Sometimes, in science, SO MUCH is written about a particular topic that it needs and deserves its own summary paper. This is called a review.
There are two options. You can directly download the PDF of the accepted manuscript by clicking the link below:
If you don’t want to read the full paper, here are the key stats & facts:
- Over 1.6 MILLION people live atop the dormant Auckland Volcanic Field (AVF).
- The AVF has 53 volcanic centres.
- Each volcanic centre was created by one eruption. A possible exception to this rule is Rangitoto, which may have had two eruptions (but maybe not).
- The boundary of the field currently spans a ~30 km tall (north – south) and ~20 km wide (east – west) elliptical area.
- Auckland’s tectonic setting is considered ‘intraplate,’ meaning we are far away from a tectonic plate boundary. Volcanism in Auckland is not related to volcanism in the Central North Island, such as that at Whakaari or Taupō.
- We have geochemistry data from 51 of the 53 volcanic centres, and can place 47 of the 53 volcanoes in age order. Comprehensive geochemical and age databases help us understand the AVF and its behaviour.
- The AVF started erupting 193,000 years ago at Pupuke. The last eruption was at Rangitoto about 550 years ago.
- Rocks melt in Auckland’s mantle starting at around 70-90 km depths to form magma.
- There is probably a bit of melt down there right now!
- We don’t know why rock is melting, but it probably has to do with how rock moves within the mantle or maybe part of the lower crust and upper mantle is peeling off under Auckland (this happens sometimes; nothing to be alarmed about). We can hopefully test these ideas in the future.
- The magma is all basaltic (< 50% SiO2) but spans a large range in chemistries. The chemical composition of the volcanoes varies, and can change during individual eruptions. These important chemical differences help tell us how the magma formed and how it rose to the surface.
- The range in chemistries is caused by the melting and mixing of three different mantle sources (three different types of mantle rock).
- The batches of magma that reach the surface are very small volume (<<0.1–0.7 cubic kilometers) compared to many volcanoes in other tectonic settings (like rift or subduction zones at plate boundaries). These ‘small volumes’ are still equivalent to about 40,000 to 280,000 Olympic swimming pools of material erupted! (calculated using the archived Weird Converter)
“London Olympic Swimming Pool for the London 2012 Olympics” by Sum_of_Marc is licensed under CC BY-NC-ND 2.0
- In some cases, the chemistry of the magma seems to control the resulting volcano size.
- Magmas rise at different speeds (sometimes fast, sometimes slow) depending on its chemistry and depth. In the mantle (> 25-30 km), magmas rise fast or slowly. Then they probably collect and pool at the base of the Earth’s crust (around 25 – 30 km) and stay there for up to several years before rising up through the crust rapidly.
- We don’t know what triggers the magma pooled at the base of the crust to rise.
- There are no patterns in where or when Auckland’s volcanoes erupt within the field, nor how big the eruptions are.
- Sometimes the time gap between eruptions (repose period) is less than 100 years, sometimes it is 13,000 years.
- The number of eruptions with time (eruption frequency) increased 60,000 years ago. We do not know why.
- Once the magma reaches the surface and erupts, it produces volcanic hazards. The volcanic hazards in Auckand are: hot currents of ash, steam, and gas (base surges — a type of pyroclastic flow), lava flows, flying rocks (ballistics or bombs), ash fall, gas, earthquakes, shockwaves, and volcanic tsunami. Not all hazards occur during every eruption.
- The hazards an eruption produces depends on where in Auckland it erupts. Magma volume also plays a role.
- Because Auckland is a wet place, and hot magma + water = explosion, most of the time (83%), Auckland eruptions start with an explosive phase (phreatomagmatism). During explosive phases, destructive and fatal base surges reach up to ~6 km from the vent in all directions.
- After this phase, sometimes a magmatic phase with lava flows occurs as the vent dries out.
- In dry areas such as central city Auckland, eruptions start with a magmatic phase and do not experience an explosive phase.
- Auckland eruptions form small landforms near their vents such as maars (explosion craters), tuff rings, tuff cones, and scoria cones.
- Since we do not know when or where the next eruption will occur (and since location affects the types of eruption hazards and their impacts), we are using all the above information to inform 8 eruption scenarios to plan for eruptions in all locations.
Any questions? Put ’em in the comments or contact me.
This blog post summarises: