Cavern Geology

Lesson 3: Types of Speleothems

Objective

Students will learn to identify several types of speleothems that are commonly found in limestone caverns.

Background Information

Continuing with the limestone cavern example, the terminology used in this lesson will reflect this cavern type. However, the names of speleothems are defined by their shape regardless of the material they are created from.
As mentioned in Lesson 2, formations begin when calcium bicarbonate enters the cavern and the chemical balance of the solution alters, forcing the calcium carbonate to deposit on the surface of the cavern ceiling or wall it entered through. The path of the watery solution from there is what determines the shape of the eventual speleothem as it deposits calcite along the way.

Listed below are some of the most common forms of speleothems and a brief description of how each is created. There are several other speleothem formations found in caverns around the world. Some are very rare, found only in unusual conditions.

Soda Straw - Formed when the solution first enters the cave from the ceiling and the calcite crystallizes around the drop forming a ring before the drop falls. As drop after drop continues to enter the cave at the same point, the water will then travel through the rings deposited before them, building ring upon ring at the end of the last one. Over time, the result is a thin, hollow tube that resembles a straw.

Stalactite - When a soda straw is forming, the solution will also deposit calcite along the inside of the tube as it travels through it. Eventually, the straw will become solid. At that point, as the solution continues to want to enter the cave at the same point, it is forced to the side of the tube, then runs down the outside. Over time, the result looks like an icicle.
Stalagmite - As the watery solution falls from the ceiling, it lands on the floor or a ledge where it splashes or runs over an area, leaving the remaining calcite in its wake. The continued deposits build into a mound.