There is one question that has been bugging me lately. Why are there two types of eruptions in the Reykjanes Peninsula? Slow and fast. I have talked about this before, in here. Basically eruptions can be classified into two broad categories depending on how fast the maximum eruption rate is, which clusters into two end-members, 10 m3/s and 1000 m3/s. A very significant difference!
The fissure swarms of Brennisteinsfjöll and Tindaskagi produce slow fissure eruptions, which I have also called “wandering eruptions”, because of how I think a certain vent was active for periods of days/months and then the eruption shifted somewhere else, in a way resembling the eruption of Surtsey. The characteristics of these eruptions indicate that the eruption rate was around the boundary of tube-fed and channel-fed eruptions, about 5-10 m3/s. The current Fagradalsfjall/Geldingadalir eruption is of this type.
Reykjanes also has shield volcanoes. Dyngja. This type of eruptions are very voluminous, lasting years, decades, and maybe even centuries. The rate is probably around 5 m3/s or less. They are related to the slow fissures, possibly being just a scaled up version of typical slow fissures or simply a second stage that some of them develop. Some of the dyngja eruptions had multiple vents that each was active for a lengthy period of time, so that they may have been fissures. For example, Hallmundarhraun, the youngest shield, had 4 long-lived vents. Dyngja eruptions also seem more abundant near Brennisteinsfjöll and Tindaskagi. But they show up everywhere too though.
The other style is like a flood. A ~1000 m3/s effusion I’d say, by comparing with the similar eruptions of Krafla Volcano. Reykjanes, Krýsuvík, and Hengill fissure swarms practice this particular style. Intense but short. The area around the fissures is briefly submerged into a giant pool of silvery lava, and a long wall of fire shoots 50-200 meters into the sky. The 1000 m3/s peak doesn’t last long though, this could be over in a few hours. It slowly wanes afterwards.
The opposing end members are as different as the day and the night, as different as basalt lava flow eruptions can get. But why?
A matter of structure
I initially thought it could have to do with the primitiveness of the magma, how deep it comes from, and how pristine it is. I started checking magnesium/iron ratios. But no. There was no strong correlation between primitiveness and eruption type. It is true that primitiveness increases towards the west, towards Langjökull, but this seems to be due to the increase in crustal thickness. Eruptions in this whole area come from the MOHO, base of the crust, and the MOHO is deepest at Langjökull. This could give the wrong impression that shields are more primitive because the greatest concentration in the whole of Iceland is in Langjökull. This I saw later. Initially I checked only the area around Hengill and the different eruptions span the same range of primitiveness.
The clue was somewhere else however. It was crystals. Apparently shields may have higher crystal contents. One of them, Lambahraun, has a crystal content of 25-75%, really high. Some of the shields even erupt picrite basalts, like Vatnsheiði, a shield next to Fagradalsfjall. Pricrite is a term used for the highest olivine crystal contents. As far as I know picrite doesn’t happen in the fast fissure eruptions of Reykjanes. It turns out however that there is not too much data on crystal content so I couldn’t really find whether this correlation was really strong or not.
What could crystals mean? A mush. A mixture of molten rock and solid crystals, where the crystals form the greater percentage, is called a crystal mush. And this could explain A LOT. And when things really make sense that is good.
Imagine a wet sponge with water slowly dripping from it. Now imagine the sponge is a crystal mush, the water is the magma, and it is dripping upward because it is driven by buoyancy. You get an slow eruption. Normally this wouldn’t happen because the mush is trapped under the crust, but if a leak has formed, a dike, then you can drip all you haven’t done in hundreds, or thousands of years. The slow fissure eruptions and shields would be feeding from the mush. Magma would need to be slowly extracted from the pores between the crystals.
Fast fissure eruptions would follow the classical model. Magma comes from a magma chamber. This time it is more like popping a water balloon. A chamber is ready to unleash hell because its magma is all placed within the same space that can drain out rapidly when the leak happens. It doesn’t drain entirely, just until pressure drops low enough.
The geochemistry between the two types would not be too different because it is really the same magma. What changes is whether this magma is gathered in one place, or distributed over the sponge,
What are the implications of this idea? It means the magma from the Fagradallsfjall eruption is coming straight from the source. The decompression melt itself. Not stored. Melted and erupted right away. Mid-ocean ridges are underlain by areas of decompression melting. The mantle rises up to form new crust, and because the solidus point is lower at lower pressures, you will get partial melting, and therefore the mush or magma sponge.
The Mid-Ocean Ridge
Why should we favour this idea? An idea based on incomplete data due to the lack of it. First because as much as I’ve tried to look for all possible answers, you do need the magma sponge. The nature of the magma sponge could be many, it could be a mush, or perhaps it could also be a group of thin sill intrusions. But you do need something that releases the magma “drop by drop”, or otherwise you would get the lava flood.
It does match however with the distribution of dyngja eruptions in Iceland. They are found under the mature mid-ocean ridge segments, from Reykjanes to Langjökull, and from Vatnajökull to Tjörnes. They are lacking in the off-rift volcanism like the Snaefellsnes Peninsula. The Eastern Volcanic Zone also lacks them. It is a very young rift, the very powerful volcanoes Hekla, Katla, Torfajökull, Grimsvotn and Bárðarbunga are cutting through the crust like they were slicing through a cake, producing massive, fast fissure eruptions, and creating new Mid-Ocean Ridge, but which it isn’t ripe just yet.
Shield eruptions are known to have peaked during deglaciation, This makes sense. The deglaciation would have augmented decompression melting in the mantle magma sponge. And who would have been affected? Feeding from the magma sponge the shields would be the first to be benefited from these changes.
The size of an eruption has a reason. For example Holuhraun was so big because the summit of Bardarbunga underwent caldera collapse. The large calderas can feed large eruptions. Fast fissure eruptions of Reykjanes could be limited by the small size of the magma pockets from which they feed and the great depths that makes it impossible for them to collapse. But the magma sponge is almost limitless, so big that each segment runs through half of Iceland and beyond, continuously producing new melt. Shield eruptions can be in fact the largest ones in Iceland, even when they do not come from caldera collapses, they reach up to 50 km3, outperforming even the giant fissure eruptions of the Dead Zone.
Future of the Fagradallsfjall eruption
I still think we are going to see new vents open, even if it takes time. But I have already said this before. So instead. What does it mean to come from a magma sponge? The sponge is big, yes. But Fagradallsfjall still can only tap a certain portion of it, and melt production isn’t as big as it was during deglaciation times. Some say we will get a shield. I’d say a 1 km3 shield would be a realistic possibility. A really big shield like 9 km3 Hallmundarhraun, 25 km3 Skjaldbreiður, or 48 km3 Eríksjokull, seems too much for Reykjanes. This is something that can still happen in Langjökull. But a large shield hasn’t erupted in Reykjanes in over 5000 years, since Brennisteinsfjöll’s Leitahraun. Deglaciation was a while ago.
A cubic kilometre is also a common size for slow fissure eruptions, probably. The last eruptions of Brennisteinsfjöll and Tindaskagi were about this size. In that case the eruption could last a few years. Good for tourism! Bad for optical fibre cables, I guess.
We are watching the creation of new ocean crust. Something hard to find anywhere that is not submerged under a few kilometres of water!