In the last week, a steadily increasing number of earthquakes have occurred on the Reykjanes
peninsula of Iceland.
Unlike several previous earthquake swarms, ground uplift was also detected meaning that
this sequence was not tectonic in origin.
Rather, it was caused due to a new intrusion of magma at depth.
Unlike the two previous episodes of magmatic uplift, one of which resulted in a volcanic
eruption, this was not occurring adjacent to the Geldingadalir volcano.
Rather, this new uplift is centered 9 kilometers west of this vent at the site of the Svartsengi
geothermal power plant.
As a result, the alert level of the Reykjanes volcanic system was finally raised from green
to yellow on May 16th and an uncertainty phase was declared.
A new volcanic eruption in the immediate future is now quite probable and this time abundant
structures are at risk.
A mere 3 kilometers to the south of this uplift is the city of Grindavik, which has a population
of 2,500.
Volcanism within Iceland is dominated by the fact that the nation sits on a divergent plate
boundary under which a hotspot that supplies molten rock to the surface is located.
Here, the Eurasian Plate is spreading apart from the North American plate at a series
of spreading centers on the island.
One of these spreading centers is located on the Reykjanes peninsula, which has resulted
in the creation of 4 loosely defined volcanic complexes which each span an area of several
hundred square kilometers.
Each of these centers produce infrequent eruptions during intense periods of rifting which allows
magma to intrude shallower into the crust, eventually erupting on the surface through
what is known as a fissure eruption.
This is exactly what occurred during early 2021 which resulted in the 6 month long Geldingadalir
eruption which is considered to be a part of the Krysuvik volcanic system.
Due to the northeast trend of many faults on the peninsula, it can be difficult to determine
whether a sequence of earthquakes is tectonic or magmatic.
Thus, to determine if a magmatic intrusion is truly ongoing, we need to see if ground
uplift is occurring.
Intrusions of magma cause the ground to uplift much like a mole digging a tunnel in a grass
yard.
Small amounts of ground uplift have been ongoing north of Grindavik since March 17th, 2022.
However, the rate of uplift greatly accelerated on April 28th, hinting at a change in the
overall volcanic system.
Since then, the ground has been pushed upwards by as much as 3.3 centimeters.
Although it is occurring over a broad region, the uplift is clearly centered near the geothermal
power plant, potentially because a pathway for shallow magma already exists there.
While it represents the most likely site for an eruption to occur, an eruption is also
possible where the most numerous cluster of recent earthquakes have been centered.
Regardless, this latest intrusion of magma which is the 3rd to occur since the beginning
of 2020 does not necessarily signify that an eruption will occur.
A useful comparison to the modern Reykjanes peninsula is the Krafla fires eruptions which
occurred between 1975 and 1984.
During that period 18 magmatic intrusions occurred, but only 6 resulted in new volcanic
eruptions.
Thus, we could reasonably put the odds of a new volcanic eruption north or northwest
of Grindavik as at least 33% which is quite high.
I need to note that if an eruption does occur, there is a likelihood that it might have an
explosive component to it unlike what occurred at Geldingadalir.
Of the 16 recorded volcanic eruptions in the last 12,000 assigned a volcanic explosivity
index, only 5 were rated as a 0 like Geldingadalir's eruption.
Other episodes were assigned VEIs of 1, 2, 3, or even 4.
A VEI 4 eruption would be quite devastating, indicating an eruption at least 10% the size
of the 1980 eruption of Mount Saint Helens.
While an eruption of this magnitude is highly unlikely, it is quite likely that a new eruption
would cause minor ashfall to occur.
Such a new eruption would create a chain of cinder cones, explosion craters, and spatter
cones along a northeast trend of activity.
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