Rain mixes with carbon dioxide in the atmosphere as it falls to the ground and then picks up more of the gas as it seeps into the soil. The combination is a weak acidic solution that dissolves calcite, the main mineral of karst rocks. The acidic water percolates down into the Earth through cracks and fractures and creates a network of passages like an underground plumbing system.
The passages widen as more water seeps down, allowing even more water to flow through them. Eventually, some of the passages become large enough to earn the distinction of "cave". Most of these solutional caves require more than , years to widen large enough to hold a human. The water courses down through the Earth until it reaches the zone where the rocks are completely saturated with water.
Here, masses of water continually slosh to and fro, explaining why many caverns lay nearly horizontal. Hidden in the darkness of caves, rock formations called speleothems droop from the ceilings like icicles, emerge from the floor like mushrooms, and cover the sides like sheets of a waterfall.
Speleothems form as the carbon dioxide in the acidic water escapes in the airiness of the cave and the dissolved calcite hardens once again. The icicle-shaped formations are called stalactites and form as water drips from the cave roof. Stalagmites grow up from the floor, usually from the water that drips off the end of stalactites. Columns form where stalactites and stalagmites join. Sheets of calcite growths on cave walls and floor are called flowstones.
Other stalactites take the form of draperies and soda straws. Twisty shapes called helictites warp in all directions from the ceiling, walls, and floor. All rights reserved. Formation Process But most caves form in karst, a type of landscape made of limestone, dolomite, and gypsum rocks that slowly dissolve in the presence of water with a slightly acidic tinge.
Photograph by john spies, National Geographic Your Shot. Share Tweet Email. Many cavities occur at various depths in a cave system due to the continual seepage and flow of the mildly acidic water through the deposits, while underground rivers may eventually carve their way through a mountainside, creating openings and entrances to the outside.
Many beautiful structures — including stalagmites and stalactites — form inside caves as carbonic acid, carrying limestone, drips through cave roofs and onto their floors. Structures inside a cave may require millions of years to develop. Flowstones are speleothems deposits of calcium carbonate on the walls or floor of a cave formed from a gradual flow of water over a relatively broad area. Essentially, water reacts with carbon-dioxide to form carbonic acid.
It then seeps slowly through the roof of the cave, depositing calcium carbonate, which hardens and builds up over time to form a stalactite. Stalagmites are corresponding formations on the floor of caves to stalactites. Stalagmites rise from the floor in a build-up of calcium carbonate over time, from mineral-bearing water dropped from the roof of the cave. Plants die and decompose in a process that produces carbon.
The rainwater itself turns into carbonic acid. Water wants to go down. Just as rivers on the Earth's surface flow toward the sea, Orndorff said, caves are pipelines for water to move from one place to another. If the water takes a fairly direct route, you can end up with what are called pit caves vertical shafts stretching straight down into the rock. If the water takes a more circuitous route, you get a horizontal cave system.
Mammoth Cave, in Kentucky, a horizontal cave, is the longest cave system in the world , stretching for at least miles kilometers beneath the surface of the Earth. However, some caves turn this tidy formula on its head and form from the bottom up.
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