Definition of karst
Areas that are affected by groundwater dissolution,
cave complexes, and sinkhole development are known as
karst terrains. Globally, several regions are known for spectacular
karst systems, including the cave systems of the Caucasus,
southern Arabia including Oman and Yemen, Borneo,
and the mature highly eroded karst terrain of southern
China’s Kwangsi Province.
The formation of karst topography begins with a process
of dissolution. Rainwater that filters through soil and rock
may work its way into natural fractures or breaks in the
rock, and chemical reactions that remove ions from the limestone
slowly dissolve and carry away in solution parts of the
limestone. Fractures are gradually enlarged, and new passageways
are created by groundwater flowing in underground
stream networks through the rock. Dissolution of rocks is
most effective if the rocks are limestone, and if the water is
slightly acidic (acid rain greatly helps cave formation). Carbonic
acid (H2CO3) in rainwater reacts with the limestone,
rapidly (at typical rates of a few millimeters per thousand
years) creating open spaces, cave and tunnel systems, and
interconnected underground stream networks.
When the initial openings become wider, they are known
as caves. Many caves are small pockets along enlarged or
widened cavities, whereas others are huge open underground
spaces. In many parts of the world, the formation of underground
cave systems has led to parts of the surface collapsing
into the caverns and tunnels, forming a distinctive type of
topography known as karst topography. Karst is named after
the Kars Limestone plateau region in Serbia (the northwest
part of the former Yugoslavia) where it is especially well developed.
Karst topography may take on many forms in different
stages of landscape evolution but typically begins with the formation
of circular pits on the surface known as sinkholes.
These form when the roof of an underground cave or chamber
suddenly collapses, bringing everything on the surface suddenly
down into the depths of the cave. Striking examples of sinkhole
formation surprised residents of the Orlando region in
Florida in 1981, when series of sinkholes swallowed many
businesses and homes with little warning. In this and many
other examples, sinkhole formation is initiated after a prolonged
drought, or drop in the groundwater levels. This drains
the water out of underground cave networks, leaving the roofs
of chambers unsupported, and making them prone to collapse.
The sudden formation of sinkholes in the Orlando area
is best illustrated by the formation of the Winter Park sinkhole
on May 8, 1981. The first sign that trouble was brewing
was provided by the unusual spectacle of a tree suddenly disappearing
into the ground at 7:00 P.M. as if being sucked in
by some unseen force. Residents were worried, and rightfully
so. Within 10 hours a huge sinkhole nearly 100 feet (30 m)
across and more than 100 feet deep had formed. It continued
to grow, swallowing six commercial buildings, a home, two
streets, six Porsches, and the municipal swimming pool, causing
more than $2 million in damage. The sinkhole has since
been converted into a municipal park and lake. More than
one thousand sinkholes have formed in part of southern
Florida in recent years.
Sinkhole topography is found in many parts of the
world, including Florida, Indiana, Missouri, Pennsylvania,
and Tennessee in the United States, the Karst region of Serbia,
the Salalah region of Arabia, southern China, and many
other places where the ground is underlain by limestone.
Sinkholes have many different forms. Some are funnelshaped,
with boulders and unconsolidated sediment along
their bottoms; others are steep-walled pipe-like features that
have dry or water-filled bottoms. Some sinkholes in southern
Oman are up to 900-feet (247-m) deep pipes with caves at
their bottoms, where residents would get their drinking water
until recently, when wells were drilled. Villagers, mostly
women, would have to climb down precarious vertical walls
and then back out carrying vessels of water. The bottoms of
some of these sinkholes are littered with bones, some dating
back thousands of years, of water carriers who slipped on
their route. Some of the caves are decorated with prehistoric
cave art, showing that these sinkholes were used as water
sources for thousands or tens of thousands of years.
Sinkhole formation is intricately linked to the lowering
of the water table, as exemplified by the Winter Park example.
When water fills the underground caves and passages, it
slowly dissolves the walls, floor, and roof of the chambers,
carrying the limestone away in solution. When the water
table is lowered by drought, by overpumping of groundwater
by people, or by other mechanisms, the roofs of the caves
may no longer be supported, and they may catastrophically
collapse into the chambers forming a sinkhole on the surface.
In Florida many of the sinkholes formed because officials
lowered the water table level to drain parts of the Everglades,
to make more land available for development. This ill-fated
decision was rethought and attempts have been made to
restore the water table, but in many cases it was too late and
the damage was done.
Many sinkholes form suddenly and catastrophically,
with the roof of an underground void suddenly collapsing,
dropping all of the surface material into the hole. Other sinkholes
form more gradually, with the slow movement of loose
unconsolidated material into the underground stream network,
eventually leading to the formation of a surface depression
that may continue to grow into a sinkhole.
The pattern of surface subsidence resulting from sinkhole
collapse depends on the initial size of the cave that collapses,
the depth of the cavity, and the strength of the
overlying rock. Big caves that collapse can cause a greater
surface effect. In order for a collapse structure at depth to
propagate to the surface, blocks must fall off the roof and
into the cavern. The blocks fall by breaking along fractures
and falling by the force of gravity. If the overlying material is
weak, the fractures will propagate outward, forming a coneshaped
depression with its apex in the original collapse structure.
In contrast, if the overlying material is strong, the
fractures will propagate vertically upward, resulting in a
pipe-like collapse structure.
When the roof material collapses into the cavern, blocks
of wall rock accumulate on the cavern floor. There is abundant
pore space between these blocks, so the collapsed blocks
take up a larger volume than they did when they were
attached to the walls. In this way, the underground collapsed
cavern may become completely filled with blocks of the roof
and walls before any effect migrates to the surface. If enough
pore space is created, almost no subsidence may occur along
the surface. In contrast, if the cavity collapses near the surface,
a collapse pit will eventually form on the surface.
It may take years or decades for a deep-collapse structure
to migrate from the depth where it initiates to the surface.
The first signs of a collapse structure migrating to the
surface may be tensional cracks in the soil, bedrock, or building
foundations, formed as material pulls away from unaffected
areas as it subsides. Circular areas of tensional cracks
may enclose an area of contractional buckling in the center of
the incipient collapse structure, as bending in the center of
the collapsing zone forces material together.
After sinkholes form, they may take on several different
morphological characteristics. Solution sinkholes are saucershaped
depressions formed by the dissolution of surface limestone,
and have a thin cover of soil or loose sediment. These
grow slowly and present few hazards, since they are forming
on the surface and are not connected to underground stream
or collapse structures. Cover-subsidence sinkholes form
where the loose surface sediments move slowly downward to
fill a growing solution sinkhole. Cover-collapse sinkholes
form where a thick section of sediment overlies a large solution
cavity at depth, and the cavity is capped by an impermeable
layer such as clay or shale. A perched water table
develops over the aquiclude. Eventually, the collapse cavity
becomes so large that the shale or clay aquiclude unit collapses
into the cavern, and the remaining overburden rapidly
sinks into the cavern, much like sand sinking in an hourglass.
These are some of the most dangerous sinkholes, since they
form rapidly and may be quite large. Collapse sinkholes are
simpler but still dangerous. They form where the strong layers
on the surface collapse directly into the cavity, forming
steep walled sinkholes.
Sinkhole topography may continue to mature into a situation
where many of the sinkholes have merged into elongate
valleys, and the former surface is found as flat areas on surrounding
hills. Even this mature landscape may continue to
evolve, until tall steep-walled karst towers reach to the former
land surface, and a new surface has formed at the level of the
former cave floor. The Cantonese region of southern China’s
Kwangsi Province best shows this type of karst tower terrain.
See also CAVES.
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