Geoscience Reference
In-Depth Information
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 concen-
trated, 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).
Hydrometeorological conditions required to sustain
optimal (near equilibrium) solution rates, and flush out
dissolved carbonates, include moderate to high annual
liquid
precipitation amounts (over 1,500 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. 293). CO
2
saturation concentration at 0
o
C is twice
that at 20
o
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 13.11
).
Aggressive water
- charged with
solution potential - etches grooves (
karren
) or hollows
(
lapies
) 1-10 m deep into limestone surfaces (
Plate
network. Solution depressions 10
1-2
m deep (
dolines
)
develop and permeability is enhanced as they are widened
or collapsed by solution (
Plate 13.10b
).
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
(a)
Gorge and
dry valley
(b)
Tower and cockpit
Uvala
Polje
(with lake)
Pocket valley
Resurgent
stream
Doline
Cave
solution dolines drain water into the limestone where it
develops cave systems before resurgence along an aquiclude.
(b) Continuing surface solution weathering and cavern
collapse progressively corrode the landscape down to poljes
and alluvial plains; residual towers and cockpits are common
in tropical karst.
(
Plate 13.10c
)
. This is most dramatic in tropical
tower and
cockpit
karst (
Plate 13.11b
)
, which led to the view that
climate was pre-eminent over geology.
However, climate change inevitably enters the
argument. It is now thought that the persistence of warm,
humid climate throughout the Quaternary in many
tropical areas sustained karst development. Moreover,
geological control in climatically similar regions is evident
in contrasting karst development between the massive
white limestones (good) and weak blue limestones (poor)
belts of Jamaica. Carboniferous limestone
pavements
and
chalk
dry valleys
in northern and southern Britain
respectively represent
glaciokarst
landsystems, whose
development began during mid to late Pleistocene cold
stages (
Plate 13.10
).
Their modern form probably reflects
recent exhumation from beneath substantial Holocene
soil and vegetation cover by anthropogenic activities.
MASS WASTING
Weathering products seldom remain
in situ
for long and
products must be removed for vigorous weathering to
