At the Yellowstone supervolcano in Wyoming, many people often incorrectly think that the
volcano is overdue for a major explosive eruption.
However, there is no indication that it will erupt anytime soon.
With this being said, there is an ever-present danger within a section of the park that is
still quite deadly; hydrothermal explosions.
With little or no warning, a several hundred foot to several mile wide crater could form
in a massive explosion equivalent in energy to the detonation of several hundred kilotons
of tnt.
Such explosions have occurred 16 times in the vicinity of Yellowstone Lake during the
last 14,000 years and will certainly occur again.
Now, due to a recent scientific paper by L. Morgan and others, we have identified a potential
trigger for the largest of these events.
Since the end of the last ice age, 3 of the park's largest hydrothermal explosion craters
which each measured more than 1,500 feet wide formed in mere minutes or hours after a large
magnitude earthquake.
In other words, if a magnitude 5.5 or larger, and especially if a magnitude 6.0 or larger
earthquake occurs within 5 miles of any point on Yellowstone Lake, the area may need to
be rapidly evacuated.
Additionally, the paper noted something else which is interesting.
It dated the latest major hydrothermal explosion within the park to sometime between 1660 and
1860.
This latest explosion most likely originated at a fumarole field on the floor of Yellowstone
Lake, ejecting boulders of material over a wide swath of ground and depositing several
inches of ash in a 2 mile radius.
According to the US Geological Survey, significant hydrothermal explosions are the most likely
volcanic hazard the park will face in the next several thousand years.
On average, a crater formed by a powerful hydrothermal explosion which measures more
than 100 meters or 328 feet wide forms about once every 900 years.
The most prominent of these recent hydrothermal craters can be found on the western and northern
shorelines of Yellowstone Lake.
These include Turbid Lake which formed in 6130 BC, Duck Lake which formed in 3050 BC,
and the Indian Pond Crater which formed in 1350 BC.
However, 2 of the largest craters are buried in the depths of Yellowstone Lake.
Here, the 2900 foot wide Elliot's Crater formed 8,000 years ago, and the massive 7,300 foot
wide Mary's Bay crater formed 12,800 years ago.
Using mid range estimates, they formed in powerful explosions equivalent in energy to
the detonation of 155 and 526 kilotons of tnt, respectively.
Both of these powerful explosions were only triggered after earthquakes of a magnitude
of approximately 6.5 struck within the lake.
The older Mary's Bay crater previously represented an active hydrothermal system on the lake.
Before the major earthquake occurred, a significant amount of pressure had built up under the
lake due to the long-term heating of groundwater by superheated rock and magma deeper in the
crust.
However, at the time the waterline at the lake was approximately 50 feet higher than
its current level.
Thus, due to the greatly increased atmospheric pressure underneath the water, all of the
highly pressurized water was kept from flashing to steam.
Then, when the major earthquake occurred, large sections of the lake were uplifted by
as much as 3 feet.
This triggered scattered landslides, which led to the generation of a more than 40 foot
tall tsunami within Yellowstone Lake.
The tall tsunami eventually traveled to the natural outlet of the lake, which at the time
was the modern site of LeHardy's rapids.
Upon impact, the tsunami quickly washed away a large glacial debris fan.
This caused a significant portion of Yellowstone Lake to rapidly drain away, decreasing its
overall depth by 50 feet.
The decreased depth allowed for the highly pressurized water to finally flash to steam,
reach a critical point, and produce a powerful explosion in the case of the Mary's Bay Crater.
In the case of Elliott's Crater, there wasn't a large tsunami but rather the earthquake
caused uplift which fractured its overlying caprock, allowing for pressure to be released.
Although the lake level is much lower today than it was during these 2 prior explosion
crater forming events, a subsequent large magnitude earthquake could trigger another
powerful explosion.
However, 2 of the 5 large recent craters in the vicinity were not linked to large earthquakes,
so they can still form through gradual processes without a dramatic change in pressure.
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