Speleothems (Cave Formations)
An Underground Gem
Lehman Caves is known for its highly decorated chambers. A tour of Lehman Caves allows you to experience some of the more common cave formations, as well as some extremely rare formations that are seen in very few places on planet earth. The diversity of formations present in Lehman Caves is so great, that one would have to travel to countless caves to see everything that can be seen on a 60 minute tour of this cave system.
Scientifically, cave formations are referred to as speleothems. A speleothem is any cave decoration that formed after the cave itself finished forming. Most speleothems are formed in similar ways. All formations are forming by water saturated with dissolved calcium. The water begins on the surface, where the water passes through the soil and absorbs carbon dioxide, transforming the water into a weak carbonic acid. This weak acid then dissolves a small amount calcium from the limestone rock as it travels through cracks and pores on its way to the cave system. As the water drips into the air-filled cave, the dissolved carbon dioxide released. Because the water has lost carbon dioxide, it cannot hold as much dissolved calcium. The excess calcium is then precipitated on the cave walls and ceilings to make up speleothems. Most calcium deposited in the cave in the form of the mineral calcite (CaCO3). Slight differences to this process result in the creation of different cave decorations.
For more detailed information describing the formation of caves (speleogenesis), explore the Cave Geology webpage.
NPS PHOTO Maci macpherson
The Chemical Process
Degassing is the one main cause of calcite deposition. The carbon dioxide content of the groundwater entering the cave passage is about 250 times higher than that of the air. So when the water contacts the air, it degasses just like a soda pop does when you open it. Without the carbon dioxide, the calcite cannot stay in solution. The chemical reactions reverses.
Ca+2 +2(HCO3-) (in solution) --> CO2(gas) + H20(water) + CaCo3 (calcite)
The water continues on, but it does not carry as much calcite in solution. Another major method for deposition of calcite is evaporation of the water it is dissolved in.
Either way, calcite is deposited. Depending on the shape the calcite takes, it may be called by different names. Travertine is one name for the calcite in teh cave. Dripstone is a generic caver term that encompasses any cave decoration caused by dripping, splashing, or seeping water. Speleothems are cave decorations formed after the cave passage has formed, such as dripstone. (Speleogens are features like scallops that form in the bedrock while the cave is forming).
How Old are the Formations? How Fast do they Grow?
People often want to know how old the formations are. Broken columns have growth rings that look like tree rings, but there might be thousands of years between each periods of deposition. It is not possible to count the rings to date a stalactite. Formations in Lehman Caves have not been dated, so we can only guess by observing the current growth rates. Yet these current rates may not be similar to growth rates in the past.
Soda straw stalactites that are growing on broken formations in Lehman Caves are mostly less than an inch log. Most of these formations were broken between 1885 and 1922, or roughly a century ago. Keep in mind that because soda straws are hallow, they grow longer and at a faster rate than wider speleothems. The same amount of calcite may be deposited per year on other formations, but the change would not be as noticeable if it is spread over a greater surface area ( i.e. flowstone or large columns). Growth rates vary depending on the amount of calcite in solution and the drop rate. Size can be misleading. The largest column in the cave may be younger than a two-inch long soda straw. Conditions cave be very localized. Water can change the path it follows into the cave, so formations that are dry may someday be wet again. The amount of water dripping into Lehman Caves now does not seem to account for the large formations in rooms like the Gothic and Grand Palaces. Many formations now are dormant and probably grew in the past when the climate was wetter, possibly during the Ice Ages.