Limestone landscapes
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.
King Country limestone
Eminent sedimentary geologist Professor Cam Nelson from the University of Waikato, on site in the King Country, explains how Oligocene limestones express themselves in the New Zealand landscape. Limestones are often harder than the surrounding rocks and often stand out in a landscape, sometimes in big proud cliffs like the ones featured at this location.
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.
Dolines
This cratered landscape in South Canterbury is actually a series of dolines or sinkholes. Sinkholes, like other karst landscape features, are caused by slightly acidic water eating away at limestone over millions of years.
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) calcium bicarbonate solution
→
CaCO3(s) calcium carbonate solid
→
CO2(g) carbon dioxide
+
H2O(l) water
Karst landscapes diagram
Diagram showing some of the features and relationships associated with karst landscapes typically found in new Zealand.
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.
Features of the karst landscape
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.
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.
