The roads and buildings of New Zealand’s cities are all constructed from quarried materials. Crushed rock, sand and cement are mixed together to form concrete – the main material in most buildings. Roads too are mainly constructed out of crushed rock, the base on which asphalt is poured. Auckland’s rapid growth has placed pressure on rock supplies, and by the early 2000s many of the old quarries which excavate the lava flows of volcanic cones such as Mt Wellington and Three Kings were being exhausted.