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.