At the Yellowstone supervolcano in Wyoming, many people often incorrectly think that the
volcano is overdue for an imminent large eruption.
This is not the case, as the overall volcano is not showing at depth signs pointing towards
a large eruption in the short to moderate term future.
However, there is an often overlooked hazard within the National Park which is related
to volcanic activity which can still be quite deadly.
What I am referring to are powerful hydrothermal explosions.
Take for example the Indian Pond Lake.
This lake formed in a powerful hydrothermal explosion in 1350 BC which carved out a 1,640
foot or 500 meter wide crater in the ground.
The explosion in question ejected car sized boulders up to 9840 feet or 3 kilometers distant
and created a notable crater rim which can still be seem today.
While many more recent smaller hydrothermal explosions have occurred since, such as shown
here at the Biscuit Basin, the possibility remains of similarly sized explosions occurring
in the future.
According to the US Geological Survey, the odds of a 100 meter or larger crater forming
in a hydrothermal explosion within Yellowstone National Park is about 1 in 900 each year.
To better understand these craters, I will now emphasize the single largest crater in
the park.
Approximately 14,000 years ago, the 7,300 foot long and 4,500 foot wide Mary's Bay Crater
formed in a powerful explosion.
This crater has a highly irregular profile, owed in part to its formation occurring from
several near simultaneous explosions instead of one single explosion.
Mary's Bay Crater represents the single largest hydrothermal crater on the planet, but its
origin can be a bit confusing.
You see, hydrothermal explosions are ridiculously similar to a type of volcanic eruption called
a phreatic eruption.
In phreatic eruptions, magma or superheated gasses associated with magma directly heat
nearby groundwater.
This groundwater then flashes to steam, pressure builds, and a powerful explosion occurs.
One example of a phreatic eruption is at the Taal volcano in the Philippines on July 1st
2021.
While its eruption column primarily consisted of water vapor, it also contained small fragments
of magma and volcanic gasses.
Hydrothermal explosions on the other hand occur in areas where magma is present at depth
but it does not directly interact with groundwater.
Groundwater is heated by residual heat of magma permeating through neighboring rock,
which heats it to just below the boiling point.
This water then heads towards the surface, where it is periodically ejected through a
series of geysers.
However, if a cap forms on top of the points which let steam escape, an immense amount
of pressure begins to build.
If enough time passes, this pressure eventually becomes too great and the ground explodes
upwards in a powerful steam explosion.
While indirectly caused by volcanic activity, hydrothermal explosions are not considered
to be volcanic eruptions.
In the case of the Mary's Bay Crater, its formation relates to both stress release and
a rapid change in the overall level of Yellowstone Lake.
As of 16,000 years ago, the ice age was still ongoing, and almost all of Yellowstone was
buried by 4,000 feet thick of ice.
When this ice slowly melted away, the crust rebounded upwards, causing changes in numerous
hydrothermal systems.
When this pressure was finally released 13,800 years ago, it generated a powerful explosion
on the order of at least several hundred kilotons of TNT.
A blast wave expanded outwards from the center of the crater, water was ejected several thousand
feet into the air, and a tsunami was created on Yellowstone Lake.
As hundreds of millions of tons of rock were pulverized, large boulders were thrown high
into the air, falling back to the ground at up to 2.5 miles distant.
Although several other hydrothermal craters have formed since, none have been quite as
large.
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