Adjacent to Mexico City is the active Popocatepetl volcano, which has been near continuously
erupting since 1994.
Yet, this volcano is merely one of 8 nearby volcanoes, a number of which are lesser known.
One of these other volcanoes erupts far less frequently but has a history of very large
explosive eruptions.
The volcano I am referring to is Nevado de Toluca, which stands at an impressive 4,680
meters or 15,354 feet in height.
One of its most recent eruptions buried what is now Mexico City in 50 centimeters of ash,
meaning that such a similarly energetic eruption is possible in the future.
The Nevado del Toluca volcano can be found in central Mexico where it is 20 kilometers
southwest of the large city of Toluca.
This volcano is surprisingly old, as it began forming 2.6 million years ago.
At the time, the planet's most recent ice age had yet to start, and a volume of andesite
magma erupted onto the surface.
This initially occurred at the southern end of the volcanic complex, forming large gray
colored lava domes.
Although eruptions continued for the next 1 million years, they were relatively infrequent.
Then, between 1.6 and 1.2 million years ago, dozens if not hundreds of viscous dacite lava
flows erupted, which spread out in a rather circular shape, forming the main edifice of
Toluca.
The volcano subsequently became dormant after 1.2 million years ago, and during the ongoing
ice age glaciers occasionally grew and retreated on its summit.
Then, 100,000 years ago, volcanic activity resumed, once again building a group of overlapping
lava domes.
These eruptions primarily occurred between a series of two west to east trending faults.
These faults also happen to correlate with caldera collapse and landslide scars at this
volcano, suggesting that underlying magma used them as a path of least resistance to
reach the surface.
Nevado de Toluca's central edifice was constructed during this period, which at one point had
several secondary peaks.
However, around 42,000 years ago, the overall nature of eruptions changed.
Eruptions became far less frequent but increasingly energetic, producing plinian eruption columns.
These eruptions resulted in the complete collapse of some lava domes, which led to long reaching
pyroclastic flows.
Three such eruptions occurred 42,000, 36,000, and 28,000 years ago, each of which had a
volcanic explosivity index of a 4 or 5.
Over time, hydrothermal alteration became visible on the eastern flank of the central
lava dome complex, showing that its edifice was becoming structurally unstable.
Then, 26,000 years ago, the majority of the central complex collapsed to the east in a
massive landslide.
This landslide may or may not have been triggered by a volcanic eruption, but regardless left
the modern horseshoe shaped caldera which is visible in its summit today.
The center of this caldera soon produced more highly explosive eruptions, such as 21,700
years ago, which explosively ejected 2.3 cubic kilometers of ash and volcanic rock.
At the conclusion of a smaller eruption 14,000 years ago, a large bed of volcanic rock was
emplaced in the center of the caldera, which acted as a plug that would prevent gasses
from escaping.
Over the next two thousand years, an immense amount of pressure began to build underneath
the volcano.
Then, 12,100 years ago, the pressure was violently released, sending a plume of ash more than
42 kilometers into the atmosphere.
As pyroclastic flows raced across the landscape, an immense amount of ash was ejected.
In the city of Toluca, 1.5 meters of ash fell.
Further away in Mexico City, 50 centimeters of ash fell.
This cloud continued to drift eastwards, dumping additional amounts of ash over a vast swath
of Mexico.
Then, due to the large amount of erupted material, the volcano collapsed downwards by more than
100 meters, deepening the existing caldera.
In total, 14 cubic kilometers of volcanic rock had been ejected, meaning the eruption
had a volcanic explosivity index of a 6.
Since then, only two small eruptions have occurred, but we are unsure exactly where
they originated.
Regardless, an eruption 10,500 years ago and then again 3,300 years ago produced a small
ash layer and pyroclastic flow deposit.
Since then, no additional eruptions have occurred.
Although there is little direct evidence to suggest that another eruption will occur in
the next several centuries, there is a concern.
There is an apparent pattern of large volume explosive eruptions which occur every 14,000
years at this volcano.
Since the last era of major explosive eruptions began 14,000 years ago, it is quite possible
that Nevado de Toluca will erupt again in my lifetime.
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 patrons Alan Olsen and Kevin Connors for supporting
this channel!