Colossal Cave is primarily limestone, a sedimentary rock that forms from the accumulation of calcium carbonate-rich sediments. These sediments were deposited in a marine environment when the region was covered by a shallow sea during the Paleozoic Era.
The cave’s formation is closely to the local karst landscape. This type of landscape is characterized by the dissolving of limestone (called dissolution) and other soluble rocks by acidic groundwater. Rainwater that absorbs carbon dioxide from the atmosphere forms weak carbonic acid, which reacts with the calcium carbonate in the limestone, gradually dissolving it and creating cavities.
As water percolated through fractures and joints in the rock, it enlarged cavities, leading to the development of the cave system.
As water containing dissolved calcium carbonate drips from the cave’s ceiling, it leaves behind mineral deposits that form stalactites, hanging formations. When these water droplets fall to the cave floor, the water evaporates and the minerals left behind there create stalagmites, upward-growing formations. These formations are generally composed of calcite deposits.
Flowstone and Draperies
Flowstone formations, such as draperies and curtains, are created when thin sheets of mineral-rich water flow down cave walls, leaving behind mineral layers of calcite. These formations often exhibit intricate patterns and colors due to small variations in the mineral content of the water that has traveled different paths through the ground.
Columns and Shields
When stalactites and stalagmites grow together and eventually meet, they can form columns. Shields are flat formations that can develop on cave walls, often resembling shields or turtle shells.
The entrances to Colossal Cave are associated with fractures and faults in the limestone that have opened to the surface. These natural openings allowed water to enter the cave and contributed to the cave’s development.
Beyond stalactites and stalagmites, curtains and columns, the cave may contain other mineral deposits such as helictites, cave pearls, and cave popcorn. These strangely named formations each result from a unique mineral precipitation process.
The geologic processes that created Colossal Cave span millions of years. The limestone formed about 300 million years ago. About 80 million years ago, the Rincon Mountains the cave is located in rose. Since then the slow dissolution of limestone, with the formation of passages, and the growth of mineral formations nearly the next 80 million years to the present.
The Hohokum were the first people to visit the cave that we have some record of. They were using the cave as early as 900 CE. The cave was rediscovered by Anglos in 1879. Today there are cave tours are offered by Pima County. Reservations are a really good idea.