On November 21st, 2012, hikers in New Zealand were climbing up several trails in Tongariro
National Park.
Suddenly, a loud explosion was heard.
As everyone nearby looked around for its source, a large plume of superheated gas and ash rose
into the atmosphere, forming a several kilometer high mushroom cloud.
While this was occurring, superheated gas and ash clouds raced across the landscape,
burning everything in a 1000 meter radius.
What had just occurred was an unexpected volcanic eruption from the same complex used as a stand-in
for Mount Doom in the Lord of the Rings trilogy.
What I am referring to is Tongariro, which is the second most active volcano on the north
island of New Zealand.
The eruption which occurred on that day in 2012 was relatively small, as older eruptions
created vast explosion craters which were up to 2 kilometers wide.
The Tongariro volcano can be found in the central section of the north island of New
Zealand, where it is 116 kilometers west northwest of the city of Hastings.
Its volcanic complex consists of a group of stratovolcanoes and more than a dozen large
explosion craters which are almost all uniformly stretched out in a north-northeast trending
line.
Zooming out, this line stretches through several other regional volcanoes such as Ruapehu,
and represents a rift zone in the crust referred to as the Taupo volcanic zone.
Here, the crust is slowly stretching apart to the east and west, which allows for magma
in the mantle to travel upwards and erupt onto the surface.
The Tongariro volcanic complex began forming approximately 350,000 years ago when at the
time the Ruapehu volcano to the south had yet to begin forming.
Tongariro's first eruption occurred when a volume of this deep magma erupted onto the
surface in an explosive manner.
These initial eruptions occurred south of the modern volcanic cone of Ngauruhoe, and
formed a series of explosion craters from which small volcanic cones grew out of.
Although the initial volcanic cones resembled cinder cones, they were simply small stratocones
as when they erupted they occasionally produced pyroclastic flows.
Over the next 160,000 years, eruptions built a series of fissure like ridges with intermittent
peaks representing active vents.
These eruptions covered large swaths of land in andesite lavas, which due to their relatively
high viscosity left behind a distinctive appearance.
Then, 185,000 years ago, volcanism shifted several kilometers to the north.
The subsequent phase of activity expanded the existing volcanic ridge, but also began
constructing a broad volcanic cone.
Eventually, the summit of Tongariro became so tall that a permanent glacier became emplaced
on its edifice.
As the glacier slowly expanded in all directions during the most recent ice age, it carved
away at large segments of rock.
Eruptions continued during this period, but largely occurred underneath the glacier, resulting
in large glacial outbursts of water and highly explosive eruptions.
Around 40,000 years ago, a new volcanic cone erupted away from the same line of activity
which defines this volcanic complex.
6 kilometers to the west, the satellite volcanic cone known as Pukeonake erupted andesite lava.
Between 11,600 and 11,100 years ago, a series of highly explosive plinian eruptions occurred
to the south.
These formed large explosion craters called maars, two of which are called Upper and Lower
Tama Lakes.
The largest of these craters formed underneath what would later become the edifice of Ngauruhoe.
Beginning 7,000 years ago, this large explosion crater became the center of volcanic activity.
Eruptions began building a large volcanic cone called Ngauruhoe.
These eruptions only became more common 2500 years ago, after which near continuous eruptions
built this stratovolcano into the tallest peak of the overall complex.
During the 1900s, this volcanic cone produced vulcanian explosive eruptions which produced
pyroclastic flows 45 times.
After going no longer than 7 years without a distinct eruption at its peak, this volcanic
cone stopped erupting in 1977.
Some scientists think that this cone will never erupt again due to the period of dormancy
it has had, although magmatic earthquake swarms have occurred underneath its edifice indicating
the presence of shallow magma.
The two most recent eruptions in 2012 occurred at the Upper Te Maari Craters.
At the present, a surprise volcanic eruption could begin with little or no warning anywhere
in Tongariro.
Thus, anywhere within 3 kilometers of its edifice represents a primary hazard zone where
pyroclastic flows could reach.
However, superheated acidic mudflows known as Lahars could reach as far away at Lake
Rotoaira if an unusually large eruption were to occur.
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