MadSci Network: Earth Sciences

Re: I need some extra information on stalagtites. Could you please help me?

Date: Thu Nov 8 16:25:16 2001
Posted By: David Scarboro, Faculty, Earth Sciences, The Open University
Area of science: Earth Sciences
ID: 1004300587.Es

Dear Mikeyo,

Stalactites are features of limestone caves and caverns.  They hang from 
the cavern ceiling and resemble icicles.  They are made of a mineral 
called calcium carbonate.  Limestone itself is also made of calcium 
carbonate, and it is the properties of calcium carbonate that explain both 
the existence of the cavern and the presence of stalactites (and also 
stalagmites, which grow up from the cavern floor).

Calcium carbonate dissolves easily in water, especially water that is 
slightly acidic.  Rainwater is slightly acidic because rain dissolves the 
gas carbon dioxide from the atmosphere and becomes a very weak acid called 
carbonic acid.  When rain falls, a proportion of the water soaks into the 
soil and down into the rocks to form groundwater.  Because the rainwater 
is weak carbonic acid, the groundwater is too.  If it soaks into limestone 
it will dissolve calcium carbonate from the rock, so that the groundwater 
percolating down carries dissolved calcium carbonate with it.

On a large scale and over millions of years, groundwater may dissolve so 
much of the limestone that cavities, eventually linking up to form caves 
and caverns, will be formed underground.  If a cavern exists, groundwater 
seeping down will eventually reach the roof and drip through into the 
cavern.  Because the groundwater contains dissolved calcium carbonate, as 
each drop of water drips off the ceiling it will leave behind a tiny 
deposit of calcium carbonate.  As you can imagine, given enough time and 
enough dripping water these tiny deposits will build up to form a 
stalactite, which slowly grows down from the ceiling.  Large stalactites 
can take many thousands of years to form.

Likewise, when the drop of water hits the floor of the cavern it will 
deposit any remaining calcium carbonate it contains, and slowly 
stalagmites will grow upward.

This is the process that occurs in nature.  But I have seen stalactites, 
probably also made of calcium carbonate, growing on the underside of a 
cement sidewalk on a riverbank.  These were a few inches long and took 
just a few years to form, as they cannot be older than the sidewalk!  
Cement contains calcium carbonate, and in this case rainwater percolating 
over the sidewalk has dissolved some of the calcium carbonate and 
redeposited it as it dripped off the bottom to form these little 

There are many websites promoting commercial caverns with beautiful 
pictures of stalactites and stalagmites.  One of the most famous is Luray 
Caverns in Virginia; have a look at  If you 
click on the Classroom/History link on the home page you will find a good 
description of how Luray Caverns formed, including the process that forms 
stalactites and stalagmites.

I hope this answers your question.

Best wishes,

David Scarboro 

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