On June 12th, 2011, a series of moderate earthquakes struck in rapid succession in the desert border
area between Ethiopia and Eritrea.
The largest of these earthquakes was a magnitude 5.7, causing the ground to shake over much
of both countries.
Although it was not initially known at the time, a new volcano had just began an explosive
eruption.
These earthquakes were caused by the rapid upward movement of magma at the Nabro volcano.
Soon, a large explosion occurred at Nabro, causing a plume of ash to shoot 14 kilometers
or 46,000 feet into the atmosphere.
Part of this eruption column then collapsed downwards, releasing energetic pyroclastic
flows.
From Nabro's 1,300 meter wide crater emerged several vents which ejected large volumes
of lava.
As these lava flows advanced over the landscape, significant quantities of ash resulting from
explosive activity drifted to the south, west, and southwest.
By the time this eruption had ended, 12,000 people had been displaced, millions in damage
had been caused, and several adjacent villages were buried by more than a meter of ash.
This lengthy eruptive sequence represented the first known eruption of the Nabro volcano,
and was assigned a volcanic explosivity index of a 4.
Its longest lava flow traveled an impressive 17 kilometers distant, being composed of a
dark volcanic rock known as trachybasalt.
If we zoom out from the Nabro volcano, you can see that the vicinity is covered in other
large and highly active volcanoes such as Erta Ale which is erupting right now.
However, almost all of the other volcanoes are shield volcanoes which produce nonexplosive
eruptions.
To the contrary, Nabro is part of a chain of several adjacent volcanoes which almost
exclusively erupt in a highly explosive manner.
This contrast in eruptive styles in owed in part to Nabro's geologic setting.
For context, the Nabro volcano is located in a highly geologically active regional known
as the Afar Triangle.
It is a sort of extension of the east african rift zone, and owes its existence to the movement
of three tectonic plates.
Here, the Arabian Plate, African Plate, and Somali Plate are spreading apart from one
another, which thins the area's crust.
Molten rock in the mantle then rushes upwards to fill the empty space, which often occurs
as a series of large cracks or fissures.
From these fissures the material eventually erupts, leading to the creation of numerous
highly active volcanoes.
In the Afar Triangle, the majority of volcanoes are located on lineaments which trend to the
northwest, as this direction is perpendicular to the movement of the crust.
For example, Erta Ale is located on a ridge which contains 8 active volcanoes.
On the other hand, the Nabro volcano is located on a lineament which trends to the northeast.
This different trend likely represents an extension of the dominant rift direction in
central Ethiopia.
Eruptive activity has likely been ongoing is this area for a long time, which might
explain the highly explosive volcanism of Nabro and its neighboring volcanoes.
Zooming in on the Nabro volcano, its most prominent feature is its massive and steep
caldera whose walls are around 1,000 feet tall.
This caldera measures 8 kilometers or 5 miles wide at its longest point, and formed as a
result of Nabro's most explosive eruption.
Although we do not know when this eruption occurred, it almost certainly occurred at
some point in the last 100,000 years.
Surrounding this volcano for up to 40 kilometers in some directions is a thick bed of highly
welded rock known as ignimbrite.
This ignimbrite is attributed to Nabro's caldera forming eruption, and early estimates suggest
this material has a total volume of at least 20 cubic kilometers, indicating an eruption
rating on the 0 to 8 volcanic explosivity index of a 6.
However, it is quite likely that the vast majority of tephra from this ancient eruption
eroded away, which may have had an original volume in the 40 to greater than 100 cubic
kilometer range.
This implies that the volcano still has the capacity to produce similarly large eruptions
through an unusually massive magma chamber.
Although Nabro's latest eruption produced trachybasalt, this volcano has also produced
large volumes of trachyte and rhyolite, which formed lava domes on its flanks.
More recent eruptions have largely been confined to a series of southward facing cinder cones,
and three large summit craters.
Although the true geologic history of this volcano is unknown, what is clear is that
this volcano will continue to produce highly explosive eruptions for thousands of years
to come.
Thanks for watching!
If you would like to request a specific topic, please leave a comment below.
Additionally, I would like to thank my new patron Matt Whelan for supporting this channel.