Auckland’s Iconic Volcanoes

The City of Auckland is built on a known, and potentially active, volcanic field, with its most recent eruption only 600 years ago.

A) Imagine a modern city of more than 1.5 million people built on an active volcanic field of more than 50 volcanoes, with scientists and experts predicting that an eruption is likely within the next thousand, or, some say, even, hundreds, of years, which is a very short time span in the world of geology. Each of the existing, and now dormant and extinct, volcanoes erupted for just one period of time – the oldest volcano erupting nearly 250,000 years ago – and the eruptions usually lasted for just weeks for most of the volcanoes, with the exception of the most recent eruption, Rangitoto Island, which blew about 600 years ago, and which repeatedly spewed lava with a series of explosions. This was a particularly violent eruption, as the lava came into contact with the sea, and has been noted as the first New Zealand volcanic eruption which would have been witnessed by humans.

B) The volcanoes beneath Auckland have driven upward from a field of basaltic magma, a single source, and a wide pool of magma about 100 km below the city. This differs from other, large, volcanoes which have a central core of magma working up from below the surface, in that each of Auckland’s volcanoes is a separate eruption from the same pool, pushing through the surface in different locations, rather like porridge bubbling under heat, and unlikely to repeat in the same spot. The eruptions have tended to be small by volcanic standards, with most being less than 150 metres in height, but have grown increasingly bigger over time, with Rangitoto, the most recent, being the largest, with estimates of its volume of spewn material at 60% of the total.

C) The first volcano was recorded as Onepoto Volcano, at roughly 248,000 years ago, and the volcanic field extends from Lake Pupuke in the north to Wiri Mountain in the south, and from Mount Albert in the west to Pigeon Island in the east. Many of the volcanoes have had lava flows that run to the sea, and Auckland is built on the remains of the scoria cone volcanoes, scoria being a type of basalt rock very dark in colour. The scoria rock is pockmarked with holes where gas was trapped as the lava cooled, and the scoria rock is now widely used in road-building, landscaping, used as fill, and some volcanoes have been mined and quarried to the point where the volcanic cones have been mainly levelled, and in some cases, dug below the ground to extract the scoria. Other volcanoes, such as Mt. Eden, with its famous crater, and One Tree Hill, have become iconic landmarks of the city, recognizable by their physical attributes. Another impressive feature is the network of labyrinthian caves and tubes below the surface, and in 1942, the then-mayor of the Mt. Eden borough, R.J. Mills, put forward the suggestion that the caves could be used as war-time shelters. This was not a preposterous idea, as the indigenous people, the Maori, had used the caves as warring meeting places, and also a sacred place to hold the remains of the dead.

D) The volcanoes in Auckland are different from other types of volcanoes in New Zealand, and, indeed, other types of volcanoes around the world. Beneath Auckland lies a single field of hot rock, known as a ‘plume’, which causes some rock to melt, and the molten, liquid, rock then pushes toward the surface, creating a new volcano each time. The three main types of volcanoes found around the world are what are known as, firstly, ‘composite’ or ‘strato’ volcanoes, which are steep-sided volcanic cones formed from layers of ash and lava flows which cause the volcano to form a very high mountain of lava and rock and dust. These are some of the most famous volcanoes in the world, such as Mt. Fuji in Japan and Mt. Pinatubo in the Philippines. A second type of volcano is known as the ‘shield’ volcano, which are the types of volcanoes associated with Hawaii, with generally low sloping sides with fast-moving lava strata, one on top of the other. And the third general type of volcano is called the ‘dome’ volcano, with steep sides and thick lava, which hardens rapidly so that it does not flow great distances. Examples of these are the many volcanoes in South America, such as the Chaiten lava dome which erupted in 2009. Auckland’s volcanoes are known as monogenetic, and, as the name suggests, they come from one source, the single field of hot rock. Other examples are found in Mexico and in British Columbia, in Canada.

E) So, the question must be asked, with a projected population of nearly 2 million people living on this field of hot, and sometimes molten, rock, what are the dangers associated with this natural phenomenon? As Auckland is New Zealand’s largest centre, and the commercial heart of the country, major infrastructure disruptions to transport, communications, and commercial and industrial activity would be expected, and the potential loss of residential and commercial buildings could see the shutting down of large portions of the city, if not the entire urban area. Scientists have noted that not only one eruption is possible, but many, over a short period of time. There are indications that 32,000 years ago there were five eruptions within 50 years. In addition to the immediate hazards an eruption would bring, there are also secondary dangers, such as shock waves, volcanic gases, earthquakes (which may also precede an eruption) and tsunamis.

F) To prepare for such eventualities, there is a network of civil defence and emergency services, as well as seismic monitoring stations, including satellite imagery which detects the slightest changes in the shape and bulging of the earth in the immediate region. Experts from the Ministry of Civil Defence note that the network of seismometers, which measure earth tremors linked to the movement of magma at deep levels indicate that the seismicity in the Auckland region is very low, which makes it easier to detect the ‘subtle signs of an impending eruption’. However, all experts agree that it is not a matter of if another volcano erupts, but when. Attention now turns to predicting and pinpointing where such an eruption might occur, the likely effects of a land-based eruption or one under the seabed, a ‘phreatomagmatic’ (steam-generating) eruption, and the required steps to limit the damage. The earth is ever-evolving, and Auckland’s dramatic volcanic past, and it’s potentially volatile future is testimony to this change.