On June 4th, 2011, a powerful volcanic eruption began in central Chile.
A large explosion occurred, causing a 5 kilometer wide plinian column to shoot to a height of
10 kilometers or 33,000 feet above sea level.
As the eruption energetically continued for several days, large quantities of ash drifted
towards the east, much of which subsequently fell and buried large sections of land in
more than 30 centimeters thick of material.
While the eruption continued over the next few weeks, eruptive activity waned.
Then, 18 days into the eruption something strange occurred.
The eruptive fissures began producing flows of viscous lava, but they were not orangish
red in color.
Instead, they were solid black.
These lava flows were in fact almost completely composed of obsidian, which is a type of volcanic
glass that is typically only seen in rhyolite composition eruptions.
The reason why obsidian formed relates to how quickly the material cooled.
In the world of geology, the longer a volcanic rock takes to cool, the larger the crystals
formed in it will be.
The opposite is also true, where rapidly cooled lavas will only form miniscule crystals.
However, if the lava cools almost instantaneously upon being erupted it forms thick flows of
the volcanic glass obsidian.
Over the next 10 months, this thick obsidian lava flow would expand across the snow covered
landscape, eventually covering 16 square kilometers of ground in 30 meters or 100 feet thick of
volcanic glass.
What had just occurred was a geologic rarity that was part of the planet's largest volcanic
eruption since 1991.
In total, 1 cubic kilometer of volcanic rock was explosively ejected, meaning this eruption
was the first since 91 to be assigned a volcanic explosivity index rating of a 5.
The Puyehue-Cordón Caulle volcano is located in central Chile where it is 84 kilometers
east of the city of Osorno.
This volcano not only includes the stratovolcano which bears its name, but also a series of
young vents to the northwest and the older Cordillera Nevada caldera.
This volcanic complex began forming 314,000 years ago when a volume of basaltic andesite
lava erupted onto the surface high in the Andes Mountain Range.
Due to the abundant nearby snow and groundwater, it triggered a series of powerful explosions
which carved out a several hundred meter wide crater in the ground.
From this crater emerged a flow of viscous lava, which slowly flowed downslope in a lobe
of material.
Over time, numerous lobes of lava overlapped one another with layers of ash, and slowly
began building a central volcanic cone.
Over the next 200,000 years, volcanism occurred along a series of northwest trending fissures,
including at the site of the Cordillera Nevada shield volcano.
Then, 100,000 years ago, a powerful explosive eruption occurred at this volcano.
As a towering column of ash shot more than 40 kilometers into the atmosphere, energetic
pyroclastic flows raced across the landscape, burying everything in a 22 kilometer radius
with a thick layer of rock known as ignimbrite.
Then due to the large amount of erupted material, an 8.5 kilometer wide section of ground collapsed
downwards, forming a massive caldera.
This shield volcano subsequently went extinct, as explosive eruptions shifted back towards
the southeastern volcanic cone.
34,000 years ago, this cone began erupting a more widely varied series of lavas, ranging
from basalt to rhyolite.
While the less silica rich basalt lavas formed lengthy lava fields, the viscous silica rich
rhyolites formed lava domes.
The rhyolite and dacite magma allowed for powerful eruptions to subsequently occur in
5080 BC and then again in 860 AD, which formed the volcano's modern 2.3 kilometer wide and
150 meter deep caldera.
After the eruption in 860, every single eruption since has occurred at a series of fissure
vents to the northwest, erupting andesite, dacite, and rhyolite lava flows.
Labeled on screen are three of these flows, which erupted in 1921 to 1922, 1960, and 2011
to 2012.
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