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Hang Va Cave

The karst in the Quang Binh province of central Vietnam is notable for the extensive series of very large cave passages that drain through the blocks of limestone. Many of the caves are part of the Phong Nha - Ke Bang system. In March 2012, the British-Vietnamese Caving Expedition was fortunate to be able to explore Hang Va.

The entrance to Hang Va is a collapse in the floor of a small steep valley, with cliffs to the north. A descent of about 15m leads into a stream passage that averages 5m high and wide. The cave passage contains a stream throughout the dry season.

Downstream leads after 400m to a sump, we believe this water flows into Nuoc Nut Cave, though it has not been confirmed scientifically.

A small passage entering the Hang Va streamway can be followed up to another small sump. This is believed to carry water from the flood stream that sinks at the bottom of the Wall of Vietnam in Hang Son Doong, about 900m away.

Upstream Hang Va continues for 350m through areas of rock collapse, until a high-level passage opens about 20m above. The streamway continues underneath for a further 300m to an upstream sump. The water comes from Khe Om, a stream which disappears into rocks on the surface.

Inside Hang Va, the upstream high-level passage above the flowstone climb and traverse is much larger that the streamway beneath; it is 20-30m wide and 10m high. Initially the passage slopes up over mud floors in dry gour pools. Further along the pools are partially filled with water. Within the wet and dry gours are the numerous conical calcite formations that have been named as the Tower Cones.

THE CALCITE CONES OF THE TOWER CONE CHAMBER


The cones are spread across the entire passage where it reaches a width of about 85m. There are more than a hundred calcite cones.

They are all very steep, and they reach heights of around 2m. Many of the tops are at the same level, which is the same as the water level in the gour pool before it dried out. They are coated in soft mud, but this is generally only a few millimetres thick over the calcite. When washed clean of the mud, the cones reveal a complicated structure.

There is some uncertainty in exactly how these cones have formed. Their characteristics are comparable with some features of cones, recorded in other caves, that are either raft cones or geysermites, but many different processes have created their present structure.

RAFT CONES AND TOWER CONES


The cones of Hang Va have many features of raft cones. These calcite rafts form as a thin layer on the surface of cave pools. They sink into the pool either under their own weight when they grow too thick, or when hit by a drop of water falling from the cave roof. A repeated drip from the same place sinks a number of rafts that then build up on the pool floor to create a cone, which may reach to the level of the water surface. These are known as raft cones, because they are cones formed from calcite rafts.

Steep sided cones are more often called tower cones. It is not known exactly why some cones form very steeply, and other with a more shallow slope.

A striking feature of the tower cones in Hang Va, and those in many other caves, is the external layer of calcite that gives them a knobbly appearance and also hides any structure remaining from the original piles of rafts.

TUFA TOWERS AND GEYSERMITES


The knobbly outer surface and very steep profiles of the cones in Hang Va are similar to the features of tufa towers. These form over vents of geothermally warmed water, full of dissolved calcite, that lie in the floors of lakes and cause rapid, underwater precipitation of the calcite. Again they are exposed only when the lake level declines. Those of Lake Abhé in Djibouti are the best known examples (Waltham, 2005). Many of these rise to heights of more than 20m.

Features comparable to tufa towers but formed inside caves are known as geysermites.

The one available cross-section of a cone from Hang Va reveals a porous core with upward-pointing branches, a structure that would be expected in a growing tufa tower, or geysermite.

The different pieces of evidence for the origins of the Tower Cone Chamber appear to disagree. In general, the site appears to be a very fine collection of raft cones, perhaps best described as tower cones because of their very steep profiles. It is clear that they were formed in gour pools before the natural drainage and disappearance of the pools. But conical structures formed by rising water can be also be very similar in appearance.

To date these cones are the only example of this type of formation known in Vietnam, but new discoveries of major passages and spectacular calcite decorations continue to be made in the remarkable Quang Binh karst.

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