The vast Saharan desert is one of the most barren places on the planet.
Yet, the region also contains something unexpected, towering volcanoes.
One of these volcanoes, Emi Koussi in the nation of Chad, is not only the highest point
in the country with its summit standing at an impressive 11,204 feet or 3,415 meters
above sea level but is also one of the largest volcanoes on the planet, covering an area
of 1,236 square miles or 3,200 square kilometers.
Yet, Emi Koussi appears to be in the waning stages of its lifespan, as it only contains
one lava flow which appears to be recent.
The same cannot be said of a separate volcano known as Tousside, where numerous lava flows
spread across an area of 300 square kilometers can be easily seen from orbit.
Based on the amount of erosion present there, these lava flows in my estimate were last
emplaced in 2,000 BCE, and thus are one day in the future likely to be emplaced again.
Tousside is a fascinating stratovolcano in a remote section of the planet, existing in
the northwest section of the nation of Chad where it is 995 kilometers north of Chad's
capital city, N'Djamena.
It is still debated to this day why Tousside and other regional volcanoes exist, so here
is what we do know.
Tousside is located within a region of the crust in both southern Libya and Northern
Chad which over the past 15 million years has experienced a very high degree of uplift,
totaling more than 500 meters.
While the exact cause of this uplift is unclear, we do know that between 80 and 150 kilometers
depth below the surface that the mantle is unusually warm.
Some have even postulated that this might be a low intensity mantle hotspot, but I disagree
with that assessment.
The uplift generated by the warm mantle weakened overlying rock, creating numerous faults which
intruding magma could use as a path of least resistance to erupt.
The Tousside volcano first erupted approximately 1.5 million years ago, when a volume of basaltic
magma from a deep source intruded into the crust along a lengthy fault line before explosively
erupting onto the surface.
As a series of strombolian explosions occurred, tall lava fountains were generated, and moderate
amounts of ash were ejected in intermittent explosions.
By the time the eruption ended, a broad area had been covered in a layer of black lava
rock.
Additional eruptions during the following 1 million years occurred in a varying number
of locations.
Basaltic magma erupted from scattered vents in effusive to moderately explosive eruptions
which were scattered on the outer edge of the volcanic complex.
Meanwhile, as the years progressed, more viscous magma such as andesite, dacite, trachyte,
and then rhyolite erupted closer to the center of the volcanic complex.
During the latter half of this stage, a lava dome complex known as Timi was constructed,
representing a temporary primary satellite vent.
However, at depth, an immense amount of pressure was building in Tousside's large volume rhyolitc
magma chamber, which now contained more than 100 cubic kilometers of molten rock.
Eventually this became so highly pressurized and volatile rich that it triggered a truly
catastrophic eruption 430,000 years ago.
After a massive explosion occurred, a plume of ash shot to a height of 50 kilometers or
164,000 feet into the atmosphere.
So much heavy volatile gas was released that it fell back to the ground as pyroclastic
flows, which advanced across the landscape, scorching all vegetation and rock in a 40
kilometer radius.
As perhaps tens of millions of cubic meters of rock were being ejected largely as fine
ash each second, ash began to quickly pile up as much as 100 feet thick next to the erupting
vent.
Then, due to the large amount of ejected material, the volcano's underlying magma chamber became
partially empty.
This caused a large amount of overlying rock to collapse downwards and fill the empty space
much like a sinkhole, forming the 13.8 kilometer wide Yirrigue Caldera.
Rhyolite ceased erupting after this VEI 7 eruption occurred, as instead trachyandesite
erupted.
Another powerful eruption, likely registering in as a high end VEI 6 eruption occurred 120,000
years ago, forming the 1 kilometer deep Trou au Natron caldera.
The modern Tousside stratovolcano began erupting around the same time, forming the first overlapping
layers of dark black lava.
A powerful 1,000 foot deep maar forming eruption occurred 80,000 years ago, forming a feature
known as Doon Kidimi.
Tousside's large trachyandesite lava flows were most likely emplaced by several closely
timed eruptions between 10,000 and 4,000 years ago.
We can assume that these features are younger than 14,000 years old, as that represents
when basaltic lava domes and a volcanic cone erupted in the Trou au Natron caldera, which
look more significantly eroded.
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