Limestone is a hard, grey sedimentary rock largely composed of calcium carbonate. It was formed under the sea from the remains of organic matter such as seashells and plants. Limestone rock is characterised by vertical cracks called joints and horizontal cracks called bedding planes. Limestone is said to be a pervious rock, as it allows water to flow through the joints and bedding planes.
The main process that weathers limestone is known as carbonation. Since limestone is made up of calcium carbonate, it is a suitable candidate for carbonation. When atmospheric carbon dioxide is dissolved in rainwater, it forms carbonic acid.
When it rains, the carbonic acid comes into contact with the limestone, and some of the water percolates through the joint's embedding planes. The carbonic acid reacts with the limestone to form the soluble compound called calcium bicarbonate. This is then washed away in solution with streams and rain water gradually weathering the limestone. For more water and carbon dioxide present the higher the rate in which limestone dissolves.
Carbon dioxide is found in higher concentrations in the gaps in the soil than in the atmosphere due to soil microbes generating carbon dioxide. When rainwater percolates through the porous soil the carbon dioxide reacts with the water to produce higher quantities of carbonic acid. Some of the best developed karst landscapes are in tropical Southeast Asia and subtropical Southern China where there are high levels of rainfall. Warm temperatures and lush vegetation here results in high concentrations of carbon dioxide. Thus, the ideal conditions for carbonation to take place.
The South China cast covers 600,000 square kilometres and is characterised by pinnacle cast known as shell and or stone forest, some of the world's largest cave systems and spectacular tower cast. The development of tower cast requires a minimum of 120 centimetres of precipitation per year and an average temperature of 18 degrees Celsius. The process of water dissolving rocks like limestone is known as castification. This process produces a karst landscape.
Karst landscapes can be found all over the world and at varying altitudes. These pancake rocks at Pernikaiki in New Zealand's South Island are at sea level. They can be contrasted to this high altitude karst landscape in Sichuan, China.
There are a number of features that characterize karst landscapes. Karst landscapes are most famous for the cave systems that have been hollowed out by the action of underground streams and by carbonation and solution. Caves are characterized by formations known as speleotherms, which develop where water percolating through the rock deposits calcite over thousands of years. Speleotherms include stalactites, which hang down from the cave ceiling.
Stalagmites, which grow from the cave floor up. Sometimes a stalactite and a stalagmite will join together to form a pillar. The most common types of speleotherm is called a flowstone, which forms where water runs down the walls or along cave floors. Another speleotherm formation is ribstone. which takes the shape of a stone dam and forms where water has deposited calcite besides a pool or a stream.
These unique examples of rimstone pools in Huanglong, China, are unusual, as they have formed in an alpine valley rather than inside a cave. In karst landscapes, many rivers and streams disappear underground and flow through the limestone, carving out caverns until they reach impermeable rock layers. At this point the water will flow under the limestone until it re-emerges at the surface.
The place where an underground stream re-emerges is called a resurgence. Sinkholes or dolines are topographic depressions in the limestone. Sinkholes vary in size, from 2 to 100 meters deep, and from 10 meters to 1 kilometer in diameter. Some sinkholes are formed when a cave ceiling collapses, while others form when natural fractures have been enlarged by a solution.
Due to the permeable nature of the rock, and extensive subterranean drainage in karst landscapes, dry valleys that have no surface streams may develop. Another feature often found in a karst region is a large basin that is a flat floor, steep walls, but no outflowing stream on the surface. This basin is called a polder.
They are formed by the union of several sinkholes and can have walls up to 100 meters in height. Lake Disappear near Aotea Harbour on New Zealand's west coast is a poldra that is drained by an underground stream. When there is heavy rain, the stream does not drain fast enough so it fills up with water to become a lake.
In dry periods, the lake drains away and disappears. Sometimes the ceiling of a large underground cabin falls in to create a steep-sided gorge with a river running into the bottom. In this example from New Zealand, part of the ceiling remained to create an arch, but downstream is a steep-sided gorge. Cliffs that exist at the edge of the area of limestone are called scars. Notice that the cliffs featured here have an almost 90 degree angle and are highly jointed.
Karin is the general term used to describe the various small scale solution features of limestone. Karin features are particularly common on limestone pavement, which are the large areas of exposed limestone rock. Limestone pavement is characteristically divided into large slabs of rock called clints, separated by deep vertical fissures called grogues.
These grogues have developed by weathering and intensified solution processes along the joints in the limestone. Channels called runnels are eroded out of the limestone surface, which drains into the grogues. Pits formed by solution processes can also be found on the top of the cliffs.