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Rock, limestone and clay

by  Carl Walrond

The energetic suburban handyman working his concrete mixer was for years one sign of a growing nation laying the foundations of the future. The roads, bridges and buildings of the urban environment have been constructed with millions of tonnes of New Zealand’s abundant greywacke, limestone, and other rocks.


Raw materials

Rock (crushed into aggregate), limestone and clay are the bulk products that dominate New Zealand’s industrial mineral output. Millions of tonnes are dug, blasted and gouged from the earth each year. Since the 1950s they have accounted for between one-third and two-thirds of the value of mined and quarried materials.

Cities and roads

In many places rock, limestone and clay are quarried in massive bulk for general uses. Motorways, roads, bridges, high-rise buildings, pavements, brick houses, the foundations of buildings, and the glass in windows – all are made from quarried materials.

For a small country New Zealand has a great variety of rock formations, which lend themselves to specific uses. The most common rock, greywacke, is suited for road and construction work. Some clay deposits are good for bricks. Certain limestones are ideal for cement – one of the constituents of concrete – and they are mined and quarried the country over. Countless tonnes of concrete are used in urban construction.

Phosphate, sulfur and pumice

Many lesser minerals are also mined or quarried in New Zealand, or have been in the past. These include asbestos, dolomite, pumice, serpentine, sulfur and zeolites. Although at one time small amounts of phosphate were mined, the resources were exhausted, and phosphate is now imported for use in agriculture.


Aggregate

Aggregate, which is crushed rock from a mixture of sources, is an essential material in New Zealand’s built environment. The term aggregate includes naturally occurring collections of rock particles like beach pebbles (often used in concrete) and sands, as well as manually crushed rock. The differing shapes, sizes and types of rock can be put to a range of uses, notably in building, roading and other construction projects.

Early uses

When cultivating taro and kūmara (sweet potato), Māori mixed gravel and sand into the soil to improve drainage. To build hāngī (earth ovens), in which they would slow-cook buried food, they used stones known to retain heat. Stones were also used as sinkers for line and net fishing. If a village came under attack, the defenders would hurl stones and roll boulders onto the enemy.

European settlers made use of loose rock, gravel and sand for minor building projects, stone walls and road surfacing before imported explosives made it possible to quarry hard rock. Initially many quarries produced building stone. Then with the development of rail and road construction, and the use of reinforced concrete for building, the demand for aggregate quickly grew. By 1931 quarries were producing over 900,000 tonnes of stone or gravel for road surfacing or railway ballast.

Heavy metal

Quarrying is an explosive business. Today there are high-tech methods for calculating the quantity of explosives when blasting the rock apart. But in the 1940s no one could be exactly sure what would happen. At the Greymouth Harbour Board’s quarry at Cobden, one blast rolled a block of limestone over three times before it came to rest on the quarry floor. Nothing was unusual in this, except that the block weighed 963 tonnes – typically the largest had weighed a mere 27 tonnes.

After the Second World War the demand for raw construction materials boomed. Projects such as roading, harbours and hydroelectric dams all called for more quarry production. In 1964 aggregates overtook coal as the highest-value mined commodity in New Zealand − a position they have retained in most years since.

In the early 2000s aggregates accounted for $400 million of the country’s total mineral production of around $1 billion. The North Island dominates aggregate production, which in 2003 exceeded 34 million tonnes.

Transport costs

Because it is simply crushed pieces of rock, aggregate is high in bulk and low in value. Transport costs are therefore a major factor in their final cost, and for this reason there are many small quarries throughout the country. As proximity to the market is vital, Auckland has large quarries in suburban areas.

Quarry operators are constantly juggling the trade-off between the quality of the rock and the distance to market. If minor road works are planned in an area, the nearest suitable rock face often becomes the quarry. On the West Coast, where flooding can scour away bridge supports, small quarries can be found on both sides of a river, enabling repair work to be done quickly after a flood.

Environmental issues

In the 2000s Auckland, with an expanding population, faced a shortage of good aggregate sources. The best rock sources are lava flows. (The more common scoria, which makes up much of the volcanic cones, is riddled with small holes that reduce its strength.) However, many of the city’s volcanic cones have been quarried – Three Kings, the name given to a trio of distinct humps, looks more like half a king today. There is ongoing tension between those keen to preserve the distinct volcanic landscape and those who want cheaper aggregate.


Sources and uses

New Zealand is lucky: the rock known as greywacke, an ideal source material for aggregates, is widespread in mountain belts. Other widely used rocks are argillite, basalt and andesite. They are quarried throughout the country by drilling and blasting, and then crushed, washed and screened.

Gravel pits

The South Island’s braided rivers, which carry rocks down from the Southern Alps, have helped create a huge, accessible resource on the Canterbury Plains. Most of the weathered and softer rock has been washed away, leaving harder greywacke in the form of river gravels. These are crushed to produce angular surfaces and different sizes and shapes. Most are for bulk use, but some are not suitable for road-sealing chips as they have rounded surfaces.

Return to the stone age

Metal machinery used to crush rock has a high rate of wear and tear. In the late 1960s and early 1970s, quarryman Bryan Bartley and roading engineer Jim Macdonald invented a more durable rock crusher, based on a simple idea – the rocks themselves would break each other into smaller pieces, as they tumbled inside a drum. This reduced the need for metal components. It took 20–30 years to be recognised, but this Kiwi invention proved a worldwide success. Thousands of ‘Barmac’ rock crushers have been used on projects including the massive Three Gorges Dam on China’s Chang (Yangtze) River, and sand manufacture in the deserts of the Middle East.

Roading

Roading aggregate must comply with certain specifications. For example coarser, more angular rock chips are used to increase traction on corners, and on roads such as the Desert Road or in the Kawarau Gorge, which are likely to ice up in winter.

For road foundations clay is helpful to bind rock chips. In the layer on top of the foundation the chips are harder. The top layer of sealing chips must be resistant to chemical attack and rough, so that it does not become polished by tyres passing over it.

In Gisborne and other areas which lack more suitable rock types, limestone is commonly used as a road aggregate, giving the roads a lighter colour. As limestone is a softer rock these roads tend to wear faster.

Concrete

Aggregates are used in concrete, whose other components are cement, sand and water. To ensure the concrete is strong enough, the rocks must also be non-reactive – they must not contain minerals that react with the other constituents of cement. In New Zealand, potentially reactive rocks occur mainly in the volcanic region of the central North Island. The andesite from Mt Taranaki is also too reactive for many uses.

Bulk fill, rip rap and pea metal

Almost any rock is suitable as fill for reclaiming land from the sea. Fist-sized stones become railway ballast, and large rocks, known as rip rap, are used to contain harbours and rivers. Pea metal, made of rock chips about the size of a pea, is used extensively for drainage.

Sand

Silica sands, which are brilliantly white, have in the past been mined at Pārengarenga Harbour for making bottles and other glass products in Auckland.

In the 2000s sand for concrete had to be dredged offshore to supply Auckland’s building industry. Since 1953 about 750,000 cubic metres of sand have been mined from the entrance to Mangawhai Harbour in Northland. This was a contentious issue in the early 2000s, as residents were concerned about coastal erosion.


Limestone

Limestone, a sedimentary rock abundant in New Zealand, consists mainly of the bones and shells of tiny marine fossils made of lime (calcium carbonate). Rocks with more than 50% calcium carbonate are considered to be limestone.

Most New Zealand limestone deposits formed in the Oligocene and Miocene periods, 5–37 million years ago. Around 20 to 30 million years ago, when the climate was warmer and much of New Zealand was submerged under shallow seas, conditions were ideal for limestone formation. Not much sediment from land entered these coastal waters, and layers of shells and bones from billions of sea creatures accumulated on the sea floor. These hardened into rocks, which were eventually uplifted and now form the country’s karst (weathered limestone) landscapes. They are a dramatic sight at Castle Hill on the road to Arthur’s Pass, and form the intriguing Pancake Rocks on the West Coast. Other sites include the Waitomo caves in the King Country, and the rolling land around Ōamaru in North Otago, which is known as ‘white stone country’.

Uses

Limestone is used mainly in a finely crushed form as an agricultural fertiliser, and for roading aggregate. In the early 2000s annual limestone production was valued at around $40 million. There are two large quarries in the Waikato region, and a quarry at Te Kuiti annually produces around 50,000 tonnes of high-grade limestone. Another at Ōtorohanga produces nearly 500,000 tonnes of lime products for varied uses in agriculture, steel making, gold-ore processing, pulp and paper manufacture, and sewage and waste water treatment.

Although there is a lot of limestone in New Zealand, much of it is too hard or fractured to be used for construction. The limestone around Ōamaru is used for building. As it is relatively soft it can be cut in to blocks by huge circular blades.

Burnt lime and agricultural lime

As early as the 1860s lime was quarried and burnt in kilns; lime kilns still dot the countryside on the Otago Peninsula. Burning changes the chemical composition by driving off the carbon dioxide, to leave calcium oxide – known as burnt lime. The burnt lime was then crushed. Because burnt lime is more caustic and concentrated than agricultural lime and unpleasant to handle, it is rarely used in agriculture.

Lime gets in your eyes

Mechanisation has helped improve working conditions in dusty lime-crushing plants over the years. In the 1940s a day’s labour bagging burnt lime at Doherty’s quarry in Southland was far from pleasant. Men would tie rags across their heads to stop the sweat running into their eyes – the burnt lime mixed with sweat and momentarily blinded them.

Agricultural lime is the most common liming material used in farming. It consists of limestone crushed to a very fine powder. It is spread onto paddocks to make acidic soils more neutral and promote the activity of other fertilisers, improving pasture growth. Initially lime was simply spread by the shovel-load from drays, until machines known as lime spreaders were developed in the 1930s.

Limestone production surged in the 1960s, reaching over 3 million tonnes per annum in the early 1970s. It tumbled to around 2 million when the government removed its fertiliser subsidy in 1984. However, by 2002 production had climbed to over 4.7 million tonnes.

Today, lime works operate throughout the country, with much of their output applied to farmland. Where the limestone is hard it has to be blasted, but many deposits are soft enough for bulldozers, fitted with rippers (large picks), to gouge up the rock. Most limestone is no longer burnt, but is crushed very finely before being applied to farmland.


Cement, marble and dolomite

Limestone for cement production

Cement, used to make concrete, is a mixture of two main materials – lime and marl (a calcareous clay). Cement was imported until local cement works were established. In New Zealand it was first produced near Warkworth, and in the 1850s was used in the Queen Street sewer and other Auckland construction projects. It was not until manufacturers could produce high-quality cement that they could compete with imports.

Limestone deposits close to rail links or deep-water ports were ideal for cement manufacture. They were quarried at Whāngārei, Tarakohe in Golden Bay and Milburn in Otago. The names Golden Bay and Milburn became synonymous with cement.

In the early 2000s cement was produced at only two plants – one at Cape Foulwind, near Westport, the other at Portland, near Whāngārei. Their combined output approached 1 million tonnes per annum.

Concrete culture

For driveways, garden paths, boxing walls, or securing fence posts, concrete is the wonder material. Many Kiwi men, living in the suburbs that sprang up in the 1950s and 1960s, owned concrete mixers. These churn a combination of cement, sand, gravel and water, producing a concrete mixture ready for use. Politician Tim Shadbolt used his mixer as a campaign mascot, towing it behind his car.

Marble

Marble is metamorphosed limestone. This means that it has been buried deep within the earth and subjected to great heat and pressure, which alters and hardens the rock.

Large marble deposits are found in north-west Nelson and Fiordland. The main uses are as a building stone and, when crushed into a powder, as a filler and surface coating in various industries.

The Ngārua quarry on Tākaka Hill in Nelson produced between 15,000 and 20,000 tonnes of marble each year until 2003. In years past, Ngārua glass-grade marble was shipped from Port Motueka on the coastal scow Mamaku at the rate of 6,000 tonnes a year, bound for Auckland glass works. With its high calcium carbonate content, Tākaka Hill marble was also ideal for agricultural use – making soils less acidic, improving soil structure and assisting nutrient uptake.

Dolomite

Dolomite is essentially limestone with a high magnesium content. Mt Burnett, which overlooks Collingwood in Golden Bay, is the only source of dolomite in New Zealand. In the early 2000s, 25,000–40,000 tonnes were produced per annum, mostly for use as an additive to phosphate fertilisers.

Dolomite blocks from Mt Burnett have been barged across Cook Strait. Their distinctive brown weathering sets them apart from the rocks of Wellington, and massive blocks can be seen at Wellington Harbour, where they act as breakwaters. They are also used to prevent the Hutt River overflowing its banks.


Clay

Clay is found most frequently in flat rolling land where there has been prolonged weathering but little erosion. Forming a sticky, poor-draining soil that is the bane of many gardeners, it consists of rock that has been weathered into very small particles – a teaspoonful of the clay mineral allophane has the surface area of a rugby field. The small size and flatness of the particles is what makes clay impermeable.

Clay has a unique property that has been exploited for thousands of years. When it is fired under high temperatures the particles cement together to form the hardened waterproof material used in pottery.

Early uses

Māori did not fire clay, but they were well aware of it through their cultivation of the soil. There were over 30 Māori terms for different clays, soils and gravels.

Rock bottom

When Okurarenga at Māhia Peninsula was besieged by a combined force of tribes armed with muskets, the inhabitants began to starve. After three months they had to resort to eating a soapy clay known as uku. The place was given the name Kaiuku, ‘to eat clay’.

Some of the earliest European settlers used clay to make wattle and daub dwellings, and sod or cob houses and fences. Chimneys were made from tree fern trunks heavily plastered with clay. Māori also began to use sod to build whare paruparu (dirt houses) and walls known as takitaki.

In 1881, to stimulate local industry, the government offered £250 for the production of £1,000 worth of earthenware products. A Dunedin company won the grant and by 1887 other pottery works were producing bricks, drainpipes, chimney pots and tiles, as well as porcelain and terracotta products.

European settlers dug small clay pits, and toiled at the back-breaking task of shovelling the puggy soil. Kilns were built to fire the clay. New Zealand farms, often essentially wetlands, soon had kilometres of drainage pipes.

During the late 1800s and early 1900s brickworks appeared, with their chimney stacks and large bottle kilns – wide at the bottom and tapering upwards like the neck of a bottle. The kilns churned out red bricks by the thousand, helping to build the growing townships. The colour of bricks and roofing tiles varies according to the local clay. Banks Peninsula bricks made from clay-rich loess, a fine, wind-blown sediment found in Canterbury, are a distinctive tan colour.


Types of clay

Kaolinite

Kaolinite, New Zealand’s most common clay mineral, is formed by the deep weathering of many different rocks such as granite, schist and greywacke. It is widely quarried and manufactured into domestic brick, tile, pipe, ceramics and pottery. It is also used as a filler in rubber, bitumen and adhesives, and highly pure kaolinite is used for paper coating.

In the 2000s the largest kaolinite clay pits were in the wider Auckland area, while South Island clay pits were typically smaller operations. The biggest brick-making operation in the early 2000s was in Auckland. Small-scale pits also keep New Zealand’s potters supplied with raw material.

Making marbles

Len Castle, a New Zealand ceramic artist, first encountered clay in the 1930s as he sat under a pōhutukawa tree on Westmere beach, Auckland. He started his career early, making marbles out of clay for his classmates during the Second World War, when glass was in short supply. He would fire them in the kitchen oven, often with explosive results.

Halloysite – white clay

Under certain conditions the volcanic rock rhyolite weathers into a distinct type of clay. Known as halloysite, it has been altered by hydrothermal fluids passing through it and then weathered by rain and exposure to the atmosphere. The producers of halloysite clay from deposits in Matauri Bay, Northland, claim that it is the world’s whitest clay. The clay is indeed white and bright, and is very expensive.

The Matauri Bay operation is the largest single clay producer in New Zealand. Most of the clay is exported for the manufacture of high-quality ceramics including porcelain, bone china and technical ceramics. Silica sand, a by-product, is sold to the building industry and used in golf course bunkers. In the early 2000s there were sufficient resources for over 30 years’ production.

Bentonite

Bentonite is derived from the weathering of volcanic ash deposits. A small quarry digs bentonite from the Harper Hills, 65 kilometres west of Christchurch. This clay can absorb large amounts of water, making it useful as a lubricant in drilling operations, a medium for growing mushrooms, and a stock-feed additive.


Ceramics and pottery

Clay is often found at the site of coal seams. If it is of suitable quality and there are local markets, it can be dug as a by-product of coal mining. From the 1860s clay was fired in kilns conveniently fuelled by nearby coal. At Benhar in South Otago, the clay was used to make ceramic toilets.

Temuka pottery

Clay has been dug from the Kākahu deposits near Geraldine in Canterbury since the 1860s, and kilns have fired the clay into various products at nearby Temuka since 1868. Tiles, bricks, tableware and porcelain electrical components have all been manufactured at Temuka by a number of companies. By the late 1990s Temuka Homeware (a division of the company New Zealand Insulators) was the only large-scale producer of dinner plates and other clay kitchenware in New Zealand. At this time it was annually producing half a million pieces for the domestic and export markets. As the tableware is made from a clay similar to that used for electrical insulators, it is virtually unbreakable.

Crown Lynn

After the Second World War New Zealand’s commercial potteries flourished. Crown Lynn had its origins in the Specials Department of the Auckland brickworks, R. O. Clark Ltd, which began making tableware in the late 1930s. Crown Lynn sourced its clay from kaolinite deposits in the Coromandel Peninsula during the late 1960s and early 1970s. By 1970, 700 staff were producing 15 million wares per year, and exporting around the world. However, deregulation of imports in the 1980s brought hard times and the company closed in 1989, unable to compete with a flood of cheap imported goods.

Built to last

Hefty New Zealand Railways cups, manufactured by Crown Lynn Potteries, are now collectors’ items but were once tossed from the train windows. According to an old joke, only three things would survive a nuclear holocaust: ants, cockroaches and New Zealand Railways cups. And the first two would make it only if they were under the third.

Industrial ceramic production rose but declined relatively quickly as pre-cast concrete pipes replaced ceramic sewer pipes. As with domestic pottery, when import controls and tariffs were removed in the 1980s, many producers could not compete with cheaper imports. In the 2000s small potteries were serving the crafts market, their clay supplied by specialised companies. Other commercial potteries such as Morris & James near Warkworth have survived the competition, digging their clay on site and producing pots with distinctive bright glazes.


Phosphate, sulfur and pumice

Phosphate

As New Zealand lacks phosphate deposits, it needs to be imported. However, a small phosphate deposit was quarried at Milburn in Otago. The rock was burned, and then crushed and chemically treated. The phosphate was used as a fertiliser, replacing imported manures. The major working periods were 1920–24, with 140,000 tonnes produced, and 1942–44, with a further 53,000 tonnes of output. But the deposit was not large and the quality not high enough to meet New Zealand agriculture’s requirements.

Sulfur

The yellow mineral sulfur is found in the geothermal areas of the central North Island and Bay of Plenty. It is used to make sulfuric acid, for the manufacture of superphosphate fertiliser. Sulfur production has never been a big industry and most supplies are imported.

Domestic deposits are small and the geothermal environment where sulfur is mined is very corrosive on machinery. In 1917 the New Zealand Drug Company mined 4,392 tonnes of sulfur for use in a chemical works at Rotorua. The company employed workers to dig sulfur from fumaroles. Attempts were also made to mine sulfur from White Island in 1898–1902 and 1912–14; altogether, 11,200 tonnes were extracted. In 1993, 6,600 tonnes were mined at Tikitere near Rotorua, but in the early 2000s there was no production.

Pumice

Pumice is a very light volcanic rock that floats. When the central North Island volcanoes erupted they spurted out pumice, which is riddled with holes that were filled with gas when it formed. Rivers have also eroded and carried down pumice to create alluvial deposits near Hamilton and on the Hauraki Plains. In the late 1800s it was used as an insulation material in refrigeration. The Firth Pumice Company on the banks of the Waikato River exported this lightweight, floating rock to Australia.

Today pumice sand is dredged from the Waikato River near Mercer and quarried from rock in the central North Island. It is used as a drainage material in construction, and as an additive in potting mixes. Some stonewashed jeans are made from denim that has been washed with New Zealand pumice.


Other minerals

Zeolites

There are about 40 minerals known as zeolites. Often white or colourless, they are commonly found in volcanic rocks, much like grains in a loaf of bread. But they can also occur in massive form as bedded deposits. They are often associated with volcanic rocks that have been buried and subjected to pressure or hydrothermal alteration.

Zeolites have a unique open crystal structure with many surfaces. This provides a huge surface area for chemical exchange and absorption. Zeolites can be used to soak up oil and chemicals spills, and for cat litter.

The only site that has been mined in New Zealand is at Ngākuru near Rotorua. Ngākuru zeolites, mixed with sand, have been used to improve the growth of the turf at Westpac Stadium in Wellington.

Serpentine

After the Second World War, serpentine was added to the fertiliser superphosphate. Its free-running properties when crushed allow superphosphate to be sprayed from aircraft. It is also added to other fertilisers and applied to magnesium-deficient soils.

By 1993, 4.2 million tonnes of serpentine had been quarried in New Zealand. At that time Greenhills near Bluff, and Piopio near Te Kuiti were the main producers. But by the 2000s, many accessible New Zealand deposits had been worked out. Production also fell because of the development of granulated, free-running superphosphate. At some locations, serpentine was found to contain asbestos fibres and quarries were closed for health reasons.

Mica

Mica is a glittering flat mineral that sometimes occurs as large overlapping flakes termed mica books. The Inuit people of the Arctic are said to have used large transparent flakes of muscovite mica as window panes in their igloos. In New Zealand, individual sheets of mica rarely reached this size, but during the Second World War a small amount was mined briefly in the Mataketake Range in South Westland. In 1942 mica was considered an important war mineral, as the Allies used it for spark-plug washers in aircraft engines. In New Zealand it was also used for radio condensers. Wartime shortages led to a scramble for a new supply, but the deposits proved too small to be economic.

Asbestos

Significant asbestos deposits have been identified in two areas – the Cobb River valley of north-west Nelson, and the Red Hills in South Westland. From about 1913 until the 1950s a reclusive couple, Henry and Annie Chaffey, lived near the isolated Cobb River, where Henry Chaffey prospected for asbestos and promoted the development of the region’s deposits. After a road was built to a hydroelectric dam at the headwaters in the 1930s, the Cobb River deposits were mined on a small scale from 1941 to 1963. About 5,000 tonnes were produced.

Road to nowhere

In the early 1970s the isolated Red Hills in Fiordland were considered a likely asbestos prospect. A mining company tried to blaze a trail with a bulldozer from Arawata to the remote claim. The motley convoy included tractors, sheds on sleds and other gear. To make a path they had to use explosives in the Monkey Puzzle Gorge, and drill and blast enormous boulders. Swamps, forests, and rough ground added to the obstacles. They never reached their destination, and returned from the 200-kilometre trip without doing a single day’s prospecting.

During the 1970s several mining companies spent considerable time and effort exploring the potential of the asbestos deposits in South Westland’s Red Hills. Three horizontal tunnels, or adits, were driven into one hillside and samples were taken. While a large deposit with good-quality fibre was found, it did not yield enough asbestos and the isolated location meant that mining would not be economic.

Asbestos was used to manufacture pipes and corrugated roofing from the late 1930s. Thousands of tonnes were applied to boilers and pipes and used as a fire retardant in homes and commercial buildings. In the 1970s the health risks of inhaling asbestos fibres were exposed, and since 1987 no asbestos has been used in New Zealand products. Costly techniques are used to remove asbestos from old buildings. Care must be taken with cement sheeting used in older buildings. It should not be sanded as fibres may enter the air.


Hononga, rauemi nō waho

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How to cite this page: Carl Walrond, 'Rock, limestone and clay', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/mi/rock-limestone-and-clay/print (accessed 13 December 2019)

He kōrero nā Carl Walrond, i tāngia i te 12 Jun 2006