The state of Colorado is often incorrectly attributed to have the highest peak in the
lower 48 states as it contains the 14,440 foot tall Mount Elbert.
In reality, there is another peak in California which stands a bit taller, Mount Whitney at
14,494 feet.
However, in the recent geologic history of the United States, California didn't always
have the tallest peak.
At one point a mere 400,000 years ago, this title belonged to a peak within the state
of Arizona.
There, a 16,000 foot or 4,877 meter tall volcano was near continuously erupting, producing
numerous explosive eruptions near the modern town of Flagstaff.
The volcano in question was known as San Francisco Mountain, which had snow on its summit year
round despite the arid climate of Arizona.
With this being said, this peak is clearly no longer visible today.
So, what happened to this volcano?
Are there any remnants of this seemingly massive mountain?
The remnants of the once vast San Francisco Mountain volcano can be found 9.3 miles or
15 kilometers north of the town of Flagstaff in north central Arizona.
Today, the remnant of this once massive volcano contains several secondary peaks, the tallest
of which is Humphrey's peak, which happens to be the tallest point in Arizona.
Yet, at 12,637 feet tall, this does not compare to the prior height of this volcano.
Looking at San Francisco Mountain to the side, it becomes very clear why it no longer stands
at such an impressive height.
Note the horseshoe shaped scar which is open to the northeast.
This scar is very similar to the one produced by the 1980 explosive eruption of Mount Saint
Helens.
Thus, San Francisco Mountain was destroyed by an ancient catastrophic landslide, potentially
associated with a large volcanic eruption.
The landslide caused a majority of the volcano to slump to the east, with pieces of debris
travelling beyond 20 miles distant in some locations.
This landslide occurred sometime between 400,000 and 92,000 years ago, as the younger figure
represents the age of Sugarloaf Peak; a rhyolite lava dome which formed on top of the landslide
deposits.
For some context, San Francisco Mountain was the tallest vent of the vast San Francisco
volcanic field.
This volcanic field covers an area of 1800 square miles, and stretches from just west
of the town of Williams to 30 miles east of downtown Flagstaff.
It is an especially unique volcanic complex, as it owes its existence to a low intensity
hotspot in the mantle, much like the Hawaiian volcanic chain.
How do we know this?
The reasoning is that over time, there has been a prominent eastward trend over the course
of this field's eruptive history as the overlying North American Plate has slid in a westward
direction while the hotspot remained stationary.
This volcanic field is largely monogenetic, as the vast majority of its several hundred
vents such as SP Crater only erupted once.
However, throughout the long history of this volcanic field, occasionally magma found a
large uniform path to the surface.
This created a series of lava dome complexes similar to Lassen Peak in California, where
dozens of explosive eruptions occurred from the same vent.
Bill Williams Mountain is one example of these complexes, where its large and rounded lava
domes are still visible today.
Around 1 million years ago, this magma found a new uniform path, and began constructing
a single volcanic cone.
As frequent eruptions continued over the span of tens of thousands of years, the volcano
grew taller and taller.
These eruptions added numerous overlapping layers of lava and ash, which caused the volcano
to have a primarily gray color.
The volcano continued to frequently erupt for a period of 400,000 years, by which it
was 14,272 feet tall.
Then, activity shifted towards the north, causing a second taller volcanic cone to be
constructed.
By 400,000 years ago, this cone was between 14,633 feet and 16,500 feet in height, depending
on the cited scientific paper.
San Francisco Mountain by this time was snow covered year round, which eventually built
into a truly massive glacier, similar to those seen on Glacier Peak in Washington state.
The eventual collapse of its edifice most likely occurred due to the presence of the
taller volcanic cone adding structural instability to its eastern slopes.
Eventually, fumaroles and hydrothermal alteration caused this weak slope of the volcano to turn
bright shades of yellow and red.
A critical point was then reached, at which the volcano catastrophically collapsed to
the side.
This collapse may have occurred long after San Francisco Mountain stopped erupting, or
could have been associated with a large explosive eruption, although the former possibility
appears to be most likely.
Today, San Francisco Peak is long extinct, but a possibility remains that the larger
volcanic field could one day create another towering stratovolcano.
I hope that you enjoyed this video.
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