He korero whakarapopoto
Limestone, dolomite and marble
- Limestone consists mainly of the shells of tiny marine fossils, which are made of lime (calcium carbonate).
- Dolomite is limestone that has a high level of magnesium.
- Sometimes extreme pressure and heat turns limestone into marble.
Limestone, dolomite and marble are called carbonate rocks.
Where limestone is found
Patches of limestone are found around New Zealand. Millions of years ago, there was a lot more limestone than today. These areas reached onto mountain tops, but the limestone has eroded away in high places.
Water dissolves rock
Rainwater is slightly acidic, because it contains dissolved carbon dioxide. When it runs through soil, it becomes even more acidic. Limestone, dolomite and marble can be slowly dissolved by this acidic water. When water runs into gaps in the rock, they become larger as the rock dissolves.
Over time, as water breaks down the rock, some typical features of limestone country may develop, including:
- large underground caves
- hollows in the ground (dolines or sinkholes)
- fluted rock outcrops (karren)
- rivers and streams that disappear underground
An area with these features is called a karst landscape.
Karst in New Zealand
New Zealand has a number of karst landscapes. The most well-known are around Waitomo in the North Island, and the areas of marble around Mt Owen, Mt Arthur and Tākaka Hill in the South Island.
- Waitomo Caves are New Zealand’s most popular caves for tourists.
- Lake Disappear, near Raglan, is a large hollow drained by an underground stream. It fills up and becomes a lake after heavy rain, when the stream can’t drain it fast enough. In dry weather, the lake disappears.
- Bulmer Cavern under Mt Owen is 749 metres deep, and has about 50 kilometres of explored passages.
Traditionally, Māori used limestone caves for shelter, or for burying the dead. Karst areas are also used for recreation. Cavers explore caves, and are still discovering new passages. Black-water rafters float along underground streams on rafts made from rubber tubes. Limestone areas are also sometimes used for rock climbing.