|Mangere Mountain, L. Homer / GNS Science|
|Auckland volcanoes, GNS Science|
The magma that erupts in the Auckland Volcanic Field (AVF) is generated in a 'hotspot' about 80 to 100 kilometres below the surface. It is a very fluid type of basalt that is known to rise quickly to the surface (at up to 5km / hour) from the magma source.
|Tuff outcrop at North Head, J..Thomson / GNS Science|
This, along with the expansion of gases within the lava itself, creates extremely violent eruptions that fragment the lava into small particles and blasts them upwards and sideways from a wide, flat explosion crater. This becomes surrounded by a ring of ash. Such deposits are known as tuff (pronounced 'toof' as in 'woof'). You can see outcrops of this in Auckland, for example around the shoreline at North Head. Each individual layer represents an explosion from the vent.
|Surtsey eruption, courtesy NOAA|
|Mount Eden Crater, J.Thomson / GNS Science|
|Scoria outcrop, Mount Wellington, J.Thomson / GNS|
If the magma reaches the surface where there is little interaction with water there is a different type of eruption. This includes eruptions in areas of dry land, as well as those that start off as wet eruptions, but where the water supply near the vent gets used up before the supply of erupting magma runs out. The magma then erupts in a fountain of lava, driven up by gases within it that are expanding as the pressure is reduced.
The lava fountains might be several hundreds of metres high, with blobs of lava partially solidifying in mid-flight, and landing as scoria in a ring around the vent.
This is a bit like the froth coming out of a soda bottle once the lid has been removed. The scoria pieces and lava bombs are relatively sticky and can build the steep sided cones that are very recognisable in the Auckland landscape. The reddish colour comes from the oxidation of iron in the magma as it cools during its flight through the air.
|Lava bomb approx 1/2 m in length, Mangere Mountain|
|Takapuna lava flow, J.Thomson / GNS Science|
|Lava tree mould with bark impression, J.Thomson / GNS|
surrounded by the lava which cooled around them. The trees then burnt, leaving tree shaped holes within the lava.
|Takapuna Fossil Forest and Rangitoto, J.Thomson / GNS|
Could a volcanic eruption occur in Auckland in the future? What are the probabilities in the short to medium term and what would the impacts be? The short answer to the first question is 'Yes, definitely!' There is no reason to think that eruptions won't occur again. In order to answer the last two questions ('When?' and 'What?') it is important to get as clear a picture as possible of the history of past events, their timing, duration and magnitude, and their geographic relationship to the housing and infrastructure in the wider Auckland area.
Auckland Museum Volcanic Eruption
|Auckland City and Mount Victoria, J.Thomson / GNS|
These questions are the focus of a long term scientific programme called DEVORA (Determining Volcanic Risk in Auckland). DEVORA is led by GNS Science and the University of Auckland, and is core-funded by the EQC and Auckland Council. The first part of this programme has been to further our knowledge of the eruption history of the Auckland Volcanic Field volcanoes. What this work has shown is that there is no simple pattern that we can project to help easily forecast the likelihood of eruptions in the future. The timeline of eruptions shows them to be clustered, with large gaps between phases of relatively high activity.
|Graham Leonard, photo by Brad Scott / GNS|
Graham Leonard of GNS Science is a co-leader of the project. He comments that: "Some eruptions flare-up over what is, geologically speaking, a short period of time. For example, there can be 6-10 volcanoes erupting within a 4000 year timeframe. On the other hand, the volcanic field has also gone quiet for up to 10,000 years in the last 60,000, which is quite a long gap."
For more information about the DEVORA research have a look at this media release.