These series of hills which stand out in the landscape are remnants of a truly catastrophic
event which occurred 340,000 years ago at Mount Shasta.
Known as hummocky terrain, they were formed when a large section of Mount Shasta collapsed
in a massive landslide totaling 45 cubic kilometers of rock and moving up to 45 kilometers distant.
While this is one of the more famous localities with this type of volcanic landslide generated
landform, there is also a far more active volcano which has generated this feature during
a several thousand year old landslide; Mount Galunggung in Indonesia.
There, groupings of up to 40 meter or 131 foot high tree covered green hills dot the
landscape over an area of 250 square kilometers.
More easily visible from satellite, these features are referred to as the Ten Thousand
Hills of Tasikmalaya, although despite the name, there are not ten thousand of them.
Nevertheless, on a clear day it is quite easy to see where on Galunggung they originated
from.
As, a giant C shaped chunk of the volcano, representing 18 cubic kilometers of material
is seemingly missing.
Thus, these hills or hummocks originated from the eastern section of the volcano during
a powerful eruption in 2695 BCE.
And, the location of Galunggung's 1982 eruption provides a clue as to how this exactly occurred.
For context, the Galunggung volcano is located on the western third of the Indonesian island
of Java, where it is approximately 175 kilometers southeast of the city of Jakarta.
The Galunggung volcano is probably best known for its 1982 eruption, which was the third
largest explosive volcanic eruption within Indonesia to occur during the 20th century.
This eruption occurred at the same site of a smaller 1918 eruption, which created a 1500
meter wide lava dome.
Pressure then built up underneath this wide lava dome, before eventually bursting upwards
in a series of powerful explosions in 1982.
These explosions created a sustained plinian eruption column which shot to a height of
20 kilometers, and as part of this collapsed downwards, it created superheated pyroclastic
flows which scorched the landscape up to a distance of 5700 meters.
In total, this eruption ejected approximately 0.4 cubic kilometers of volcanic rock, meaning
it was assigned a volcanic explosivity index of a 4.
Dozens of villages were destroyed, largely from lahars and heavy ashfall, but this eruption
is best known for causing all four engines to fail on a commercial aircraft after it
being caught in the ash cloud along with another incident where 3 engines failed on an aircraft
later in the eruption.
Luckily, despite significant damage to each aircraft, everyone on them landed safely and
thus survived the incident.
Along with a similar incident caused by the eruption of Mount Redoubt in 1989, this led
to the creation of the VAAC or Volcanic Ash Advisory Center, which advises pilots worldwide
and forecasts the direction eruption plumes take to prevent a repeat disaster from occurring.
Despite how old the entire Galunggung volcano might appear, it only began forming around
35,000 years ago when a volume of basaltic andesite lava erupted onto the surface.
Through dozens of strombolian to vulcanian explosive eruptions, a gray colored stratocone
with possibly red sides due to the presence of scoria was built.
Three vents were intermittently active, emplaced along a north-northeast to south-southwest
trending line.
Eventually, a large explosive eruption carved out a 700 meter wide and 120 meter deep Guntur
Crater.
While this remained the dominant crater for thousands of years, a large intrusion eventually
occurred underneath Galunggung's eastern flank.
Just like seen at Mount Saint Helens in 1980, a large cryptodome pushed up overlying rock
by perhaps as much as 50 or more meters.
Eventually, in 2695 BCE a landslide occurred, causing the underlying cryptodome magma to
explosively erupt in a powerful eruption, simultaneously occurring as more than a dozen
cubic kilometers of the mountain collapsed.
This deposited the numerous hummocky terrain blocks, and even caused a river to divert,
leading to the creation of the Situ Gede lake.
The remnants of what was once the cryptodome have been the site of every historical eruption
since, including a powerful VEI 5 eruption in 1822, which led to several thousand casualties
through pyroclastic flows, ashfall, and lahars.
Galunggung has not erupted since January of 1984, but since 1800 has erupted on average
about once every 45 years.
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