A soda straw is a speleothem in the form of a hollow mineral tube. They grow in places where water leaches slowly through cracks in rock, such as in caves, and results in consecutive drops of water falling from exactly the same place. Calcium carbonate or calcium sulfate dissolved in the water comes out of solution and forms a ring at the water drop's edge, before it drops. Each successive drop of water deposits a little more mineral before falling, eventually building a tube. Flowstone or a stalagmite may form where the water drops hit the cave floor.

Soda straws are some of the most fragile of speleothems. They can be easily crushed or broken by the slightest touch. Because of this, soda straws are rarely seen within arms reach in tourist caves. When left alone, soda straws can grow to hang more than 20 feet.

A soda straw can turn into a stalactite if the hole at the bottom is blocked, or if the water begins flowing on the outside surface of the tube.

Links

• Showcaves.com definition and explanation (http://www.showcaves.com/english/explain/Speleothem/Straw.html)
• The Virtual Cave soda straw photos (http://www.goodearthgraphics.com/virtcave/sodastw/sodastw.html)