Guest post by Chad
As is now abundantly clear, a new eruptive cycle on the Reykjanes Peninsula has begun. While an isolated eruption did happen in the ocean off the end of the peninsula in 1783, there has not been an eruption on land since 1240, today 781 years ago.
The eruption ongoing now has ratherbroken the regular cycle of eruptions at Reykjanes. Normally those begin at Brennisteinsfjoll and go west over a few hundred years, but this vent is on Fagradalsfjall, a mountain right in the middle of the peninsula. This area would not be expected to activate for at least a century yet, but here we are, watching the camera religiously as this new volcano grows before our eyes.
The magma is being generated at a great depth, nearly 20 km or twice as deep as normal eruptions in this area begin, and by mantle that is nearly totally melted to result in a lava that has a very high concentration of olivine. This sort of basalt even has its own name, picrite, and it is the closest thing to the ancient komatiite lava that erupts on the Earth today. This has been interpreted as a possible case for a shield volcano to form at, something which has not happened here for many millennia, but I think there are rather more profound implications for this. The magma is fluid and extremely hot, well over 1200 C, and that is enough to allow both physical and thermal erosion of the interior of the conduit, which will serve to only allow more magma to flow, in a compounding effect. Direct mantle eruptions are rare, but one that is currently ongoing if hard to observe is going on under the ocean near the island of Mayotte near Madagascar. It is not particularly intense, but since it began in mid 2018 some 5 km3 of lava has erupted to date.
The magma for eruptions like this is created by decompression melting as the pressure drops, a self feeding cycle that is limited only by the rate of mantle flow to the area. It is a self sustaining machine, the Hell Machine as I like to call it.
Hell Machines are a rare sort of eruption. Like said above they are perhaps best categorised as some sort of hybrid between a shield volcano and a lava flood fissure eruption. The eruption rates may be sustained at a relatively high rate for a long time, well above the rate seen in the formation of shield volcanoes but also never getting to the colossal rates seen in the big fissure eruptions, at least not for any long duration.
Mayotte is an example of a Hell Machine that is erupting now, it is in the deep sea and it may be that most such eruptions are, but there is a historical eruption like this that occurred on land and it offers some tantalising clues.
On the 1st September 1730 an eruption began on the island of Lanzarote, in the Canary Islands. It was relatively large, creating a cinder cone that was ultimately named Caldera del Cuervos, as well as over a few months the similarly sized Caldera del St Catalina and Pico Partido cones. This constitutes what would have been a typical if somewhat larger than average eruption in the Canary Islands, but we now know this first eruption managed to set off the Hell Machine. Eruptions continued into the next year, though it had declined from the initial high rates as expected, the flows were still fed at an impressive volume to sustain over a year. Submarine eruptions well off the west coast and going up all the way to just inland beginning in June of 1731 marked the completion of the hell machine, it was now in full working order, and eruptions marched eastwards back onto land to flood even more of the island. Observations after this are poor as few inhabitants stayed beyond this point but the eruptions continued into 1736, erupting some 5 km3 of lava, constituting the large majority of the volume of all eruptions in the Canary islands in recorded history. Lanzarote has no active central volcano, and has not possessed one for millions of years, the eruption of 1730-1736, or Montanas del Fuego as it is known locally, was fed out of the mantle directly by extensive melting under the base of the crust. Perhaps most notably, all the eruptions from the late Pleistocene and Holocene on Lanzarote before this were on the northern part of the island, the area where the Montanas del Fuego eruption occurred had not been volcanically active for possibly several tens of thousands of years or more, a certain similarity it does share with Fagradalsfjall.
The first sign of a hell machine is that the eruption is persistent and erupts out of the mantle directly, something clearly the case already. An eruption should also be a rifting event, where the rift is kept open by new magma, and this also seems to be happening, as a dike has formed within the crust going from less than 2 km depth down to at least 7 km, with possibly many feeding points below this that are not resolved. The very high temperature of the lava will also serve to enlarge the pathways, something that may take longer than a week to become evident but which is expected.
So far our eruption is quite small, even tiny, but it has a big future. The first obvious sign could be that the vent increases in output, or that it stays constant but another vent opens. We may already be seeing this, as the vents are rather a lot larger than they were to begin with, and the output appears to have increased. Deflation is not observed along the dike either, which would suggest the eruption is being fed by continued decompression melting in the mantle, a process that may increase accordingly with the eruption rate and set the hell machine in motion.
What will become of the Reykjanes peninsula if this occurs? Perhaps the most direct consequence is that land within perhaps 10 km of Fagradalsfjall is potentially at risk. There is no danger to Reykjavik but Grindavik may suffer if not from the lava then from the gas emission. The effect it will have on the normal Reykjanes cycle is also unknown, most typically the section of the transform fault at Brennisteinsfjoll is the first to go but this time around the section going through Krysuvik was set of by the recent intrusion, leaving Brennisteinsfjoll in a precarious situation. Decompression melting at Krysuvik may well have already begun now in preparation to its awakening, and Brennisteinsfjoll could wake up rapidly following its eventual quake. Eruptions here will resemble those from the Reykjanes Fires, but taking a back seat to the lava flood that is just beginning now at Fagradalsfjall. We are watching history creating itself.
Perhaps this will not come to fruition, it may turn into a shield instead, or even stop. But with all the signs it is looking like the future is grand for the new little volcano.
Chad – 2021