Beginning on October 17th, a series of earthquakes struck the northeastern edge of the unusually
shaped Herðubreið volcano.
These quakes were quite suspicious as they occurred at a depth that was plausible to
represent a shallow intrusion of magma.
That is, a depth of around 5 kilometers.
This was made all the more unusual by the fact that the Herðubreið volcano which they
occurred beneath is not considered an active volcano.
Instead, for all intents and purposes it is considered an extinct volcano which last produced
a volcanic eruption more than 11,650 years ago.
Then, on October 22nd at around 9:40 pm local time, another far more energetic series of
earthquakes occurred in rapid succession.
The largest of these earthquakes was a magnitude 4.1, originating just at the edge of Herðubreið's
northern flank.
As of the writing of this video, the earthquake series has consisted of more than 550 earthquakes
and is still ongoing.
Due to the presence of an ongoing magmatic intrusion at the nearby Askja volcano and
the recurrence of earthquakes at this flat topped volcano referred to as a tuya has caused
several people to suggest that what is ongoing is another magmatic intrusion.
However, although the hundreds of still ongoing earthquakes could theoretically represent
a new magmatic intrusion, perhaps representing a sill or dike originating from the nearby
Askja volcano or maybe even being part of a reactivated magma chamber, I personally
do not think that these earthquakes are magmatic.
For context, the nation of Iceland is almost completely volcanic in origin, having originated
from volcanism caused by a hotspot in the mantle which somewhat closely intersected
with a divergent plate boundary.
On the island, the Eurasian Plate is moving eastwards and the North American Plate is
moving westwards.
This creates a series of cracks, rifts, and fissures, allowing for magma to intrude into
the surface at a path of least resistance, forming a chain of volcanoes.
Herðubreið, like other regional volcanoes exists in one of these rift zones being a
mere 25 kilometers northeast of the active Askja volcano which is currently at a yellow
alert level.
To the east and west of the Herðubreið volcano are distinct rift areas and fissure swarms.
To the east is the Kverkfjöll fissure swarm and to the west is the Askja fissure swarm.
Whenever one of these two segments produces movement, it causes increased strain at Herðubreið.
This then causes pressure to build up, subsequently being released via movement of a left lateral
strike slip fault.
Note how suspiciously perfect the quakes are as being along a straight line.
In this portion of Iceland, magmatic intrusions tend to result in earthquakes over a somewhat
broad area along a line.
On the other hand, tectonic earthquakes also occur along a line but are more clustered
than their counterparts over a narrower region.
Thus, although the earthquakes in the current swarm are largely occurring at 3 to 9 kilometers
depth, they did not occur alongside any notable ground deformation as shown in this insar
image.
If you are wondering how this unusually shaped volcano formed, this is how.
When magma erupts into a thick ice sheet, it initially forms a layer of material which
looks much like a dome out of pillow lava.
However, if eruptive activity occurs for long enough, it can breach the top of the overlying
glacier.
As eruptive activity continues, the heat melts significant volumes of ice, creating a thick
glacial lake around the volcano.
After the glacial lake develops, eruptive activity shifts to a more explosive phase,
depositing a rock type known as hyaloclastite.
Over time, the erupting vent which now has an unusually steep profile breaches the surface
of the lake, causing eruptive activity to change once again.
Instead of being highly explosive, more effusive to moderately explosive eruptive activity
occurs, sending lava flows to fill the area previously occupied by the lake.
The movement of the lava is partially constrained by the presence of the glacier, forming an
incredibly steep slope at its edge.
After the glacier retreats, the highly unusual volcanic feature becomes visible for all to
see.
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