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Notable geological value of Halong Bay

Halong Bay Vietnam

Halong Bay

The landscape of Halong Bay is nothing less than spectacular. As far as the eye can see, islands and islets loom grey, blue and green on the horizon. The sea, emerald and azure, is deep and mysterious. The sheer walls of limestone, grey and black from the weather and topped with vegetation, support a diverse ecosystem. And despite being used by humans over a long history, the area is remarkably well-preserved.

First inscribed by UNESCO as a naturally significant site worth preserving in 1994, Halong Bay had the honour of being recognised again in the year 2000 for its geological and geomorphic features.

While this recognition is relatively recent, Halong has been eons in the making. Interestingly, the Halong Bay we know today is young — geographically speaking — having been shaped around 6,000 years ago by rising sea levels.

For billions of years, the deep water of the bay was lined with a layer of fine mud and sediment one kilometre thick that didn’t shift, even with rising and falling sea levels. It was in the Carboniferous period — during the late Paleozoic Era and some 300 million years ago — that the heavy tectonic activity caused the area to shift and uplift.

The shallower and warmer water of the bay formed the limestone that makes Halong so unique.

Limestone, largely composed of calcium carbonate, is a soft rock made from mud, sediment, plants and organic matter and is porous, allowing moisture to seep through. It is easily eroded by water, including rainfall, ground water and the ocean. When water, for example rain, comes in contact with the limestone, acid is formed. The water percolates in the cracks of the limestone and gradually dissolves the limestone and washes it away. The tropical climate of Vietnam — and South East Asia — creates the perfect conditions for this erosion and weathering to occur.

During the Jurassic and Cretaceous Periods — around 200 million years ago — the area shifted and uplifted again and became mountainous highlands, exposing the limestone to the elements. The limestone eroded to shape the karst landscape of Halong Bay, forming millions of small caverns and canyons. Flooding caused the bay to collapse and, over time, the limestone dissolved and crumbled into the sea, turning caves and tunnels into deep valleys between rising columns of rock.

The karst landscape for which Halong is known for consists of clusters of conical peaks (Fencong karsts), isolated tower features (Fenglin karsts), and lakes that occupy drowned dolines — a key feature of Fencong karsts. Weathered pillars and towers, some 100 metres in height, rise out of the sea, with the erosion forming stunning arches, caves and grottos. Travellers to Halong Bay can also marvel at the abundance and diversity of caves — phreatic, old karstic foot caves, and marine notch caves — the cool, subterranean beauty formed over millions of years by unusual geomorphological processes.

Halong Bay’s value is essentially as a living museum, where the full range of karst formations are displayed on a large scale. It is the most complete and extensive example of its type in the world, and supplies geologists with a rich supply of data on karst formation over time, and in a complex geoclimatic environments.

Continued UNESCO recognition cements the uniqueness and value of Halong Bay as a site of geological significance. And it’s no wonder. Halong is an ode to the power of Mother Nature’s artistry as a sculptor.

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