Well-meaning geologists ended up confusing plenty of folks when they named stalactites and stalagmites. Both of these similar-sounding structures—typically formed in limestone caves—are capable of stretching over 27 feet in length. But what's the difference between them, and how do such strange ornaments grow in the first place?
Let’s clear up the terminology. Here’s a helpful (and widely-used) phrase you can use to sort out which is which: “Stalactites hold tight to the ceiling and stalagmites might touch the ceiling.” In other words, stalactites form on the roofs of caves and dangle downward like rocky icicles. Stalagmites, in contrast, are based on the floor and stretch upwards, only occasionally coming into contact with the overhanging ceiling.
An alternate memorization method goes as follows: “stalactite” is spelled with a “t,” as in “top.” “Stalagmite” uses the letter “g," as in “ground.”
According to the Oxford English Dictionary, these two terms descended from the Greek word stalaktos, which means “to drip." This is because trickling rainwater is responsible for the conical objects’ formation. When rain seeps through limestone, the water extracts carbon dioxide gas from the rock. What results is a weak carbonic acid that penetrates the stone and deposits a patch of calcite on the caves’ roof. As the water continues dripping, more and more calcite is added to the spot, eventually producing a lengthy stalactite.
But what about stalagmites? There’s a reason these are generally found directly underneath stalactites—all that dripping water has to land somewhere, after all. When a drop finally hits the cave floor, it deposits even more calcite there, this time in an unassuming mound. The liquid keeps dripping off the tip of a stalactite and the lump keeps rising, leaving us with a surging stalagmite. To call this process gradual would be a gross understatement. In limestone caverns, the usual growth rate is under 10 centimeters per millennium.
It’s also worth noting that both stalagmites and stalactites belong to a larger geological group known as “speleothems." This is an extensive family of differently-shaped mineral formations that also includes globular “cave popcorn” and stunningly-beautiful “flowstones." Additionally, lava is occasionally involved in stalactite creation, leading to some strange-looking results.