What you are looking at is a spectacular volcanic crater in Iceland, measuring 558 feet or 170
meters wide and 180 feet or 55 meters deep.
Due to the abundance of volcanic deposits, you might think that this feature known as
Kerid Crater is a maar aka a volcanic explosion crater caused by shallow magma flashing overlying
water to steam and generating an upwards explosion.
And, for many years it was once thought that this feature was indeed a maar of a regional
volcano.
Yet, by looking at two maars with the name Viti, one at the Askja volcano, and the other
at the Krafla volcano, we can see that Kerid Crater is not a maar.
Instead, at Kerid Crater we can see a series of scarps in the landscape which look reminiscent
of those seen at calderas which have undergone major subsidence.
Yet, Kerid is too small and associated with too insignificant of a volcanic eruption to
be considered a caldera.
Instead, it has a different origin which makes this fascinating feature that formed only
a few thousand years ago a sort of geologic oddity.
For context, Kerid Crater is located 51 kilometers east-southeast of the city of the Reykjavik
in southwestern Iceland, where it is located adjacent to Route 35.
While Kerid is the most noteworthy volcanic feature in the vicinity, it is not the only
regional vent as a broader active volcano exists in this region.
Here, the Grímsnes volcano covers an area of 64.5 square kilometers, encompassing 12
different volcanic centers which each only erupted once, largely forming a series of
cinder cones in what were generally but not always moderately explosive eruptions.
These eruptions occurred in a brief geologic timespan between 9,000 and 5,000 years ago
due a similar reason which caused the 2021 and 2022 eruptions of the Fagradalsfjall volcano
on the Reykjanes Peninsula; rifting.
Here, the crust is spreading apart to the east and west at a rate of about 2 centimeters
a year.
The Eurasian Plate is moving eastwards while the North American Plate is moving westwards.
This allowed for magma to intrude into the crust due to the mantle plume underneath the
country, forming a chain of volcanic fields in what is known as the Western Volcanic Zone.
However, the Grímsnes volcano also intersects with the Southern Seimic Zone, which is a
chain of long strike slip faults.
These two zones act as a path of least resistance, allowing for molten rock in the mantle to
intrude into the crust, much like what happened during the eruption which formed Kerid Crater.
This magmatic intrusion began 6,500 years ago, and as it advanced upwards at a rate
of more than a kilometer a day, it caused the overlying rock to fracture, generating
hundreds and perhaps thousands of earthquakes, with the largest being perhaps around magnitude
5.0.
Intruding to shallow depths, the ground was pushed upwards by several inches, until the
ground began to smoke.
Soonafter, lava suddenly erupted onto the surface via a fissure which was 700 meters
in length.
Several fountains of lava emerged from the ground which continuously ejected material
up to a height of 100 meters.
Over time, instead of the entire length of the fissure erupting, specific sections became
dominant.
Since the erupting lava contained moderate amounts of gas, the eruption was not purely
effusive.
Thus, much like witnessed during the 2021 eruption of the Cumbre Vieja volcano on the
island of La Palma, intermittent explosions began alongside the lava fountaining which
ejected a rock type known as scoria up to a distance of several hundred meters distant.
Over time, four small cinder cone vents formed.
However, eventually the eruption reached a point where only the largest of these vents
was still erupting, the cinder cone which would eventually become Kerid Crater.
This eruption continued for several weeks, emitting more than 45 million cubic meters
of lava which flowed in masses of rope like pahoehoe and jagged aa lava up to a distance
of 4400 meters away from the cone.
Then, after the newly formed cinder cone which was covered in bright red scoria reached a
height of 80 meters, the eruption ended.
During the aftermath, lava drained away to a lower point in the crust, causing the overlying
cinder cone to partially collapse downwards almost like a sinkhole to fill the empty space
in a very shallow magma chamber.
The collapsed cinder cone experienced additional erosion over time, and was subsequently filled
by a crater lake as it dipped below the level of the area's water table.
Today, this crater is privately owned by a group but they thankfully allow tourists to
visit each year for a fee of 450 ISK per person with that fee being used to maintain the crater.
The Grímsnes volcano which Kerid is a part of has not erupted for 5,000 years, and in
my opinion appears highly unlikely to erupt again in the next 1,000 years.
Thanks for watching!
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