Arizona is a state known for its desert and scenic geologic features.
Yet, it contains several volcanoes.
Two of these volcanoes are classified as active; the San Francisco volcanic field centered
around Flagstaff, and the Uinkaret volcanic field located on the north rim of the Grand
Canyon.
Both of these produced eruptions within the last 1,000 years, and occasionally produce
swarms of earthquakes.
Thus, in the scientific community, there was a recent development.
One vent within the San Francisco volcanic field was just dated more accurately, placing
its age of formation at sometime within the last 10,000 years.
The vent which I am referring to is known as SP Crater, which contains a beautiful several
mile long fresh looking lava flow adjacent to a non-eroded, young looking cinder cone.
Since its age is within the 10,000 year cutoff used for calculating the risk posed by volcanic
systems, it means that this volcanic field has erupted not once but twice during this
timespan.
Thus, it is now a slightly higher risk volcanic system.
This video will discuss this new development, and this young volcanic feature in detail.
For some brief overview, the San Francisco volcanic field in northern Arizona is a hotspot
related volcanic field which has been intermittently active for more than 6 million years.
Over time, it has erupted a wide variety of features including the towering stratovolcano
known as San Francisco Peak which was once 16,000 feet tall, obsidian rich lava domes,
cinder cones, and explosion craters.
These several hundred vents have erupted a wide variety of lava ranging from basalt to
andesite to dacite and rhyolite.
Over time, the crust above the hotspot has moved to the west, causing features to get
younger in an eastward direction.
Before this new development, the only recent eruption from this volcanic field occurred
at Sunset Crater in 1075 AD.
It formed a large cinder cone and series of lava flows in a prolonged eruptive series.
Take a look at how young these lava flows look.
Now, moving to the west, take a look at these dark black lava flows visible from orbit.
They also appear to be quite young, as very little vegetation has grown on them and little
erosion is present.
Yet, the cinder cone they originated from, SP Crater, was considered to be 70,000 years
old.
For comparison, here is a 20,000 year old cinder cone in a different section of the
field known as Merriam Crater.
Note how it looks older than SP Crater due to the higher levels of erosion present.
The original 70,000 year old age was obtained by potassium argon dating.
However, there was a problem with this original sample.
It contained too much argon, meaning that the obtained age was likely inaccurate.
Using a basic age estimate based on the levels of erosion present, the cinder cone appears
to be 5,000 to 7,000 years old, given its environment.
This estimate largely agrees with the newly obtained age which dates SP Crater as originating
5,750 years ago, +- 250 years.
The new age was obtained via a method known as optically stimulated luminescence of quartz
xenoliths within the adjacent lava flow.
Although this test was originally done in 2012, it wasn't fully accepted by the scientific
community until this year, hence the update video.
SP Crater is a beautifully preserved cinder cone, which stands at 250 meters or 820 feet
tall, and 1200 meters wide.
It contains a central crater which is where much of its explosive activity occurred which
measures 400 meters wide.
This cone formed via intermittent small explosive activity, in what is termed a strombolian
style eruption.
However, unlike its younger cousin, Sunset Crater, this vent did not erupt basalt.
Instead, it erupted slightly more viscous basaltic andesite rock, which formed a blocky
lava flow.
This lava flow erupted over a period of several months to years, eventually covering an area
of 18.7 square kilometers or 7.2 square miles.
It flowed up to 7 kilometers or 4 miles from its source.
Although this individual vent will never erupt again, it has implications for the overall
volcanic field.
Currently, the US Geological Survey uses a weighted mathematical system to rate how hazardous
a volcano is which takes into account things such as eruptive frequency, eruption size,
and evidence of recent pyroclastic flows.
Because there are now 2 young eruptions, we can change the field's eruptive frequency
from once every 5 to 10,000 years to once every few thousand years.
With the updated tally, the field is still rated as moderate threat, but now ranks 68th
of 161 US volcanoes instead of 80th.
I hope that you enjoyed this video!
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