Limestone caves of Andamand Islands
In Baratang Island, particularly at Nayadera near Wrafters Creef, there is a huge deposit of Limestone, where some deposits are in the form of caves and the rest of it are in the form of layers or beds one upon the other. These deposits are either above the soil or under the ground (the underground portions from caves).
There are more than 300 big and small deposits of limestone, out of which only one in the form of a cave is opened for the tourists. In the cave, massive limestone formations are dangled from the ceiling, glowed from the sides and sprouted from the ground.
The Limestone cave(s)
How to reach
One has to reach Baratang Island which is a 100 kilometre drive through the narrow Andaman Trunk Road ( ATR ) from Port Blair. The drive is followed by a short vehicle ferry ride from Middle Strait to Baratang Nilambur Jetty and from there another half-an-hour ride on a speedboat. As the speedboat knifes its way through the waters of the Baratang Creek, tiny islands and endless stretches of massive, primordial tropical forests bordered by unique Mangrove vegetations move up and down into view on either side.
The speedboat is berthed at a temporary wooden jetty at Nayadera Creek. A few stepes after the landing a "Mangrove Canopy Walk" welocmes the tourists. The Mangrove Canopy Walk is maintained by the staff of the Baratang Forest Division of the Andaman and Nicobar Administration. The Canopy walk is a 240 meters long narrow bridge made of Non-timber Forest produce, winding its way through thick mangroves.
The Mangrove Canopy Walkway is designed in such a way that neither any Mangrove branch was pruned nor any tree felled during the construction of the path. The information on the boards are instructive and enlightening. One can get well acquainted with various aspects of mangroves, its morphology, salt water adaptations etc while walking on this path.
On the way to the cave(s)
The Limestone cave(s)
The mangroves swamps help in flood control, trapping of silt, storm breakers and act as coastal protection barriers. The Mangroves Canopy Walk, the information boards in Hindi and English, eco huts, sit-outs and chairs formed part of “An open interpretation centre” to provide information on mangroves, their morphology, and their uses and so on.
After getting off the bridge, one has to trudge over one and a half kilometer through dense tropical forests. At a couple of clearings were paddy fields with idyllic huts located strategically. On either side of the path there are eco-huts to take shelter when it rains, eco sit outs, benches and chairs, all made of bamboo and thick cane plats. While passing through this forest, one can see only the faint ray of sunlight filtering through the dense foliage. The “cave” comes into view after the walk through forest.
The cave is a sight to behold- massive limestone formations dangled from the ceiling like chandeliers, glowed from the sides and sprouted from the ground like short pilasters. One hung like a thick pillar from the ceiling of the cave. The serrated edges of the limestone blocks shine in the dark. There is a narrow walkway of the cave inside and constant dripping of water from the ceiling. The limestone caves at Baratang have been opened to visitors to promote eco-tourism.
FORMATIONS OF LIMESTONE CAVE
The origin of limestone caves is complex and scientists are not in complete agreement as to the sequence of their formation. Limestone is a sedimentary rock formed at the bottom of the sea. It is formed by the compression over millions of years of the gradual deposits of many ingredients such as marine life, shells, skeletons and coral. The primary source of calcite in Limestone is most commonly the marine organisms.
The Limestone cave(s)
The Limestone cave(s)
These organisms’ exuded shells that settle out of the water column and are deposited on ocean floor as pelagic ooze or in coral reefs. These produces are speleothems such as stalactites and stalagmites. The formations are pushed above sea level in many parts of the world. They have different hues, textures and degrees of purity. The solution process forms these Limestone Caves. These types of caves are believed to be the deepest and largest caves found by man. A common characteristic of this rock is that it is easily dissolved by rainwater.
FORMATION PROCESS AND CHEMISTRY
When it rains, the rain drops pickup Carbon dioxide from the atmosphere. It hits the earth’s surface and flows downward into small cracks. As it flows it passes through air pockets in the soil. These air pockets contain more carbon dioxide, which is given off by rotting plants and soluble lime from the limestone rocks. When the atmospheric Carbon dioxide gets dissolved into the rainwater. This mixture forms a mild Carbonic Acid, which is an effective solvent limestone.
2CO2 + 2H2O --------------> 2H2CO3
This acid water seeks any split in the rock formations, such as fissures, joints or faults. It gradually seeps into these weakened rocks enlarging them and dissolving Calcuim Carbonate, the main mineral in limestone. This mixture flows downward to the water table.
One of the important properties of limestone is that it is easily
a weak acidic solution when compared to other rock types.
Limestone is calcium carbonate rock, which is dissolved by water that contains carbon dioxide (mild carbonic acid), forming a calcium hydrogen carbonate solution. The equation for this chemical reaction is:
2CaCO3 + 2H2CO3 --------------> 2Ca (HCO3)2
This solution travels through the rock until it reaches an edge and if this is on roof of a cave it will drip down. When the solution comes into contact with air the chemical reaction that takes place is reversed and particles of calcium carbonate are deposited. The reverse reaction is:
2Ca (HCO3)2 --------------> 2CaCO3 + 2H2CO3
2Ca (HCO3)2 --------------> 2CaCO3 + 2H2O + 2CO2
By the continuing physical flow of rain water, and the chemical reaction of the Carbonic Acid making its way through these weakened areas, erosion results gradually over thousand of years until underground rooms, chambers or carvens are formed. There is constant seepage of ground water through the rocks. Water carries calcite, which has been leeched from the rocks. Calcite is a crystallite mineral that is the main component of chalk, limestone and marble stone. It is a natural form of calcium carbonate.
Carboniferous limestone is a sedimentary rock made of calcium carbonate, generally light grey in color and is hard. It has horizontal layers or beds with bedding planes and vertical joints. These joints are weak in the rock, which are exploited by agents of both denudation and weathering. An average growth rate of limestone is 0.13 mm (0.005 inch) a year. The quickest growing stalactites are those formed by fast flowing water rich in calcium carbonate and carbon dioxide which can grow at the rate of 3 mm (0.12 inch) per year.
Stalactites are the formations that form on the ceilings of caves, which look like icicles. Formation of stalactite begins with a single mineral laden droop of water, i.e. the water chemically combined with carbon dioxide, calcium carbonate and calcite. The water drop squeezes between the rocks and drops to the floor leaving behind a minute deposit of the dissolved calcite in the shape of a thin ring of calcite. Each subsequent drop that forms and falls deposit another calcite-ring. Eventually, these rings form a very narrow (0.5 mm dia) hollow tube hangs from the ceiling is commonly known as a ‘Soda Straw’ stalactite. Soda Straws can grow quire long, but are very fragile. If they become plugged by debris, water begins flowing over outside, depositing more calcite and creating the more familiar cone-shaped stalactite.
The water drops that fall from the tip of the parent formation, the Stalactite, deposit more calcite on the floor below, eventually resulting in a rounded or cone shaped stalagmite. Stalagmites are the formations found on the floor of caves, reaching upward to touch the stalactites hanging above from the ceiling. Unlike stalactites, Stalagmites are rounder, smoother and has no central tube. Straight formations of stalactite and stalagmites can meet and fuse to create column.
Helictites starts its growth as a tiny stalactite. This crystal structure changes its shape from cylindrical to a conal one. In some caves, Helictites structure together and form bushes as large as six feet tall. The helictites are, perhaps, the most delicate and most beautiful formations of the limestone cave cluster together and form bushes as large as six feet tall. The helictites are, perhaps, the most delicate and most beautiful formations of the limestone cave.
Inside these caves sinkholes are seen. A sinkhole is a natural depression or hole in the surface topography caused by the removal of soil or bedrock, after wash, by water. Sinkholes may vary in size from less than a meter to several hundred meters both in diameter and depth and vary in form from soil- lied bowls to bedrock-edge structures. Mechanisms of formation may include the gradual removal of slightly soluble bedrock of limestone by percolating water, the collapse of a cave roof, or lowering of the water table. Occasionally a sinkhole may exhibit a visible opening into a cave below.
FOR MORE INFORMATION:
1. Principal Chief Conservator of Forests, Andaman & Nicobar Islands, Van Sadan, Haddo P.O. Port Blair- 744102 (India), Phone No. 91-3192-233321. Fax No.- 91-3192-230113.
2. Conservator of Forests (Southern Circle), Andaman & Nicobar Islands, Haddo P.O. Port Blair-744 102 (India), Phone No. 91-3192-230152. Fax No.-91-3192-230152.
3. Divisional Forest Officer, Baratang, Andaman & Nicobar Islands, Phone No. 91-3192-279524, Fax No. 91-3192-279524, email id: firstname.lastname@example.org
Text courtesy: Shri. C.R Mallick, Divisional Forest Officer, A & N Administration. Published by the Directorate of Tourism, A&N Administration, Port Blair, Ph- 03192-232694 email: email@example.com