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
characterized 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
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
joints and bedding 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. The
more water and carbon dioxide present the higher rate in which limestone
dissolves. Carbon dioxide is found in higher concentrations than 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 Karst covers 600,000 square
kilometers and is characterized by pinnacle karst known as shillin or Stone
Forest. Some of the world’s largest cave systems and spectacular tower karst. The
development of tower karst requires a minimum of 120 centimeters of
precipitation per year and an average temperature of 18 degrees Celsius. The
process of water dissolving rocks like limestone is known as karstification. This
process produces a karst landscape. Karst landscapes can be found all over the
world and at varying altitudes. These pancake
rocks at Punakaiki 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 Speleothems which develop where water percolating through the rock deposits calcite over thousands of years. Speleothems include stalactites
which hang down from the cave ceiling, stalagmites which grow from the cave
floor up. Sometimes a stalactite and stalagmite will join together to form a
pillar. The most common types of Speleothem is called a flow stone which forms
where water runs down the walls or along cave floors. Another Speleothem formation is rimstone which takes the shape of a stone
dam and forms where water has deposited calcite besides a pool or stream. These
unique examples of rim stone pools and 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 reemerges at the surface. The place where an
underground stream reemerges is called a resurgence. Sinkholes or dolines are
topographic depressions of the limestone. Sinkholes vary in size from two to 100
meters deep and from ten meters to one kilometer in diameter. Some sinkholes are
formed when a cave ceiling collapses, while others form where natural
fractures have been enlarged by 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 has a flat floor, steep walls but no out flowing stream on
the surface. This Basin is called a polje. They are formed by the union of several sinkholes and can have walls up to 100
meters in height. Lake Disappear near Aotea Harbor on New Zealand’s west
coast, is a polje that is drained by an underground stream. When there is heavy rain,
the stream does not drain fast enough so it fills up with waters to become a lake.
In dry periods the lake drains away and disappears. Sometimes the ceiling of a large
underground cavern falls in to create a steep-sided gorge with a river running
in the bottom. In this example from New Zealand parts of the ceiling remains to
create an arch, but downstream is a steep side of 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. Karren is a general term used to describe the various small-scale
solution features of limestone. Karren features a particularly common on
limestone pavement which are large areas of exposed limestone rock. Limestone
pavement is characteristically divided into large slabs of rock called
clints separated by deep vertical fissures called grykes. These grykes 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 grykes. Pits formed by solution processes
can also be found on the top of the clints.