Donna Barclay, age 15, of Huntsville, Alabama, for her question:
What are sinkholes?
Sinkholes and puddles have one thing in common. Where you find one of them you are likely to find others round about. Some sinkholes also are brimful of water but they cannot be trusted. They may drop ten feet or 20 feet or even more right straight down to the bottom.
In certain regions these sneaky booby traps are strewn around all over the ground. tae have them in Florida and Kentucky, Virginia and Tennessee. In one part of Indiana there are hundreds of them to the square mile. Near Carlsbad Caverns in New Mexico, one sinkhole is 4,000 feet long, 625 feet wide and 350 feet deep. This fellow is big enough to be noticed and, unless you are daydreaming, you are not likely to walk into it unawares. But most of the earths sinkholes are merely a few feet wide or even smaller. What's more, their open tops may be camouflaged with overhanging vegetation. As you walk innocently along, before you know it you may find yourself on the bottom of a deep, steep hole in the ground, groaning with broken bones.
Booby traps, as a rule, do not aim to play fair, but with sinkholes we have one thing on our, side. They tend to occur only in certain rock formations. Their favorite rock is limestone, and in order to do their best or their worst they also need the help of a moist, showery cli¬mate. When conditions are right, lots of rain seeps down and saturates the pores in the soft, spongy limestone. The ground water is near the surface. Next we need a bit of chemical activity to nibble out these deep holes in the ground.
Rain tends to dissolve carbon dioxide from the air and still more from decaying materials in the ground. The watery mixture becomes a solution of carbonic acid. Acids tend to corrode, and this one chews away at the limy chemicals in limestone. The ground water sinks and seeps, saturates and per¬ colates, lapping at the limestone and dissolving its chemicals. Wherever there are underground pores and pockets, they grow bigger. Where there are fine cracks downward from the surface, the ground water licks away their sinkholes for Friday, December 27, 1968 sides. In time they become deep, steep sided sinkholes. Then conditions are just right for the Job, it takes about 2,000 years to carve a sinkhole one foot deep. But remember, that bed of limestone and that showery climate have been busy for at least a million ,years. The neighborhood booby traps have had time to grow very deep and very plentiful.
Geologists have a special name for sinkhole territory. They call it karst topography, because one of the world's worst areas is near the town of Karst in Yugoslavia. Karst topography also tends to have a lot of under¬ground caves and caverns, chewed out by the same chemical process. All sink¬holes, you might say, are really topless caves, and many of them really were caves until their roofs collapsed.