Professor Cam Nelson is on site at the Mangapohue Stream in the King Country. Cam explains how the karst landscape in this region has developed from the slow dissolution of limestone rock as a result of exposure to slightly acidic rainwater. The end result over millions of years is a karst landscape with features such as sinkholes, caves, natural bridges and fluting.
PROFESSOR CAM NELSON
A karst landscape forms from dissolving rock, in this case, dissolving limestones. Most landscapes result from the rain, the Sun beating down on them and physically destroying them, but a karst landscape forms from the dissolution, the dissolving of rock, the dissolving of limestone in particular. And here we have an example of karst landscape that’s formed in an old cave system, we’re only about 20 kilometres to the west of the Waitomo Caves, and here is a much smaller ancestral cave system which is formed in the Mangapohue Stream, which is flowing below us here.
The cave system behind us has collapsed in its roof and so it’s now open to the skyline, but right here at the Mangapohue natural bridge, the roof of that cave system still survives, and these are called natural bridges in limestone terrains.
Rainwater is slightly acid. It actually picks up CO2 from the atmosphere, and that makes a mild carbonic acid out of the rainwater. And then when the rainwater actually enters into the soil zone, it can also pick up additional acidity from the organic acids associated with the decay of vegetation and the respiration of plants. So the water that’s flowing through the limestone can be quite acidic, and that acid will dissolve the limestone, breaking the calcium carbonate that limestone’s made up of into its various components. In particular, CO2carbon dioxide is released in that process as the water passes through the limestone. So the acidity of rainwater is certainly the active control on the production of karst features.
There’s a whole raft of terminology associated with karst topographies ranging from the large-scale cave systems to features such as natural bridges, sinkholes, fluting or lapiez weathering
And then the things that go with a karst topography that precipitate calcium carbonate. Again, once it’s dissolved, it can build up in its concentration in the water to such an extent that you can actually start to precipitate calcium carbonate again in things like stalactites and stalagmites – stalactites hanging from the ceiling, stalagmites growing from the ground. There are drip stones where dripping water rich in calcium carbonate starts to reprecipitate the dissolved calcium carbonate. There are flow stones that run down the walls and produce travertine deposits. In fact, stalactites and stalagmites, basically the calcium carbonate that’s precipitated, we wouldn’t really call it a limestone, we call it travertine – it’s still a calcium carbonate-rich rock. So there’s a myriad of these different sorts of micro-karstic features.
Chris Hendy, University of Waikato