How do stalactites and stalagmites form in caves?

Answer 1

Much of the water that percolates into the cave from above first acquires carbon dioxide from the soil and then uses it to dissolve calcium carbonate (by carbonation) from the rocks and sediment it encounters on its way to the cave. Cave air, in contrast, contains a comparatively low amount of carbon dioxide. When the dripping water contacts the cave air, it therefore releases carbon dioxide gas to the air. This degassing of carbon dioxide from the water essentially reverses the carbonation process, causing the water to precipitate calcium carbonate. Eventually, enough calcium carbonate is deposited in this way to form a stalactite, stalagmite, or other depositional cave feature.