The precise erosional forms which the karst takes depends on many variables. The mechanical structure and chemical composition of the rock, the local climate and temperature range, and the amount of vegetation and rainfall a region has all determine how fast a carbonate landscape erodes. Karst along a seaside is quite different than that inland, and tropical karst does not resemble karst in temperate or sub-Arctic zones. Landforms in zones with earthquake activity are quite different than that in quiet zones, or places where mountain-building have turned the layers of carbonate rock on edge. Some karst is formed as a result of sulfuric acid welling up from below instead of carbonic acid percolating down from above. Some places in the American West are a mix of the two processes.
But the karst of the Missouri Ozarks is almost textbook, and is characterized by well eroded rolling hills, deep hollows, springs, caves, sinkholes, losing streams, natural bridges, and tunnels.
Missouri, especially south of the Missouri River, has all the natural resources to make, (in the
words of Jerry Vineyard, one of our best known geologist-cavers) a wonderful cave
factory.
In order for temperate karst to form, there must be sufficient layers of carbonate rock (in
Missouri, anywhere from none to thousands of feet thick); adequate rainfall (about 45 inches
annually); a reasonable vegetative cover to provide humus and carbon plant debris (oak-hickory
forest and grassland over much of the state); suitable entrances to the bedrock (faulting and
dipping from the Ozark uplifts and seismic activity); and a variable climate (bored with our
weather? Just wait a few minutes.)
Karst is formed when rainwater picks up carbon dioxide from the air, and dead plant debris in the soil, then percolates through cracks dissolving the rock. The bedrock becomes saturated with water at some level, and dissolving continues as the water moves sideways along bedding planes (horizontal cracks between rock layers) and joints (or fractures) in the rock itself. These conduits enlarge over time, and move the water, via a combination of gravity and hydraulic pressure, further enlarging the conduits through a combination of solution and abrasion of water on the surrounding rock.
Eventually, much of this water under pressure reaches the surface of the land as a spring. A spring may emerge high on a cliff, at the base of one, or even forced upward from below the level of the surrounding surface streams, depending on nature of the surrounding rock, and the altitude of the groundwater level, with respect to the base level of the controlling stream in a drainage area. Often in Missouri, springs have little relationship to surface drainage, because so much of our water movement is actually groundwater movement. In some areas of the Ozarks, more than 70% of all water goes underground via karst processes.
Once beneath the surface, the groundwater is unevenly distributed through the karst bedrock.
This "reservoir theory" accounts for the varying levels of spring flow, which are related to rainfall,
but not necessarily on an immediate basis. Tom Aley, a well-known hydrologist, divides spring
flow into base flow and surge flow. Base flow is that which is more or less constant, and is stored
in deep levels, whereas surge flow is that which more immediately related to local rainfall.
Because the water is contained in varying conduits such as tubes, caves, cracks, joints and these
are interconnected at varying elevations, the sources of groundwater for a spring may vary
depending upon the amount of water already in the system, the level of the outlet, and the amount
of water entering. Aley illustrates this state of affairs with his "milk-jug" theory of spring flow--by
placing milk jugs with varying amounts of water in them, and interconnecting pipes at random
levels, he demonstrates that adding more water to the system will cause varying results based on
many variables. The water "table" in southern Missouri is much more like a pockmarked and
tunnelled minefield than anyone's table.
As groundwater levels in an area drop, more and more of the underground passage becomes air filled. When it is sufficiently air filled, springs become cave entrances, passable by humans. Other voids never develop a natural opening, and are intersected by drilling, notably of wells looking for water. At this point, due to changes in chemical equilibrium underground, the resulting caves begin to fill with dissolved mineral, called cave deposits or speleothems. Caves may refill with water or continue to dry out, or even cycle several times as water levels change.
Erosion continues underground, and eventually a cave hollows enough for the roof to thin, and the cave collapses. Such cave collapse may actually unroof the cave if it is near enough to the surface, or simply form a slump in the level of the land. In either example, a sink forms. Natural bridges and tunnels can be formed as resistant remnants of a cave collapse, or independently, if a block of bedrock becomes cut off from the main land mass, and it is hollowed out by wind, ice wedging, and rain.
Many karst areas have poor soil, and do not retain water easily, allowing it to go directly underground. Sinks also act as "swallow holes" for rainwater; some sinks take water under certain conditions, and resurge it at others. These reversible sinks, called estavelles, are among the curiosities of karst. Some springs in the Ozarks are periodic, or "ebb and flow" springs, whose discharge can be measured to rise and fall independent of local rainfall. Many theories, but no one knows why, for sure. Another oddity of Missouri karst is the karst window, where one may look into a cave or water filled sink below, but getting down there is another matter entirely.
Karst makes for beautiful scenery, but it is very vulnerable to groundwater pollution, due to ease of water flow. Natural filtration is nearly non-existent in karst. To make matters worse, the use of cave conduits as natural sewer lines, and sinkholes as garbage dumps in small towns and rural areas puts the local drinking water supplies at risk. It is only recently that these problems are being addressed. Urban expansion in karst areas often means the building of houses on land which cannot support them and problems with septic tanks, underground pipeline breaks and landfills.
Some of the most beautiful sites in Missouri are a result of karst processes, as are some of our most pressing land use problems. Since the undeveloped land seemed to have managed very well, it is up to people to use some sense when they go to develop it, and take its unusual character into account.