Geological Setting and Cave Formation
To understand the geology leading up to Onondaga Cave, it is helpful to first understand the general geology of the region. About 1.4 billion years ago, the area that is now home to Elephant Rocks and Taum Sauk Mountain state parks was dominated by giant volcanoes, some with calderas up to 15 miles across. Explosive eruptions threw out lava and ash, which became the rhyolites that define the region today. The prominent granites in the area are magma chambers that cooled into a solid long before ever reaching the surface through erosional processes. These Precambrian igneous rocks formed what is called the basement layer, a foundation upon which all other rocks in the state were deposited. In Onondaga Cave State Park, this basement layer is roughly 1,000 to 1,500 feet below the cave.
Approximately 520 million years ago, the Ozarks region was lowered in elevation due to tectonic action, and the area was inundated with shallow seas. These seas began laying down the material that, over vast amounts of time, would form the sedimentary rocks found in the Ozarks today. About 490 million years ago, a large deposit of “lime slime,” basically the calcium-rich mud at the bottom of the ocean at the time, was laid down. This lime slime hardened into limestone, and later weathered into dolomite – a mineral that is similar to limestone but contains a large amount of magnesium. This dolomite, later called “Gasconade dolomite,” is the rock unit in which Onondaga Cave is formed.
Sometimes, what goes down must come back up, and that is what has happened to the Ozark Plateau. After being lowered by tectonic forces, the region rebounded into an uplift, giving new life to old streams, which now enjoy eroding down the rocks and soils. In the area around Onondaga Cave, the rocks laid down 440 million to 280 million years ago have eroded away, and several hundred feet of rock that used to sit on top of the cave is now gone, turned into sand and gravel and washed into the rivers and oceans. Today, the surface rock in Onondaga Cave State Park is Gasconade dolomite, and dolomite is the perfect host for caves! Some time ago, water got into this dolomite, and Onondaga Cave was born.
Most estimates date the actual formation of Onondaga Cave to the last few million years. However, scientists quickly run into some problems when trying to accurately time-stamp a cave's beginning, a process called speleogenesis. Since a cave is a hole in a rock, it is, by definition, that which is not there. Thus, any evidence of the cave’s origin is destroyed. There is also the question of when a cave can even be considered a cave, especially since no one was there to see it in its very beginning stages. All that is known is the general rate at which dolomite dissolves in rainwater. Going from that, scientists can somewhat estimate a cave's age. Nevertheless, due to the wide array of variable conditions over several million years, any accuracy beyond a rough estimate is currently an unsolvable scientific mystery.
There are lots of different types of caves in the world: glacier caves, lava tubes, sea caves and solution caves, to name a few. Missouri, being the host of lots of dolomite and limestone, has been a breeding ground for solution caves over the past couple of eras. But what makes these rocks so special? Limestone (CaCO3) and dolomite (CaMg(CO3)2) are perhaps some of the most common cave-forming rocks in the world because of their carbonate (CO3) content, which imparts a basic pH. This means that rainwater, made slightly acidic by CO2, can dissolve the rock over time as it takes advantage of joints, fractures, faults and other natural pathways through the rock. This leads to a type of topography known as karst, which includes features like caves, sinkholes, losing and gaining streams, and karst towers, which can be seen in southern China.
As water infiltrates and percolates through the rock, it creates voids and often leaves behind an insoluble residue, called “cave clay.” Two kinds of erosion are at work here: chemical dissolution, usually performed by standing water under the water table, and mechanical erosion, such as the grinding away of the rock by cave streams. Typically, solution caves begin completely under the water table – in the phreatic zone – and end up with air space above the water table, in the vadose zone. Most people will only ever see these vadose caves; only cave divers get to see caves under phreatic conditions. However, visitors to any of Missouri's large springs – such as Alley Spring or Round Spring – can at least see the entrance of a water-filled phreatic cave!
Because chemistry is king in karst systems, there is the possibility for some interesting phenomena associated with cave formation. Once a cave enters the vadose phase and air space is present, speleothems can form. Speleothems are those deposits that everyone knows and loves: stalactites, stalagmites, flowstones, columns, curtains and canopies. But how do these things form? By the time water has percolated to the ceiling of an air-filled cave room, any CO2 in the water droplets degases into the atmosphere. The laws of chemistry say that when CO2 leaves, CaCO3 must also leave, as losing one creates an imbalanced increase in the other. The CaCO3 (calcite) comes out of solution, and, depending on whether the water is dripping, flowing, splashing or sitting in still pools, it can make stalactites, stalagmites, flowstone, cave coralloids, lily pads and many other formations!
The most commonly recognized speleothems are stalactites, which stick tight to the ceiling. Under them, one can usually find stalagmites, which stalag-might reach the ceiling one day. Soda straws, which appear as small hollow tubes, are baby stalactites. Water passes through these tubes and drips out the bottom. If these tubes clog up, water has to go outside the straw, and a cone-shaped stalactite starts to take shape. Columns occur when a stalactite and a stalagmite meet and span from floor to ceiling.
The aforementioned structures are all known as dripstones, because they are formed by the dripping action of water. Flowing water can form speleothems called flowstones, which look like melted wax canopies. Dripstones and flowstones make up the major speleothem groups that most people are familiar with. However, when you really start looking, you can find all sorts of specialty speleothems, such as cave shields, blisters, helictites, bottlebrush spar, stalactiflats, boxwork, rafts and rootsicles.
Speleothems can come in many shapes, sizes and even colors. Each color reflects some impurity in the calcite, which is clear in its purest form. White speleothems have incorporated air into the calcite crystals, not unlike the white spot in the middle of ice cubes, which is air trapped in the ice crystals. Red or brown speleothems may have some clay or soil mixed into the crystal structure. Yellows and oranges come from organic acids like tannic acid, which was traditionally used to "tan” hides. These organic acids are found in decaying vegetation on the forest floor and find their way into the cave with surface water. Dull black colors are typically a coating of manganese oxides. In some places in Onondaga Cave, there is bright blue-green, the result of copper oxide from coins staining the calcite. This color may sound pretty, but subjecting something that is alive to any amount of copper is a great way to kill it, so keep your coins in your pockets!
How long do these formations take to grow? Some sources may say they take 100 years to grow an inch. Well, to give a more precise estimate, research on the development of speleothems has shown that they can grow anywhere from 0.09 to 39.0 inches per 100 years (Dreybrodt, 1999). Of course, this range does include a possible rate of 1 inch per 100 years. There are many variables that go into speleothem formation, including temperature, humidity, mineral content and pH of the water, drip rate, and flow rate. With all these factors changing all the time, it is easy to see why it is so difficult to pin down an accurate growth rate for one particular speleothem. One study in 2001 radioisotope dated (U/Th) a stalagmite from Onondaga Cave and found that it had an average annual growth rate of 0.05 millimeters (or 0.002 inches) per year over a 27-year span and later 0.7 millimeters (0.03 inches) per year over a 29-year span. The stalagmite was about 27 centimeters (10 inches) tall, and its base was dated to approximately 13,110 +/- 260 years of age (Denniston et al, 2001).
Like most caves in the region, Onondaga Cave began in the phreatic zone, as water sitting in the pore spaces between sand grains in the middle of a large block of dolomite. The water dissolved the rock surrounding these spaces, formed a small void and began to slowly move along preferential conduits. This void grew and merged with other voids, creating larger rooms that are now the top few feet of the cave. Eventually, the water table dropped, and air space appeared in the cave. This allowed water to flow and mechanically erode the cave walls. Several iterations of streams formed and meandered throughout the cave, lengthening the passage and then abandoning their channels as they found paths of lesser resistance. Several of these abandoned channels still exist today – now as dry passages, far away from the current cave stream. Clay was built up and washed out by the stream. Eventually, an opening was made in a valley wall, and for the first time, the cave was connected to the outside world.
It is through this opening that the current cave stream emerges as a spring at the base of a bluff. As the cave passage continued to deepen, speleothems began filling in the voids. Over many millennia, water eroded passages, dripped from the ceiling and walls, and built up large canopies and stalagmites. Yet all this went unnoticed by appreciating eyes until the summer of 1886, when some adventurous millers decided to poke their heads into a water-filled crack at the base of the bluff by their mill. And the rest is history.
Dreybrodt, W., (1999) Chemical Kinetics, speleothem growth and climate. Boreas 28, pp 347 – 356.
Denniston R.F., Gonzalez L.A., Asmerom Y., Polyak V., Reagan M.K., Saltzman M.R. (2001) A high-resolution speleothem record of climatic variability at the Aller∅d-Younger Dryas transition in Missouri, central United States, Palaeogeography, Palaeoclimatology, Palaeoecology 176, pp 147-15