We scientists here in Auckland have an…issue. And it’s a rather large one. Specifically, it’s about 260 meters (850 ft) tall and about 5.5 km wide*. Many people (including myself) go out of their way to visit this problem, and enjoy hiking on it and checking out its lava tube caves.
Of course, I’m talking about Rangitoto Volcano. Rising above the Waitemata Harbour and viewed from nearly everywhere downtown, it is the most well known icon of Auckland, other than the Sky Tower. It is a great example of a small shield volcano. Anyone can look at it and know that it is a volcano. We all love it for its beauty, charm, and lovely pohutukawa forests that erupt (!) with red blooms at Christmas each summer.
Yet, there are times when we, as scientists, see its darker side. The issue is that in the AVF (that’s the Auckland Volcanic Field, use it and impress your friends), it seemingly came out of nowhere. It represents a departure from almost everything we know about the volcanic field. And that causes a problem.
When volcanologists predict what may happen next in a volcanic area, they use a variety of tools. For volcanoes, or volcanic fields, in which we haven’t seen an eruption first-hand, we study what is left behind after eruptions–the deposits, like lava or ash layers preserved in lake muds. And the Rangitoto deposits tell us that there are several ways that it is different from the rest of the volcanoes in the field:
- What is easily seen, even to non-geologists, is that Rangitoto is BIG. Bigger than every other volcano in the AVF, by far. It makes up ~41% of the total volume of the entire field**.
- Though all the AVF volcanoes are made of basalt, the chemistry type of the lava produced during the last eruption at Rangitoto is different from most of the rest of the volcanoes in the field. Rangitoto’s last eruption produced subalkaline lavas while the lavas from the other volcanoes all have alkaline chemistry. This is odd.
Thanks to new research that you may have seen in the news, it may have erupted several times, over about 1,000 years or more. We don’t see evidence that any other volcano in the field did this.
[Update 2020: this finding has now been refuted by a few studies and several lines of evidence. At most, Rangitoto erupted twice: once about 620 years ago in AD 1397 and another time 570 years ago, in AD 1446. However, the time break between these eruptions could have been much shorter than 50 years, perhaps 1-10 years. These two eruptions could be two phases of one eruption, and quite normal for the AVF.
The latest ideas indicate that Rangitoto may just be like the rest of the field in terms of how often it erupts: just once. Because it is the most recent eruption, we are able to get better age control and a finer understanding of volcanic processes. As a consequence, we may be detecting two phases of one eruption. Other volcanoes in Auckland may have undergone similar dual-phases, but since they erupted longer ago, we cannot ‘see’ these two phases to distinguish them from one another.]
- It’s the youngest volcano in the field.
The field’s behaviour has us scratching our heads. What is likely to happen next, given that the last volcano to form is so different from the rest of the volcanoes? Will the next volcano be even bigger than Rangitoto? Will the next volcano also erupt off and on for 1,000 years or more? Or will Rangitoto erupt again?! We don’t know if this is the start of a new trend or just a one-off. All we can do is learn as much as we can about the field, and especially Rangitoto, and prepare for each scenario.
So the next time you look at The Problem in the Harbour, er, Rangitoto, admire it for not only its beauty, but its ability to mystify even the scientists who study it.
*Data from the Smithsonian Institution’s Global Volcanism Program webpages.
**Data from Kereszturi, G., Nemeth, K., Cronin, S.J., Agustin-Flores, J., Smith, I.E.M, Lindsay, J., (2013) A model for calculating eruptive volumes for monogenetic volcanoes – implication for the Quaternary Auckland Volcanic Field, New Zealand. Journal of Volcanology and Geothermal Research 266, 16-33.