
1-5
m long in thick
limestones, as the karst landsystem matures. Continuing solution finally widens dolines
or collapses cave systems to the point that resurgent rivers appear in a residual
landsystem. This is most dramatic in tropical
tower
and
cockpit
karst (see Colour Plate 12
between pp. 272 and 273), which led to the view that climate was pre-eminent over
geology.
