Limestone

The chemical weathering of limestone has created of some of the Earth's most unusual and interesting landscapes.

Limestone consists mostly of calcium carbonate, which is not soluble in pure water. However, if the water contains carbon dioxide dissolved from the air or from organic material, it becomes a weak carbonic acid. This acid reacts chemically with limestone, turning it into calcium bicarbonate, which is readily soluble.

Limestone Caves

Limestone is riven by joints (vertical cracks) and bedding planes (horizontal cracks), which allow water to percolate easily through it.

On the surface, chemical erosion enlarges some cracks into funnel-shaped sink or swallow holes. Streams sometimes enter these holes and plummet downwards. The water may then travel hundreds of miles under the ground, dissolving away vast caverns.

Sometimes, so much limestone is removed that the roof of a cavern collapses. If the cavern is near the surface, a gorge is formed.

Eventually, however, the underground water collects into streams and reemerges at the base of the limestone.

Some caves are extremely deep.

Drops of water, highly charged with calcium carbonate, often hang for some time on a cave roof before falling. While they hang, some of the water evaporates and a film of calcium carbonate is deposited on the roof.

If the water always drops from the same place, successive films grow into rock icicles, or stalactites.

When the water drops onto the floor, similar deposits form in upward-growing stalagmites.

Eventually, they may join and form pillars of calcium carbonate. The speed of growth varies. Some can take 2,000 years to grow only one centimeter.

However, a stalactite in Ingleborough Cave, in Yorkshire, England, grew by 7.6 centimeters in only ten years.

In some caves, fringed curtains of calcium carbonate form when water drips down from an irregular crack in the roof of a cave. Water flowing across a cave floor may deposit flowstones, which look like sheets. Some calcium carbonate deposits look like flowers.

Karst Landscapes

Most limestone landscapes have a distinctive surface character, called Karst topography, after the carst district in the Dinaric Alps of southern Europe.

Because water percolates so readily into limestone, the surface is often dry and plants grow only in hollows where clay accumulates.
Bare limestone surfaces are divided into blocks, called clints, separated by eroded joints, called grikes.

Karst scenery occurs in the Cockpit Country of Jamaica and in the North American Kentucky plateau, which has more than 60,000 sinkholes, hundreds of caves and over 350 miles of underground passageways in Mammoth Cave National Park.