Magma sponge

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.

Sandfellshæð, a lava shield of the Reykjanes Peninsula, formed 14,000 years ago. From Google Earth.

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.

Wikimedia, by Julien.leuthold.

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.

Trölladyngja, a shield volcano formed in one huge eruption. From Wikimedia, Heidi Soosalu.

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.

Skjaldbreidur shield volcano. Wikimedia, from Ingeborg Breitfeld.

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.

A magma chamber that can be visited on Iceland, from the Brennisteinsfjöll eruptions 1000 years ago. The image looks up from inside the open volcanic conduit of Thrihnukagigur. A lift use to move people up and down the 120-meter drop is visible in the image. Wikimedia by Dave Bunnell.

We are watching the creation of new ocean crust. Something hard to find anywhere that is not submerged under a few kilometres of water!

549 thoughts on “Magma sponge

  1. Very nice active flows on the Visir close-up webcam now.

  2. Maybe it is something that everyone has noticed already but there are almost no earthquakes in the whole greater Reykjanes area, I just checked it, theres not even a slight line along the rift when the eruption started.

    Seems to me this is pretty well proof the eruption is an open hole in the crust now, the dike is solidified, now it is just a mostly vertical conduit. One thing I want to know is if inflation is observed anywhere, the conduit is going to melt itself wider and that will increase the eruption rate if there is excess pressure.

  3. I’m just plodding through the usual Iceland drumplot charts.
    Am I the only one who finds the very noticeable tornillos in the Krisuvik chart over the last couple of weeks something to feel…slightly…bothered about?

    • More magma into the system? Earthquake activity started near Krýsuvík at the beginning of this volcano-tectonic episode.

  4. There is a very strong outflow from the crater now, looks like the flow rate has increased because there is also a lava flow going down into natthagi up on the hill, and the lava there is still slowly rising.

  5. Attached image shows increase in Natthagi in 19-hour period to 5 hours ago (then fog!): Gap between the pink lines is the increase, yellow line is estimate of exit elevation line.

    Flow is a little over halfway from 60m line to 65m on the elevation map. Only increased 1.5 meters, but also was somewhat less than a day. Maybe 1.5 meters to exit plus a little depth above that to flow into Litla-Borgarhraun, Maybe 24-36 hours from now? Rough calculations suggests flow into Natthagi was 10m^3/s.

    • Big caveat is new surface flows in Geldingadalir and Meradalir. Is it increased flow, or a blockage near Nar of the tube to Natthagi? Too foggy to see what is happening in Natthagi though the very rapid streams out of the cone do appear more voluminous than 12 m^3/s. Furthermore the latest flows into Meradalir started before the fog and Natthagi rose during that time.

      • There is a continuous spattering and upwelling to at least a few tens of meters high in the crater, and a fast moving surface flow. Natthagi seems to still be active too, and I did only just notice that one of the skylights to meradalir is spattering, which suggests it isnt a blocked tube there but rather actually one filled to its limit… Not sure how long we should expect this episode to last but it is for sure an increase in flow rate and a significant one at that, probably at least double, maybe that signal Clive talked about was an early precursor.

    • 1.5 meter thickening in one day requires about 7 m3/s, in my estimation. we can make a very rough guess that half the flow ends up here. That would give a total flow of 14 m3/s. It may be far off, but it indicates that what we see can be done with a constant eruption rate

      • Was only 19 hours, because after that Langihryggur camera too foggy for my map drawing, would guess around 9 m^3/s. I’m doing a little rounding given the inherent uncertainty in all dimensions. Day before yesterday unrounded was 11 m^3/s.

        I think the vast majority, 75%+, has been going to Natthagi until recently, almost no accumulation in days along the channel to Natthagi and only small amounts in Meradalir. I don’t think Nar’s output increased significantly, maybe 12-14M^3/s. Until about 18 hours ago when the new surface flows raised questions, thus my second comment.

    • That was excellent – and I spotted the “Crimson Permanent Assurance” boardroom bit (the short feature that accompanies “Monty Python’s The Meaning Of Life”) and it took on a whole new meaning.

      People aren’t wearing enough hats.

  6. I wish there was a view of where the new lava river is going. It is going to flow into natthagi but there isnt a view of it.

    I expect though if this keeps up the overflowing of the road will be in a matter of days, with an ocean entry by this time next week.

  7. Looks like the flow rate is most definitely higher, not only is there surface flows but the fountains are higher too. Higher fountains would mean more volatiles, and unless the magma composition has changed a bit more volatiles would mean higher effusion rate. The lake was not spattering this much the other day, now it is bordering on true fountaining again.

  8. Natthagi has seen basically nothing added in 12 hours, ending six days of filling. That frees up a substantial flow for elsewhere, probably the Meradalir flow. I think the Natthagi tube did indeed get blocked. If so, it could be several weeks before the last little bit in Natthagi gets filled and a rapid advance toward the road begins. Maybe Nar’s rate has increased, but if so, it is incidental, rather than the cause of the surface flows appearing.

    • Not really sure, there is lava on the hill. Might not be rising because the flow has to fill in further up the valley than before. The lava river that recently began though is flowing to natthagi anyway so if it is long lived we will see the ocean entry soon.

      Thing is the surface flow is really strong, the channel is open and sustained far beyond the vent itself, and is moving very fast too. It is hard to judge properly but the cone back in the beginning of May looked like it had a similar level of outflow at the peak of a fountaining stage, only now it is continuous and ongoing for many hours, as well as at least one major tube being filled to capacity.

  9. At 14:23 local time, there is thick brown smoke at the walls above Natthagakriki. A digger is in action at the moment, but has the wall been breached?

    • Possibly a second berm to reinforce the first?

      Should have posted this yesterday, Gutn Tog’s videos show this is what’s happening.
      And more height is being added today, Sunday. They’re a bit more serious about this barrier.

  10. Dragons mantra: “be nice” 😁.
    Meanwhile, interesting drumplot south of Langarjökul.

  11. To whom it may concern…

    After getting an email alerting me and a few other Admins it seems like I have to issue a warning here.
    We have one main rule (and a couple of small ones), the one that tends to get people banned like no tomorrow is this one:
    “BE NICE!”
    It is our rule number one.

    It is simple, and we have after 10 years found that it covers 99 percent of the problems. We have used it less than 10 times in 10 years.
    This is not that we are kindhearted, we are rather grumpy crusty old Admins in here. Instead it is you the readers and commenters that are fabulous.

    Today was one of those days that we had to issue a stern warning. It is the only warning that will be issued, so tip-toe extremely lightly from now on. Next instance and there will be skidmarks on the other side of the door.
    The comment in question (up above) was emptied out from the original comment, and there are two separate Admins comments shown in Italics in it.

    Now, rule number two. If you have any opinions on the moderation and have been issued a warning. Use the email, we do not discuss moderation actions in here. We ban people who do that.
    So, no comments on this comment please.

    Pinned the post using some true Dragon Magic and time travel! Yes, admins of Volcanocafé can time travel.

    • Future travel!
      It’s like 07:00 to 08:00 on the 19th in what is here the now!

        • Precisely. Real chilli sauce, not Tabasco. Not too hot though, no macho points to make.

          • I didn’t think dragons ate vegetables ….
            I am afraid this is a rather anarchic group.

          • Me thinks ketchup would be far less burning, if you catch my drift!


  12. Sunday
    20.06.2021 17:59:19 63.920 -22.220 1.1 km 4.0 50.5 3.3 km NE of Fagradalsfjall

    • Magnitude M 4.0
      Date time 2021-06-20 17:59:19.0 UTC
      Location 63.92 N ; 22.22 W
      Depth 1 km
      Distances 29 km SSW of Reykjavík, Iceland / pop: 118,000 / local time: 17:59:19.0 2021-06-20
      14 km NE of Grindavík, Iceland / pop: 2,800 / local time: 17:59:19.0 2021-06-20

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