The city of Phoenix is known for being surrounded by a vast desert and its associated unusually
hot temperatures.
However, a mere 1.5 million years ago, this city was also in close proximity to a series
of erupting volcanoes.
One of these vents formed a small shield volcano which is today located 7 miles east southeast
of the Palo Verde nuclear power plant.
Known as the Arlington volcano, this shield volcano-cinder cone hybrid formed in a lengthy
effusive eruption most similar to volcanic activity today seen at Kilauea in Hawaii.
Due to its proximity to the Gila River, it likely dammed the river during its eruption.
Additionally, there are several other areas to the south and southwest where volcanic
vents of similar age did dam the overall river, forcing it to repeatedly change course.
These vents are all part of a single volcanic complex referred to as the Sentinel Plain
volcanic field.
The Sentinel Plain volcanic field covers 240 square miles of desert in Arizona and is primarily
located west of the city of Gila Bend.
It contains a series of more than two dozen separate volcanic centers which are composed
of shield volcanoes, cinder cones, and tuff rings.
The reason why this volcanic field exists relates to the area's geologic setting.
In this section of Arizona, the crust is slowly stretching apart due to extensional forces.
This stretching has created a series of large fault lines which represent a weak point in
the crust.
Thus, any magma at shallow depths from the mantle uses these as a path of least resistance,
causing lava to erupt onto the surface.
This volcanic field began erupting 3.3 million years ago, when a several hundred foot long
fissure which trended to the northwest opened in the ground a few miles south of the modern
interstate 8.
As a fountain of lava was ejected high into the air, the roar of volcanic activity became
prominent.
This lava then fell back to the ground and slowly advanced downslope.
Since the area was rather flat, lava flows moved at less than a mile per hour.
After a several month long eruption, a new shield volcano with a summit 100 feet higher
than the surrounding landscape was constructed.
Over the next several hundred thousand years a half dozen additional eruptions occurred,
once again building more small shield volcanoes.
While the aforementioned eruptions were almost entirely nonexplosive, a single vent had a
highly explosive component to it.
What I am referring to is the Sauceda tuff cone.
This vent consists of a cinder cone which formed in its center surrounded by a 1700
foot wide explosion crater.
It formed when an intrusion of magma heated nearby groundwater, causing it to flash to
steam.
This caused an immense amount of pressure to build, resulting in a powerful volcanic
explosion which formed the more than 1,000 foot wide crater.
To the southwest of the Arlington volcano is another volcano whose outline can be prominently
observed from satellite.
Known as the Gillepsie volcano, this shield volcano formed 2.3 million years ago, a mere
70,000 years after Arlington formed.
As a ten thousand foot long lava flow advanced east toward the ancient Gila River, it soon
interacted with it, generating a series of small explosions.
Before long, a 100 foot thick lava flow had dammed the river, causing a large mass of
water to build up behind it.
This dam would eventually burst, sending a massive flood downriver.
As volcanism continued to the southwest, long periods of inactivity lasting tens of thousands
of years occurred between individual eruptions.
As of 2.5 million years ago, the Gila River flowed 15 miles south of its modern path where
it cut through a thick black field of basaltic lava.
A subsequent eruption during this timespan from a vent near the Theba shield volcano
dammed the river, causing the Gila River to divert to the north, where it remains today.
2.3 million years ago, another lava dam formed to the north which was 200 feet thick.
Intermittent cinder cone forming eruptions continued until 1.1 million years ago, when
the overall field became extinct.
There may be younger lava flows which have yet to be dated, but there are no indications
that this volcanic field will ever erupt again.
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