A scientific look on the karst process of Halong Bay

04 Aug 2015
A scientific look on the karst process of Halong Bay, photo by Bhaya Cruises

Maybe for you, Halong Bay is just a vast bay with water and limestone, amazingly beautiful but that’s all. But actually, it has been involving in a complex geological karst process since ancient time to create such Halong Bay as today. Knowing how it all become will definitely make you cherish more the critical value of geology and geomorphology of the bay.

1. What is karst?

The carbon dioxide (CO2) in the atmosphere dissolves in water then meets the positive hydrogen ion (H+) to form carbonic acid. Acid contacts the limestone and dissolves some substances in stone, causing the corrosion phenomenon. Natural products of karst process are caves with stalactites, stalagmites and underground rivers.

2. Halong Bay – a perfect karst process over 500 million years of evolution

From 570 to 500 million years ago, Halong Bay is a huge area of land and limestone. Pass by such that long period, Halong Bay has witnessed different ancient geographical conditions before becoming the bay as today. Take a look at this detail table to see which amazing phases that Halong Bay has experienced.

DateGeologic periodEvents
570M - 500M BCbeginning of the Cambrian PeriodThe area, which now forms Halong Bay, was basically mainland, submitted to a process of rain erosion.
 end of the Cambrian PeriodThe area was flooded, commencing the existence of Halong Bay.
500M - 400M BCOrdovician and Silurian periodsThe area of north-east Vietnam was almost like a deep sea, submitted to the constant activity of tectonic plates.
 end of the Silurian PeriodIt underwent a phase of inverse-motion that created mountains deep under the water.
420M - 340M BCend of the Silurian Period and throughout the whole Devonian PeriodThe area was subjected to powerful forces of erosion from the hot and dry climate. At this point, Halong was part of a wide mainland that comprised most of today's South China Sea and Chinese continental shelf.
 end of Devonian PeriodDue to tectonic activity, the Halong area and the entire north-east region were raised from the depths
340M - 240M BClater Carboniferous and Permian periodsThe formation of the limestone layer more than 1,000 m thick. A shallow and warm sea reformed, which existed for approximately 100 million years. It created two kinds of limestone: the Cat Ba layer of the early Carboniferous period (450 m thick); and the Quang Hanh layer of the middle Carboniferous and the early Permian period (750 m thick). These two layers constitute the majority of the islands of the Bay.
67M BCend of the Cretaceous PeriodHalong Bay existed in the environment of a high mountainous mainland due to the influence of strong mountain-forming phases.
 middle of the Paleogene PeriodThese motions remain continuous and stable, while strong processes of erosion began, and after millions of years, a form of semi-highland topography took shape. The continuation of this erosion has progressively cut the highlands into blocks with altitudes similar to today's mountains.
26M - 10M BCNeogene PeriodThe development of the Halong depression.
2M - 9,000 BCPleistocene epoch of the Quaternary periodThe process of erosion began dissolving the limestone-rich region of Halong, after that, forming the limestone plain was most active
68,000 - 9,000 BCmiddle and late Pleistocene EpochPeriod when the caves and grottoes of the area formed.
 early Holocene EpochThe islands of today's Halong Bay are basically remnants of these mountains, flooded. Rainwater flowed into crevices in the limestone that had formed from tectonic activity. This steady erosion constantly widened the cracks, eventually creating today's formations.
9,000 - 5,000 BCHolocene EpochThis period is notable for the advance of the sea.
5,000 - 2,000 BC The movement of the sea reached its peak and forming today's Halong Bay.
2,000 - 1,000 BC With the sea in a steady process of recession, Halong culture began to develop.
 beginning of the late Holocene EpochThe level of the water once again increased, forming a marshy floor of canals and streams, and creating the water marks that can be seen on the stone cliffs of today.

Table Source: Wikipedia

Nowadays, the karst process keep occurring, the corroded bottom of every islets on the bay is a proof for that.

From 570 to 500 million years ago, Halong Bay is a huge area of land and limestone. Pass by such that long period, Halong Bay has witnessed different ancient geographical conditions before becoming the bay as today.

3. The diverse system of islets and caves

If you pay a little attention to the islets, you may realize that they can be divided into 2 different kinds: one is the islets group, having the cone shape, they are called Fengcong karst; the other is the single islet in the tower shape, which is called Fenglin karst.

On the other hand, caves are separated into 3 basic groups. The first group is the ancient underground caves, forming from the rainwater going through the land’s cracks. Sung Sot Cave, Thien Cung Cave, Dau Go Cave are belong to this group.

For the second group, the land is corroded horizontally to form the caves so that you can see their entrances are nearly horizontal like a mouth. Typical caves are: Trinh Nu Cave, Bo Nau Cave, Hang Trong Cave ...

The third group is called cleft cave, formed by chemical corrosion of sea water with abrasive effects of tide and waves, creating a tunnel through the stone as you can see at Ba Ham Lake or Luon Cave...

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