Geology and Minerals
In geology, frostwork is a type of speleothem with acicular ("needle-like") growths almost always composed of aragonite (a polymorph of calcite) or calcite replaced aragonite. It is a variety of anthodite. Frostwork can also be made of opal or gypsum. In some caves frostwork may grow on top of cave popcorn or boxwork.
Lake Tai.
Caldeira Volcano.
Text from Wikipedia; plate via Wikimedia Commons. Text CC BY-SA 4.0; plate freely licensed (see Commons). Source record. Images and catalogue data are reproduced from open-access collections.