Story: Limestone country
Page 2 – Karst
Water dissolves rock
Limestone and other carbonate rocks (dolomite and marble) are highly soluble in rainwater. Rainwater contains dissolved carbon dioxide, which makes it a weak acid known as carbonic acid (H2CO3). It becomes even more acidic as it percolates through soil, because there is also carbon dioxide in gaps in the soil – up to 100 times more than in the atmosphere.
This percolating water is very corrosive towards limestone. As it seeps into fissures in limestone, it gradually enlarges them by dissolving the rock.
In this process, the relatively insoluble calcium carbonate changes into calcium bicarbonate, which is much more soluble. This change is shown in the formula:
CaCO3 + H2O + CO2 = Ca(HCO3)2
(limestone + water + carbon dioxide = calcium bicarbonate)
The term karst referred originally to the limestone landscape of the Karst area, near Trieste around the Italy–Slovenia border. Large rivers disappear underground, and there are many caves, enclosed depressions, fluted rock outcrops, periodic lakes, subterranean rivers, and large springs. In Roman times the region was called the Carsus. In the 19th century, when it was part of the Austro-Hungarian empire, the word was changed to karst.
Over time, water erodes narrow fissures in limestone, and they can develop into very large caves. The land surface can become pocked with hollows, known as dolines or sinkholes.
This process of dissolving rock and transforming the landscape into sinkhole country with caves and underground rivers is known as karstification. It is named after the Karst area, around the Italy–Slovenia border. Names of individual landforms in this area are now standard scientific terms.
- Dolines are enclosed depressions up to a few hundred metres in diameter (American and engineering literature also calls them sinkholes).
- Polje are large flat-floored depressions that often flood (named after the word for a field in Slavonic languages).
- Karren are fluted rock outcrops.
- A ponor is a place where a stream sinks underground (sometimes called a swallow hole or stream-sink in English).
Not all areas of limestone develop karst features. These usually form where the limestone or marble is relatively pure, rainfall is high and the land is hilly.
Karst in New Zealand
New Zealand has a number of karst landscapes. The most well-known are around Waitomo in the North Island and the marble country of Mt Owen, Mt Arthur and Tākaka Hill in the South Island.
Some plants are specially adapted to grow in limestone soils, including two threatened limestone wheatgrasses (Australopyrum calcis subspp. calcis and optatum). In 1997 a new fern species, Asplenium cimmeriorum, was described growing around cave entrances and on limestone walls at Waitomo and in the Ōpārara valley, north of Karamea on the West Coast.
Formation of karst rocks
The main rocks in New Zealand’s karst landscapes are limestones and Ordovician-age marble (490–443 million years old). During the mid-Oligocene to early Miocene periods (32–22 million years ago), limestones were deposited over most of the Northland peninsula and along the east and west coasts of what are now the North and South islands.
Over the past 10 million years there has been active tectonic deformation along the boundary of the Pacific and Australian plates. In the last 5 million years, the land has been uplifted. Erosion has reduced limestone outcrops to discontinuous patches, widely distributed along the length of the country.
Marble from the Ordovician Period is found mainly in north-west Nelson, although there is also some in Fiordland.
There are also limestones from the Miocene (23.8–5.3 million years ago), Pliocene (5.3–1.81 million years ago) and Pleistocene (the past 1.8 million years) periods, especially along the North Island’s east coast. These tend to be poorly cemented shelly limestones.