As it turns out, it is very difficult to count volcanoes. When I started with DEVORA in 2008, I was told that there were 49 volcanoes in the Auckland Volcanic Field. Now, there are more, but exactly how many are there? Well, that depends on who you ask.
The honest answer is that we don’t know. That may sound funny–I mean, we are supposedly smart scientists, shouldn’t we know? Our lack of consensus on this isn’t due to our need for lessons from The Count from Sesame Street, but because we all use slightly different criteria to define a volcano. This question is actually quite good to ponder, because it forces us to define what a volcano actually is, and has implications for how we think the field behaves.
When pressed, we usually give an estimate: about 50 – 55 volcanoes. Like I said, in 2008, when I started with DEVORA, I was told that there were 49 volcanoes. Since then, the Grafton volcano was rediscovered under the Med School in early 2011, bringing the number to 50. Later in 2011, Boggust Park, Puhinui Craters, and Cemetery Crater were added to the mix, and some of us now say that the volcano count has been increased to 53. I personally like to say that there are 53 volcanic centres, because that is how many groupings of volcanic features I count, but someone else might see 55 volcanoes, and someone else still may count only 50.
How could the range be so wide? Here are a few reasons why we can’t seem to settle on a firm number:
- Some volcanoes have several vents, indicated by craters or scoria cones, within a small area. This is normal for volcanoes, but how close does a vent have to be to others to be counted as a part of the same volcanic centre? Where is the cut off, distance-wise, for this? The perfect example is Puhinui Craters. These three small craters were discovered in 2011. They are within a small distance of one another, so I’m on board with calling them one volcanic centre, and NOT three individual volcanoes. There is a precedent for this: several AVF volcanoes visibly have more than one vent or crater, and are considered one volcanic centre (One Tree Hill, Three Kings, Mt Eden). They probably erupted within the same time period–nothing contradicts this idea so far. But this isn’t the end of the decisions we have to make to figure out how to count this centre: Puhinui Craters are also close to McLaughlin’s Hill. Are Puhinui Craters a part of McLaughlin’s Hill?? Some of us think no, and increased the volcano number count to 53, and some of us might say yes and keep the number as-is at 52. Still others count each crater as a separate volcano and say that there are now 55.
- We generally think of Auckland volcanoes as one-shot deals, so if a volcano erupts in almost the same spot more than once, forming a new crater, after some time gap, is it technically a new volcano, or is the old one active again? Rangitoto falls into this category: we can see at least three vents and know that at least two eruptions occurred about 50 years apart. For now, until other evidence surfaces, I consider this to be one volcano, with at least two eruptions. You can use the examples of Purchas Hill and Mt Wellington to see why this can get complicated: these are considered two volcanoes, but are well within the same distance as Rangitoto’s craters are of one other. Also, these two volcanoes may have even been active at the same time. Geochemically, these two volcanoes are very different, so that is why we call them two volcanoes and not one. But is that the correct thing to do?
Hopefully you can see how hard it can be to decide whether or not a crater or scoria cone is an entirely new volcano or part of an existing one.
Will we discover more volcanoes?
Maybe, but probably not! A thorough analysis of Auckland has been done now. There may be more volcanoes that have been buried or eroded away that we cannot see and therefore have gone unrecognized, but there are likely to be very few of them–we would likely see basaltic ash layers in lake cores from several ‘mystery’ eruptions if so. Our ongoing investigations of lake cores and new entries to our borehole database could shed more light on this, however.
Why does it matter?
The number of volcanoes we count does actually …count. We can use it to determine, statistically, how likely future eruptions are based on what has happened in the past. If every ‘volcano’ represents one eruption (except for Rangitoto!), and if we know how old each centre is, we can start to examine how active the field has been over time and start to pinpoint patterns and what may happen during future eruptions.
So the debate about the number of Auckland volcanoes will continue until scientists can agree. Perhaps some day we will settle on a number. Until then, you will likely see the count range from about 50 to 55 (not to mention some other wild guesses out there). And now you know why!