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TAG Speleothems

TAG refers to an area of heavy concentration of caves located at the junction of the states of Tennessee, Alabama, and Georgia, USA. This page features only cave formations found in the TAG region. The formations here are absolutely magnificent. The following article was written by me and appeared in the Spring, 2001 issue of the Upper Cumberland Grotto's Newsletter, The Winged Messenger.

Depositional cave formations are known collectively as speleothems. The term speleothem refers to the way the mineral was formed, not what it is composed of. Speleo refers to "cave" and "thema " refers to deposits. Therefore speleothems are formations that were created from mineral deposition in caves. They are found in a variety of familiar forms including stalactites, stalagmites, soda straws, flowstone, dripstone, and other's. Many people find viewing speleothems the most enjoyable aspect of their caving experiences. Formations also are a favorite theme for cave photographers. This article attempts to provide the reader with a simplified explanation of the genesis of depositional cave formations. The information presented in this article primarily refers to the types of speleothems encountered in limestone caves in the TAG (Tennessee, Alabama, and Georgia) region. The ever present rock bluffs located in Tennessee's Highland Rim and Cumberland Plateau are almost invariably limestone. Sea floors and reefs left behind from invading and receding seas over millions of years deposited calcite (CaCO3) sediments that eventually formed the limestone layers that contain our caves. Limestone is calcite (CaCO3) rock. This is important because two of the three most common types of speleothems in TAG are calcite formations. These are standard calcite deposits (i.e.: common dripstone stalactites and flowstones) and aragonite (white, lacy, "chandelier" formations that are more rare and found in association with regular calcite). Interestingly, calcite and aragonite are polymorphs of each other, meaning that they are chemically the same (CaCO3) but their crystalline structures differ. Aragonite is even capable of converting into regular calcite over time. The third most common type of speleothem is a twice hydrated calcium sulfate (CaSO4-2H2O), commonly known as gypsum. Calcite formations are formed from the simple release of carbon dioxide gas (CO2) from water as it seeps out of limestone into an airspace (i.e.: a cave). Water picks up CO2 as it percolates through organic matter in the soil. It can pick up CO2 until it has concentrations up to 90% higher than the concentration of CO2 in normal air. This increased concentration of CO2 gives the water increased ability to dissolve and absorb calcite as it percolates through the rock layers. Eventually the water becomes saturated with calcite. When this water percolates out of the limestone and into a cave area, the low air concentrations of CO2 in the cave air in proportion to the higher CO2 concentrations in the water causes CO2 to "degas" out of the water, similar to gas escaping from a soft drink when the container is opened, but much more subtle. As the CO2 is released from the solution, the water loses it's ability to hold the dissolved calcite and it reforms and "falls" out of the solution as a crystalline deposit as the water passes by. Whether the water is flowing, dripping, seeping, splashing, or gently spilling over a basin determines the nature of the calcite deposits left behind. Aragonite formations are more rare than calcite "drip" and "flow"formations but occur in association with them and share a chemical structure, therefore they are included with calcite formations as the most common speleothems in TAG. Color variations in calcite formations are primarily influenced by the organic materials in the substrate above the cave, but can also be influenced by the presence or absence of metallic ions from ores in the substrate. Gypsum is the third most common speleothem in TAG and is easily recognizable in caves as gypsum crusts, flowers, needles, hairs, beards, crystals, and stalactites. Gypsum occurs when the carbonate (CO3) portion of calcite (CaCO3) is replaced with the sulfate ion (SO4)2- and two water molecules (H2O) yielding calcium sulfate (CaSO4) +2 H2O's. This can happen several ways but a common occurrence takes place when calcite and CO2 laden water is introduced to the sulfate ion as the water percolates through the earth. Sulfate is highly reactive and can readily substitute itself for carbonate in the presence of water. When this calcium sulfate solution enters a cave environment the formation created is not made in the same way as calcite deposits. Gypsum (CaSO4-2 H2O, or calcium sulfate) is left behind as a product of evaporation of water, not deposition from water. This happens simply when the calcium sulfate rich water seeps into a cave passage and evaporates, leaving behind the mineral compound in one of it's recognizable forms. This is relatively common in TAG in the form of gypsum crusts on cave ceilings and walls. Gypsum flowers also form this way, but differ in that a flower has parallel strands of calcium sulfate crystals all growing from a common center point. Water seeps out of the rock and pushes the older crystal out of the way as the new crystal forms beneath it. This growth pattern can be confirmed by observing a white gypsum flower that is capped with a darker layer that matches the substrate the flower grew from. The curves in gypsum flowers occur when the growth rate is uneven in the crystal strands, forcing the "petal" to one side. Gypsum needles grow directly from soil that is saturated with calcium sulfate rich H2O. As the humidity in the cave environment drops, the soil is unable to hold as much H2O and it evaporates out of the soil. Gypsum literally grows out of the soil as the water evaporates yielding crystals that can look like frost heave, transistors, and crystal needles that can reach over three feet in length. Examples of all of these amazing crystals and formation can all be seen in select caves in TAG. I hope I have helped to take some of the mystery out of how all of those beautiful speleothems formed in caves. I tried to simplify the information as much as possible while attempting to maintain accuracy. This information is not intended be comprehensive but to present a general account of speleothem genesis. The important things to remember are that TAG caves are formed in calcite rock (limestone), therefore the many amazing formations that we see in the caves are primarily composed of calcite and calcite variations. Drip and flow formations are generally deposited in caves and gypsum formations are primarily left behind as a product of evaporation. All of this is possible because water gains CO2 as it percolates through the earth allowing it to dissolve and absorb calcite. When water loses CO2 it loses some of it's ability to hold dissolved minerals and they are left behind as beautiful speleothems.

Sources:1) Hill and Forti, Cave Minerals of the World, 2nd Edition, National Speleological Society, 1997 2) Skinner and Porter, The Dynamic Earth, 3rd Edition, John Wiley & Sons, Inc., 1995 3) Howard, Mike and Darcy, Introduction to Crystallography and Mineral Crystal Systems, Internet source, URL: http://www.rockhounds.com/rockshop/xtal/part2.html, 1998

All photos taken and copyrighted by me, Jay Greene, 2001, unless otherwise noted.

This is a basic flowstone formation. Water has flowed and deposited calcite for years, leaving behind what you see.

This formation was formed in the same way as the flowstone in the previous photo, but this one has been colored by impurities during formation.

A cluster of drip formations that have created stalactites and draperies.

This photo features helectites and soda straw formations. The soda straws are rapid-growing drip controlled formations while the helectites are composed of a combination of both drip formations and crystalline angle-controlled growths. As water evaporates slowly, calcite can grow as crystals in any direction and then occasional heavy water flow causes drip formations to grow in conjuction with and on top of the angular helectites. This is purely my own theory of helectite growth. I have heard and read other theories but did not agree completely with any of them. Personal observation has led me to develop the theory presented here.

This rather impressive helectite is quite well known by active cavers in the region where it is. It is over 30cm (12 inches) long and located in a heavily encrusted, crystalline passage. Photo copyrighted by Kristen Bobo, 2001.

This photo features a very thin drip formation know as a veil. The left-hand side of the photo shows drapery/bacon strip flowstone. This formation is very, very delicate.

A close-up photo of more veil speleothems. The delicate nature of the formation is evident in this picture.

Nora Dickins and Kristen Bobo pose under soda straws and a large "bacon strip" veil.

A shot looking straight down onto the top of a stalagmite that has five different points of water drip. The orange color appears to be either a recent addition to the water flow, or possibly it is composed of a metal ion, such as iron (Fe) that is so heavy that is falls out of solution immediately and leaves it's coloration only where the water hits the surface.

A cluster of soda straw formations. Soda straws grow rapidly and are typically hollow and composed of relatively pure calcite. "Rapidly" is a general term when talking about speleothems, but I have seen soda straws growing out of concrete bridges and concrete cave gates that were several inches long when the concrete they were growing on was less than 20 years old. That is fast compared with the thousands and perhaps hundreds of thousands of years necessary to create the larger formations featured in the first two photos on this page.

Gypsum is calcite that has a carbonate ion (CO3) replaced by a sulfate ion (SO4) with water as a by-product. The sulfate comes from sulfur present in the organic substrate above the cave or as a mineral already dissolved in water that enters the cave through other sources, such as flooding or underground streams. When a substrate (dirt in this photo) is saturated with ions (salts) and water evaporates, crystals composed of the salts will typically grow right out of the substrate. This photo shows gypsum (CaSO4) "transistorites" that have grown out of dirt. They are approximately three cm tall (1.5 inches).

A different form of calcite, aragonite, is composed of the same chemical structure as calcite (CaCO4) but it is a stereoisomer, meaning that it's crystal form is exactly opposite that of calcite. This makes calcite grow in large, obvious crystals and aragonite grow in lacy, intricate crystals. Aragonite is also called cave coral or frostwork, depending on how large the formation is, where you are, and who you are with. Speleothem nomenclature is a rather inexact science.

This photo shows calcite growing in it's typical rhomboid crystalline shape. This formation is about 11cm (5 inches) from left to right.

This is another form of gypsum, called selenite. This type of selenite is typically known as "gypsum needles" in the TAG region. Selenite can grow in perfectly straight "needles" as in this photo and it is also found in interesting, round, rose-bud shapes in the Mojave desert of the southwest USA.

This is an evaporative crystal growth of gypsum growing on a cave wall.

A cluster of gypsum cyrstals. Also found in the TAG region.

A cluster of gypsum needles that are so thin as to almost be considered gypsum hair or angle hair. But angle hair is slightly more delicate as the next photo demonstrates.

Gypsum "angle hair."

These are extremely large gypsum flowers as evidenced by the finger in the photo for scale. The crystals that grow in TAG can rival those of any region in the world.

A bifurcated and nearly symmetrical gypsum flower.

A perfectly shaped, beautiful gypsum flower.

Cave coral or cave popcorn, depending on who you ask. Probably formed by water dripping near the area and splashing back onto the wall. A very unusual grouping of shapes either way.

More cave popcorn. This group could be purely evaporative or a combination of evaporative and drip/splash.

An absolutely beautiful example of drip and flow formations. The entire cave this speleothem is in glistens just like this photo.

This photo shows the active formation of speleothems. The flowing water is leaving behind stalagtites where it drops off of the ledge. A stalagmite is being formed at the base of each stream of water. Small, delicate cave popcorn is being formed on the wall behind the water from splashes. I have seen this formation at different times of the year and can verify that the water flow varies from a splasing torrent to a barely perceptable drip.

Water can deposit impressive rimstone dams under certain circumstances. Once the process starts and the water trickles over the edge, the CO2 is able to leave solution faster and deposit the calcite in predictable and ever increasing rings.

A rimstone ribbon has formed across this pool, creating a beautifully intricate dam.

Rimstone can also form on top of existing formations. This stalagmite features a beautiful rimstone pool on it's entire top. It is possible that the entire stalagmite grew as one large rimstone, but it seems unlikely.

Rimstone pools are know in some countries as "gour pools." This photo features a micro-gour pool composed entirely of crystalline calcite.

I call this a "bird's nest" formation. It is simply a small stalagmite with interesting, spherical knobs all over it.

These are gypsum "christmas trees" that are growing straight down out of the mud in a cave near my home. Each crystal is about 5cm long.

These are typical drip formations. The colors are from mineral and/or metallic ion content in the water that deposited the calcite.

These drip formations were photographed in the same area of the same cave as the previous photo.

I have only seen these interesting "brain" formations in one cave. They are like huge cave pearls, cememted together except I don't think that they are cave pearls. The objects in this photo are totally under water. There is a small bubble over the larger formation that indicates the water level.

These formations are somewhat similar to the ones in the previous photo. Both were formed in rimstone pools. My theory is that the water in the rimstone (gour) pools spilled slowly over the edge of the dam, leaving a portion of it's calcite behind as CO2 evaporated at the interface of the dam, the water, and the air. Some of the precipated calcite stayed on the edge of the rimstone dam but some also seeped back into the pool, until the pool became so super-saturated with calcite that small pebbles of calcite crystals in the pools acted as catalysts to more cystals. Crystals continued to fall out of solution and cling to the existing "balls" of calcite until these very unusual little balls formed. Again, this is an unproven theory of mine.

This gour pool has a thin layer of calcite raft floating on it's surface. The rafts bring the calcite into contact with the existing speleothem creating the white growths at the water interface.

This is a straight down view of a stalagmite and micro-gour pools.

This photo is a close-up of a stalagmite that shows what appears to be fungal growth on the surface. The tiny, clear hairs and the larger, white growth in the center are definately organic and living on the surface of the speleothem and are not an actual part of it. This raises the question of the possibility of biological factors being involved in the formation of some particular types of crystalline growths, like the ones in the next photo.

Could these crystalling growths have started as fungal hairs that trapped calcite from passing water? This theory has not been properly studied and a grant could probably be acquired for just such a project if someone were interested.

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