Due to a simultaneous combination of ideal factors such as thick, pale, grey, and strong limestone layers, which are formed by fine-grained materials; hot and moist climate and slow tectonic process as a whole; Hạ Long Bay has had a complete karst evolution for 20 million years. There are many types of karst topography in the bay, such as karst field.
Hạ Long Bay is a mature karst landscape developed during a warm, wet, tropical climate. The sequence of stages in the evolution of a karst landscape over a period of 20 million years requires a combination of several distinct elements including a massive thickness of limestone, a hot wet climate and slow overall tectonic up lift. The process of karst formation is divided into five stage is the formation of the distinctive do line karst. This is followed by the development of fengcong karst can be seen in the groups of hills on Bo Hon and Dau Be Inland. These cones with sloping side average 100m in height with the tallest exceeding 200m. Fenglin karst is characterised by steep separate towers. The hundreds of rocky islands with form the beautiful and famous landscape of the Bay are the individual towers of a classic Fenglin landscape where the intervening plains have been submerged by the sea. Most towers reach a height of between 50 and 100m with a height to width ratio of about 6. The karst dolines were flooded by the sea becoming the abundance of lakes that lie within the limestone islands. For example, Dau Be island at the mouth of the Bay has six enclosed lakes including those of the Ba Ham lakes lying within its fencong karst. The Bay contains examples of the landscape elements of fengcong, fenglin and karst plain. These are not separate evolutionary stages but the result of natural non – uniform processes in the denudation of a large mass of limestone. Marine erosion created the notches which in some places have been enlarged into caves. The marine notch is a feature of limestone coastline but, in Ha Long Bay, it has created the mature landscape.
Within Ha Long Bay, the main accessible caves are the older passages that survive from the time when the karst was evolving though its various stages of fengcong and fenglin. Three main types of caves can be recognized in the limestone islands (Waltham, T. 1998):
- Remnants of old phreatic caves
- Old karstic foot caves
- Marine notch caves
The first group of caves is old phreatic caves which include Sung Sot, Tam Cung, Lau Dai, Thien Cung, Dau Go, Hoang Long, Thien Long. Nowadays, these caves lie at various high levels. Sung Sot cave is on Bo Hon island. From its truncated entrance chambers on allege high on the cliff, a passage of more that 10m high and wide descends to the south. Tam Cung is a large phreatic fissure cave that developed in the bedding planes of the limestone dividing the fissure cave into three chambers. Lau Dai is a cave with a complex of passages extending over 300m opening on the south side of Con Ngua island. Thien Cung and Dau Go are remnants of the same old cave system. They both survive in the northern part of Dau Go island at between 20 and 50m above sea level. Thien Cung has one large chamber more that 100m long, blocked at its ends and almost subdivided into smaller chambers by massive wall of stalactites and stalagmites. Dau Go is a single large tunnel descending along a major set of fractures to a massive choke.
The second group of caves is the old karstic foot caves which include Trinh Lu, Bo Nau, Tien Ong and Trong caves. Foot caves are a ubiquitous feature of karst landscapes which have reached a stage of widespread lateral undercutting at base level. They may extend back into maze caves of stream caves draining from larger cave systems within the limestone. They are distinguished by the main elements of their passages being close to the horizontal and are commonly related to denuded or accumulated terraces at the old base levels. Trinh Nu, which is one of the larger foot caves in Ha Long Bay with its ceiling at about 12m above sea level and about 80m in length, was developed in multiple stages. Bo Nau, a horizontal cave containing old stalactite deposits, cuts across the 25o dip of the bedding plane.
The third group is the marine notch caves that are a special feature of the karst of Ha Long Bay. The dissolution process of sea water acting on the limestone and erosion by wave action crates notches at the base of the cliffs. In advantageous conditions, dissolution of the limestone allows the cliff notches to be steadily deepened and extended into caves. Many of these at sea level extend right though the limestone hills into drowned dolines which are now tidal lakes.
A distinguishing feature of marine notch caves is an absolutely smooth and horizontal ceiling cut through the limestone. Some marine notch caves had been not formed at present sea level, but old sea levels related to sea level changes in Holocene transgression, event to Pleistocene sea levels. Some of them passed preserved the development of old karstic foot cave in mainland environment or preserved the remnants of older phreatic caves. One of the most unusual features of Ha Long Bay is the Bo Ham lake group of hidden lakes and their connecting tunnel – notch caves in Dau Be island. From the island’s perimeter cliff a cave, 10m wide at water level and curving so that it is almost completely dark, extends about 150m to Lake 1. Luon cave is on Bo Hon island and extends 50m though to an enclosed tidal lake. It has a massive stalactite hanging 2m down and truncated at the modern tidal level. It has passed though many stages in its formation.
The karst landscape of Ha Long Bay is of international significance and of fundamental importance to the science of geomorphology. The fenglin tower karst, which is the type present in much of Ha Long Bay, is the most extreme form of limestone landscape development. If these karst landscapes are broadly compared in terms of their height, steepness and number of their limestone towers, Ha Long Bay is probably second in the entire world only to Yangshou, in China. However, Ha Long Bay ha also been invaded by the sea so that the geomorphology of its limestone is lands are, at least in part, the consequence of marine erosion. The marine invasion distinguishes Ha Long Bay and makes it unique in the world. There are other areas of submerged karst towers which were invaded by the sea, but none is as extensive as Ha Long Bay.