A cave is simply a natural hole in the earth that is big enough to admit the entry of a human being. Most caves are found in landscapes of limestone, dolomite and gypsum because these rocks are easy for water to dissolve. This type of rock is called karst. The caves are formed when water seeps through cracks and dissolves the rocks. Over time, this creates rooms and passages. Some of these passages meander for miles through the earth, and some may be so narrow that a person needs to crawl through them. Some rooms in caves are enormous and filled with interesting formations. When lights are aimed at these formations many are brilliantly colored.
Formations and features in caves
Types of formations in caves include stalactites, which hang down from the cave ceiling like huge icicles. There are also pillars called stalagmites, which are similar to stalactites but rise up from the floor. These formations are called dripstones. Sometimes, they meet, fuse and form great, stony columns.
Other features include speleothems, which also hang down from the ceiling, cover the walls in sheets or sprout up from the cave floor. Flowstone forms rocky sheets on the walls and floor of the cave, and types of speleothems called helictites twist and turn all over the walls, floors and ceilings. Some are tiny, delicate spikes and others are as large as moose antlers.
How do caves form?
At first, the cave is filled with water as it dissolves away the rock over thousands of years. Then, the water level drops and air is allowed to enter, though water still drips in through cracks in the ceiling and the walls. This water is full of calcium bicarbonate and is slightly acidic. As it evaporates, carbon dioxide gas is released and calcium carbonate is deposited as a solid called calcite. Calcite helps form the dripstones and other features inside the cave.
Less common caves are lava caves, which are formed near volcanoes. They arise when molten lava hardens at the surface while magma flows beneath it. This leaves a small cave with a thin crust of hard lava for a roof. Ice caves start as ordinary limestone caves. When the temperature drops to below freezing, ice forms and accumulates. Sea caves are cut into rocky cliffs by the ceaseless energy of the ocean waves.
Other things found in caves include cave pearls, which look like real pearls and are found in shallow pools. Balloons are little pouches filled with gas. People really don’t know what causes them. Moonmilk is made up of fine crystals deposited on the wall. It has a cream cheese texture when it’s wet and is crumbly when it’s dry. Chandeliers are stalactites made out of gypsum, with crystals that grow out to the side like crystals found in real chandeliers. Popcorn are clusters of knoblike growths. They are very common in caves but no one is sure how they form. They can be found in the air or under water.
- Mammoth Cave is in central Kentucky and has its own river, waterfalls and lakes as well as spectacular formations.
- The Caves of Altamira in northern Spain are the site of famous prehistoric paintings. Archaeologists believe these paintings are at least 20,000 years old. The Lascaux Cave in southwestern France is also famous for its cave paintings.
- Carlsbad Caverns in New Mexico is the home of the magnificent stalactites and stalagmites.
- The Blue Grotto on the the island of Capri is a sea cave famous for its almost luminous, turquoise blue water. Fingal's Cave in Staffa, a Scottish island, is also a famous sea cave.
- Waitomo Cave in the New Zealand’s northern island is famous for the glowworms that live on its ceiling.
- Gouffre Berger, found in southwester France, is one of the deepest caves in the world and between 1953 and 1963 it was considered the deepest. It is 3,681 feet deep and contains an underground river that joins several lakes.