By far, the most famous speleothems are the icicle-shaped stalactites and stalagmites. Lesser-known varieties include floor-to-roof columns and draping flowstones. And then there's cave popcorn.
Typically made of calcite, gypsum or aragonite, cave popcorn is named after its distinctive shape. This kind of speleothem consists of tightly clustered nodules measuring anywhere from 0.7 to 3.9 inches (2 to 100 millimeters) in diameter apiece. If you squint your eyes, the bumpy bundles look sort of like ossified movie theater popcorn. Yum.
"Cave popcorn usually occurs in wet areas of the cave where water can flow on the [cave] surface," Boze says. "Many of the longest cave systems tend to be drier, with some notable exceptions, and these dry areas tend to be less decorated. However, in the wet areas, cave popcorn is a common feature, usually indicating a wet environment and air flow."
Boze says many different geologic mechanisms can create cave popcorn. "It is most commonly formed when water fills the pores of a rock, and air flows over it," he explains. "Other common environments can include dripping water, in which the drips may cause popcorn to form in a radius around the drip sites." It's even been known to form underwater, at the bottoms of cave floor pools.
And the popcorn doesn't always emerge in isolation. It often develops on or around other kinds of speleothems. For example, "Billy Clubs" are stalactites that've been coated in cave popcorn. Examples of these may be found in South Dakota's Black Hills region — where, according to Boze, the caves tend to be big, wet and somewhat breezy. "Together these factors form really good conditions for popcorn to occur," he says.