Halong is 1553 sq. km with 1969 islands of various sizes, of which 989 have been named. The islands in Ha Long Bay are mainly limestone and schist islands most lying in the two main areas: the southeastern part of Bái Tử Long Bay and southwestern part of Hạ Long Bay. These islands represent the most ancient images of a geographical site having a tectonic age of from 250 million to 280 million years. They are the result of many times of rising and lowering processes of the continent to form a karst. The process of nearly full erosion and weathering of the karst created the unique Hạ Long Bay in the world. In a not very large area, thousands of islands with different forms look like glittering emeralds attached to the blue scarf of a virgin. The area where many stone islands concentrate has spectacular scenes and world-famous caves and is the center of Halong Bay Natural Heritage, including Halong Bay and a part of Bái Tử Long Bay.
The area is recognized as the World Natural Heritage that is the area of 434 sq. km with 775 islands. It looks like a giant triangle with Ðầu Gỗ Island (in the west), Ba Hầm Lake (in the south) and Cống Tây Island (in the east) as its three angle points. The nearby area is the buffer area and areas classified as national beauty spots in 1962 by the Ministry of Culture and Information.
HALONG FORMATION & DEVELOPMENT:
Development of Halong Bay
Halong Bay is located along the boundary between Vietnam and China at 20o North latitude, approximately 160km to the east of the capital Hanoi. It is part of the Gulf of Tonkin, an extension of the southern Sea of China and covers an area of 1,566 km2 within which 1,969 islands rise out of the water.
As a characteristic development of the full tropical karst formation, these islands appear as steeply rising limestone mountings with a distinguished crest shape which can reach heights of between 50 and 100m, some even as height as 200m. the limestone originates from the Carboniferous and Permian eras. During the Caledonian mountain formation 300 million years ago, northern Vietnam was part of a region consisting of alternating mounting chains and deep oceanic depressions in the shape of waves. Lime with thickness between 2,400 and 2,600 m was deposited in these depression. The later geological history of this region is very complex and still unclear in many details. The fact is that the lime deposits were subjected to a series of Tectonic phases of lifting and sinking whereby they were also distorted and cocked. Deviations in the sea level during phases of transgressions and regressions also affected the development of the shape. It is certain that major portions of Halong Bay were dry during the ice ages. Fossil valleys witnessing a significantly lower sea level during the ice age pervade the ocean floor between the islands.
Which factors are responsible for development of the wealth of shapes which present themselves today? An important prerequisite is the fact that the limestone formation was lifted above the level of the receiving bodies of water at some time because the latter always provides a local base for corrosion. This means that when the limestone formation rises vertical dissolution operations can occur, ending wherever the crevices in the limestone are filled with water (phreatic zone). The upper limit of this phreatic zone corresponds to the level of the receiving bodies of water – expressed in simple form. The limestone islands which one sees today are basically the remains of a once mighty and extended limestone formation, the majority of which was dissolved by corrosion. For this reason the limestone formation continues as a “trunk formation” below the surface of the water – rarely more than 10 m. Limestone strata, once located above this, have disappeared.
Another important factor determining the intensity of corrosion to a decisive degree also contributed to formation of this tropical karst – the climate. It can be assumed that intensive corrosion occurs particularly at high temperatures and high precipitation rates such as under climatic condition that exist today. However, it may possibly have been even more distinguished in earlier phases of the earth’s history, for example in the Tertiary period.
Interestingly the intensity of corrosion appears to be the decisive criterion responsible for the development of the full karst formations. It is recognize at many points in the Halong Bay that the wealth of shapes developed almost completely independently of the stratification of the stone.
Karst Formation: Karst – surface shapes in limestone resulting from dissolution.
Generally the term “karst” subsumes all surface limestone formations, whose development results from chemical dissolution processes. Rainwater always contains a certain amount of acids determined, among other factors, by the omnipresence of carbon dioxide in the atmosphere and also particularly in the soil air. When rainwater enriched with acids comes into contact with limestone, the later is dissolved until a chemical balance is reached, namely the saturation of the water with lime. The visible result is “cavities” in the landscape – hollow formations – which form the typical wealth of shapes in karst depending on local factors of tectonics, composition of the stone and climatic conditions.
This richness of shapes is always clearly visible because dissolution of the lime also occurs deep in the subsurface of the formation. When water seeping into the fragmented subsurface of the formation mixes, additional carbon dioxide going beyond the saturation rate is released which is again capable of dissolving the limestone. This chemical process known as “mixing corrosion” is also responsible for a further characteristic formation in karst regions – caves.
The complex interaction of precipitation, temperature, rock and tectonics provide for the development of a wealth of shapes which is simultaneously scientifically interesting and fascinating. The Dinaric karst, the Moravian karst and in Germany, the Swabian Mountain karst are well known examples of intensively reasearched landscapes which are also appreciated by visitors and tourists. However, the karst as distinguished in its full tropical karst formation at present in the Halong Bay is particularly attractive.
Richness of karst shapes in Halong bay
In Halong bay we find the typical richness of shapes characterizing tropical karst such as are also present in other tropical countries such as Mexico, Jamaica, Cuba, Thailand, Malaysia, Indonesia and China. Witnesses of the once extended and contiguous limestone formation are the numerous caves of which Thien Cung, Dau Go, Sung Sot, Tam Cung and Bo Nau are the most familiar. Some of these caves are significantly above sea level and therefore developed at a time that the affected limestone strata were located at the phreatic level. As a result of tectonic increases and decreases in the sea level and the subsequent vertical corrosive erosion processes, these caves were frequently cut open making them visible along the walls of the karst towers.
If one rides through Halong Bay in a boat, a large number of typical minor karst shapes can be recognized. Where the limestone is flat, more or less round lapis (“Trittkarren”) occur, small, usually circular depressions which are separated from one another by frequently razor – sharp ridges. The steep walls of the krast hills are, by contrast, subdivided by longitudinal extending ridges (“Rillenkarren”). Both of these formations are the result of dissolution and can also be found outside the tropics. “Trittkarren” result during the initial stage, frequently through collection of products from weathering, such as plant residues which provide for intensive corrosion of the local areas whose depths continues to increase in time. “Rillenkarren”, by contrast, result from dissolution of lime by the rainwater flowing down the slopes shows that karren on the slope of the full karst formations are not distinguished everywhere. Fresh fracture zones interrupt the ridge structure at individual points, testifying to active morpho – dynamics. In fact distinctively intensive denudation dynamics on the slopes of the full formations are typical for tropical karst. The fractures are activated from the notches formed everywhere in Halong Bay. These have developed in the tide variation area at the base of the full karst formations as a result of limestone break – off resulting in the characteristic steepness of the slopes. As a matter of principle, these notches formed by dissolution are present not only in the full karst formations surrounded by the sea but also present at the base of many terrestrial karst towers.
The influence of the sea water on the morpho – dynamics of the karst formation is shown by the dissolution of the notches as well as by the formation of so called foot caves. These are present in Halong Bay, particularly in the area of the Ho Ba Ham Islands as well as at other points – here usually in less spectacular form. Their development is also assumed to be associated with the phenomenon of mixing corrosion, whereby the conjuncture of clevis water resulting from precipitation from the inside of the limestone hills and the sea water releases additional dissolution forces which are capable of creating hollow cavities – some of which can even be navigated with boats.
In the caves themselves as well as at individual points in the outer area on the slopes of the full karst formation; sintered shapes can be recognized, some forming mighty stalactites. Such exterior stalactites, attest to high limestone turnover; lime dissolution and lime precipitation are located close to one another spatially. In comparison to the karst formations in Phang Nga Bay in southern Thailand, these visible precipitation processes in the form of exterior stalactites in Halong bay are, however, not very prevalent – an indication that the lime exchange and dissolution intensity decreases in proportion to the distance from the Equator.