Nelson-Denny Practice Test 6

Rock That Dissolves: How Karst Landscapes Form

¹Most rock wears away by being broken into fragments, but limestone can also be carried off in solution. Limestone is made mainly of calcite, a form of calcium carbonate. Pure water dissolves calcite only very slowly. Rainwater, however, picks up a little carbon dioxide from the air as it falls, and it picks up far more as it soaks through soil, where decaying plant matter and the respiration of roots keep carbon dioxide levels high. The result is a weak carbonic acid that reacts with calcite and carries the calcium away as dissolved ions. Landscapes shaped chiefly by this kind of chemical weathering are called karst, after a limestone plateau on the border of Slovenia and Italy where such features were first studied in detail.

²Dissolution does not proceed evenly through the rock. Limestone is laced with joints and bedding planes, cracks along which water can move. Water flowing through these openings slowly widens them, and a wider opening carries more water, which widens it faster still. Over thousands of years this feedback concentrates the flow into a few enlarged channels, while neighboring cracks remain narrow. Some of those channels eventually grow into caves large enough for a person to walk through.

³At the surface, the most characteristic karst feature is the sinkhole, a closed depression into which water drains. Some sinkholes form gradually, as the bedrock beneath a patch of soil dissolves and the ground slowly sags. Others form suddenly, when the roof of an underground cavity, weakened over a long period, gives way. Collapse sinkholes often appear during droughts or after heavy pumping from wells. In both cases the water that had filled the cavity and helped hold up its roof has been withdrawn, leaving the roof with less support.

⁴Karst regions often have surprisingly few surface streams. Rain disappears into sinkholes and fissures, and whole rivers may vanish underground at points called swallow holes, only to reappear miles away as large springs. Because the water moves through open conduits rather than through the tiny spaces between grains of sand or gravel, it travels quickly and receives little natural filtering. A pollutant spilled near a sinkhole can therefore reach a spring or a well within days. Many towns in karst country depend on such springs for drinking water, which makes protecting the land surface unusually important.

⁵The same chemistry that hollows out a cave can also decorate it. When water that has seeped through the rock drips into an air-filled cave, it releases some of its dissolved carbon dioxide into the cave air, which holds much less of the gas than soil does. Losing carbon dioxide makes the water less able to keep calcite in solution, so it leaves behind a thin film of mineral. Drop by drop, such films build stalactites hanging from the ceiling and stalagmites rising from the floor. Growth rates vary widely, but a formation may lengthen by only a small fraction of a millimeter in a year.

⁶Karst thus illustrates a principle that runs throughout geology: a process need not be powerful to be important. A weak acid, acting on a common rock over very long spans of time, can drain rivers, open caverns, and determine where people can safely build and draw water.