stalactites and stalagmites
Weak carbonic acid, formed when rain absorbs carbon dioxide from the atmosphere, dissolved tiny amounts of calcium carbonate from limestone rock, as it percolates down through the rock (1). If the water enters a previously eroded cave (2), it forms drips (3). Some of the dissolved carbon dioxide escapes from the water, and the calcium bicarbonate in the water is precipitated as a calcite known as travertine, consisting of calcium carbonate. They form rising stalagmites (4) and descending stalactites (5) from which water drips.
A stalactite and stalagmite just beginning to meet and form a column. Credit: US National Park Service.
Stalactites and stalagmites are rocky structures found growing downward from the roof (stalactites) and upward from the floor (stalagmites) of caves formed in limestone. (Remember, stalaCtites hang from the Ceiling and stalaGmites grow up from the Ground.)
Rainwater percolates through the rocks above the cave and, as it contains atmospheric carbon dioxide, can dissolve calcium carbonate en route. On reaching the cave, the water drips from the roof to the floor; as a drop hangs, some water evaporates, leaving a little calcium carbonate as calcite on the roof. Repetition forms a stalactite; and evaporation of the fallen water on the floor forms a stalagmite.
On occasion, the rising stalagmite and descending stalactite fuse to form a column. Stalactites, stalagmites, and columns are called dripstone because they are deposited by dripping water.