Gary Perras, age 12, of St. Catharines, Ontario, Canada, for his question:
HOW ARE CAVES FORMED?
Most famous limestone caves in the United States are those at Carlsbad Caverns in New Mexico. Other outstanding caves include Floyd Collins Crystal Cave in Kentucky, the Luray Caverns in Virginia, the Wayandotte Cave in Indiana and the Mammoth Cave in Kentucky. Famous sea caves include the Blue Grotto on the Isle of Capri in Italy and Fingal's Cave on the Scottish island, Staffa. Limestone and dolomite caves are formed in two stages. First, water dissolves large sections of rock and then air comes in and dripstone develops inside.
There are two types of dripstone: stalactites and stalagmites. These are rocks which are formed by dripping water and either hang on the ceiling of a cave like an icicle or stand on the floor of the cave like a tall cone. Which is which? Just remember the " tite " in stalactite and you'll always recall that it's the stalactite which is holding on tightly to the ceiling of the cave.
A cave will first start to form when water seeps through cracks in rock. Slowly the rock is dissolved along the cracks, and the small particles are carried away by the water. It takes thousands of years, but eventually rooms and passages are formed. During the early stages, the water completely fills all of the underground chambers.
When the flow of underground water changes its pattern, the level will drop and air will enter the caves. Water may continue to seep into the rooms through the cracks in the rock. The dripping water carries dissolved calcium bicarbonate and, as the water evaporates or as its temperature rises, carbon dioxide gas is released from the solution and solid calcium carbonate is deposited as the mineral calcite.
Lava caves are formed in a different way. As molten lava from a volcano hardens, it cools first from the surface while liquid lava continues to flow beneath the hardening crust. Quite often this forms a roof over hollow areas.
Sea caves are formed as waves pound rocky shores.
The ice cave is still another type of cavern. It first developed as an ordinary limestone or dolomite cave, but then ice formed after the temperatures dropped, freezing all of the water and turning the underground chamber into a natural refrigerator. When a stalactite hanging from the ceiling of a cave meets a stalagmite rising from the floor, a column of stone is formed. It takes nature thousands of years to perform this bit of magic.
One famous cave, the Shanidar Cave in Iraq, has provided shelter for people for about 100,000 years. People are still living in it today.