Vietnam has an exposed carbonate rock area covering 20% of his total territory, i.e. almost 60000km², which distributes mainly in its North and Northwest. The latter (including Son La, Lai Chau) is one of the most important karst areas, where carbonate rock extends 400km long from the border with South China to east coastal zone in SE direction with a width ranging from 5 to 40 km. Ha Long Bay which has been listed as the Word Heritage by UNESCO in 1994 is located in Northeast of Vietnam, Quang Ninh Province, where there is a beautiful karst landscape (isolated peaks) on the sea (Photo 1). Vietnam has a humid tropical climate heavily influenced by Southeast Asia monsoon. As a consequence climate in Northwest Vietnam is characterized by rich precipitation (annual rainfall 1413mm), higher annual mean temperature (21° C) and average air humidity (80%). Geologically, wide spread carbonate rocks have deposited since Pre-Cambrian to Holocene with some pure limestone strata 2000m thick and experienced strong foldings and faultings. Middle Triassic (including T₂dg: Dong Giao Formation and T₂lmt: Muong Trai Formation) limestone which mainly distributes in Northwest Vietnam, is the most predominant karst strata. Karstification in northwest region is benefited from local geological, climatic, tectonic and stratigraphical factors. Denser vegetation and thicker soil cover in this region also make karstification more intensive.

Northern Vietnam (including Northwest Vietnam) is located at the juncture of two active tectonic belt in global scale, the Mediterranean (Tethys) and the Circumpacific. This region is characterized by successive phase of rifting and collision, as a result most carbonate rocks suffered from folding, faulting, cracking and under influence of differential neotectonic movements. Some hot springs outflow in active tectonic belts, such as Muong Tai - Pi Toong, Son La Province and Kenh Ga, near Ngo Quan, Ninh Binh Province. This zone is uplifted towards northwest and went down towards southeast. In consequence of that, we can find many types of karst landscape, which are formed by autogenic, allogenic agent or both of them (such as karst underground water, surface water, salt water or rainfall). Main karst morphological types in Northern Vietnam change from northwest to southeast regularly. The transition from Karst Plateau (Moc Chau, Son La, altitude 620-940 m), Peak Cluster Doline (transition zone between Moc Chau and Muong Khen, or between Son La and Da River, alt. 650-1100 m), Peak Forest Valley (from Muong Khen to Ngo Quan, alt. 160-70 m), to Isolated Peak Plain (Ngo Quan northwards, alt. Below100m, Photo 2) is very clear. This sequence of karst landscapes is quite similar with those of South China, where the same spectra of karst morphological change can be recognized from Guizhou Plateau to Guangxi Plain, but Vietnam enjoys its unique characteristics, such as a broader isolated peak plain along its coastal zone, more arable karst land on the karst corrosion plateau and less or beginning of rock desertification in karst area.

Besides the macroscopic karst landforms as mentioned above, a series of minor karst features have also been seen in some karst areas.

(1) Sharp microkarrens and karrens have been seen in every karst area, especially near Mai Chau district where karren develops along carbonate rocks bedding plane that is almost in a vertical dip angle. Coastal karrens are very typical in the Ha Long Bay, Quang Ninh Province, where karst karrens are in two types, rock surface karren and sea water corrosion notches (Photo 1), the latter is probably formed by the mixing corrosion process of salt and fresh water.

(2) Karrenfield are often found in open valleys where thin soil cover exists and assemblage of karrens looks like "stone sea".

(3) Through Valleys are formed by allogenic input when underground cave systems are impossible to drain the whole water coming from non-karst areas.

(4) Blind Valleys are also formed by allogenic input but underground cave systems are possible to drain the whole water coming from non-karst areas.

(5) Poljes are widely developed in different karst zone. Most of them are not larger than 100 km², usually from 3-5 km² to 40-50 km², and range in elevation from less 10 m in coastal zone to over 1500m at Sin Ho. The genetic types of poljes are mainly as follows: border polje, structural polje and base level polje.

Vietnam has superior condition for karstification, such as rich rainfall, higher annual temperature, thicker soil and denser vegetation cover, thick and pure carbonate rocks that were modified strongly by tectonic movement, and rich allogenic water coming from non-karst areas or surface river. Therefore, more intensive karstification are observed in northern Vietnam.

(1) Higher carbonate hardness. The HCO₃^- concentration of karst springs in Vietnam ranges from 4.60 to 8.57mmol/l, but it is usually lower than 6.00mmol/l in Southwestern China.

(2) Very thick travertine tableland (Photo 3) shows active karstification in geological period. Along the Road No.6 returning Hanoi, a continual old travertine tableland can be seen from Yen Chau village (256km to Hanoi, 300m in elevation) to Du Lang village (211km to Hanoi, 700m in elevation). It is supposed that some over saturated karst water flowed out with CO₂ outgassing and tufa deposition. Then regional uplifting made travertine become tableland which distributed on steep slope and higher position.

(3) The hydrochemistry of calcareous tufa deposits. Near Du Lang village as mentioned before, the hydrochemical content of a karst spring water changing quickly as it flows down the steep slope, i.e., the HCO₃^- and pH value are 8.57mmol/l and 6.40 at the point 50m above the highway, but change rapidly to 6.61mmol/l and 7.55 at the road side. Much tufa deposits on the way of karst water downflow. We also measured soil CO₂ concentration at the side of this spring where soil layer is loose and dry. The data (8000ppm in 10cm deep) shows that higher soil CO₂ content is very favorable to karstification.

Karst is a fragile environment that will be very difficult to recover if once deteriorated. In South China, some serious karst environmental problems already happened and restricted local economic development. The cases are taking place in Northwest Vietnam karst area.

(1) Deforestation, soil erosion and rock desertification. Such problem is in its initial stage in NW Vietnam. It comes from the fragility of karst environment (very thin soil, and leakage of surface water) and unreasonable land use, i.e., to cultivate land on steep slope (steeper than 25° ) without terracing, to destroy forest for farming. More than 10 years ago, the vegetation cover in NW Vietnam is better, and then the forest began to degenerate and slight rock desertification occurred in some places (Photo 4) owing to the construction of Hua Binh Reservoir, and more convenient traffic. Unreasonable land use led to the loss of soil, the leakage of surface water, and accelerating siltation on major rivers that will endanger the Hua Binh Reservoir, and the being planned Pa Vinh Reservoir on Da River.

(2) Drought and Flood. Many underground streams can not only bring about leakage of surface water and drought on one hand, but also serious flood on the other when they are not able to drain away excess water during heavy storm, such as the disaster happened in Son La town on July 27,1991.

(3) Water Pollution. This case already took place at Mai Son polje near Son La, where a sugar manufactory is putting some wastewater into the karst aquifer, leading to the pollution of underground water.

(1) The 5000 years BP sea level notches about 10m in elevation on many isolated karst peaks north of Ngo Quan are very informative for sea level change study (Photo 5). The same phenomenon can be seen on present sea surface at Ha Long Bay.

(2) The cave relics of 12000-7000 years BP Prehistoric man at Dong Nguoi Xua, are very similar with those of Zhengpi Cave in Guilin, China, i.e., human skeleton were buried with shell layer more than 0.5m thick. It shows that they had the similar culture and living style.

(3) The redissolution of speleothem is popular in many caves, such as in Hong Son Moc Cave near Moc Chau, the older speleothem (e.g. flowstone and stalagmite) was dissolved and covered by a new one again (Photo 6). It indicates that paleoclimate had a multiperiodic change.

Photo 1 The isolated Karst Peaks on the sea and the present sea water notches, taken at
              Ha Long Bay, Quang Ninh Province

Photo 2 Typical tower karst on the Isolated Peak Plain northern Ngo Quan, Ninh Binh

Photo 3 An old travertine tableland distributes between Yen Chau and Du Long village,
             19km in  distance along the highway No.6, 320m difference in  elevation

Photo 4 Forest degeneration and the beginning of rock desertification following
             unreasonable land use, taken near Mai Chau village. The similar case can be
             seen from Mai Chau to Moc Chau

Photo 5 The 5000 years BP. sea level notch on isolated peak north Ngo Quan, about 10m in elevation

Photo 6 The redissolution of speleothem, the older speleothem was dissolved and covered by
              a  new one, taken in Hong Son Moc Cave near Moc Chau

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