Cave Formations

 Cave formations are created when acid reacts with limestone or a rock containing 80% or more calcium carbonate. These formations are found on the walls, ceilings and floors of caves. Cave formations are called speleothems, from the Greek word "spelaion",cave and "thema" meaning deposit (Robertson, 2004). A number of conditions need to be present for speleothems to form within a karst enviornment. First the types of rocks within and surrounding the cave need to have an 80% content of calcium carbonate which is usually limestone, dolomite or a similar type of rock. The bedrock also needs to be highly fractured or jointed so the water can flow through or follow these joints or fractures. The bedrock also needs to be relatively close or at the surface. The fourth requirement for Karst and spelethem formation is a relatively moderate annual rainfall (>500 mm). The final requirement to form the speleothems is vegetation cover. Vegetation enhances the Karst process by producing more available acids. A few variable factors including humidity, temperature and air flow through the cave also play an important role in speleothem formation.

 Caves and their features form when rainwater follows the cracks or joints in the rock material, usually limestone or dolomite. The rainwater combined with carbon dioxide forms a weak acid called carbonic acid. This weak acid once in contact with the limestone begins to dissolve the limestone. This process slowly creates larger and larger crack and joints. The acid can remain at a consistent level, but often is strengthened due to increased amounts of carbon dioxide absorption from vegetation and soil surrounding the area. As more and more limestone is dissolved large tunnels, networks of tunnels and joints, and actual caves become established. Once the caves and network of tunnels have formed different types of cave formations begin to evolve. Many different speleothems are common in caves including stalactites, stalagmites, columns, rimstone pools, cave pearls, and baconstrips (Figure 1) (The Legendary Black Water Rafting Company, 2004).

Figure 1 : Stalactites on the ceiling of the cave including many soda straws. (The Legendary Black Water Rafting Company, 2004)

 Just one drop of water on the ceiling of a cave is all that is needed for a stalactite to start forming. Each drop contains a small amount of dissolved limestone that has been acquired from flowing through the cracks and joints of the bedrock. Once this drop is hanging suspended from the ceiling some of the contained carbon dioxide escapes the droplet. Due to this carbon dioxide escaping, the droplet can’t hold as much limestone so a thin external ring is formed. After the drop falls, a small layer is left as a residual. After multiple drops have fallen the dripstone forms a hollow stalactite. These stalactites are called soda straws. As growth continues the soda straw regularly becomes plugged by the deposition. The limestone rich droplets are now forced outside of the soda straw creating the droplets to leave a small “paper trail” of limestone. This results in the cone shaped dripstone. Average growth rates for dripstones (stalactites) are about ½ inch for every 100 years (Figure 2a and 2b). When multiple soda straws or dripstones join together a baconstrip is formed. These features usually form along a joint where multiple dripstones and stalactites can form.

Figure 2 a and 2b: Both photos are examples of the many soda straws that are present at Crystal Cave which is located near Spring Valley, Wisconsin.

 

 Stalagmites are formations that are created from the ground up. These formations form from the drops that have fallen from stalactites or dripstones. Even though the drops left some dissolved limestone with the stalactite (dripstone) some still remains in the droplet. As this droplet falls and hits the bottom of the cave or tunnel the droplet scatters. This process allows more carbon dioxide to be lost hence another dripstone formation on the cave floor has begun to form. After many drops have landed on the same spot a stalagmite has formed (Figure 3).

Figure 3 : The left center feature is a well defined stalagmite located in Crystal Cave.

 

 Columns form after thousands if not millions of years of stalactite and stalagmite formation (Figure 4). When both of these two formations finally grow into one another a column is formed. This as stated earlier can take a very long time and in some cases never happens due to cave or environmental changes occurring.

Figure 4 : This Crystal Cave column is roughly 25 cm tall and 10 cm wide.

 

 Cave pearls form when dissolved limestone within droplets falls into thin pools. Cave pearls require rotation to coat the sand grain or rock fragment, which is the building block of a cave pearl. Due to the water’s tension on the sand grain or rock fragment an even coating takes place. Eventually the cave pearl sinks to the bottom of the pool where the pool’s water gives them their high gloss finish. The average size of a cave pearl is smaller than a marble, but occasionally they grow larger than baseballs. When the pearls are taken out of their pools and allowed to dry they turn to powder (Chadick, 1999).