Limestone caves

Limestone cave near Hobart, Tasmania, Australia

Limestone is amazing stuff.  We use it all the time and often are not aware of it.  It is a favourite crushed rock of gardeners; I have some of it in my dog run area.  If your lawn is too acidic you can "sweeten" the soil by adding lime, a derivative of limestone.  It is found in antacids and chalk.  The biggest use of limestone though is in making concrete.

Concrete is a mixture of lime, sand, stones, and water.  The proportions are modestly specific, but when done right produces an incompressable solid which we build skyscrapers and roadways out of.  The lime is known chemically as calcium oxide (CaO).  If you take limestone (CaCO3) and heat it up enough you blow off carbon dioxide and get lime (CaCO3  ------>  CaO  +  CO2).  The great thing about this is that it is easy; you can do it at home with a propane torch and some limestone rock.  The problem is that it produces carbon dioxide, and like we need more of that.

The other wonderful thing about limestone is that it can be dissolved.  If you take a piece of limestone and put it in a bowl of vinegar it will eventually dissolve.  To do it justice you will need a lot of vinegar and a small piece of the rock.  If you really want to be impressed you can use a strong acid like sulfuric acid (battery acid), but it is dangerous and not a great idea.  This ability to be dissolved by acids is what leads to cave development.

Limestone is a sedimentary rock and is laid down on ocean floors from the compressed exoskeletons of dead sea creatures.  It also precipitates out on its own under certain conditions.  The compressed sediments eventually form limestone, although there tend to be a lot of impurities.  When geologic uplift occurs we get limestone beds above sea level.  The top of the rocky mountains are, for example, largely made out of limestone.  If slightly acidic water comes in contact with it, the rock gets slowly eaten away. 

It turns out that water naturally becomes acidic on its own.  The air contains compounds which, when water is added, become acidic.  That is where acid rain comes from.  We make rain more acidic when we pollute the air by adding sulfur dioxide and carbon dioxide.  Combustion in cars causes the formation of nitric acid.  Plants produce organic acids which get into the soil and can be leached out, with the water percolating down into the water table.

It is all these acids that eat limestone away.  Water drops suspended from the ceiling carry tiny amounts of dissolved limestone in them.  If that water evaporates it leaves a deposit - stalactites.  Water drops hitting the floor cause lumps to form and grow over time - stalagmites.  Then you get all kinds of other cool structures forming as well.

I have always loved caves.  Our recent trip to "The land down under" allowed us to visit an amazing cave down there.  It has been one of the things I have really wanted to do.  We have caves here in Canada but they are small and relatively undeveloped because the glaciers wiped out ones that existed before them.  There are some amazing ones in the States I would some day really like to see.

www.ericspix.com