Stalactites and stalagmites are geological formations that occur when water dissolves limestone and other minerals. The mineral-rich water seeps through cracks and into a cave. In the right conditions, the minerals in the water can precipitate out and be left behind as thin white rock coatings on the ceiling or floor of a cave.
Over thousands of years, these coatings can build up in layers. The formations that build up in this way are called speleothems. Speleothems on cave ceilings are called stalactites and those on cave floors are called stalagmites.
The beginnings of a stalactite
The jars hold a solution of Epsom salts. The mineral solution is carried along the string to its lowest point. When the water evaporates, it leaves the minerals behind. Each drip deposits more minerals – growing the stalactite. A stalagmite is forming under the stalactite. Dye has been added to the solution to aid observation of the solution’s movement along the string.
This activity models the process of speleothem formation. Like real speleothems, the stalactites and stalagmites in this activity form drop by drop. However, they form in a matter of hours and days rather than thousands of years!
In this activity, students use an Epsom salts solution to model the formation of stalactites and stalagmites in cave systems.
By the end of this activity, students should be able to:
- recognise that Epsom salts dissolve in water – forming a solution that is a mixture of both substances
- recognise that mineral compounds remain when water in the solution evaporates
- discuss how this activity models the formation of stalactites and stalagmites
- discuss the limitations of this activity in modelling the formation of stalactites and stalagmites.
Download the Word file (see link below).
Related content
A model is a representation of an idea, object or process that describes and explains phenomena that cannot be experienced directly. Find out more about scientific modelling.
The articles Carbonate chemistry and Limestone landscapes provide in-depth information regarding the chemical processes of cave, stalagmite and stalactite formation.
The article Why caves matter explains what speleothems – stalactites and stalagmites – can tell us about the past.
Useful link
Younger students often confuse the concept of dissolving with melting or disappearing. Visit Melting and dissolving for pedagogical information on how to help students transition to scientific views.
