What you are looking at is one of the planet's most impressive examples of a fascinating
geologic formation.
Known as a caldera, this specific feature called Crater Lake in Oregon formed a mere
7700 years ago when a 12,000 foot or 3,660 meter tall volcano completely collapsed due
to one of the planet's largest recent explosive eruptions.
This drained the volcano's underlying magma chamber, leading to the ground overlying the
volcano collapsing downwards, which formed a massive depression in the ground, much like
a sinkhole.
Other calderas can be found around the planet and originate in eruptions which are typically
at least ten times larger than the 1980 eruption of Mount Saint Helens.
One such caldera within Africa is unusually massive, and formed during an eruption when
a volcano taller than Mount Kilimanjaro completely collapsed.
This ancient snow covered 19,700 foot or 6,000 meter tall volcano is now the site of a massive
depression in the ground known as Ngorongoro Crater.
It is today one of the best national parks for viewing savannah wildlife in Africa, containing
thousands of wildebeest, zebras, elephants, flamingoes, and numerous lions.
The Ngorongoro Crater is located in north central Tanzania, where it is centered 60
kilometers southwest of the unusual Ol Doinyo Lengai volcano.
This caldera measures 21 kilometers long and 18 kilometers wide and has a very prominent
caldera rim.
This caldera rim displays an adjacent drop of 1,000 feet, representing the edge of Ngorongoro's
depression.
Looking around the area we can see other similarly massive albeit slightly smaller calderas which
formed in the last several million years, hinting at the explosiveness of eruptions
in the region.
The reason why the Ngorongoro volcano formed relates to a large rift zone in eastern Africa.
There, the crust is spreading apart to the east and west in several locations in what
is termed the East Africa Rift Zone.
This allows for molten rock in the mantle to travel upwards into a thin section of the
crust, eventually erupting on the surface via a fissure eruption.
The initial fissure eruption occurred from a northeast facing fissure 2.25 million years
ago, erupting fluid basaltic magma directly from the upper mantle.
As fountains of lava shot several hundred meters to the sky, the fissure continued to
propagate and expand.
Eventually, molten rock solidified on the active fissures, causing freshly erupted material
to only erupt at a single central vent.
Over tens of thousands of years, a broad shield of black volcanic rock was constructed.
However, the underlying magma slowly became more viscous, causing its composition to change
from basalt to trachyandesite.
This resulted in explosive eruptions, a few of which created lava domes.
Since this magma was now more viscous, it did not flow as far, causing the overall volcano
to change in shape to the steep profile typical of stratovolcanoes.
As the volcano grew taller and taller, explosive vulcanian and pelean style eruptions became
increasingly common.
Reaching a height of more than 15,000 feet, the composition of magma changed once again
to trachyte.
A warning sign of events to come occurred when Ngorongoro began erupting large rhyolite
lava domes on its summit and flanks.
This type of lava is typically only seen from volcanoes that produce the world's largest
explosive eruptions, and this was no exception.
Before the major eruption, Ngorongoro was almost 20,000 feet in height.
Then, 2.038 million years ago, a large volume of trachyte composition magma intruded into
a smaller rhyolite magma chamber.
The two compositions of magma mixed, causing an immense amount of pressure to build.
A plinian eruption thereafter occurred, sending a plume of ash to a height of 164,000 feet
or 50 kilometers into the atmosphere.
Part of the eruption column then collapsed downwards, producing energetic pyroclastic
flows which burned everything in a 50 mile or 80 kilometer radius.
To understand just how thick these flows of ash were, one location 31 kilometers distant
which was upwind of the eruption was buried in 20 meters of ash.
Then, due to the large amount of erupted material, a 21 kilometer wide section of ground collapsed
downwards by in some locations more than 5000 meters, forming a massive caldera.
In total, an estimated 365 cubic kilometers of volcanic rock was ejected meaning the eruption
was about the size of a low end supereruption.
In the aftermath, minor lava flows occurred in the center of the complex, one of which
built Engatati Hill out of trachyte 2 million years ago.
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