The state of Arizona contains several volcanic fields which are classified as
active. One of these can be found in the northern part of the grand canyon,
while another can be found around Flagstaff. Both of these volcanic fields have erupted in the last
1,000 years, with the most recent eruption occurring at Sunset Crater in 1075 AD.
However, these are not the only volcanoes in the state which could erupt again in the near future.
At the Arizona Mexico border, there is a third, lesser known volcanic field known
as Pinacate. This video will discuss this active volcano, and mention what its future hazards are.
The Arizona portion of the Pinacate volcanic field can be found 19 miles west of the western
edge of the Organ Pipe Cactus National Monument. Although the vast majority of this volcanic field
is in Mexico, there are still about a dozen recent volcanic cones north of the border.
This volcanic field does not exist due to plate subduction or a hotspot. Rather, it exists due
to a weak spot in the earth's crust caused by its proximity to two tectonic plates spreading apart.
These two tectonic plates have caused Baja California to separate away from
the rest of Mexico, which generated volcanic eruptions similar to those
seen at mid ocean ridges. As a result, the volcanoes in this field follow a northward trend
similar to the direction of the pacific plate. The Pinacate volcanic field began forming
4 million years ago. Over the next several million years, numerous eruptions of basalt,
trachyte, and andesite lava occurred. These eruptions built hundreds of
different monogenetic eruptive features. Then, 1.2 million years ago, a shield volcano
began to form about 20 miles south of the US border. Slowly, dozens of eruptions built up this
large low prominence volcano. Individual eruptions created new cinder cones or erupted lava onto the
surface through fissures. Eruptions on this shield volcano ceased 100,000 years after they began,
leaving behind tallest point in the Pinacate volcanic field.
32,000 years ago, the largest explosive eruption in this field took place. An area
that used to host a freshwater lake violently exploded and formed a 1 mile wide and 800 foot
deep crater. Pyroclastic flows from this event reached more than 5 miles away and
the ash cloud likely reached 30,000 feet. Between 16,000 and 8,000 years ago, two
different eruptions produced cinder cones and long distance lava flows. One of these cinder cones,
known as La Laja, produced a lava flow which covered 4 square miles with fresh basalt lava.
Interestingly, these recent eruptions contain both peridot and labradorite crystals in the cooled
rock. These yellow labradorite crystals can be up to 4 inches wide and are sometimes facet grade!
Although no eruptions have been confirmed since this period, there have been at least
three reports of recent eruptions. The most recent of these reports claimed that there
was an eruption in 1928, and then another three day long eruption at the end of 1934.
So, what hazards does the Pinacate Volcanic Field pose in the future?
Using the rating system developed by the US Geological survey, the Pinacate volcanic field
would rate as a low threat volcano. Eruptions from this field occur on average about once every
8,000 years, so an eruption in the next century is unlikely. If an eruption was to take place,
it would occur as one of two eruption styles. The most likely style of future eruption would
create a cinder cone. This type of low intensity explosive eruption would generate lava flows which
could flow up to 5 miles from the erupting vent. Cinder cone eruptions typically last for a few
years, and represent more than 95% of eruptions in Pinacate. The second type of volcanic eruption
would not erupt any magma, and create a large explosion crater. These craters are known as
maars, and occur when groundwater at depth is flashed to steam and generated an explosion.
If this style of eruption occurred, it would generate pyroclastic flows and ashfall which would
effect a radius of a few dozen miles. Overall, I do not expect an eruption from this volcanic
field in the next 1000 years, and place the odds of an eruption in that timeframe at about 12%.
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