Limestone is a sedimentary rock (rock laid down as layers). It consists mainly of shells of tiny marine fossils made of lime (calcium carbonate, CaCO3). All limestones also contain some sand or mud.
Rocks with more than 50% calcium carbonate are classified as limestones. With less than 50%, they are called a calcareous sandstone or mudstone.
Limestone outcrops are scattered in patches along the length of New Zealand. They occur mainly east and west of the main mountain ranges – not high in the mountains.
There is a good reason for this. Most limestones are made of shell fragments and lime muds, originally deposited in shallow seas. They were compressed and cemented together after sediments (such as siltstones and sandstones) were deposited on top. Then tectonic forces folded, fractured and uplifted the limestone. Areas that were uplifted first were eroded first. Limestone that once extended across mountain tops has been worn away, but remnants often survive on areas that were not lifted as high (for example Castle Hill in Canterbury).
Sometimes limestone also contains magnesium carbonate. The double carbonate of calcium and magnesium is called dolomite – CaMg(CO3)2. In New Zealand, dolomite is only found in the small area of Mt Burnett, near Collingwood in Golden Bay.
When limestone is subjected to considerable pressure and heat, it recrystallises into marble. Fossils disappear in the process, but chemically the rock remains the same. Marbles occur in a discontinuous belt up to 7 kilometres wide for about 90 kilometres in north-west Nelson, extending from Mt Owen and Mt Arthur through to Tākaka Hill.
Limestone, dolomite and marble are collectively known as carbonate rocks. Carbonate rocks with more than 10% sand, silt or mud are considered impure. Karst develops best in pure limestones and marbles. In New Zealand most karst is found in rocks with less than 5% impurities.
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.
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.
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.
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.
The rate at which limestone dissolves depends on the amount of rainfall and the concentration of carbon dioxide in the water.
In the open atmosphere, carbon dioxide has a concentration of about 0.03% by volume. However, in gaps in the soil, concentrations are often 2%, and can even reach 10%. As rainwater runs through soil, its carbon dioxide content increases. A hundredfold increase in carbon dioxide concentration means that limestone will dissolve about five times as fast.
In terms of rates of weathering, the amount of rainfall is even more significant than carbon dioxide concentration. The wettest places in the world have the fastest rate of karstification (formation of limestone caves and other features). As New Zealand is relatively wet, there is plenty of water to dissolve karst rocks.
These rates are moderately high by world standards.
Most dissolving of limestone happens just beneath the soil. This is where carbon dioxide is generated by soil microbes, so percolating water has its highest level of carbon dioxide. Some 90% of dissolving can occur in the top 10 metres or so of the limestone outcrop. The heavily corroded rock layer beneath the soil is known as the epikarst.
Fissures are widest near the surface, and taper with depth, usually reaching about 10 metres. Rainwater drains in much more easily than it drains out, and after heavy rain, water accumulates at the base of the epikarst. This water can remain there for months, eroding the rock.
Karstification makes limestone both more porous and permeable, so it has a great capacity to store groundwater and to let water flow through. Porosity is a measure of the gaps (pores and cracks) in the rock; permeability measures the ease with which water can flow through these gaps.
There are three types of porosity.
Porosity is highest in the youngest rocks, including the Pliocene shelly limestones of Hawke’s Bay and the Wairarapa (5.3–1.81 million years old). In the older, widespread Miocene–Oligocene limestones (32–22 million years old), the rock’s primary porosity is usually less than 2%. In the marble country of north-west Nelson, it is almost zero.
High secondary and tertiary porosity is found in the Miocene–Oligocene limestones that contain plenty of caves. Tertiary porosity in marble, where water enlarges fractures into caves, helps form aquifers (underground layers of water-soaked rock). The classic example is aquifer-fed Waikoropupū Springs in the Tākaka valley, New Zealand’s largest freshwater spring.
Caves are an integral part of limestone country. Sometimes they are too narrow for humans to enter, or access is blocked by soil. In the few million years that New Zealand carbonate rocks have been subjected to karstification, there has been ample time to develop large caves.
Many kilometres of cave passages have formed in the marble country of north-west Nelson. The total explored length of passages in the Ellis Basin system on Mt Arthur is close to 29 kilometres. The total in Bulmer Cavern beneath neighbouring Mt Owen is almost 51 kilometres. Some of these passages originally formed below the water table, but are now high and dry. They are more than 700,000 years old.
Nettlebed Cave on Mt Arthur has a total depth of 889 metres, and Bulmer Cavern is 749 metres deep.
In limestones in other areas, caves are shorter and shallower. The longest in the Waitomo region is Gardner’s Gut Cave at 12.2 kilometres. Other long caves are Honeycomb Hill Cave (13.7 kilometres) and Megamania (14.8 kilometres), both north of Karamea on the West Coast.
Cave formations known as speleothems develop where percolating water deposits calcite (calcium carbonate) over thousands of years. These include the well-known icicle-like stalagmites (which grow from the ground up) and stalactites (from the roof down), as well as fragile straws and odd-shaped helictites (twisting, curling straws).
Flowstone floors develop where water seeps over the rock, leaving calcite deposits that look like flowing water. Rimstone dams form where water has deposited calcite on the edge of a pool or stream.
Rivers and streams that disappear underground and then re-emerge in another spot are common in karst landscapes. The place where a river re-emerges is called a resurgence.
On Mt Arthur the water table lies about 900 metres beneath the upper slopes. Its level is controlled by the main outflow spring, the resurgence of the Pearse River. Divers in the Pearse Resurgence have found that flooded passages reach downwards well beyond the current limit of exploration (177 metres below the water surface). The marble continues to an unknown depth.
The level of the water table changes with the season and with storms. In some karsts, such as at Cave Creek near Punakaiki on the West Coast, the level can change by up to 75 metres. This presents serious safety hazards for explorers in Xanadu Cave, part of the Cave Creek groundwater system.
Māori used limestone caves for shelter and burial, and drew pictographs on the walls. Modern cavers have explored many subterranean passages, especially around Waitomo, Mt Owen and Mt Arthur. New discoveries continue to be made. Black-water rafting (rafting through cave streams) was first developed in Ruakurī Cave near Waitomo in 1987. Some limestone outcrops, such as those at Castle Hill in Canterbury, are also important rock-climbing locales.
The marble landforms around Tākaka have inspired some impressive landscape works by New Zealand artists, including Colin McCahon’s 1948 painting ‘Takaka: night and day’. Painter Leo Bensemann also took the karst landscapes around Tākaka, his birthplace, as a theme.
Karst landscapes also attract revellers. Starting in 1997, and for a number of years after, a huge New Year’s Eve dance party known as The Gathering was held at Canaan Downs on top of Tākaka Hill. The saucer-shaped depressions (dolines) were used as amphitheatres.
Before rainwater drains underground, it flows across and corrodes rocky outcrops on the surface. Outcrops form into sculptures of vertically fluted rock and widely opened joints, collectively known as karren. These are particularly common above the treeline, where soil and plants are thin or absent. One of the best places to see these landforms is from the main road on Tākaka Hill, between Nelson and Golden Bay. Karren also develop beneath soil and vegetation, where they are smoother.
In some areas, bedrock has been scoured by glaciers and stripped of loose debris, leaving a flat surface. Joints have opened up and been cut downwards by the dissolving force of water. The resulting landforms are called limestone pavements. Large areas of these, especially above the treeline, are called karrenfields. These are common on Mt Owen and Mt Arthur. Mt Owen’s karrenfields featured in the film trilogy The lord of the rings.
The Waitomo Caves are New Zealand’s most popular tourist caves. Wai means water in Māori, and tomo is a hole in the ground – a word that cavers have adopted for potholes or shafts. In 1887, Tāne Tinorau and Fred Mace floated on a raft of flax flower stalks into the Waitomo glow-worm cave. The caves soon became popular with tourists, who were guided by local Māori. In 1905, the government’s Tourist Department took over the caves’ management.
A karst landscape’s surface is often pocked with closed depressions, especially the bowl- or saucer-shaped hollows called dolines. These are usually up to 100 metres in diameter and tens of metres deep. When dolines take up all the available space, the surface looks like an enormous egg tray, and is known as polygonal karst. At Waitomo, the polygonal karst has 55 hollows per square kilometre.
Sometimes the ground over a cave collapses, forming a crater at the surface. These are known as collapse dolines. In New Zealand they are sometimes called tomos, although this term also applies to collapse pits in soil.
Streams that flow from other rocks onto neighbouring limestones often dissolve the limestone and sink underground. Their valley may finish abruptly, sometimes against quite a steep slope. These are known as blind valleys.
Lake Disappear, near Raglan in the western North Island, is a large enclosed hollow (polje) drained by an underground stream. The lake fills after heavy rain, when the flow of incoming water is too great for the stream-sink to drain. In drier weather, it empties and disappears.
Lipyeat, Moira, and Les Wright. Delving deeper: half a century of cave discovery in New Zealand. Waitomo: New Zealand Speleological Society; Christchurch: Hazard, 2003.
Potton, Craig, and Andy Dennis. Images from a limestone landscape: a journey into Punakaiki–Paparoa region. Nelson: Craig Potton, 1987.
The New Zealand cave atlas: North Island. Waitomo: New Zealand Speleological Society, 1988.
The New Zealand cave atlas, volume 2: South Island. Waitomo: New Zealand Speleological Society, 2004.
Williams, P. W. ‘Karst hydrology.’ In Waters of New Zealand, edited by M. Paul Mosley, 187–206. Wellington: New Zealand Hydrological Society, 1992.
Williams, P. W. ‘Karst in New Zealand.’ In Landforms of New Zealand, edited by J. M. Soons and M. J. Selby, 186–209. Auckland: Longman Paul, 1992.