Limestone caves, where most stalactites and stalagmites are found, are mainly composed of calcite,
a common mineral found in sedimentary rocks. Calcite molecules are made
of calcium and carbonate ions, and are referred to as CaCO3, or calcium carbonate.
When rainwater falls over a cave and trickles through rocks, it picks
up carbon dioxide and minerals from limestone. If we add water, carbon
dioxide and calcium carbonate together, we get this equation:
H20 + CO2 + CaCO3 = Ca (HCO3)2
Ca (HCO3)2 is known as calcium bicarbonate,
and the water carries the substance, basically dissolved calcite,
through the cracks of the roof of a cave. Once water comes into contact
with the air inside the cave, however, some of the calcium bicarbonate
is transformed back into calcium carbonate, and calcite starts to form
around the crack. As water continues to drip, the length and thickness
of the calcite grows, and eventually a stalactite forms on the ceiling.
It can take a very long time for most stalactites to form -- they
usually grow anywhere between a quarter-inch and an inch every century.
It doesn't end there -- we haven't talked about stalagmites yet. Of
course, stalagmites don't just emerge from the ground. The water
dripping from the end of a stalactite falls to the floor of a cave and
deposits more calcite into a mound. Soon enough, a stalagmite will form
in a conelike shape. This is why you usually find stalactites and
stalagmites in pairs, and sometimes they'll even grow together to form
one big column. There are many limestone caves around the world famous
for their displays of dripstone, including Carlsbad Caverns in New
Mexico, Timpanogos Caves in Utah, Mammoth Cave in Kentucky and Jenolan
Caves and Buchan Caves in Australia.
And if you ever forget which one's which, there are several classic
memory tricks to get stalactites and stalagmites straight. Here are a
Stalactites have to hold on "tight" to the top of the caveStalactites hang from the ceiling like ladies' tightsStalactite has a "t" in it, as in "t" for "top"Stalactite has a "c" in it, as in "c" for "ceiling," and stalagmite has a "g" in it, as in "g" for "ground"