The solubility depends on the pressure the magma is under. Magma contains certain volatile substances which are dissolved in the melt, the main ones being CO2, SO2 and water. But why can’t the lava just come out steadily? The answer probably is gas. Layer builds upon layer adding height to volcanic edifice. Each peak of activity leaves a new layer of rock. The eruption may pause completely, shutting off the active lava flows. Sometimes fountains burst from the lava lake and spill lava in every direction.

The reasons why so much material has built up near the vent is that the eruption rates have been very variable. Making the shield with gushing and fountaining. 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. The eruption could stop tomorrow and end up being a very modest one, but it could also last years, or decades. How big it gets will depend on how long it keeps going. It could already be considered that Ragnar is a shield volcano, because of its growth mainly by overlapping overflows. Ragnar however, seems to me, is being much more enthusiastic about its upward growth, by piling up countless small overflows around its prominent cone. The only exception was Húsfellsbruni, which did behave a little more like Fagradalsfjall. 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 way Ragnar is rapidly rising the surrounding ground by erupting many short flows on top of each other is also unusual. The magma is perhaps very gas rich, as it comes from a considerable depth. This seems related to the great intensity of the fountains that constructed the currently active cone. 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. Its size is enormous, over 100 meters tall. The most striking aspect of the eruption is Ragnar, the name that some have been using for the currently active vent. This could have consequences regarding the amount of magma available in the area. Several markers of how primitive the magma was turned out to be remarkably high.Īdditionally 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. It rises up from a depth of 15-20 km in the mantle. Scientists who have studied the lava erupted from the volcano have reported its remarkable characteristics. The first sign that the eruption was special was the deep origin and primitiveness of its magma composition.

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. 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. 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. 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. To watch the birth and evolution of a new volcano is a rare opportunity. It has become increasingly clear to me that the current eruption is walking down the second path. The other was that the eruption would become a shield volcano, a dyngja. One was that it would turn out similar to the eruptions of the Brennisteinsfjöll volcanic system that took place 1000 years ago. 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.

Shield volcanoes diagrams