Looking back to when Fagradalsfjall eruption started, I wrote a post about the Reykjanes Fires, where I speculated about how the eruption could end up being like. I mentioned two main possibilities. One was that it would turn out similar to the eruptions of the Brennisteinsfjöll volcanic system that took place 1000 years ago. The other was that the eruption would become a shield volcano, a dyngja. It has become increasingly clear to me that the current eruption is walking down the second path. A shield volcano in the making.
To watch the birth and evolution of a new volcano is a rare opportunity. I find it quite interesting. This is why even after writing three posts about the Fagradalsfjall eruption I still have so many questions and so much to say about them. This post will go about shield volcanoes and their varied types of activity, these I have divided into three “stages”, a simplification of the complex evolution of these volcanoes. I should say that by shield volcano I will be referring to those that are formed in one unique eruption, the term monogenetic is used in such cases. The contrary would be polygenetic, which are the volcanoes that go through many eruptions and long dormancies, so that they remain active for thousands or millions of years.
Why is Fagradalsfajall so different?
The first sign that the eruption was special was the deep origin and primitiveness of its magma composition. Scientists who have studied the lava erupted from the volcano have reported its remarkable characteristics. It rises up from a depth of 15-20 km in the mantle. Several markers of how primitive the magma was turned out to be remarkably high.
Additionally this eruption has broken a very long volcanic dormancy of ~800 years in the entire area of the Rekyjanes Peninsula and the Western Volcanic Zone of Iceland, since the Reyjanes Fires and the massive Hallmundarhraun eruptions took place. This could have consequences regarding the amount of magma available in the area.
The most striking aspect of the eruption is Ragnar, the name that some have been using for the currently active vent. Its size is enormous, over 100 meters tall. Typical eruptions of the Rekjanes Peninsula make cones that are 10-20 meters high, some may perhaps reach up to 50 meters, but as far as I’ve seen none of the eruptions that belong to the 1000 yrs old Reykjanes Fires have created any structure anywhere as tall as Ragnar already is, despite some of them being superior in volume. This seems related to the great intensity of the fountains that constructed the currently active cone. The magma is perhaps very gas rich, as it comes from a considerable depth.
The way Ragnar is rapidly rising the surrounding ground by erupting many short flows on top of each other is also unusual. The eruptions of the Reykjanes Fires, both the slow and fast fissure eruptions, generally send the lava flows far away, making thin, vast sheets. The only exception was Húsfellsbruni, which did behave a little more like Fagradalsfjall. Ragnar however, seems to me, is being much more enthusiastic about its upward growth, by piling up countless small overflows around its prominent cone.
It could already be considered that Ragnar is a shield volcano, because of its growth mainly by overlapping overflows. How big it gets will depend on how long it keeps going. Shields come in all sizes. The eruption could stop tomorrow and end up being a very modest one, but it could also last years, or decades. Many of Iceland’s monogenetic shield volcanoes are more than 500 meters tall, and many kilometres wide, with the very few largest of them reaching volumes of up to 50 km3, being formed in what must have been centuries-long eruptions.
Making the shield with gushing and fountaining.
The reasons why so much material has built up near the vent is that the eruption rates have been very variable. Sometimes fountains burst from the lava lake and spill lava in every direction. Afterward it goes quiet. The eruption may pause completely, shutting off the active lava flows. Each peak of activity leaves a new layer of rock. Layer builds upon layer adding height to volcanic edifice. But why can’t the lava just come out steadily? The answer probably is gas.
Magma contains certain volatile substances which are dissolved in the melt, the main ones being CO2, SO2 and water. The solubility depends on the pressure the magma is under. If pressure is reduced then the volatiles can fight their way out of the liquid and make gas bubbles. As magma rises towards the surface the enormous load of rock above it will be reduced and gas will go free, this will fill the conduit with pressurized gas bubbles, readily expanding, exploding onto the surface in a roaring jet of gasses, carrying along magma fragments. This is what happens in a lava fountain.
I think it can be agreed that the fluctuations in the volcano’s activity are caused by how the gas is released from the magma, as it is the gas what drives the fountain. The exact mechanism may be up to discussion. Here however I will present my views on the two types of activity pulses that Ragnar has demonstrated, gushers, and fountain episodes.
Gushers came in cycles 8-9 minutes long. These happened earlier in the eruption, while now are no longer being observed. These seem to have had to do with the lava pond above the vent acting like a lid. The pond contains magma that falls from the fountains and the bubbling, it has lost most of its gas and as such it is quite dense, it weighs down on the conduit below. Fresh magma is bubbling in the pond. If a batch of fresh magma punches through the whole thickness of the lake it will open a hole in the lid. The hole then reduces the weight acting on the conduit, the gassy magma expands and shoots through the opening, rises in a mighty wave that spills in every direction of the cone, or it sprays bits of magma high up into the air. For a few minutes the hole is open. At some point the pond collapses over the hole and keeps in line the magma column. Eruption pauses. Then magma builds up under the pond until it punches again. This cycle repeats over and over.
What evidence is there to back up this hypothesis? Gushers started at about the same time the lava pond was formed. As the cone grew higher and the pond deeper the gushers stopped, presumably magma could no longer punch through the increased thickness of lava. The 10th of July lava broke from the base of the cone and lowered the height of the lava pond and for a time the gushers returned. Now it is up to the rim again and they no longer happen.
Lava domes that occur in many volcanoes of the world also seem to behave in a somewhat similar manner. A lava dome can be unstable so that the sides often collapse. When part of a dome breaks away this reduces the load on the magma beneath. And what happens? The dome will often explode to bits. A vulcanian eruption it is called. Magma in a conduit is a ticking bomb that is being contained by the weight from above, if the weight is reduced the gas bubbles may go wild. Boom!
Presently Fagradalsfjall has settled into a very different mode of eruption. Fountaining episodes or “paroxysms”. The activity pauses for about 24 hours. Then activity rises gradually to a peak and an abrubt end follows, also lasting about 24 hours in eruption. I know this type of eruption all too well. Not just Fagradalsfjall, but Mauna Ulu, Pu’u’o’o, Etna, Pacaya, Villarica, Shishaldin, Pavlof, Fuego, have all behaved this way too, and I could really go on and on, naming every active basaltic stratovolcano in the world. With Pu’u’o’o they were termed fountaining episodes. For Etna and Fuego the term paroxysm has been used instead. Etna has been doing frequent paroxysms this year, and so has Pacaya, and probably other volcanoes I do not know of.
This is the way it happens, and it applies to all volcanoes that perform paroxysms. At first the magma in the conduit starts rising and nucleating gas bubbles. It may start to overflow a little from the lip of the crater, or maybe there are some small explosions or spattering which increase in frequency. The eruption picks up speed progressively. Why? When gas bubbles nucleate the magma in the conduit becomes lighter, so that the magma below rises more quickly and nucleates more bubbles, faster than they can escape through the walls of the conduit, more and more the magma becomes increasingly porous until it becomes a exploding jet of gas, a towering column of fire. At some point the magma column lowers enough for the walls of the conduit to collapse inward and clog the vent with rubble. The eruption ends quite abruptly. Black clouds of dust may occur as the walls come down into the collapsing pit. The volcano will now go dormant. However the rubble is weak, so as magma recharges the volcano, it will rise up through the debris, then another paroxysm may bloom into life.
Each volcano will do them in a different way, and even separate conduits that occur within a same volcano will have a characteristic pattern of doing paroxysms. The conduit width is very important, limiting how much speed the rising magma can achieve. Friction against the conduit walls reduce the speed. A tight narrow opening will allow less speed. Mount Etna for example has several summit craters that perform paroxysms, and among them Voragine, the big central one, has usually done the most violent ones. The Southwest Crater instead is young, formed in 1971, and started with small, low intensity events, but has been gaining strength, so that now in 2021, after 50 years of frequent activity, it has practically come to match the power of Voragine in performing eruptions. Each episode will make a new layer of material. With so many paroxysms Etna has grown 30 meters higher this year. Ragnar is not the only volcano growing up!
The storage of the volcano is also important. Fagradalsfjall has no proper reservoir in which to accumulate magma for an eruption so each episode can only throw out a little amount material, but this also means the incoming supply cannot be contained anywhere so that it has to come out to the surface, the episodes will follow each other quickly. At Fagradalsfjall there is an episode every 1-2 days. The Pu’u’o’o eruption of Kilauea during its first years would do much more voluminous paroxysms than Fagradalsfjall, but these happened about a month appart. Pu’u’o’o could release and store a much greater amount of magma that was collected within the summit of Kilauea.
In a way the occurrence of intermittent eruptions in Fagradalsfjall shows that the eruption has evolved beyond a simple fissure. Fissure eruptions do not make fountain episodes. Etna does not make paroxysms when it erupts from its flank. Etna’s paroxysms occur when it erupts from the four summit craters, the ones that remain semi-open and erupting over and over again, the ones that may have occasional explosions and puffs of gas, that also collapse into pits, and sometimes they do paroxysms.
Fagradalsfjall has evolved beyond its fissure stage of the eruption. Pu’u’o’o and Mauna Ulu, the satellite shields of Kilauea, are satellite volcanoes which have been observed from their birth to their demise. They went through three successive modes of activity. This is a simplification of their highly complex stories, but nonetheless I think useful because it can be generalized to other shield volcano eruptions, including Fagradalsfjall. A shield may go through one, two, or the three stages.
Stage 1. The fissure eruption.
How does a new volcano form? Well if no conduit exists then a conduit is needed. Dykes and sills are the basic form of conduit. If one such intrusion reaches the surface it will open one or more fissures, therefore every new volcano will start from a fissure, and this will of course be stage 1 in the making of a new one.
Pu’u’o’o started with a dyke intrusion and fissure eruption that lasted 20 days, in January 1983. During this stage a large amount of magma was intruded underground creating a dyke. The intrusion acted as a conduit channelling magma from other intrusions that exist within the East Rift Zone, and that are in turn connected to the summit storage of Kilauea, so that a pathway was established from the summit of Kilauea to Pu’u’o’o, part of which already existed, part of which was completed by the dyke.
Another example is Surtsey, 1963-67. An example of a different end-member of fissure eruption. As I have mentioned in some of my articles fissure eruptions can go faster or slower. Pu’u’o’o was a fast one. Kilauea is a mature volcano with a well developed plumbing that allows rapid transport of magma to the eruption site. Surtsey was a new volcano that must have had it much more difficult to get the magma, since it erupted slowly over a period of 3.5 years. The entire eruption of Surtey consisted of a fissure opening phase, with activity shifting between various fissure vents which erupted from the seafloor, and some of them formed islands. It wandered from one place to the other.
Despite being a fissure eruption, Surtsey did build a shield volcano, or better said two small shield volcanoes. Surtur and Surtungur. These two vents erupted enough material to form an island and erupt onto dry ground. They became effusive and started producing gusher events. Yes, like those of Ragnar. Lava would burst spectacularly from lava ponds and send lava overflows rushing into the ocean at up to 70 km/hour. These gushers built two twin shields, each 90 meters height.
Going back to the Fagradalsfjall eruption it was preceded by a lengthy intrusion, that went for 3 weeks before breaching the surface, on March 19, and establish a pathway for magma to flow from the depths of the earth to the surface. Of the multiple fissure vents that opened, Fissure 5, or “Ragnar”, was the most successful. Ragnar went on to erupt with an open channel that fed an aa flow and rubbly pahoehoe. Afterward came the gusher events. Initially gushers started like high fountains which went into the lava channel, but over time they took the form of spectacular surges which went down many sides of the cone. The surges added layer over layer. Ragnar’s shield growth had commenced.
The first pause of Ragnar’s activity on June 28, I take it as the start of the second stage.
Stage 2. Intermittent eruptions from the central vent
When an eruption pauses and the conduit becomes clogged, the magma may need a new opening. If lava rises up to the surface and erupts again without the need of a dyke then it can no longer be considered a fissure eruption. When Pu’u’o’o started Episode 2, or when Ragnar came back to life on June 29, they did not need a new dyke, instead they re-established the previous conduit, and as such they went beyond a fissure and entered a new mode of erupting. From now on the magma column may disappear back into the ground. The crater may look empty and dead. However magma rises up through the rubble and the volcano is reborn. This happens repeatedly. Pu’u’o’o had 46 fountaining episodes, not counting the initial fissure stage. I‘m not sure how many Ragnar has done already, they are happening frequently, every 1-2 days.
Pu’u’o’o had the storage of Kilauea as a reservoir to hold magma. It could erupt a lot, and very fast. Initial episodes erupted just over 100 m3/second, but the intensity grew systematically so that the final fountain episodes were erupting at more than 1000 m3/second.
Other eruptions of Kilauea that involved high fountains, Kilauea Iki 1959 and Mauna Ulu 1969-74 also increased the intensity of the fountains over time. This seems to have had to do with a widening of the conduit, which reduced friction against the walls, and allowed the magma to achieve greater speeds. This widening was well recorded with Mauna Ulu. In just about a year the conduit of Mauna Ulu grew from being a 1-4 m wide dike, to 100 meters wide double cylindrical pipes. By this time the eruption was into “stage 3”. Fountains had shut off much earlier. Perhaps because the conduit was so wide that it could no longer build enough gas to burst into a jet.
When a conduit is very wide, and the magma fluid, the top becomes a lava lake which is in permanent circulation, moving up and down, the places where the magma comes up usually have a still surface, those spots where magma sinks into the depths have spattering that releases gas into the air. A convecting lava lake degasses efficiently, which is perhaps the reason why they are never seen to burst into high fountains. Convecting lava lakes are for example the pre-2018 Halema’uma’u of Kilauea, the former lake of Nyiragongo, Erta Ale, Masaya, or the 4 former lava lakes of Ambrym.
But that is what may happen in “stage 3”. Stage 2 instead consists of eruptions from a central pipe that feature fountains. Fagradalsfjall doesn’t possess magma storage which places a limit on how big or fast the episodes can be. This makes many small episodes that are rapidly rising up the ground next to the vent.
Many of the largest shields of Iceland seem to be made many surface streams of both pahoehoe and aa lava which issued from the top of the volcano and ran downslope. They seem to have been formed by many episodes of eruption. This seems similar to Fagradalsfjall now. The large shields often have a height of ~500 meters above their base.
The shields of Iceland are most of them many thousands of years old, and are not well preserved, but I have found several monogenetic shields in Arabia that are similar, and very young, perhaps historical. One the biggest ones is Jabal al Qidr, located in the Harrat Khaybar volcanic field. Jabal al Qidr is 300 meters tall. It is formed mainly by black flows of aa lava. They grade upslope into pahoehoe flows, that form a more silvery collar around the crater of Jabal al Qidr. The flows probably flowed in smooth rivers near their source but were rapidly shattered into clinkery aa as they rushed down the steep slopes of the shield.
Stage 3. Sustained effusion from flank vents
Both the Mauna Ulu and Pu’u’o’o satellite shields of Kilauea entered a final phase of eruption during which the main conduit fed radial dykes, which resulted in flank vents. These openings in the flanks erupted slowly but steadily, and fed lava tubes that reached very far away, often pouring lava into the ocean. The main side-vent of Mauna Ulu was the Alae shield, the lava that continually flowed underground between the two locations started eroding the ground and eventually formed a small canyon 530 meters long and 40 to 60 meters wide, floored with hot, steaming rubble, through which lava could sometimes be seen.
Pu’u’o’o also had numerous flank vents throughout its 35 year long eruption, and practically all the volume of the eruption was produced by the flank vents not the summit. The long steady eruptions eroded the flanks of Pu’u’o’o too. The erosion formed a complex of pit craters known as Puka Nui.
The top of Pu’u’o’o and Mauna Ulu would feature an open convecting lava lake, or else the conduit was roofed over and erupted from many small openings, ponds, and spatter cones, which sometimes produced small fountain episodes that were like miniature versions of the episodes in their second stages. When the volcanoes drained, the top collapsed into a pit crater, sometimes quite deeply. One collapse of Mauna Ulu led to a months-long dormancy, afterwards however the magma had no trouble in rising up through the rubble, quietly without a single earthquake, nor any sign of its movement, and resumed its activity.
All or almost all shield volcanoes of Iceland do show extensive tube-fed flows which form an apron around the main cone. Usually it is not clear though, if these came from the summit, or from flank vents. Or if their exact timing in the eruptions. There is one shield, Kollóttadyngja, which has a 2 km long lava erosion feature, consisting of a chain of pits. It looks like this chain possibly marks the pathway from the shield to vents on the lower southern side, where vents issued lava tubes which in turn reach 25 kilometres away. This activity seems to be have been among the latest events of the eruption. It is hard however to reconstruct an eruption based only in its morphology.
Some shield volcanoes, like those of the Snake River Plain in the U.S., do show clearly that first a central shield was constructed from many small overflows, and then later lava erupted from secondary openings along the lower flanks of the shields and they fed a voluminous apron of inflated pahoehoe, with lava transported through tubes inside the flow.
Another shield volcano in island of Santiago, in the Galapagos Islands, features many radial spatter ramparts, dykes that propagated from the central conduit tore cut open the slopes and in turn feed lava tubes and flows.
Jabal al Qidr, in Saudi Arabia, has a group of flank vents on its northern slope. They formed towards the later part of the eruption. These vents however did not erupt slowly, instead they produced rapid floods of lava. Sheet pahoehoe flows that must have been formed violently. One possibility is that they were a series of catastrophic drainings of the lava lake at the top of the shield.
There is another vent 8 kilometres north of Jabal al Qidr, which looks of about the same age. It erupted mainly tube-fed flows that reached distances of up to 50 kilometres from the vent, and must have formed at very low eruption rates that were held steadily for a long time. This vent could have been a flank eruption of Jabal al Qidr, but this is hard to know for sure.
If the Fagradalsfjall eruption goes on for long enough it is possible that it will go into this mode of activity. Although it would be best for people who live near the volcano that it reaches no further than stage 2. Tube fed flows may reach very far away and cause destruction in populated areas.
This is quite a complex eruption that we are seeing unfold, and there might be a lot to be learn about how volcanoes grow. and how their plumbings work. It is hard to make any estimate of its duration, it could end tomorrow or it could last decades. Shield volcanoes do come in all sizes. We shall see. As much as the fog and the night allow.