When people think of Arizona, they tend to think of our cactus filled deserts and the
Grand Canyon.
They do not think of volcanoes.
Yet, Arizona contains three active volcanoes, 2 of which have erupted in the last 1,000
years.
Of the several volcanoes within the state, none are quite as impressive, active, or unique
as the San Francisco volcanic field.
This volcanic field has been erupting for millions of years, and contains hundreds of
volcanic vents surrounding the town of Flagstaff.
One of these vents, Sunset Crater, last erupted only 950 years ago.
As such, it may only be a matter of time before Arizona has another volcanic eruption.
This video will discuss this volcanic field, and mention what its future hazards are.
The San Francisco volcanic field encompasses a vast area in north central Arizona.
It covers 1800 square miles, and stretches from just west of the town of Williams to
30 miles east of downtown Flagstaff.
This volcanic field is especially unique, as it was formed by a 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.
The San Francisco volcanic field began forming 6 million years ago, when magma first reached
the surface just west of the town of Williams.
For the first several million years, the vast majority of eruptions occurred at isolated
vents which erupted once and then never erupted again.
However, sometimes, the magma would pool at depth in an area, resulting in a single larger
volcanic edifice.
The first example of such a long term volcanic edifice is Bill Williams Mountain, which formed
3.5 million years ago.
This volcano contains dozens of overlapping lava domes made out of viscous dacite and
rhyolite lava.
The eruptions which formed this volcanic complex were highly explosive, and as a result there
is an abundance of ash in the area.
However, these were not the only lava domes to form.
One notable lava dome formed about 1.5 million years ago, and grew to 6400 feet wide.
The reason why this lava dome is fascinating is that it contains an abundance of large
obsidian crystals, which formed as part of a large rhyolite lava glow.
This lava dome is the best area to collect black obsidian and mahogany obsidian in all
of Arizona.
By 900,000 years ago, magma related to the underlying hotspot began building a large
magma chamber about 11 miles north of the modern town of Flagstaff.
Then, a series of violent explosions shot lava and ash high into the sky.
The eruption which had just occurred was the first of hundreds to occur from a singular
vent, which would subsequently create a towering stratovolcano called San Francisco Peak.
By 550,000 years ago, this stratovolcano stood at 15,500 feet tall, making it the tallest
point in the continental United States.
However, this height did not last.
530,000 years ago, the volcano exploded in a catastrophic eruption.
This eruption caused the towering stratovolcano to collapse to the northeast in a massive
landslide.
Today, the tallest point in Arizona, Humphreys Peak, sits on the northwest end of a prominent
horseshoe shaped caldera.
Since this collapse, a number of notable vents have formed.
400,000 years ago, rising magma interacted with a lake on the surface, causing it to
flash to steam and explode.
The remnants of this explosion formed Colton Crater.
71,000 years ago, a tall cinder cone formed in a moderately explosive eruption.
This eruption formed SP Crater out of basaltic andesite, along with a several mile long well
preserved black lava flow.
However, the most famous and most recent vent to form in this volcanic field is Sunset Crater.
Sunset Crater is a 1000 foot tall cinder cone which formed in a several decade long explosive
eruption which was witnessed by Native Americans in the area in 1075 AD.
This eruption rated on the volcanic explosivity index as a 4, and coated the vicinity with
several feet of ash, thus forcing the local populace to relocate.
Today, the well preserved lava flows, and associated features such as this red spatter
cone can be visited in the Sunset Crater National Monument northeast of Flagstaff.
Today, the San Francisco volcanic field is still active, as indicated by earthquake activity
at depth.
In 2009, a swarm of earthquakes occurred adjacent to Sunset Crater, indicating the movement
of magma beneath the surface.
With this being said, it is unlikely that any of the existing vents, including Sunset
Crater, will ever erupt again.
Instead, the next eruption will create a new cinder cone in a mildly explosive eruption.
The main threat from such an eruption would be long distance lava flows and ashfall, which
could effect areas up to 8 miles away.
Luckily, the next eruption will most likely occur in a sparsely populated area to the
northeast of Flagstaff which I have outlined on screen.
The odds of a future eruption are about 1 in 7,000 each year.
As a result of these hazards, the US Geological Survey has designated the San Francisco Volcanic
Field as a moderate threat volcano.
I hope that you enjoyed this video!
If you would like to request a specific topic, please leave a comment below!
Additionally, if you wish to support this channel, consider becoming a patron on patreon.