Why The Eruption At Fagradalsfjall Is Unprecedented

 The eruption that began in Iceland's Reykjanes peninsula in March of 2021 has for the most part shocked scientists, volcano enthusiasts, and tourists alike. The volcano, it seemed at the time, was (or still is, depending on who you ask) part of the Krýsuvík-Trölladyngja system, which was experiencing its first magmatic eruption in some 800 years. The conventional wisdom at the time was that this was a relatively small eruption that would probably only last a few weeks. This has been proven wrong, the question now is why?

One need not look much farther than Iceland itself. Icelandia is a submerged continent who's geological history is incredibly complex. Until recently, Iceland was thought to be a volcanic hotspot underneath an oceanic spreading ridge. It turns out, that not only is Icelandia a continent, it may have existed since before the ridge, been submerged, only to split and have a hotspot form under it in the last several million years. 

This is significant, since original, ancient continental crust would provide more protection/insulation from magmatic intrusion, but since the continent of Icelandia drifted into (or was split in two by) an oceanic rift, the geology is unique. On one hand you have very thick crustal regions, and in the split middle, you have a thin layer of hardened material before you reach a mantle source, in some areas.

This explains in part the presence of stratovolcanoes on Iceland, as typically a stratovolcano is constructed by means of recycled or at least conglomerate material - mantle sourced, primitive magmas, mixing with more advanced, modern crustal material, which incorporates both H2O and organic components leading to explosive, tephra generating eruptions.

 Without any crustal 'recycling', you'd expect the entirety of Iceland, or at least most of it, to display mostly Hawaiian-style hotspot eruptions much like the current eruption at Geldingadalur. 

Back to Iceland's newest eruption, it has not gone according to conventional science, however there is precedent. 

A few days ago, Fagradalsfjall volcano had a brief period where ash was emitted, in what was thought at the time to be the end of the eruption. A similar event occurred at Kilauea, on the big island of Hawaii, at Pu'u'O'o crater. Magma had drained down rift, rapidly draining the small magma chamber under the spatter cone of Pu'u'O'o, and quickly, the roof of the chamber caved in, generating a rather interesting plume of ash, gas, and vapor. While magma has not yet returned to Pu'u'O'o, it can be reasonably assumed that it may, given the right circumstances.

Fagradalsfjall, however, is brand new! This is a magma dyke that has come from quite a depth, far deeper than any extant magma chamber in Iceland. It is not a magma chamber, rather it is a crack that goes down far enough to a mantle source, to build a magma chamber ABOVE land. It is building a new shield volcano. A Shield volcano forms layer-by-layer. The most famous example of which is probably Mauna Loa, the largest active volcano by volume in the world, on the big island of Hawaii.

Mauna Loa likely started life out just as Fagradalsfjall, as a minor crack in the ground, slowly growing and expanding, issuing fluid lavas, until a massive, shield-shaped mountain formed. Over millions of years, volcanoes like Mauna Loa grow their profile, building their own reservoirs of magma, and over time, building their own magmatic plumbing systems. The older they get, the less likely magma injections lead to eruptions (at least for most non-hotspot shields). 

Hotspot shield volcanoes, like Mauna Loa, however, can grow to immense proportions, and last for millions of years. While it is highly unlikely that the new volcano at Fagradalsfjall will be as dynastic as Mauna Loa, it is far, far too soon to tell.

What is clear, and what I will predict is that Fagradalsfjall (or whatever the official name for it will be in the future) is likely to be a very long eruption, with minor pauses, sometimes even months, in between. But this volcano is likely here to stay, providing Iceland with a bit of tourism excitement, but also questions in regards to how ti handles the eruption and disruption to infrastructure.

Already, Iceland has fought this volcano. It has constructed several dirt and rock berms by which to redirect the lava flows away from roads, homes and infrastructure. While the first two berms constructed did fail, several others appear to have learned from the previous berms, and have held. One is preventing the lava from spilling into a valley to the SW of the main vent, while the other is preventing lava from advancing towards the South Road highway, and several farm houses. 

Whether this is going to work or not is unclear, but one thing is certainly notable - Iceland is the only country that I am aware of to successfully fight and redirect lava flows when they threated critical infrastructure. This is quite an impressive feat, and civil engineers should take note. 

Today's activity at Fagrdalsfjall seems to consist mostly of lava-lake activity within the main vent, with some occasional overflows and large strombolian explosions. A new pattern has emerged in the past several days, from an easy-effusiveness to violent lava lake activity, followed by periods of outflow, then massive overflows. This is then typically followed by a period of perceived inactivity (although we cannot tell what is going on under the crater, it is assumed that 70% of the eruption and effusion is occurring out of view in underground lava tubes). 

The resumption of activity is typically begun by gas emissions, followed by the gradual appearance of lava fountaining, and eventual resumption of overflows. The time is not consistent between events, so it is unclear whether this is just a random 'sputter' or if the volcano is building up to something else, or building to an end. I suspect this will keep going on for several years, if not decades. But only time will tell.