The state of Arizona is rightfully considered to be one of the hottest states in the United
States, hence its mostly arid desert climate.
In these deserts, temperatures can reach up to 50 degrees Celsius or 122 degrees Fahrenheit.
Because of this fact, you might be stunned to learn that around 10,000 years ago, the
state contained one of the southernmost glaciers in the entire country.
This glacier was once more than a mile in length, and at the time was around 295 feet
or 90 meters thick.
Today, these glaciers are no more, having completely melted away more than 9,000 years
ago.
Yet, remnants of their existence can be found through moraines and glacial cirques aka depressions
carved by their movement.
These cirques can be found on the western section of the former San Francisco Mountain's
Inner Basin along with the edges of its collapse crater walls.
And, outlined on screen in red is the furthest extent these glaciers reached.
The longest glacier was a little more than 2.5 miles in length, which helped to carve
out steep valleys via a mass of ice which was as much as 750 feet thick.
These glaciers at a minimum existed only above 9,250 feet above sea level, with four originating
adjacent to Humphrey's Peak.
Before I describe why exactly these glaciers formed in Arizona and when they existed, I
am going to first remind my audience why such a large and fairly isolated mountain exists
north of the city of Flagstaff.
Although Arizona is generally not well known for this, the state contains two active volcanic
fields which have both erupted in the last 1,000 years.
One of these volcanic fields consists of more than 600 vents of various shapes in sizes
scattered across an area of around 1,800 square miles.
This field has erupted on average about once every 4,500 years for around 6 million years,
creating a grouping of cinder cones, maars, and lava dome complexes, with volcanism possibly
being related to an underlying hotspot in the mantle as vents on average get older to
the west, matching the direction of tectonic plate movement.
Between 1 million and 400,000 years ago, the majority of eruptive products occurred at
a single vent instead of multiple vents which largely only each erupted once.
This formed a towering stratovolcano much like Mount Saint Helens which at the end of
its lifespan was the tallest peak in the lower 48 U.S. states, rising to a height of 16,000
feet.
However, around 400,000 years ago, this volcano collapsed in a massive landslide, carving
out a giant C shaped crater to the east northeast, leaving behind an assortment of scattered
remnant peaks the tallest of which today is Humphreys Peak.
Although it is quite likely that the original San Francisco Mountain Peak volcano once was
topped by thick glaciers due to its height around 500,000 to 400,000 years ago, this
cannot be proven due to the original deposits if they once existed having been obliterated
by the massive ancient landslide.
While the highest valleys on the mountain would become the site where glaciers would
grow, they also began growing within the inner valley directed to the east-northeast.
This is because despite being at a lower elevation, the valley was kept at a colder temperature
than other terrain in Arizona which was at similar elevations.
Whereas Humphrey's Peak now has an average temperature of around 33 degrees Fahrenheit,
around 20,000 years ago during the last glacial maximum it averaged 16 degrees Fahrenheit.
As, during the Ice Age, Arizona was on average as much as 14.4 degrees Fahrenheit or 8 degrees
Celsius colder than it is today.
The vast deserts did not exist, as instead grasslands and woodlands existed in their
place.
This makes sense when you consider that this temperature difference would be like the climate
of the city of Phoenix which is desert becoming like the modern climate in the city of Payson
which is surrounded by dense forests, not a desert.
During the last 400,000 years, glaciers were not always present on San Francisco Mountain,
having only existed for 46% of the time as the climate naturally fluctuated over long
timescales.
Thus, glaciers expanded on this peak in Arizona between 375,000 and 340,000 years ago, 278,000
to 252,000 years ago, 190,000 to 135,000 years ago, and 70,000 to 10,000 years ago
During their peak, you could theoretically have gone skiing or snowboarding down the
mountain any time of the year for as much as 60,000 years straight, as at their furthest
extent these glaciers covered a total of 3.4 square miles, consisting of 1.4 cubic kilometers
of glacial ice.
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