What is a karst area?

Karst is an area of land made up of limestone. Limestone, also known calcium carbonate, is a soft rock that dissolves in water. As rainwater penetrates slowly through the rock, it slowly erodes. Karst landscapes can be worn away from the surface or dissolved from a weak point inside the rock.

Watch this short video which explains how chemical weathering occurs in limestone resulting in a number of distinctive features both on the surface and underground.

The name of this type of topography comes from the Karst region in Yugoslavia, but we find karst topography all over the world. It is now used to refer to any similar topographical region throughout the world. In fact, ten percent of Earth’s surface is covered in karst landscape! Karst regions occur throughout North America, as well as in Central and South America, Europe, Asia, Africa, and even Oceania (Australia and nearby islands). Malta is also a karst region. In karst regions we find a number of features such as limestone pavements with clints and grykes, swallow or sink holes as well as caverns with stalactites and stalagmites. All these features can be seen in the diagram below.

Limestone is a sedimentary rock that is made up of horizontal blocks called bedding planes and vertical cracks called joints. Limestone is also a permeable rock which allows rain water to pass through these joints and cracks. As it made of calcium carbonate it dissolves with rain water resulting in chemical weathering.

Swallow or sink holes: These are characteristic of karst landscapes, and are places where the surface collapses in on itself, creating a funnel-shaped hole in the ground. This is the direct effect of chemical weathering whereby the water dissolves the limestone. Most swallow holes are cave entrances to cave systems with stalactites, stalagmites and pillars. Sometimes rives flow through these caves and where the water comes out again to the surface as springs is known as resurgence. Below you can see an image of a swallow hole.

Limestone Pavements: Water seeps through cracks and joints in the permeable limestone. As it passes through the rock it dissolves the rock enlarging joints and bedding planes through chemical weathering. As a result of this, on the surface, cracks are widened and deepened to form grykes. The exposed blocks of limestone which stick up are called clints and the resulting pattern of blocky rock is called a limestone pavement.

Stalactites, stalagmites and pillars: These pointy features are found underground, inside caves and formed over thousands of years. They are also the result of water seeping through the rocks and in so doing dissolving the rock. Limestone is dissolved because rainwater is a weak carbonic acid. Karst underground features form when chemical weathering is very active.
Watch this video showing clearly how caves, stalactites and stalagmites form.

Calcite the building block of limestone is carried down through cracks in the ceiling. The dripping water leaves behind traces of calcite, which slowly builds up on the ceiling until a stalactite takes shape, hanging down like an icicle. Water from the end of the stalactite leaves more calcite in a pile on the cave floor, and pretty soon a cone-like stalagmite forms. That’s why stalactites and stalagmites are usually found in pairs. Sometimes they grow together to form a pillar or column.

Now try to identify the features marked on the diagram. Write the name of each feature in the correct label.
Choose any three of these features and explain how each one formed.

Now work out all the activities on the worksheet entitled Limestone worksheet. Worksheet is available in the resources section.