The Message In The Magma: The Geldingadalir Eruption Site Is Growing—What Have We Learned?

On March 19th, 2021, the world’s gaze turned to Iceland as fissures opened in the Geldingadalir valley—the first eruption on the Reykjanes peninsula in over 800 years.

Initially, scientists predicted the volcano would erupt for a matter of days or weeks. This was quickly proven wrong as its flow grew stronger and stronger, starting as a light trickle of lava and morphing into the pulsating geysir-like monolith it is today. Now it’s expanding—daily and rapidly. In fact, by the time you are reading this, it’s very likely the lava might have taken over the nearby road.

At the same time it has wreaked seismic havoc on southern Iceland, the volcano has given scientists an unparalleled opportunity for research. An accessible erupting volcano on the doorstep of a capital city is like winning the geological lottery and, in the months since the initial eruption, Geldingadalir has become a pilgrimage site for international scientists angling for a first hand reading on a new volcano.

While it’s clear big revelations surrounding the new eruption site are on the horizon, the first scientific news to emerge concerned the unusual source of the magma in the volcano, so it’s here where we will begin our exploration into the source of Geldingadalir—with the geochemistry team that was first on the scene at the eruption.

Photo by Art Bicnick

The uniform

“It’s unique in many ways,” geochemist Sæmundur Ari Halldórsson explains. He sits in his office at Háskóli Íslands, where the geochemistry department has all but dropped everything in order to study the new volcano. “In comparison to many other Icelandic units, lava flows and things that I’ve studied, it’s pretty remarkable,” he smiles.

Sæmundur pulls up a few graphs looking at various chemical tracers and element ratios in their samples from Geldingadalir in comparison with other famous volcanic units, such as the 2014-2015 Holuhraun eruption. For our purposes, the actual things they are measuring aren’t particularly important. It’s rather the comparison of other eruptions to Geldingadalir that is eye-catching. In fact, the differences between the trajectories of the Geldingadalir samples from the others are so stark that they actually prompt an audible “Wow!” from me.

“The 2014-2015 Holuhraun composition is very uniform throughout,” Sæmundur says, pointing at the relatively consistent line of dots marking the Holuhraun samples in the various graphs. “Bear in mind that when Holuhraun formed, a landmass the size of Manhattan was generated. So in six months, a new Manhattan was formed in central Iceland. It’s remarkable. But essentially you could go from one place to another—the Lower East Side to the Upper East Side—and pick up any basement rock and they’d be the same,” he explains.

“Same with the 1783 Laki eruption. We often talk about Laki-size eruptions, meaning enormous, some say even flood basalt events, but it’s the same. The rocks are consistent. But our new fellow here does this,” Sæmundur says, gesturing towards the Geldingadalir data points, which are anything but an ordinary line.

A remarkable shift

While the geochemical data points within the parameters Sæmundur is studying from the other eruptions are remarkably uniform, the data points from Geldingadalir almost look like a logarithmic scale. Not that the word ‘logarithmic’ is applicable here—it’s just to help you visualise the sharply curved trajectory of Geldingadalir’s data points in comparison to the consistency of the other volcanoes. So unlike those of Holuhraun or Laki, the geochemical makeup of the rocks at Iceland’s newest volcano has changed as the eruption has powered on—which is what shocked Sæmundur and his team.

“In comparison to many other Icelandic units, lava flows and things that I’ve studied, it’s pretty remarkable.”

“But what’s remarkable is not only the degree of variance, it’s also the systematics,” he continues, pulling up more graphs on the composition and timescale of the eruption. “The onset of the eruption is the 19th or 20th of March and you move through March and not much happens. Then April kicks in and you see the shift in compositions.”

But what does a shift in composition actually mean? Well, it varies by element. Take something like magnesium, one of the major elements in a rock, typically making up around 5-10% of it, and these particular changes aren’t so meaningful. But when you look at more trace elements, especially highly incompatible ones, more conclusive information can be extrapolated. There’s hardly any of these elements in the rocks and they are extremely sensitive to the arrival of new melts, so if the melt is consistent and from the same source, the ratio of these shouldn’t change. Therefore a notable change in the ratio of these elements, such as what scientists are observing at Geldingadalir, means something significant.

Looking at the ratio of radiogenic lead isotopes, which have extremely long half-lifes, is even more damning. “Nothing should change this,” Sæmundur says, of the jump in these data points in the graph. “In order to change this ratio, you first need a long time, we’re talking about hundreds and millions of years— you’re talking about large scale chemical fractionation events.”

“So to go from here to here over a few weeks in one eruption indicates [this] is driven by mantle processes,” he concludes. “And that’s the main message. For us, this is remarkable.”

Mantle processes?

The mantle makes up 84% of the Earth’s total volume. While it appears solid to us, within the geological time scale, the mantle moves as a viscous fluid, with the rocks bringing heat from the Earth’s core to its surface via convection. A mantle plume is an anomalously hot selection of material that rises from the core-mantle boundary to the surface, which forms hotspot volcanic regions—places like Hawaii and Iceland. Iceland, though, is unique, in that it’s also located on the Mid-Atlantic Ridge, a divergent plate boundary, which adds another layer to its complex volcanism.

So taking this all into account, just how unique is it to find a volcano that pulls up such deep magma in Iceland? Is Geldingadalir an anomaly?

“The geological record really is screaming at us, ‘Hey! I’m back again.’”

“We’ve surely seen this before. There are a few localities outside of the Reykjanes peninsula where we have magmas this primitive, very likely being extracted from Moho-located reservoirs,” he explains. ‘Moho’ refers to the boundary between the crust and the mantle. “But again, regardless of from what angle you approach the problem—if it’s from a geophysical, geochemical, or volcanological perspective—it’s the deeply derived character that’s really remarkable and unique here.”

Sæmundur chooses the Krafla volcano as a point of comparison. “From 1975 to 1984, there were repeated eruptions over a period of nine years at Krafla, but they were largely derived from a fairly shallow crustal magma reservoir,” he says. “Essentially you had magma coming in from below—obviously, it’s all formed by partial melting of the mantle—but they accumulated and were stored in a crustal reservoir. For how long? We don’t know but probably for a fairly short time and then were transported most likely laterally in the crust before erupting.”

The same occurred during the Holuhraun eruption. “There was clear evidence for storing of magma for considerable time in the years preceding the Holuhraun eruption,” Sæmundur relays. “But again, even though it was fed from a fairly deep crustal reservoir at about ten kilometres or so, it’s still crustally located. It’s within the crust.” He pauses. “You see the precursor. The magma comes together, it mixes, it homogenises and ultimately there is the eruption of one liquid that has uniform composition.”

But there’s the rub—Geldingadalir’s composition is anything but uniform, which indicates that it’s not from a stored homogenous crustal reservoir. “Here it’s different in every way,” he smiles.

The realm of speculation

So what does this mean? Did one magma source dry up only to be replaced by an even deeper magmatic source in the mantle? What exactly can we derive from the available information?

Well, it’s only been a few months of collection and analysis, so it’s here that we enter the realm of speculation. The data is coming in as fast as Sæmundur and the rest of the geochemistry team at Háskóli Íslands can handle it, but it’s too early to make concrete statements.

“Well, a first order interpretation of a ratio change like this is consistent with the arrival or pulling of melts which are extracted deeper,” Sæmundur explains. So does that means more than one source branching off?

“A heterogenous source clearly plays a role. Essentially a mantle underneath that is undergoing melting is not a single source. It’s a source that’s highly variable, that has lots of history,” he continues. “You’re pulling up melts. You’re extracting melts. You’ve depleted one residue and so forth. It has a complex history that reflects millions of years. So one of the things we are playing with is how do you bring this together?”

And this is what the team is now working on, Sæmundur explains, though any results are far from ready.

Hey! I’m back!

In terms of predicting the next steps of the eruption, Sæmundur can’t give any concrete answers, but he does point out that the Reykjanes peninsula does have rifting episodes, and looking at the historical patterns, we are due for a new one.

He’s referring to the age of settlement, from 800 to 1,100 years ago, when Iceland was a rather busy place, geologically speaking. In fact, there were around 20 eruptive events similar to this one during that time frame. “And we know from the geological record that the Reykjanes peninsula repeats itself. It repeats events of this magnitude,” he explains. “So the geological record really is screaming at us, ‘Hey! I’m back.’”

“So to go from here to here over a few weeks in one eruption indicates [this] is driven by mantle processes…”

But it’s still hard to place Geldingadalir within the historical context of the Reykjanes peninsula’s eruptions. It acts differently from the surrounding systems, which is something Sæmundur and his team noticed the moment they began studying the eruption.

“We realised very early on that this was unique, that it stood out in comparison to other recent eruptions. It resembles the big shield volcanoes,” he explains. Shield volcanoes are known for their fluid lavas, which aren’t particularly viscous, leading to large volcanoes that resemble shields—hence the name. Despite shield volcanoes being typical for divergent plate boundaries and hotspot locations, the majority of Iceland’s volcanoes are not of the shield variety—in fact, it’s been thousands of years since Iceland experienced one.

It’s still those volcanoes we could look to for more information on Geldingadalir’s future. “So if we want to find a unit in the area close to the eruption site that best resembles it, it’d be the large shield volcanoes. So then you can speculate, what does this imply for the duration of the eruption?” Sæmundur questions.

Photo by Art Bicnick

Geldingadalir, the golden goose

Regardless of how long the volcano erupts, or whether it takes over the road or not, the eruption at Geldingadalir is still a golden goose for scientists. Seemingly safe and easily accessible from the city, it’s the new Mecca for those who study the earth or those that just want to get closer to the mantle than ever imagined. Already, scores of researchers from Iceland—like Sæmundur and his team—and from across the globe have arrived at Geldingadalir, eager to get closer and closer to the history of our planet. It’s unfathomable just what a wonder the information provided by Geldingadalir will be for science—both worldwide and on this little volcanic island.

“Despite decades of studies looking at the eruptions on the Reykjanes peninsula, we still know so little, so this is really an eye-opener,” Sæmundur smiles. “And jeez! It’s just here in our backyard.”

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