Within the central Andes Mountain Range in the largest magma chamber on the planet.
Stretching across vast swaths of Bolivia, Chile, and Argentina, the "Antiplano-Puna
Magma Body" contains more than 500,000 cubic kilometers of molten rock.
Much of this molten rocks is relatively stationary at a depth of between 10 and 20 kilometers.
However, on occasion, large sections of magma of dacite composition have traveled up to
the surface, producing supereruptions.
One of these recent supereruptions occurred in Chile, producing what is now recognized
as the fifth largest volcanic eruption in the entire history of the planet.
The volcano which produced this supereruption is known as La Pacana, which today still shows
minor signs of activity.
Although this supervolcano is best classified as extinct, its caldera contains two low temperature
hot springs, indicating that magma still remains far beneath the surface.
The La Pacana supervolcano is located in far northeastern Chile, where it is 306 kilometers
east northeast of the city of Antofagasta.
On the edge of La Pacana is the highly active Lascar stratovolcano, which derives its magma
from the same at depth massive magma chamber.
The massive caldera of La Pacana measures 52 kilometers long and 35 kilometers wide
and can be easily seen from satellite, as its gray ash covered layers starkly contrasts
with the orange top layer on its caldera rim.
Volcanic activity began at La Pacana approximately 11 million years ago, when a series of andesite
stratovolcanoes erupted in the area.
Over time, these gray colored volcanic centers changed, as the underlying magma became more
silica rich.
As the composition of newly erupted lavas changed from andesite to dacite, and then
to rhyolite, eruptions became more and more explosive.
Then, a long period of little activity occurred, as the volcano seemingly became dormant.
Underneath its edifice, a large and localized magma chamber continued to build.
Then, 4.2 million years ago, this pressure was released in a catastrophic eruption.
As magma interacted with glacial cover in the region, it generated a series of explosions,
carving out a several kilometer wide crater.
Clouds of superheated gas and volcanic rock emerged from this crater, and spread out in
all directions, even travelling up mountains.
As the surrounding landscape was buried in more than 30 meters of ash, the eruption seemingly
paused.
For some context, 100 years in the geologic record is a very short timespan.
Thus, when looking at core samples of this volcanic complex, we note a very tiny gap
between two large caldera forming eruptions.
Whether this gap represents an hour, a week, or 100 years is unknown.
What is known is that in a very short time in the geologic timescale after the so called
Toconao ignimbrite was deposited, an even larger supereruption began.
This larger eruption created a 50 kilometer high plume of ash alongside pyroclastic flows
which buried everything in an 80 kilometer radius.
These flows incinerated everything and traveled up to 120 kilometers distant, depositing along
the way a superheated rock known as ignimbrite.
As a large quantity of ash fell across the high Andes, a wide swath of the volcano which
had just erupted collapsed downwards, forming an enormous caldera.
While the first phase of the eruption ejected 180 cubic kilometers of volcanic rock, the
second phase erupted a total of 3,500 cubic kilometers of material.
Over the next 2 million years, viscous magma which remained in the shallow crust erupted
onto the surface, forming a group of 7 large lava domes.
These dacite and rhyolite lava domes are all aligned along two implied trends of activity,
and occurred alongside the uplift of the resurgent dome.
An additional 8th lava dome exploded in a spectacular eruption at one point, forming
a 3 kilometer wide caldera.
After this occurred, volcanic activity at the La Pacana supervolcano ceased.
Since then, other ignimbrite forming eruptions fed by the underlying Antiplano Puma magma
chamber have occurred in the region.
One of these, Laguna Caldera, erupted 1.98 million years ago with a magnitude identical
in size to the 1815 eruption of Mount Tambora, which caused the year without a summer.
Although still quite large, this eruption was 23 times smaller than the caldera forming
supereruption of La Pacana.
At the present, we are seemingly in the middle of a long term ignimbrite flare-up, which
is currently undergoing a long pause in activity.
Thus, while La Pacana will not erupt again, other volcanic centers in the region are likely
to once again produce cataclysmic supereruptions when this flare up resumes.
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