The vast deserts of Utah are well known for their impressive scenery which are largely
dominated by unique sandstone and other sedimentary rock formations.
Yet, largely hidden away are also a series of dormant and active volcanoes with one or
more of them being in my opinion more likely than not to produce a volcanic eruption at
some point in the next 10,000 years.
The last such volcanic eruption in the state originated at a little known feature known
as the Black Rock Desert volcano in the 14th century, where a mildly explosive eruption
generated a 300 foot high volcanic cone which covered a 14 square mile area in black basaltic
lava.
Looking at other vents in the Black Rock Desert volcanic field such as Crater Bench and Black
Rock, we can see that similar mildly explosive eruptions have dominated many vents in the
last few million years.
Yet, these eruptions relied on a relative lack of ground or surface water in part to
become not very explosive.
However, Utah was not always this way.
We can see this due to the vast salt flats found across the western half of the state.
These flats resulted from a single massive ice age era lake which once was approximately
the size and volume of Lake Michigan, containing more than 5500 cubic kilometers of water spread
across an area of 21,000 square miles.
Due to the occurrence of shallow submarine volcanic eruptions that were widely documented,
such as the 2021 to 2022 eruption of the Hunga Tonga Hunga Ha'apai volcano in the nation
of Tonga, it is now fairly well known that the mere presence of water causes a volcanic
eruption to become far more explosive.
With this in mind, here is the outline the Black Rock Desert volcanic field has produced
volcanism within the last 6 million years in red, and the Lake Bonneville outline in
white.
And now, here are two unusually large and raised volcanic explosion craters that measure
2100 and 3200 feet across respectively.
These features are considered to be explosively formed tuff cones that erupted 14,000 and
15,500 years ago respectively.
The larger of these is known as Pahvant Butte.
It measures 2 miles wide and stands at an impressive 1,115 feet or 340 meters above
the surrounding desert landscape.
From satellite, you can see three distinct features from the eruption which formed it.
First we have the volcanic edifice itself which is somewhat circular but also has an
irregular profile, containing light brown tuff and darker black basaltic ash.
Zooming out there is a brownish gray area, representing pyroclastic flow deposits which
reached a maximum distance of 2.8 miles away.
And third, we have the dark black region representing ashfall deposits which despite being more
than 10,000 years later remain up to 7.7 miles from this volcanic vent.
Due to its dispersion, we know that the wind was blowing towards the northeast, suggesting
that the relevant volcanic eruption occurred during the summer.
Approximately 15,500 years ago, a basaltic dike intrusion began moving upwards in the
crust, generating earthquakes along the way.
This dike eventually found its way into a regional fault line, which is loosely disconnected
with a series of south trending faults that would be the site of the next 2 volcanic eruptions.
Reaching shallow depths, the magma heated overlying rock, which then caused the bottom
of a small section of the 300 foot deep Lake Bonneville to start vigorously bubbling.
This continued to accelerate until the lava contacted the bottom of the lake, triggering
a series of powerful explosions which fragmented the lava into fine fragments that were dispersed
into the water and the air, where they rapidly cooled.
This process continued for several weeks, causing a layer of loose ash and basaltic
lava fragments to pile up in a circular rimmed tuff cone.
Part of this then reached the surface, allowing a large ash plateau to form.
The vents in Pahvant Butte shifted multiple times, causing different smaller craters to
form.
Just barely above the lake surface, the eruption was in its most explosive phase, probably
reaching on the 0 to 9 volcanic explosivity index a rating of a high 3.
Pyroclastic flows travelled across the water up to a distance of nearly 3 miles.
As a white and black plume continued to be generated in a surtseyan eruption, the largest
tuff cone vent formed.
This would continue to grow for another few months until finally a large volcanic island
in Lake Bonneville had been constructed which was breached to the southwest due to a landslide
that occurred.
Thanks for watching!
If you would like to request a specific topic, please leave a comment below.
Additionally, I would like to thank this channel's patrons on Patreon and channel members on
YouTube!