Lesson 2: Speleothem Construction
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.)