Students will learn to explain the process by which underground rock formations grow.
Geologists refer to cavern formations as speleothems, derived from the Greek words spelaion meaning cave and thema meaning deposit (in Europe they may be referred to as concretions). Speleothems are crystalline formations that grow by continual deposits. Since the definition of “cavern” is a cave formed in a soluble rock with the ability to grow speleothems, the composition of a speleothem will depend on which soluble rock the cavern is formed in. For instance, gypsum caverns will have gypsum formations and so on. For the purpose of this lesson plan, we will concentrate on the most common cavern type – limestone.
As cold rainwater falls it picks up oxygen and carbon dioxide in the air. Once it is on the ground and seeping through surface soils it may pick up additional carbon dioxide from decaying plants. This solution of water and carbon dioxide forms a weak acid called carbonic acid. As this acid continues to make its way into the earth, following small cracks in the rock, it erodes small amounts of limestone. The dissolved limestone (calcium carbonate) is added to the solution, now creating calcium bicarbonate (the mixing of two carbon compounds).
In the process of becoming calcium bicarbonate, the solution becomes somewhat pressurized and warms up. When it reaches the open air, the pressure is relieved by releasing the carbon dioxide. In doing so, it can no longer hold the calcium carbonate and must release it as well. (The release of the carbon dioxide is similar to what happens to a soda pop when it gets warm and goes flat. Notice the similarity in the names carbonation and carbonic acid) When the calcium carbonate leaves the solution, it crystallizes into a calcite circle around the water drop. There is often still some calcium carbonate left in the water. When it drops and hits the floor, the warmth created from the mpact forces the rest of the calcite to leave the solution. Over many thousands of years, layers of calcite deposits build up on the ceilings, walls, and floors of the cavern forming a variety of formations. The shape of speleothems is determined by the different ways in which water drops into the cavern. Dripping water will form different shaped formations from splashing or seeping water. (Some of the most common speleothems and how they grow are discussed in Lesson 3.)
Several factors effect the rate of growth for cave formations. The temperature outside, which affects the rate in which plants decay in the soil (amount of carbon dioxide in the soil), and the amount of rainfall are the two most important factors. From year to year the growth rate may change, even from one formation to another in the same area of a cavern. Therefore, there is no definitive answer to the question “how fast do they grow?” However, a feasible growth rate for some formations might be 1 millimeter every 100 years.
Aragonite is a heavier and denser form of calcite. Some formations have been found to have alternating layers of calcite and aragonite. The purest form of calcite and aragonite deposits is clear to a translucent white. Most speleothems have a solid white appearance. When formations have hues of other colors such as red and green, it is due to other minerals and organic materials that were on the surface which dissolved and were brought in as part of the calcite solution.
Small, slightly separated areas of a larger cavern are referred to as grottos. The term grotto also refers to a highly decorated natural or artificial cave. In antiquity these natural caves were regarded as dwelling places of divinities.
The crystalline structure of speleothems can be very porous. This can be a problem if people touch them. The natural oils in our skin will rub onto the formation leaving a light coating of oil. While one touch has minimal effect, many deposits of oil over time prevent any further growth. When calcium solution runs over the formation it normally leaves a layer of calcite behind. However, the human oils create a coating that the calcite cannot cling to. In a sense, the formation is now dead. We can see this same effect when washing a car that is waxed – the water beads up and runs off. Whereas, when washing a car that is not waxed, water tends to cling to the car. It is very important that people do not touch any surface in a cavern. (See the People & Caverns “Conservation” lesson plan for an activity related to this subject)
All of the Sierra Nevada Recreation Corporation caverns (Boyden, Moaning, California and Black Chasm) are profusely decorated with aragonite and calcite speleothems.
A crystal growing experiment can be a great demonstration of how speleothems grow. Included in this lesson plan is an experiment that can be adapted for the K – 4 grade levels. In addition, students can do a simple sugar crystal growing experiment, or grow popcorn rock (kits are sold in our Nature Gift Shops).
Explain to the students that this experiment will show how water can deposit minerals to create cavern formations, also known as speleothems. Explain that this process in a cavern takes thousands of years. The students will be creating their formations by using a salt-based chemical compound. This will speed up the process by using concentrated solutions and the sun’s help in evaporation.
Lead the students through the following demonstration or instructions:
Dissolve as much of the washing soda/Epsom Salts as you can in very warm water. Arrange two jars on a windowsill with a saucer in between. Fill them about half way with the supersaturated solution.
Tie a weight (such as a paperclip)to each end of a piece of yarn. Soak the yarn in the solution. Place the yarn so that each end is well inside the water solution in each jar and the middle is over the saucer. Make sure that the loop of the yarn is hanging lower than the water levels inside the jars. (See diagram on included worksheet).
Put just a touch of dry washing soda on the saucer.
Leave the jars for several days.
The solution in the jars will wick along the yarn, then drip off at the low point in the middle onto the saucer. Deposits should build up on the saucer and hang from the yarn and may eventually connect to form a “column.”
Set up the second set of jars at the same time. Follow the same procedure except use the baking soda instead of the washing soda. Within 48 hours, students will notice the crystals look very different by comparison. (They will look like a delicate cavern flower.)
Label the first set of jars A and the second set B. (A different drop of food coloring in each of the solutions, when mixing, will make it easier for younger students to discuss differences.) Have students check the experiment daily and record the results on the “Growing Cavern Crystals” worksheet included.
Using the same experiment as described above, students can observe effects on the deposition process by manipulating some of the variables, such as varying solutions by concentration and by environment (light and dark locations, warm and cool, or sealed and open). Have students predict what might happen. After results are recorded and compared, ask the following questions:
Which conditions were the most favorable for growth?
What do these conditions have in common?