Story: Rock, limestone and clay
Page 4 – 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’.
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.