Halong Bay Geology: How Was Halong Bay Formed?

A Million-Year Journey

The formation of Halong Bay dates back over 500 million years. Over the millennia, the bay has undergone numerous changes that have culminated in the stunning natural wonder we see today. The magnificent ancient marine limestone formations once stood at heights exceeding 1,000 meters. However, extensive erosion over millions of years has shaped them to their current stature.

Recent research indicates that the Halong Bay as we recognize it today originated around 8,000 years ago during a geological period known as the Middle Holocene Transgression. This era marks a 'marine transgression' when sea levels rose significantly higher than they are today, leading to altered shorelines and widespread flooding.

Limestone mountain in Halong Bay.

Halong Bay, along with its adjacent areas, is primarily composed of thick limestone, mirroring the famous limestone karsts that rise majestically from the bay's waters. The limestone found in Halong Bay began its formation during the Carboniferous period, approximately 340 million years ago. At that time, the Earth experienced much warmer and wetter conditions, resulting in significant carbonate sedimentation in the oceans due to extensive swamps. This process led to the gradual hardening and thickening of limestone over millions of years, achieving a thickness of about 1,200 meters, which forms the iconic karsts we admire today.

Thanks to millions of years of erosion caused by rain and ocean waves, Halong Bay is also home to unique valleys and an intricate network of caves. Among the oldest caves are the Phreatic Caves, typically found at higher elevations. These caves include the renowned Sung Sot cave, meaning "Surprise Cave." Sung Sot is celebrated as one of the largest and most awe-inspiring caves in Halong Bay.

Sung Sot (Surprise) cave

Geological Feature

Another remarkable geological feature within Halong Bay is the Ba Ham lake system. The Ba Ham lakes are a cluster of underground lakes interconnected by concealed caves, formed solely through the process of sea erosion. Above these lakes hang stunning stalactites - formations that develop when water infused with calcium bicarbonate drips from the ceilings of limestone caves. Over thousands of years, these deposits of calcium bicarbonate accumulate, creating perfect pairs of stalactites and stalagmites.

Kayaking through Ba Ham Lake (Photo: Shutterstock)

Experts, including Dr. Tran Duc Thanh from the Haiphong Institute of Oceanology and Dr. Tony Waltham, a geologist at Nottingham Trent University, assert that Halong Bay holds "international significance and is critically important for the science of geomorphology." The karsts found in Halong Bay are an exemplary demonstration of the evolution of a limestone landscape over extensive periods, emphasizing the unique marine transgression that has shaped these formations. It is crucial that we recognize and conserve this heritage site as an invaluable component of global natural heritage.

Source:

• 1. Thanh, T. D., Can, N. & Nga, D. D. 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