Electrical Aptitude Practice Test 8

Caves Carved by Water

¹Many of the world's caves were not dug by animals or blasted by people. They were dissolved. These solution caves form mainly in limestone, though they also form in marble, dolomite, gypsum and halite, or rock salt. Water works its way along joints (fractures), faults and layers in the rock, slowly dissolving it and carrying it away.

²The water does this work with the help of carbonic acid, a very weak acid that forms when carbon dioxide dissolves in water. Some of the carbon dioxide comes from the air and some from the breathing of animals, but most of it is picked up as rain and melting snow soak through the soil. The slightly acidic water then seeps downward until it reaches the limestone below.

³At first, the water creeps through tiny cracks. As the cracks widen, more water can flow through them, and streams begin to carry sediment that scours the rock and enlarges the passages. Bigger streams form bigger passages. Even so, the process is extremely slow. A passage with an active stream grows by at most about 1 millimeter per year, which would make a passage about 1 meter wide in a thousand years. Many caves are hundreds of thousands to millions of years old.

⁴The same chemistry that hollows out a cave also decorates it. When dripping water reaches the open air of the cave, it releases carbon dioxide, much as a bottle of soda does when it is opened. The water can then no longer hold as much calcite, the mineral that makes up limestone, and the calcite is deposited again on the ceiling, walls and floor. A stalactite begins as a hollow "soda straw": as a drop hangs from the ceiling and loses carbon dioxide, a thin ring of calcite forms around its edge. Water that still carries dissolved calcite when it drips to the floor can build a stalagmite.

⁵These formations, called speleothems, grow at different speeds. Two important factors are the temperature outside the cave and the amount of rainfall. Warm weather speeds the decay of plants and animals, which adds carbon dioxide to the soil, and more rain carries more dissolved calcite down into the cave. Pure calcite is white, so the red, orange and black shades of many formations come from iron and other minerals.