Page 206 - Fundamentals of Geomorphology

KARST LANDSCAPES 189

by the presence of magnesium in dolomites. Evaporites, (Jennings 1971, 32). The distinction would be artificial
including gypsum, are much more soluble than lime- in most rocks, but in karst rocks solution ordinarily
stone or dolomite but carbon dioxide is not involved assures a clear division between the bedrock and the
in their solution. Gypsum becomes increasingly solu- regolith.
ble up to a maximum of 37 C. It is deposited as warm
◦
water cools sufficiently and when evaporation leads to Slow mass movements
supersaturation.
Soil and regolith on calcareous rocks tend to be drier than
they would be on impervious rocks. This fact means that
Silicate rocks
lubricated mass movements (rotational slumps, debris
Active sinkholes, dolines, and cave systems in quartzite slides, debris avalanches, and debris flows) are less active
must be produced by the excavation and underground in karst landscapes. In addition, there is little insoluble
transport of rock. As quartzite has a very low solu- material in karst rocks, and soils tend to be shallow, which
bility, it is difficult to see how such processes could reduces mass movement. Calcium carbonate deposition
proceed. One possibility is that, rather than dissolving may also bond soil particles, further limiting the possibil-
the entire rock, it is necessary only to dissolve the cement- ity of mass movement. Conversely, the widespread action
ing material around individual quartz grains. Quartz of solution in karst landscapes removes support in all
grains have a solubility of less than 10 mg/l, while amor- types of unconsolidated material, so encouraging creep,
phous silica, which is the chief cement, has a solubility block slumps, debris slides, and especially soilfall and
of 150 mg/l. With the cement dissolved, the quartzite earthflow. As a rider, it should be noted that piping occurs
would become mechanically incoherent, and loose grains in karst soil and regolith, and indeed may be stimulated
could be removed by piping, so eroding underground by solutional processes beneath soils and regolith cov-
passages. Alternatively, corrosion of the quartzite itself ers. Piping or tunnelling is caused by percolating waters
might produce the underground karst features. Corro- transporting clay and silt internally to leave underground
sion of quartz is a slow process but, given sufficient conduits that may promote mass movements.
time, this process could open underground passages. To
be sure, some karst-like forms excavated in quartzites Collapse
of the Cueva Kukenan, a Venezuelan cave system, con-
sist of rounded columns some 2–3 m high. If these had Rockfalls, block slides, and rock slides are very common
been formed by cement removal, they should have a in karst landscapes. This is because there are many
tapered cross-section aligned in the direction of flow. bare rock slopes and cliffs, and because solution acts
All the columns are circular, suggesting that corrosion as effectively sideways as downwards, leading to the
has attacked the rock equally on all sides (see Doerr undercutting of stream banks.
1999). Also, thin sections of rocks from the cave system
show that the individual grains are strongly interlocked Fluvial and hydrothermal processes
by silicate overgrowths and, were any silica cement to be
removed, they would still resist disintegration. Only after Solution is the chief player in cave formation, but cor-
the crystalline grains themselves were partly dissolved rasion by floodwaters and hydrothermal action can have
could disintegration proceed. significant roles. Maze caves, for instance, often form
where horizontal, well-bedded limestones are invaded
Slow mass movements and collapse by floodwaters to produce a complicated series of criss-
crossing passages. They may also form by hydrothermal
It is expedient to distinguish between collapse, which action, either when waters rich in carbon dioxide or when
is the sudden mass movement of the karst bedrock, and waters loaded with corrosive sulphuric acid derived from
the slow mass movement of soil and weathered mantles pyrites invade well-jointed limestone.