For those who have missed the previous 3 parts and the prequel. Here are the links.
The nuclear background
To be able to understand large eruptions you need to understand a bit about nuclear weapons. In this case there are several reasons why we need to do analogies with the darker side of physics and the volcanoes that we love to learn about.
But first we have to become archaeologists. Some of you might have noticed that when using carbon-14 dating you get a result with the time set to BP (Before Present), and if you have studied archaeology you will know that this means the year 1950. One might think this is just an arbitrary date, but it is not. This is the last year where you can do any atomic dating due to nuclear fallout from aerial nuclear weapons testing. It will take about 100 more years before it is possible to test our present time. Imagine, from the year 1950 up to 2100 there will forever be a hole in the chronology.
Now, those with a bit of pangeant for Cold War history will believe that the spike in the picture above comes from the Tzar Bomba test, nothing could be more false. The Russian Sakharov-designed nuclear bombs were much better designs than the American Ulam-Teller bombs. The Tzar Bomba used up 98 percent of the available fissionable material, the largest American bomb (Castle Bravo) only used up 13 percent. For all points and purposes every American hydrogen bomb up until the late sixties was a dud, albeit effective enough to kill people on a horrifying scale.
The highest point of nuclear material fallout is in 1963, and that date is one of the two used to calibrate ice core samples. It is easy enough to find. The other year used to calibrate the measurements is the Lakí event. The main reason for this is that those two show up in every ice core drill-sample in the northern hemisphere. It is a pretty handy tool for scientific comparisons between different glaciers and for calibrating timelines within the respective ice core.
The Lakí fallout trace is in two parts. First you will find an ash layer, the second part will be emplaced on top of the ash, and that is a sulphate layer, residual acidity from the sulphurous gasses released by the eruption that both stayed airborne for a longer period, and also was released up to a year after the eruption had stopped.
Calculating Nuclear Fallout
As any good scientists Volcanologists steal what they need from other scientific disciplines. In physics we steal a lot from the mathematical department. So, it is befitting that the Volcanologists raided us for the formulations to calculate nuclear fallout patterns when they needed to calculate volcanic fallout. The difference is normally so small that it should not matter really. But there are differences, some small and some so large that it becomes a problem.
First of all, as a physicist we differentiate nuclear dust (any dust that became radioactive during the detonation) and true radioactive byproducts from the detonation. The first part will behave like your average volcanic ash. It will drift around for a while depending on size and how high it was lofted into the atmosphere. The second part is basically atoms of fusionable or fissionable material that was not used up in the detonation. This will drift for a much longer time.
This is not so important really; it will though give a negative margin of error. Volcanic fallout normally travels shorter than the model gives at hand since you do not have traceable individual atoms.
There is though a real problem when using the model. The basic model was developed for the original small scale fission bombs used over Hiroshima and Nagasaki. When we started to detonate the far larger hydrogen-bombs it was rapidly discovered that the formulations had less and less with reality to do. This was foremost a problem with the dirty American Ulam-Teller bombs.
Let us now for a moment get back to the picture in the beginning. The top of the mushroom cloud is at 56 kilometers height, and that is a full stratospheric injection of fallout material. At those heights any particles will pass several times around the globe before they drift to the surface.
We now have to answer a question. What is it that lifts ash up into the higher parts of the atmosphere? Most people believe it is the explosive force that lifts ashes up into the atmosphere, but that is not true. What lift the ashes are convective currents of air. In simpler terms, ashes ride upwards on the hot air rising.
The Tzar Bomba was an almost instantaneous event where a fireball 10 km across was created, and it was at its hottest as hot as the core of the star. In an instant flash of terrible beauty humanity had made a star on planet Earth. It created a cloud of ash that rose higher than anything in human history, but it contained only what it lifted during the first few minutes, and for a couple of months it drifted across the entire northern hemisphere before falling down, almost uniformly spread.
A volcano has a much lower power output than that, but if the volcano is big enough, and heats a large enough area and the eruption last for a sufficient time it will sooner or later rival, or even surpass, the amount of ash ejected into the higher atmosphere. The height is more limited, however.
From Ashes to Ashes
The Skaftár Fires in its initial stage caused an 1800 meter high fire curtain, and it also heated a large area. This 1200 degree air and gasses sucked in air from all sides and created a tremendous upwelling of heated gas and air mixture. This created a bubble of denser than normal air to form above the eruption and inside the bubble, ash, dust, and shards of tephra was lifted. Anything heavy quickly fell down again as rubble, but the finer particles smaller than 2 millimeter hung there suspended at an altitude ranging from 13 to 18 kilometers, reaching the upper troposphere and lower stratosphere. The suspended particles were then moved onwards on the higher air currents at high speed.
The particles then quickly started to fall downwards as the heated air and gas cooled at altitude, and they fell according to size and weight. What is interesting is the time it took. Before this it was considered to have happened rather fast and according to the basic function of the nuclear fallout model due to the assumption that the atmospheric injection stopped at a maximum altitude of 10 km (boundary level between troposphere and stratosphere).
How do we then know that the ashes reached a higher altitude? We know this from the fact that we can find ashes in ice core samples across the northern hemisphere. The difference is that the heavier and larger particles are gone in the Greenland samples, but not entirely in the Svalbard samples. Incidentally, that also gives us the direction it travelled.
We know that the particle count in Svalbard is on average 250 particles per square meter, and that sums up to less than a gram per square meter. That does not sound a lot, until you start a bit of calculation. Now you say that less than 1 gram per square meter is not a lot to hang up in a Christmas tree. Only problem here… there are a lot of square meters if you sum up the polar zone and the temperate zone in the northern hemisphere. This is not a mathematics site, so the heavy math was done for you on Mathematica. The result is that the weight given by the mathematical model is 255 000 000 000 kg. With an average density of 3 000 that gives 0.085 cubic kilometers of rock injected high in the troposphere and lower stratosphere, recalculated into dense rock equivalent. Please note, this estimate assumes a uniform distribution of ash. In practice, close to eruption there would have been much more, and further away much less. But some of the ash stayed aloft long enough to circle the globe and end up in the Greenland ice. Fragments have been found in Ireland. During the eruption, ash falls were reported from the Faeroe Islands.
It is fairly uncommon to find volcanic tephra in ice cores in Greenland. The ash that is present is mostly from rhyolitic, explosive eruptions. The Öraefajökull AD 1362 eruption covered Greenland in ash. In contrast, ice-core tephra from basaltic eruptions is extremely rare: Laki was exceptional in managing to deposit some, and even for Laki, only a few fragments have been found (But this dominance of rhyolitic ash is only true for the holocene. Tephra in Greenland from the ice age is mostly from basaltic eruptions! Whether this was due to different atmospheric circulation or different eruption styles in Iceland is not well known.)
From sulphates to sulphates
The ash did not stay aloft for long. But now came the next pollutant. After the ash came the smell. In the Svalbard ice core mentioned above, the ash is the lowest level, and above it is a layer of sulphate particles. These came down as late as 6 to 12 months after the ash. This delay is for two reasons. Most of the high-altitude ash was ejected early on in the eruption when the eruption rate was highest. The later events also started explosively, but they were not as energetic. Therefore, the ash was lifted less high and came down closer to the vent. One reason for the initial explosions, by the way, is that the graben where Laki erupted was wet. There may even have been lakes. The lava erupted into water, with predictable results. Later in each eruption the water had evaporated, explosions ceased and effusion took over.
But sulphur is continuously emitted while lava flows, changing to sulphate in the rising plume. The sulphate ebbed and flowed with the rest of the eruption, but it kept going. Some of the sulphate stayed in the lower troposphere and mainly affected Iceland. But some went much higher, in the upper troposphere and lower stratosphere. These aerosols stayed aloft far more easily than the ash, and the sulphate kept coming long after the ash had gone. And so, the sulphate traveled easier and lasted longer. Some even reached the southern hemisphere. There was still significant sulphate over 10 kilometers up during early 1784. As Laki waned, so did the sulphate: very little was emitted after November 1783. But for 5 months, the North Atlantic was engulfed in a toxic haze. High latitude eruptions may be worse than tropical ones: they spread their gas over a smaller area (the northern temperate zone is smaller than the tropics) and so can give high concentrations. But Laki would have been bad either way: this was not a normal eruption: it was a mini flood basalt. The gas output was enormous.
Iceland itself was very badly affected. The sulphate fog killed the grass, and fluorine killed the cattle. Acid rain burned holes in leaves and left skin wounds on people and animals. Birch trees died across Iceland. 80 percent of sheep and half of all cattle died. This caused a lack of food: it is called Iceland’s haze famine. And it was followed by one of the worst winters Iceland has ever had. The population of Iceland declined by 21% because of this.
The haze was quickly noticed in Western Europe: ‘On many days after the 24th June, in both the town of Groningen and countryside there was a strong, persistent fog … very dense and accompanied by a very strong smell of sulphur.., many people in the open air experienced an uncomfortable pressure, headaches and experienced difficulty breathing‘. (Brugman) The leaves fell off the trees, making it look like October rather than June. Parish records show a significant excess death rate that summer in the east of England: more people may have died from Laki in England than in Iceland. However, the timing suggests this was not due so much to the bad air but rather to diseases later that summer and during the exceptionally cold winter. Europe’s death toll is an indirect rather than direct effect from Laki.
The haze appeared in mid June. There was high pressure over central Europe. The sulphate had been transported in the upper troposphere by the wind, and the downward airflow in the high pressure brought it to ground level. By July, the haze covered much of the temperate northern hemisphere, including China and Alaska. A second wave of the ‘dry fog’ came in August.
The effect on the climate was complex. Europe had heat and drought, with temperatures 3 degrees above normal. Unusually severe lightning was reported from England in July and August. But Iceland was cold, Moscow had June snow and Japan had such cold weather that the rice harvest was affected. Sulphate haze absorbs sunlight. If the sulphate is at ground level, this can heat the air and cause higher temperatures. This may be what happened in Europe, although it is clear that the atmospheric circulation was also important. The blocking high over Europe may even have been unrelated to Laki. But stratospheric sulphate heats the stratosphere, and as light can only heat once, the ground cools. And indeed, even in Europe and Eastern North America, after the heat came the cold. The winter of 1784 was extreme. As Laki faded, Europe froze.
Carl and Albert