Out of the 12 supervolcanoes which have produced a voluminous explosive eruption rating as
a mid or upper end 7 or higher on the volcanic explosivity index in the last 2 million years,
one specific volcano is largely forgotten.
As, in southern Ethiopia approximately 205 kilometers south of the capital city of Addis
Ababa is a massive depression in the ground which has more than 200 meter or 656 foot
high cliffs.
Bordered on its east are a series of solid black lava flows which achieve their color
through an abundance of the volcanic glass known as obsidian.
This complex is known as the Corbetti Caldera, and last produced a volcanic eruption approximately
2400 years ago.
The Corbetti Caldera measures 15.0 kilometers long and 11.0 kilometers wide, which has an
easily distinguishable resurgent dome structure in its center.
Yet, despite the size of this vast caldera, it does not even closely compare to the size
of a much larger structure which is visible if we zoom out on Google Earth.
There, is the much larger Awasa Caldera which approximately 500,000 people live within today.
It measures 40.1 kilometers long and 28.5 kilometers wide, containing the large city
of Hawassa which is adjacent to Lake Awasa in its center.
Both of these calderas are considered to be part of the same volcanic complex, and we
can consider it to be active not simply because an eruption occurred from it during the Holocene
epoch but because since 2009 a section of the Corbetti Caldera has been uplifted by
6.6 centimeters or 2.6 inches a year, indicating the intrusion of approximately 100 million
metric tons of magma into the volcanic complex at a depth of approximately 7 kilometers.
The reason both of these calderas exist is that in this section of the continent, the
African tectonic plate is splitting in two.
Here, in what is known as the East African Rift, the crust is spreading apart to the
east and west.
This spreading thins the underlying crust and creates a path of least resistance for
material in the mantle to intrude into via a series of rift valleys.
Approximately 2 million years ago, a large series of fault lines developed, which shifted
the regional strain into the center of the rift valley.
Over time, this allowed unusually large volumes of magma to pool, which due to the interaction
with the surrounding crust and the partial melting which sourced the original melt was
highly silica rich.
This is why rhyolite composition lava lobes can be seen along with abundant obsidian.
Although the exact age of the oldest vent in the overall complex is unclear, an educated
guess would place it at around 2 million years old.
Before the first major caldera forming eruption occurred, there was likely a series of rhyolite
lava domes and lengthy basalt lava flows which constructed a loosely jointed volcanic edifice
which sent the majority of lava flows downslope to the west.
After approximately 900,000 years of moderate eruptive activity, something caused a massive
release in pressure and volatiles materials in the underlying massive magma chamber.``
A powerful explosion then occurred on the surface, sending a plume of ash quickly to
a height of 50 kilometers or 164,000 feet.
Part of the eruption column then collapsed downwards, travelling across the landscape
to a distance of as much as 90 kilometers away.
Then, due to the large volume of erupted materials which were ejected over the span of perhaps
100 hours, a large mass of overlying rock collapsed downwards to fill the empty space
in the previously full magma chamber, forming a massive caldera.
In total, 1,000 cubic kilometers of volcanic rock was ejected, meaning the eruption had
a volcanic explosivity index of an 8.
A rhyolite shield volcano was likely thereafter constructed on the northwest rim of this caldera
which would later be destroyed in another voluminous eruption which formed the Corbetti
Caldera 182,290 years ago.
In the aftermath of this latest caldera, lobes of viscous rhyolite lava erupted, which through
numerous eruptions formed another rhyolite shield volcano.
In the last 30,000 years, numerous lobes of glassy obsidian lava erupted on the eastern
section of the Corbetti Caldera with the most recent of such occurring 15,000 years ago.
At least two explosive eruptions have occurred in this complex since the end of the last
ice age, which constructed the Biftu Tuff Cone in 5,300 BCE and the Wendo Koshe Cone
in 280 BCE.
Although there is not an immediate threat of an eruption occurring from this volcano,
a paper referenced on screen suggests that this volcanic complex appears to produce an
eruption about once every 300 to 400 years on average at least during the last 2300 years.
Such future eruptions are most likely to have a volcanic explosivity index in the range
of a 0 to 3.
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