David MacDonnell, age 10, of St. Catharines, Ontario, Canada, for his question:
How are stalagmites formed?
Stalagmites are created in nature's own underground art galleries. In certain cool moist caves, they rise up from the floors in graceful stone stalagmites, with pointed tops. In the same caves we also find stalactites, hanging like icicles from the ceilings. The stone walls are sculptured with artistic festoons and elaborate scrollwork.
These handsome stone structures usually are found in caves that have formed in thick deposits of limestone. They also occur in dolomite, which is metamorphic limestone, changed by tremendous heat and pressure. Both these rocks contain forms of calcium, soft minerals easily dissolved in water. Usually the climate of the region is fairly moist and rain water continuously seeps through the ground.
Inside, the caves are cool and damp. Usually they are dark and always there are soft sounds of dripping and trickling water. As it seeps, drips and trickles through the limestone type rocks, it dissolves molecules of calcite and carries them away. Often the bedrock is submerged completely under water that erodes deep cracks and enlarges them to form tunnels and gigantic holes. A system of caves is hollowed into the bedrock.
This basic building job changes when the ground mater drains away and air enters the hollows. Seeping rains continue to drip drip through walls and ceilings. Among other dissolved minerals, they carry calcium bicarbonate. As this water trickles into the caves, it is exposed to the air and some of it evaporates. Among other things, it releases carbon dioxide which reacts with dissolved calcium compounds to form calcium carbonate. This mineral is deposited as solid calcite.
The water drips and trickles, forming tiny streams and thin films, somewhat like slowly melting snows. As it evaporates, molecules of solid calcite are deposited on the malls, floors and ceilings. Layer by patient layer they build the sculptured formations that decorate the interiors of limestone and dolomite caves.
These formations are called dripstone because they are created by the dripping water that deposits them and also models their shapes. Films of moving water create the festooned scrolls on the cave walls. Trickles from the roof build the spiked stalactites that hang like stony icicles from the ceilings. Some of the drips from above land on the floors. As this moisture evaporates, it deposits a pile of solid calcite that forms a spikey stalagmite.
As a cave grows older, its stalactites from the ceiling and its stalagmites from the floor grow longer. Often they form in opposite pairs from the same leak in the ceiling. In time, a stalactite and a stalagmite may meet and form a single column. Some caves are decorated with groups of handsome columns from floor to ceiling.