Hard volcanic rocks, formed as lava flows, are widespread in the northern part of the North Island. There are also scattered occurrences on the eastern side of the South Island, especially near Christchurch and Dunedin. When labour was relatively cheap in the 19th and early 20th centuries, these rocks were used for construction. Trimming and dressing hard volcanic rock is time-consuming, and considerable skill is needed to lay these rocks, so the use of volcanic rocks declined after the First World War.
Banks Peninsula and the hills around Dunedin are both large extinct volcanoes that were built up in the Miocene Period, 10–13 million years ago. Numerous quarries were opened, and different rock types were tested as building stones. Christchurch probably has more stone buildings than any other city, partly because of the lack of timber in the surrounding area, as well as the availability of stone. Lava used for building stone in Dunedin is locally known as bluestone.
Fine-grained, dense black basalt from the western slopes of the Port Hills has been the dominant stone used in walls, steps and paving of Christchurch buildings since the mid-1860s, when a tramway was built to carry it into the city. Its source was a small volcanic cone, now quarried away. The quarry is closed, so Halswell stone can now be obtained only by recycling it from demolished structures.
Tuff and breccia
Tuff is the geological name for ash that has erupted from a volcano and accumulated in the surrounding area. It is usually soft and easily worked, but most tuffs that have been quarried for building stone do not weather well, and are porous.
Volcanic breccia consists of fragments of rock in a matrix of ash that has been ejected from a volcanic vent. A large area of volcanic breccia is found around Port Chalmers in Otago Harbour, and has been quarried for use in a number of South Island buildings.
Ignimbite is formed by huge volcanic eruptions that violently eject large amounts of pumice and ash. Pyroclastic flows – dense, gas-charged clouds of ash – pour across the ground at high speed, depositing a sheet of pumice and ash. The resulting rock is called ignimbrite. Because the pyroclastic flows are ejected at high temperature, the resulting rock is often partly welded. Sheets of ignimbrite cover the central part of the North Island, but it has only been quarried for building stone in a few areas.
By 2005 the only working quarry was at Hinuera valley, near Lake Karapiro. Hinuera stone is cream, buff, or orange-brown, and contains fragments of pumice in a fine-grained matrix. It is hard enough to be used as a building stone, but soft enough to be cut, and is quarried in a similar way to Ōamaru Stone.
Ignimbrite was quarried from near Putaruru in the mid-20th century, and used in a number of government buildings around the country including Wellington’s old Dominion Museum (now part of Massey University).
Plutonic rocks (granite)
Often grouped under the term granite, plutonic rocks have crystallised slowly at depth from magma. They are typically coarse-grained, with a variety of colours and textures.
Granite has symbolism as the hardest rock, implying permanence that lasts for centuries and through civilisations.
Imported plutonic rocks are widely used for polished decorative panels and facing stone. Local varieties have not been greatly used because material tested over the years has generally proved too weathered or broken. Most potential sources are in inaccessible parts of the South Island, and the cost of quarrying and trimming hard rock has been too great.
Only one plutonic rock type has been widely used. Coromandel granite (technically a tonalite or diorite) has been quarried from the northern end of Coromandel Peninsula, and used in many public buildings as a base course or as steps, and in monuments.