How caves form
Caves are formed by natural processes such as the action of rainwater, waves, glaciers or lava (molten rock).
The most commonly explored caves are those where the underlying rock is limestone or marble. As water trickles through the soil and down to the rock, it absorbs carbon dioxide from the atmosphere and air gaps in the soil. This makes the water mildly acidic, so that it slowly dissolves calcium carbonate, the main constituent of limestone and marble. Over millions of years this eroding action can hollow out underground caverns, tunnels and streams stretching many kilometres. Water often still flows through them, and the streams can rise quickly with heavy rainfall above ground.
Where are New Zealand’s caves?
About 300 North Island and 250 South Island caves have been mapped. Most are in two regions:
- The King Country in the North Island
- North-west Nelson–West Coast in the South Island.
However, caves are dotted around the country, usually where there is underlying limestone or marble.
Sea caves, lava tubes and schist hollows
Not all of New Zealand’s caves are in limestone or marble. Sea caves form in many types of rock along the coast, Auckland’s volcanic cones have tunnels known as lava tubes. These form when molten rock runs out of a lava flow, leaving walls and a roof which solidify to form a tunnel. In Otago, weathering in schist creates hollows. But most of these caves are shallow and do not form the huge, three-dimensional mazes found in limestone and marble.
Stalactites and other formations
Water that creates caves by dissolving limestone and marble also deposits calcium carbonate. This results in structures known as speleothems. The most well-known are stalactites (spears that develop as water drips from the cave ceiling) and stalagmites (spears that grow upwards from the cave floor). Crystals and other delicate structures such as cave coral also form.
Speleothems take thousands of years to form, and cavers use red tape to mark areas that are delicate and off-limits.
Possum in the pot
In 1966 cavers exploring a pot (a natural shaft) in the marble of Mt Owen reached the bottom at 53 metres, where they found a stranded possum. Expedition leader ‘St Francis’ Hardman put it in his pack and released it at the top. The cave was named Possumboot Shaft.
Organisms that have adapted to living in caves are often unique. Most life occurs close to the entrance, where there is still some light. In New Zealand most of these life forms are invertebrates (animals without backbones). Adaptations to life in the dark include no eyes or reduced eyes, a lack of pigmentation (camouflage is not needed), and long hairs or antennae to pick up vibrations from other unseen creatures (prey and predators).
It is common to find glow-worms – the luminescent larvae of the gnat Arachnocampa luminosa. A species of fern, Asplenium cimmeriorum, has been found at cave entrances around Waitomo and north Westland.
Over thousands of years, birds such as moa have fallen into some caves. Their bones, found on cave floors, have proved very helpful in identifying many extinct New Zealand birds.