Limestone is a sedimentary rock that is made mostly of calcium carbonate in the form of the mineral calcite. It is unusual in that is often made from living matter, such as seashells and coral skeletons. Coal is another sedimentary rock made from organic matter.

If you put limestone in pure water, nothing will happen because limestone is insoluble in water.

However, when carbon dioxide from the air or from organic material dissolves in water, it forms carbonic acid. As this acidic water runs through cracks in limestone, the carbonic acid will react with the limestone to from calcium bicarbonate, which does dissolve.

As time goes by, in a process known as “chemical weathering”, the limestone will wear down. Weathering changes the shape of the limestone, forming some of the most spectacular landscapes on Earth.

Karst Landscapes

Landscapes formed by the chemical weathering of limestone or other rocks, such as gypsum, are known as Karst landscapes. They often have very dry surfaces because water percolates through the limestone very easily. Plants grow only in hollows, where there is moist clay.

Karst landscapes are full of caves, sinkholes and underground streams.

Water that seeps in cracks in the landscape breaks it up into blocks and creates unusual shapes.

You can find Karst landscapes in the North American Kentucky Plateau, Jamaica’s Cockpit County and the Yorkshire Dales of Great Britain.

Limestone Caves

Caves formed by the weathering of limestone are among the most spectacular features on Earth.

As water percolates through cracks in limestone, the cracks enlarge. Some of them become funnel-shaped holes. Water entering these holes can travel hundreds of miles under ground.

Limestone caves will often have drops of water on the ceiling. This water is full of calcium carbonate. As the water evaporates, leftover calcium carbonate remains on the cave roof. If drops of water always fall from the same place, calcium carbonate deposits will grow until they form “icicles” of rock, known as stalactites, that hang from the ceiling.

Stalagmites are limestone “icicles” that grow upward from the floor. They form from drops of water on the floor, rather than the roof.

Eventually, a stalactite and a stalagmite may join to form a pillar.

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

Eventually, they may join and form pillars of calcium carbonate.

Calcium deposits in caves can form other shapes as well.  If water drips from an irregularly shaped crack in the roof of the cave, the calcium deposits form a “curtain”.

Flowstones are sheetlike deposits that left by water flowing across the cave floor.

Calcium deposits may also appear in the shape of flowers.

Do you live in or near a Karst region or have you ever visited one? Have you ever explored a cave? Share your experiences in the comments below.