Kōrero: Rock, limestone and clay

Whārangi 2. Aggregate

Ngā whakaahua

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.

Me pēnei te tohu i te whārangi:

Carl Walrond, 'Rock, limestone and clay - Aggregate', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/mi/rock-limestone-and-clay/page-2 (accessed 14 November 2019)

Story by Carl Walrond, published 12 Jun 2006