In Iceland, a volcanic eruption still appears imminent at the Fagradalsfjall volcano while
another volcano to the southwest known as Eldey just had its alert level raised to
yellow. Additionally, we now know how much magma is involved in the ongoing intrusion southwest of
Keilir; approximately 12 million cubic meters of material. This is only negligibly larger than the
amount which preceded the August 2022 eruption that lasted for a brief 19 days. Therefor,
if a new volcanic eruption was to occur, we could expect it to last for a similar length
of time. Or, it could last longer, as perhaps a sustained pathway for magma in a deeper chamber
might be created, allowing for a months long eruption to occur. We simply do not know yet.
All of this activity began when magma which had been pooling at depth under Iceland's populated
Reykjanes peninsula began shooting towards the surface. Since then, more than 7,000 earthquakes
have occurred as the intrusion of magma got shallower and shallower. By early July 6th,
this intrusion which had likely started at a depth of 12 kilometers in the crust was now only at a
depth of approximately 1 kilometer or 3,280 feet, while also having a length of approximately 2800
meters. While the rate of earthquakes have indeed been decreasing since day 2 of the earthquake
swarm, this does not necessarily indicate that a volcanic eruption won't occur, and here is why.
For context, the crust on the Reykjanes peninsula is approximately 15 kilometers thick. In this
crust is a fairly large body of magma at a depth of 12 kilometers. This magma seemingly
reactivated in early 2021, and supplies molten rock towards the surface via magmatic intrusions,
some but not all of which result in new volcanic eruptions on the peninsula.
As magma in these intrusions pushes upwards through the crust, it cracks overlying rock,
generating earthquakes along the way. Yet, when the latest intrusion seemingly passed
a depth of 1500 meters, the number of earthquakes drastically dropped. The primary reason for this
drop off is the top 1500 meters of the Reykjanes peninsula is more absorbent of differing stresses,
creating a sort of aseismic or dead zone. Thus, as magma moves past this threshold,
the rate of earthquakes decreases, even though it could still be advancing upwards. Possibility two
is that the intruding magma contacted a body of leftover partially liquid magma from the 2021 and
2022 eruptions of this volcano, which absorbed the stress for now, and will eventually resume
its upward motion, breaching the surface and erupting. Possibility three is the least likely,
as it could truly be that the new magmatic intrusion both entered an aseismic zone before
stopping its upward mobility, which if true would mean a volcanic eruption is not about
to occur. Such a scenario occurred in late December of 2021 to early January of 2022 when
a magmatic intrusion ceased its upward movement at a mere depth of 900 meters below the surface.
Luckily, for now, ground deformation appears to be ongoing at high rates,
with individual sensors detecting rapid ground shifts by as much as 18 centimeters or 7 inches
in a horizontal manner as rock moves to the side to make way for intruding molten rock.
During periods of time following swarms of earthquakes, it is not uncommon for other
regions within a 100 kilometer radius to also produce smaller intensity earthquake swarms.
This becomes all the more common when magma is involved, as several thousand earthquakes
causes stress on sections of some lengthy fault lines to be released, all the while,
placing more stress on more distant fault lines. This is most likely why a new earthquake swarm
began at a primarily submarine volcanic area known as Eldey which consists of two small
islands and numerous fairly hidden submarine vents. No magma movement or unusual ground
deformation was detected at this offshore volcanic complx, yet, right before midnight on July 6th,
a swarm of 400 earthquakes occurred, including a magnitude 4.5 quake. While this seismic activity
is almost certainly not magmatic in origin at Eldey, the slight possibility that it could be
magmatic was enough to raised its alert level from green to yellow on July 7th.
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