Over the history of Iceland, the country has been the site of a number of very large and
destructive explosive volcanic eruptions.
However, the most significant volcanic eruptions within Iceland were not strictly explosive,
but rather, effusive through vast areas of tundra being covered in molten rock.
A recent example of one such large volume effusive eruption occurred at Laki in 1783.
During a timespan of 8 months, 140 vents along a 25 kilometer long fissure output a true
flood of basaltic lava.
By the end of the eruption, 565 square kilometers of land had been covered in 14.7 cubic kilometers
of molten rock.
This eruption had widespread consequences and is blamed in part for causing the French
revolution.
However, Laki's eruption was not the only such "mini flood basalt" style eruption to
recently occur in the region.
In fact, the three largest effusive eruptions of the last 10,000 years all occurred in the
same section of Iceland.
The largest of these eruptions originated from a lengthy fissure of the Bardarbunga
volcano, and covered a whopping 950 square kilometers of land over the span of a year.
So, why has this seemingly small section of Iceland produced such unusually large eruptions?
Will a similar eruption occur again?
This video will answer these two questions and discuss the group of largest effusive
eruptions during human history on the planet.
For some context, the specific region of Iceland I am referring to is a 40 kilometer wide and
75 kilometer long section of land located between the Grimsvotn and Katla volcanoes.
Zooming in on this area, you can observe many other elongated fissures which originated
from eruptions similar to or larger than the 1783 eruption of Laki.
So, why are there so many fissures here?
In Iceland, the crust is slowly spreading apart to the east and west in what is called
a divergent plate boundary, as the Eurasian plate is separating from the North American
Plate.
This splitting creates large and elongated cracks in the crust which allows for magma
to erupt from them onto the surface, creating a fissure eruption.
However, if you look around Iceland you can find other elongated fissures such as on the
Reykjanes peninsula.
Yet, incredibly large volume eruptions do not occur there.
While these two areas both represent active spreading centers, the so called eastern volcanic
zone is most active for a single reason; it is closest to the supply of underlying molten
rock, the center of the underlying Icelandic hotspot in the mantle.
This hotspot is centered between the Bardabunga and Grimsvotn volcanoes, but large fissure
eruptions do not occur there as the crust is unusually thick in that location.
In contrast, the area with numerous fissure eruptions has an unusually thin section of
crust, meaning the area represents a path of least resistance.
Five of the regional volcanoes have produced voluminous fissure eruptions, although only
three of these have produced large effusive eruptions since the end of the last Ice Age.
In 6650 BC, Bardabunga covered 950 square kilometers in lava, burying what is now the
town of Selfoss.
In 935, Katla produced the Eldgja eruption, which covered 800 square kilometers in molten
rock.
Then, in 1783, the Laki eruption covered 565 square kilometers in superheated lavas.
Thus, we can estimate the frequency of these large eruption as occurring about once every
3,000 years.
More such large fissure eruptions will occur again and cover vast swaths of Iceland in
molten rock.
Thus, I would strongly recommend against building any future cities in this area of Iceland
which I have outlined on screen.
Of course, these lengthy fissure eruptions are not true "flood basalts" as those are
far more voluminous.
As an example, a typical true flood basalt eruption will release 350 cubic kilometers
of lava, whereas the largest of the Holocene mini flood basalts only produced 25 cubic
kilometers of rock.
Regardless, these fissure eruptions represent a scenic geologic wonder which are unparalleled
elsewhere on the planet.
I hope that you enjoyed this video.
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