Missouri, and the Missouri Ozarks are known nationwide for the profusion of Caves, Karst and world class freshwater Springs, so many, in fact, that we might be classified as a Cave Factory.
Why is this so? Without going into a dissertation on geogenesis, climatology and plate tectonics, the answer is: we have the right rocks, plenty of rain, and just enough seismic activity to create the proper conditions for cave development.
Billions of years ago, an igneous layer of basement rock upwelled in this area, creating the Ozark uplift--a "bubble" of rhyolites, felsites and granites which are still exposed at the heart of the Ozarks in the St. Francois Mountains. This basement layer has never been totally still-- movement still occurs in the form of earthquakes along the New Madrid and other faults throughout this region. But, for the most part, these earth movements are infrequently strong, leaving long periods of slight movement. Enough to crack rock layers, but not catastrophic.
After the igneous rocks cooled, the surface in this area eroded and subsided. Conditions on earth were such that huge, shallow seas inundated much of the Midwest, depositing layers of sedimentary rocks, largely dolomites and limestones, occasionally mixed with sandstones and shales. These formed in tilted layers over the harder igneous rocks, sort of like layering a piece of cheese over chunky meat on a sandwich. Uplift continued, intermittently bringing these sedimentary layers above the water, and allowing them to erode in the air, as well as crack the layers themselves. Four major periods of uplift have been documented in the last 600 million years.
South of the Missouri River, and in the bluffs along the Mississippi, conditions for the formation of caves have been good for much of this time. (Northern Missouri is not so blessed with caves (although there are a few) because, the rock layers lie flatter, contain much more sandstone and shale, and because it has been buried and scraped by several of the glacial ice sheets which covered much of North America.) Geologists have discovered paleokarst (old karst) or fossil caves and sinks among the rock layers which currently exist.
Karst landforms are created by the erosion of carbonate base rocks like limestones and dolomites. Karst is more correctly a process, not a type of landform or topography. In brief, the carbonate part of limestone (calcium carbonate) or dolomite (calcium magnesium carbonate) is chemically dissolved by carbonic and other weak acids which form as a result of rainwater mixing with carbon dioxide and other carbon-based chemicals which are decomposition products of leaf litter, and other decaying vegetable matter. Rainwater, turned into this weak carbonic acid, enters the ground, percolates through the soil, and enters cracks in the carbonate rocks, dissolving them as it proceeds downwards.
Karst begins as these vertical cracks enlarge, forming joints. Eventually, these joints hit a horizontal crack, either between different rock layers, or formed as a result of seismic activity. The water continues horizontally, eroding as it goes. At this point, cave development occurs in a totally wet environment, either at or below the average standing water level of the area. When one of these underwater conduits hits a valley wall and emerges, it becomes known as a spring. Much of Missouri karst is still at the water filled stage, as feeder conduits for our wonderful springs.
When the average water level drops, due to the draining of the karst by springs, or overall erosion of a region, these underground conduits become partly, and then completely, air filled. When they become somehow accessible to humans, they are known as caves. More mineralized water entering an air filled chamber gives off some of the carbon dioxide into the cave air. This causes a change in the acidity of the water, and a change in its ability to transport minerals--the calcium which it picked up in dissolving rock then is also released, and forms speleothems, or cave deposits--stalactites and stalagmites, flowstone and cave coral, and tens of other shapes.
The vertical cracks which initially form the cave continue to enlarge, perhaps spreading out into other rock layers above the cave. When the soil above the cave has been washed away through the cave, or these overlying voids collapse, sinkholes form on the surface. Sinkholes can also form in cave floors, sending cave development to a lower level. Eventually, a cave will collapse entirely, unroofed by sinkhole formation.
Most Missouri Caves are formed in Cambrian, Ordovician and Mississippian Age dolomites and limestones, with an average rock age of 500 to 300 million years old. Most large Missouri Springs are in Cambrian and Ordovician dolomites and limestones, with an average age of 600 to 400 million years old. No one can tell exactly how old a cave or a spring itself is; most guesses range from tens of thousands to several million years for most of these features.
(c) 1995 Jo Schaper