Nishinoshima is in many ways the perfect volcano, it is constantly doing firsts, and spectacular and unusual things. Normally volcanologists would gather nearby and play lip-banjo at its antics. But since it is far out into the ocean and is so inaccessible most miss this beauty of a beast.
In November and December of 2013, it gave us both the birth of a new little island, and an island merger as the old island and the new one merged together. After this eruption ended in November of 2015 scientists landed to start observing how life would return to the island. It was at that time believed that the volcano would rest for quite some time before erupting again.
The volcano proved them wrong as it returned to a state of eruption in April of 2017, an eruption that lasted into August of 2018. Since this eruption was smaller than the first eruption scientists once more returned to the island to observe life take hold.
After all, smaller secondary eruptions are not unheard of in the life of a volcano. Surely Nishinoshima would need a geologically sized nap after all this activity, especially since the eruptions comparatively had not been small by any measurement.
Problem is just that a third eruption started again on the 6th of December 2019, and this eruption was larger than any of the two previous eruptions. Not only was it larger, it also kept going at a steady state rate for months building more new land than what had previously existed in total.
This should have lifted a few eyebrows, volcanoes rarely if ever increase in eruptive size as closely spaced eruptions occur, and even more rare is eruptions that does not diminish over time.
Come June Nishinoshima decided to take things even further as it increased the eruptive rate with an order of magnitude.
What started in June moved Nishinoshima out of the realm of ordinary eruptions into the major league. Nishinoshima was never a small volcano to begin with, but nobody expected it to take up the competition with big hitters like Grimsvötn in Iceland, Kelud in Indonesia and Kilauea in Hawaii.
Currently the ash-plume is ranging between 3km and 8.3km, and the happy little fire-fountains are 500 metres tall.
There are two ways to count volcanic prowess, the first and most widely used is the Volcanic Explosivity Index (VEI) Scale. As the name implies it measures how explosive an eruption is over time.
The VEI-scale is useless for effusive eruptions, so there we use the very simple solution of measuring the total output of lava over time. This is the realm of Icelandic volcanoes, with a few interludes from Hawaii.
If we stick to this millennium we find that the two largest eruptions are Kelud (0.8km3 tephra) and Grimsvötn (0.9km3 tephra) making them into borderline VEI-5s, both are though counted as very large VEI-4s since both Icelandic and Indonesian volcanologists are made out of stern stuff.
Finding Nishinoshima trailing those two at a third spot is quite a surprise. Grimsvötn and Kelud did though reach those levels a lot faster than Nishinoshima, but it is still not a small feat.
If we instead look at the amount of effused lava counted in cubic kilometres we will find that Nishinoshima is doing quite well against other big hitters during the current millennium.
At third spot we find Kilauea with 1 to 1.2 cubic kilometres of magma erupted during the last eruption.
Calculating effused lava at Nishinoshima is a bit harder since most of it end up in the ocean. Thankfully, there are naval charts so we can calculate how much must be infilled at a bare minimum to build up new land.
And as landmass increases it takes more and more lava to come up to surface level. An absolute minimum is 1.2 cubic kilometres with a highest solution at 1.8 cubic kilometres.
Currently every new square metre of land needs to overcome a depth of 200 metres or more. Add to that sub-aerial over-layering and the numbers tally up quickly.
If the eruption continues for a bit longer it will transcend even Holuhraun that is currently in the lead with its 1.8-1.9 cubic kilometre eruption.
Nishinoshima is classified as a back-arc subduction volcano sitting beside a spread-centre Graben known as the Ogasawara Through.
Volcanoes of this type are known for there intermittent explosive volcanism, most often the eruptions are small. But if the dormancy period is long, they can go big with VEIs of 4 or even an occasional VEI-5. This is though very rare.
What they do not do, is erupt like a combination out of hell of a main-arc subduction volcano combined with a major mantleplume volcano.
And not even those do progressively larger eruptions in a short timeframe, or eruptions that ramp up over a long time. They tend to go big, dwindle and go for a prolonged snooze.
So, as June came around, I started to really think about what the heck was going on. In the end I came up with a solution, but it is so out there that I will probably be laughed at.
But, if it quacks like a duck and walks like a duck, it is probably a duck. I am up for being bloodied by Occam’s Razor.
If you would stand on the top of Mount Nishinoshima looking due East out over the ocean… Well, yes you would be epically and immediately dead, but let us disregard that piddly problem. What you would be looking out over is the 3000-meter-deep Ogasawara Through, a spread-centre Graben, that Nishinoshima is precariously perched on the edge of.
Further out you would see a couple of small fly-speck islands, those are main-arc subduction volcanic islands. Beyond them is where the subduction is taking place and you have a subduction abyss reaching down almost 6000 meters.
Ever so slightly north of east of Nishinoshima is where the deepest point is, and the spot where the subducting oceanic crust has the steepest angle.
This is a bit of a problem in and of itself as subduction volcanism goes, steep angles of subduction is generally a bad thing for back arc volcanism since the melt would go pretty much straight up, short-pitching for back-arc volcanism to form.
In other words, this means that there should really not be a duck around since it would starve to death in a volcanic dessert.
Let us instead look at the Graben, those are after all known to on occasion to cause decompression melt as they spread. In the case of the Ogasawara Through it is spreading slowly, so it is a bit of a barren source, and it would produce minor volcanism at best.
If we instead look ESE from Nishinoshima we find that the angle of subduction of a bit more shallow and that an oceanic ridge is being pushed down into the mantle, this part could at least be a part of the solution where the eruptive material originated from.
What it does not explain is this weird increase in time of the eruptive strength and the ever less viscous material that is effused.
In the end I was so out of options to explain the eruption that I started to grasp for straws, pondering how mantle upwellings form and how they transition into becoming full-blown mantleplumes.
So, let us toy with the idea that the more explosive material is coming from that oceanic ridge subducting, it would at least explain the explosive part.
Let us also ponder the idea that the Graben is producing some nice decompression melt. And finally let us assume that the steep angle subduction is working as a scoop tugging at the mantle at depth causing a mantle upwelling, and that this is creating a shallow proto-plume burrowing downwards.
How does a mantleplume look like when it is born?
We do not know what it would look like, we can just assume that it would be messy. And mess we have aplenty. One thing is though for sure, it would leave measurable signs.
This is about when I asked Andrej Flis to plot the available data to see what we would find, and if there would be even a small shard of evidence pointing towards my idea being correct.
The birth of a plume
As a plume is born you get an increase in sub-crustal pressure causing localized fracturing, and as the fracture becomes critical an eruption will start, but the fracture will need to develop over time, and for a long period the build up of mantle pressure will be larger than the release through eruptions.
This part of the process would take tens, if not hundreds, of thousands of years. Time enough for a chain of volcanoes to build up, behaving quite as more normal volcanoes do.
Below the crust both pressure and melt would start to build up as the mantle started to convect deeper and deeper, this would be visible as a low velocity zone below the crust. In the beginning it would be like a wave under the crust.
But as the mantle convection wave takes the step towards plumeliness there would be a distinct root forming at 100 kilometres depth or more. If you find that you have a case that something is well and truly going on down in the mantle.
And once upon a while the torch is well and truly changed into a blow-torch and it starts to cut up the crust above and a LIP form out in the ocean, creating new continents.
Next comes eruptions larger than they should ever be in normal circumstances and that are more frequent and long-lasting than anyone would expect.