At this location in California, if you were to dig straight down you would eventually
hit an active body of magma at a depth of approximately 3.7 miles or 6 kilometers.
On the surface, gas from this deep magma is actively being released via this very small
boiling hot spring, with both the heated water and gasses this vent is emitting resulting
in the emplacement of yellow native sulfur mineralization on adjacent rocks.
This vent is one of several which can be found at a volcanic complex in California which
towers above every other regional peak.
What I am referring to is the impressive active volcano known as Mount Shasta, whose summit
reaches an elevation of 14,179 feet or 4,322 meters above sea level.
While only weak degassing of fumaroles is present at this volcano today, do not be fooled,
as this peak is the third most active volcano in terms of the number of eruptions in the
last 11,650 years in the entire lower 48 U.S. states, only after Mount Saint Helens and
Mount Rainier.
Between the end of the last ice age 11,650 years ago and the year 1250, Mount Shasta
has erupted a stunning 22 times, with these eruptions ranging from small in scale all
the way to large and devastating, with some greatly changing the nearby landscape.
This is quite prominent via the number of young and fresh looking lava flows especially
to Shasta's west and north-northwest.
Additionally, the landscape around Shasta contains other fascinating secondary features,
including a series of hundreds of closely spaced large hills which are each up to 600
feet in height.
This is the story of the tallest and statistically most likely volcano in California to produce
the state's next volcanic eruption.
For context, Mount Shasta is located in far northern California where it is centered about
57 miles north of the city of Redding.
While many people think of Mount Shasta as only a single stratovolcano, it is in fact
much more than this.
As, while you might only think that products from Mount Shasta can only be found on its
central cone, in reality, material which used to compose the edifice of this volcano can
be found as much as 36 miles away in the town of Ager.
Additionally, upon closer inspection not only does Shasta contain a series of flank lava
domes and cinder cones, but it contains a vast secondary eruptive center known as Black
Butte and a secondary peak known as Shastina.
These two aforementioned features are fairly new, as they completely formed within the
last 11,000 years.
Long before Mount Shasta began to form, a very similar volcano stood in its place which
was constructed between 600,000 and 360,000 years ago.
However, as time progressed, the western half of this volcano began to turn shades of red
and yellow, indicating that hydrothermal alteration was changing hard lava and ash rock units
into unstable clay.
Eventually, this alteration reached such a point that all at once 360,000 years ago,
45 cubic kilometers of the mountain collapsed in a massive landslide.
Within 20 minutes, this landslide had covered approximately 285 square miles of land, where
material largely moved as several hundred foot thick blocks, forming large hills that
we today refer to as hummocky terrain.
The modern Mount Shasta only began to grow much later around 250,000 years ago, and since
then its eruptive activity has followed a pattern.
Namely, Shasta erupts with a cluster of about 10 moderately to highly explosive eruptions
across a period lasting 500 to 2,000 years, only to be followed by 3,000 to 5,000 years
of very few if any eruptions occurring.
Over the next 239,000 years, repeated vulcanian eruptions deposited numerous layers of ash
along with overlapping lobes of blocky and viscous primarily andesite lava flows.
Shasta's Sargents Ridge Cone formed first, and was later followed by the Misery Cone
which became the dominant center of eruptive activity for a time.
Then, approximately 11,000 years ago, a powerful volcanic eruption occurred which was 84% larger
than the 1980 eruption of Mount Saint Helens.
This eruption notably generated sizable volumes of pumice while also incinerating large swaths
of forest up to a distance of 8 miles from its summit cone, which retain a gray tint
to this day.
The aforementioned eruption along with a few subsequent smaller eruptions during the next
few hundred years eventually led to the creation of a brand new 13.5 cubic kilometer lava dome
complex known as Shastina.
Only 100 years after Shastina stopped erupting, activity temporarily shifted 7.2 miles to
the west-southwest, building a 2400 foot tall dacite lava dome known as the Black Butte.
Activity then shifted back to create the new Hotlum cone around 8,000 years ago, with eruptions
from this vent over the next several thousand years occasionally emplacing flank lava flows
but largely producing ash rich eruptions.
The last two such eruptions from this volcano occurred in the early and mid 13th century.
While we do not know when Shasta will erupt again, the rate of recent eruptions suggests
a yearly chance of an eruption is about 1 in 530.
This is the hazard map for Mount Shasta were an eruption to occur, and due to the presence
of glaciers along with heavy ashfall any future eruption would likely bring, the main threat
would be long distance lahars that have the potential to severely damage certain areas
up to 36 miles away from the volcano's summit, although pyroclastic flows that could travel
several miles would also be a danger.
These hazards are why Mount Shasta ranks as the 5th highest threat volcano in the United
States, ranking in as a very high threat volcano.
As a final note, I would like to thank my new patron Sérgio Sancevero for supporting
this channel!