Cave traits: common features?

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Caves contain various speleothems, such as stalactites and stalagmites, which grow slowly over centuries or millennia. Other formations include soda straws, helictites, and flowstone. These structures are created by dripping water depositing calcite or gypsum.

The caves of the world contain numerous beautiful and mysterious formations for anyone brave enough to venture into them. There are the obvious ones – stalactites and stalagmites – but also soda straws, lava flows, columns, tents, helictites, stone dams, cave corals and many more. These formations are called speleothems, from the Greek spelaion for cave and thema for deposit. The cave formations grow slowly, about an inch a year or less, taking centuries or millennia to create the amazing shapes we see today.

The best known rock formations are the stalactite and the stalagmite. Stalactites form from soda straws, hollow, elongated tubes of calcite (calcium carbonate) formed when water drips from a hole or crack in the ceiling. Every drop of water deposits a little calcite and it builds up over time. The material is called dripstone. When a soda straw gets big enough, it becomes a stalactite. A stalagmite is a complementary cave feature that forms on the ground under a stalactite. It is formed by the calcite-laden droplets that drip from the ceiling. An easy way to remember the difference between the two is that stalactites cling to the ceiling, while stalagmites might reach to the ceiling. When a stalactite and a stalagmite meet, they become a column. Columns can get quite impressive, with the height being 10m or greater.

Many cave formations are all based on the same principle of slowly dripping water depositing calcite, or sometimes gypsum, which is calcium sulphate, in complex shapes over long periods of time. You can observe this effect yourself by mixing water with sand, cupping it in your hands, and letting the water drip. The sand will pile up quickly, creating a small tower.

Another variant of the classic stalactite is the helictite, a stalactite that forms so slowly that subtle chemical or physical changes have caused the crystalline structure of the deposited calcite to alter, slowly twisting it. Helictites appear as cylinders or spiral needles.

When water trickles through an elongated crack or runs down a slope on the ceiling as it falls, it can create curtains and drapes, delicately folded cave formations that resemble their namesakes.
When calcite-rich water flows down a wall, it is called flowstone. Sometimes this water flows down a vertical drop in a cave and is called a waterfall. Sometimes, instead of flowing, water under hydrostatic pressure is slowly forced out of a crack in the wall or floor, creating a structure called a shield.

There are many other beautiful cave formations: cave beads, clouds, rafts and raft cones, rimstones, shelfstones, bottlebrushes, canopies, and more. To look at images of these features, find them using your favorite image search.

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