Scientists said that islands have a tectonic age of about 300 millions years, Halong Bay formations processes started when the stuffs like: eroded earth, marine organisms, minerals fall down and deposit in layers upon layers on the bottom of the ocean. These processes carry out until a chemical reaction and chemical deposition slowly turn those layers into limestone. Then after the underground movement in the Earth crust which happened about 200 millions years ago, those layers were lifted up, broken into islands and islets with their shape like what we see today.
Caves are formed by various geologic processes. These may involve a combination of chemical reactions, water erosions, tectonic forces, atmospheric influences, and even digging.
The most common formation of caves is dissolution. Dissolution begins when acidic water reacts chemically to the limestone bedrock and being absorbed by the soil into the ground. On the way traveling down, acidic water keeps reacting chemically with calcium carbonate in the rock which result a space. When the space becomes larger and larger the water can flow through, as it flows it erodes. Physical erosion washes away rock and sand then forms an underground stream. Finally over hundreds of thousands of years or even millions of years the cave is formed.
Stalactites and stalagmites:
Stalactites are formed by the deposition of calcium carbonate and other minerals, which is precipitated from mineralised water solutions.
Limestone is calcium carbonate rock which is dissolved by water that contains carbon dioxide, forming a hydro calcium carbonate solution. This solution travels through the rock until it reaches an edge and if this is on the roof of a cave it will drip down. Before dripping down, it tend to cling into droplets, this get the solution deposited when it react chemically with air and other minerals.
An average growth rate is 0.13 mm (0.005 inches) a year. With fast-flowing water rich in calcium carbonate and carbon dioxide, these can grow at 3 mm (0.12 inches) per year.
Every stalactite begins with a single mineral-laden drop of water. When the drop falls, it leaves behind the thinnest ring of calcite. Each subsequent drop that forms and falls deposits another calcite ring. Eventually, these rings form a very narrow (0.5 mm), hollow tube commonly known as a “soda straw” stalactite. Soda straws can grow quite long, but are very fragile. If they become plugged by debris, water begins flowing over the outside, depositing more calcite and creating the more familiar cone-shaped stalactite.
The best way to experience the spectacular beauty of Halong Bay formation is to take part in a and kayaking tours.