A Million-Year Journey
Halong Bay , a natural wonder of magnificent beauty, has a history that spans over 500 million years. Through countless changes and millions of years of erosion, the bay has transformed into the breathtaking landscape we see today. Once towering at over 1,000 meters tall, the ancient marine limestone rocks have been gradually reduced in size.
According to researchers and respected geologists, Halong Bay as we know it was formed approximately 8,000 years ago during a period known as 'The Middle Holocene Transgression'. This refers to a time when the sea level rose significantly, resulting in higher shorelines and widespread flooding.
Limestone mountain in Halong bay.
The prominent limestone karsts found in Halong Bay and its surrounding areas are primarily composed of thick limestone. This limestone formation began during the Carboniferous period, around 340 million years ago. At that time, the Earth was much warmer and wetter, and carbonate sediment from large swamps accumulated in the oceans. Over millions of years, this sediment gradually hardened and thickened, resulting in the formation of the 1,200-meter thick limestone karsts that mesmerize visitors today.
Thanks to millions of years of erosion from rain and the sea, Halong Bay is also home to unique valleys and an impressive maze of caves. The oldest of these caves, known as 'Phreatic Caves', are predominantly found at higher levels. One of the most renowned among them is the aptly named 'Sung Sot' cave, meaning 'Surprise cave'. Sung Sot cave is not only one of the largest but also one of the most magnificent caves in Halong Bay.
Sung Sot (Surprise) cave
Geological Feature
Another captivating geological feature of Halong Bay is the Ba Ham lake system. These underground lakes are interconnected through hidden caves and have been entirely shaped by sea erosion. Above these lakes, hanging stalactites can be found. Stalactites form when water containing calcium bicarbonate drips from the ceiling of a limestone cave. Over thousands of years, these deposits accumulate, resulting in the formation of perfect stalactite and stalagmite pairs.
Kayaking through Ba Ham lake (Photo: Shutterstock)
According to Dr. Tran Duc Thanh from the Haiphong Institute of Oceanology and Dr. Tony Waltham, a geologist at Nottingham Trent University, the landscape of Halong Bay holds "international significance and is of fundamental importance to the science of geomorphology." The karsts found in Halong Bay serve as a magnificent example of the development of a limestone landscape over millions of years. The extreme sea erosion caused by the marine transgression makes Halong Bay truly unique. It is imperative that we appreciate and preserve this invaluable part of our world heritage.
Source: Thanh, T. D, N. Can and D. D. Nga. 1997. Coastal development of Do Son - Ha Long Area during Holocene. In: Marine Geology and Geophysics. Sci and Tech. Pub. House. Ha Noi. p. 199 - 212
