Reykjavík Grapevine - jún. 2021, Blaðsíða 9

9 The Reykjavík Grapevine
Issue 06— 2021
“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 volca-
nic regions—places like Hawaii and Iceland.
Iceland, though, is unique, in that it’s also
located on the Mid-Atlantic Ridge, a diver-
gent 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 Gelding-
adalir an anomaly?
“We’ve surely seen this before. There
are a few localities outside of the Reyk-
janes 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 prob-
lem—if it’s from a geophysical, geochemi-
cal, 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—obvi-
ously, 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 precur-
sor. The magma comes together, it mixes,
it homogenises and ultimately there is the
eruption of one liquid that has uniform
composition.”
But 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 specula-
tion
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 avail-
able 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 later 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 rift-
ing 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 speak-
ing. 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.’”
But it’s still hard to place Geldingadalir
within the historical context of the Reyk-
janes peninsula’s eruptions. It acts differ-
ently 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 volca-
noes 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 volca-
noes. So then you can speculate, what does
this imply for the duration of the eruption?”
Sæmundur questions.
A 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 Sæmun-
dur 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 Geld-
ingadalir will be for science—both world-
wide 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.”
"The geolog-
ical record
really is
screaming at
us, ‘Hey! I’m
back.’"