Limestone is one of the most common sedimentary rocks found in New Zealand. Eye-catching features such as caves, sinkholes and spectacular skyline landscapes are often associated with limestone formations. These landforms have developed through the interaction of rocks, water and climate.
Over time, all surface rocks are subject to weathering and erosion, mostly of a mechanical nature. In the case of limestone, it is the chemistry of the calcium carbonate component that plays an additional role in giving rise to the often spectacular features that are found with limestone formations.
Limestone – the fizzy rock
The percentage of calcium carbonate in limestone makes it vulnerable to dissolution in dilute acid due to the basic nature of the carbonate ion.
| CO32-(aq) carbonate ion | + | 2H+(aq) acid solution | → | CO2(g) carbon dioxide | + | H2O(l) water |
When strong acid is placed on a sample of limestone, a fizzing reaction takes place.
| CaCO3(s) calcium carbonate | + | 2HCl(aq) hydrochloric acid | → | CaCl2(aq) calcium chloride | + | CO2(g) carbon dioxide | + | H2O(l) water |
Rainwater is weakly acidic due to the presence of carbon dioxide in the air.
| CO2(g) carbon dioxide | + | H2O(l) water | → | H2CO3(aq) carbonic acid |
When weakly acidic rainwater comes into contact with limestone, it slowly reacts with the calcium carbonate present. This reaction produces a solution of calcium bicarbonate.
| H2CO3(aq) carbonic acid | + | CaCO3(s) calcium carbonate | → | Ca(HCO3)2(aq) calcium bicarbonate solution |
Over time, it is this reactivity with rainwater and groundwater, resulting from rainfall, that helps to shape and mould limestone formations into the dramatic landscapes we see.
Karst landscapes
The word ‘karst’ comes from a German word used to describe the stony limestone landscape present in the north-western Dinaric Alps in the country known as Slovenia. It is the distinctive surface and underground landforms such as caves, fluted rock outcrops and dolines or sinkholes that the term ‘karst’ implies.
Limestones that are relatively pure and hard with a high percentage of calcium carbonate are the ones that tend to develop the best karst features. In New Zealand, the largest areas of karst are in the Port Waikato-King Country area, north Westland and north-west Nelson. Typical karst features include caves, fluted rock outcrops and dolines or sinkholes.
Caves are always found in karst environments since they provide the passageways for underground water drainage. Caves often develop stunning features such as stalactites, stalagmites and columns. The formation of these structures is the reverse of the dissolution of the calcium carbonate with acidic rainwater.
| Ca(HCO3)2(aq) | → | CaCO3(s) | → | CO2(g) | + | H2O(l) |
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Fluted rock outcrops have been formed as a result of the chemical action of rainwater on the limestone. As the rainwater flows over and down the limestone outcrop, it erodes the surface, resulting in the outcrop taking on a grooved or fluted appearance.
A karst landscape’s surface is often dotted with bowl-shaped hollows called sinkholes or dolines. They vary in size but are most often up to 100 metres in diameter and tens of metres deep.
Skyline features
Spectacular skyline landscapes are also found with limestone formations. Weathering and erosion along with surface uplift have resulted in features such as escarpments, cliffs and deep gorges.
