Our guest speaker was geologist Dr Bruce Hayward NZOM who spoke to us about the volcanic field upon which Auckland city sits.
Auckland is a city built on top of and out of its volcanoes. Scoria from many of the volcanoes has been used in our roads and has ballast for our railway lines. St Andrews Church one of Auckland’s oldest is constructed from bassault quarried from Auckland volcanoes.
Bruce told us that there are about 53 volcanoes in Auckland with an age range between 200,000 years old and 600 years old.
Only a few volcanoes in the Auckland Volcanic region remain more or less fully intact. Only one has not been quarried and several including Mount Smart, Little Rangitoto (adjacent to Upland Road) Elletts Mountain amongst others have been obliterated through quarrying.
Auckland’s volcanic landscape has been derived from three styles of volcanic eruption, explosive eruption where magma coming to the surface meets cold groundwater creating explosive blasts and pyroclastic surges. Bruce commented that these pyroclastic surges would kill everyone within 3 to 4 km of an explosion site.
Orakei Basin, tank farm, Onepoto Basin and Lake Pupuke are examples of these explosion craters.
The second type of eruption is a dry eruption where magma comes up through the throat of the volcano with a heavy guest mix as it comes out of the throat of the volcano the lava gas mix explodes and cools forming scoria.
This is described as fire fountaining. Bruce commented that Mount Eden is an example of a fire fountaining vent. Included as examples of such volcanoes are at its outset are Mount St John and One Tree Hill.
The third type of volcano is an abusive eruption whereby lava pushes through the less stable scoria surrounding a fire fountaining vent and flows downhill low-lying areas. The speed and character of lava depends on its temperature and viscosity.
Fast flowing lava is called Pahoehoe lava from the description given to it in the Hawaiian Islands and slow moving lava, which looks remarkably like a slow moving, but hot, rock pile, is called A’a lava.
The distance lava flows is dependent on topography. The longest lava flow in Auckland is that which terminates the end of the Meola Reef. Until recently it was thought that lava flow originated at Mount Eden. Recent research undertaken at the Auckland University has disclosed that the source was in fact the lower parts of Mount John. Mt Eden a later volcano simply sits on the old lava flow.
Bruce told us that all Auckland volcanoes are “one-off”. That is to say they erupt only once and then become extinct. This does not mean that the Auckland volcanic field is extinct rather it is dormant and is likely to spring into life at any time between next week or at the end of 5000 years. This is because there is no pattern that can be discerned in Auckland’s eruptive history. So where the next event will occur is anyone's guess.
Bruce noted that there is a web of sensors around the Auckland volcanic field designed to give warning, most likely to 3 days, of lava movement leading to an eruption so that those within 4-5 km of the predicted epicentre can be given the opportunity to evacuate. If a vent were to appear in a closely populated part of Auckland an evacuation would be an interesting exercise. The picture of the vent and lava flow above is taken from the recent eruptive event in Hawaii. That event lead to several thousand people being evacuated and several hundred houses being destroyed by the lava flow generated by the main vent as it flowed into the sea.
Bruce's address was well received by those at the meeting especially as many of us live on the sides of Auckland's Volcanos or on the old lava flows.
