The vast Saharan desert is one of the most barren places on the planet.
Between the numerous sand dunes is an occasional oasis with a supply of water.
However, there is something slightly more unexpected in the Sahara, volcanoes.
The Sahara contains more than a dozen active volcanoes which are easily spotted from satellite
due to their dark black coloration.
The most famous of these unusual volcanic systems is located within the country of Chad,
where it serves at the highest point in the country.
The volcano in question is called Emi Koussi, which is one of the most fascinating volcanoes
on the continent.
The Emi Koussi volcano is located in northwest Chad, where it is 300 kilometers south of
the border with Libya.
It is part of a vast dark colored volcanic plateau which forms much of the Tibesti Mountain
Range.
Volcanic activity has been ongoing intermittently in this section of Africa for approximately
28 million years.
The source of this activity is a hotspot located deep in the mantle beneath the continent.
Due to the unusually slow movement of the overall African tectonic plate, the hotspot
does not form a uniform chain of activity.
Rather, eruptions occur almost at random through regional crustal weak points, forming a series
of widely spaced volcanoes.
Emi Koussi is classified as a very unusual type of volcano called a "pyroclastic shield".
From satellite, this volcano can be viewed in its entirety, as it measures more than
75 kilometers across.
This vast volcano resembles a warrior's shield with a gentle slope across its edifice, which
averages between 4 and 5 degrees.
However, Emi Koussi is not an ordinary shield volcano.
You see, it never truly erupted a large volume of magma.
Instead, its gentle slopes are primarily made up of thick beds of ash called ignimbrite.
All of this ash was deposited in large volume eruptions which had a low eruptive column
despite its intensity.
Since the ash ejected during these eruptions did not travel high into the atmosphere, it
fell back to the ground and dispersed over a more narrow region.
Individual eruptions are responsible for distinct layers measuring up to 100 meters thick around
the volcano.
The Emi Koussi volcano began forming 2.42 million years ago, when a volume of magma
traveled towards the surface in this section of the Sahara.
Due to the composition of this rising magma, the eruption which subsequently occurred was
highly explosive.
As lava erupted onto the surface, several large explosions took place.
The vast majority of ash from this eruption fell in a 100 kilometer radius, thus forming
the beginnings of this pyroclastic shield.
Over the next 1 million years, thousands of similar eruptions occurred, thus building
the majority of the volcano's edifice.
One day, an unusually large eruption occurred.
As pyroclastic flows raced across the surrounding landscape, a tall column of ash was carried
into the atmosphere.
Due to the extreme volume of material erupted, a 10.5 kilometer wide section of the volcano
collapsed downwards by several hundred meters, thus forming a caldera.
Over the next 100,000 years, numerous eruptions built lava domes and volcanic cones within
this caldera.
Then, 1.3 million years ago another massive eruption occurred.
This eruption led to the creation of a second slightly wider caldera to the south.
Since then, the vast majority of eruptions at this volcano have occurred within the latest
caldera.
Later, a third caldera was formed measuring 3000 meters wide and 720 meters deep.
Other eruptions formed explosion craters termed maars, due to the interaction of rising magma
with groundwater.
On rare occasions, lava did breach the surface, leading to the construction of several cinder
cones.
The most recent eruption from Emi Koussi occurred just southwest of the caldera complex an estimated
4500 years ago.
A small sustained eruption led to the construction of a cinder cone, from which emerged a 3 kilometer
long lava flow.
Since then, no additional eruptions have occurred.
Today, we know that Emi Koussi is still active due to the presence of a series of hot springs
on its southern flank at a village called Yi-Yerra.
When this volcano erupts again, it will most likely be large, and lead to widespread destruction.
The main threat from such an eruption would be pyroclastic flows, which could in theory
travel more than 30 kilometers away.
In the event of a smaller eruption that forms a cinder cones, small settlements built into
the flanks of the volcano would be placed at risk, as lava flows have been documented
traveling up to 20 kilometers away from some ancient eruptions here.
Using the US Geological Survey method of rating volcanoes, Emi Koussi would be classified
on the upper end of the moderate threat category.
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