When people think of Arizona, they don't tend to think of volcanoes.
Yet, the state contains several active volcanoes, such as Sunset Crater in Flagstaff which only
erupted 950 years ago.
In the last 2 million years of the state, volcanoes have erupted in locations ranging
from the Arizona-Mexico border to the Grand Canyon, and even the southwest corner of Phoenix.
One of these young volcanic fields is located in the eastern section of the state near the
town of Show Low.
There, a series of 405 vents consisting of explosion craters, lava domes, and cinder
cones can be found.
These numerous vents make up the Springerville Volcanic Field, which at one point was one
of the most active and largest volcanic fields in the entire United States.
The Springerville volcanic field is the third largest volcanic field in the United States,
being only smaller than the San Francisco Volcanic Field near Flagstaff and the Medicine
Lake volcanic field in California.
This complex in a series of monogenetic vents spread across an area of 1,158 square miles
or 3,000 square kilometers.
Monogenetic vents erupt once and then never erupt again as the supply of magma to the
volcanic complex is weak and intermittent, so no singular definable magma conduit for
molten rock develops.
The Springerville Volcanic Field exists due to a 1.7 billion year old scar in the Earth's
crust.
It is part of a chain of volcanic fields which includes a supervolcano that it stretched
along a northeast trending line.
This line represents the boundary of an ancient plate collision where one Proterozoic continent
subducted underneath another, which has left a permanent weak zone in the crust.
In the more than 1 billion years since, this scar known as the Jemez lineament has remained
as a weak point in the crust, offering a path of least resistance for any magma deep in
the crust to travel upwards and erupt onto the surface.
The Springerville volcanic field began forming 2.1 million years ago when a volume of magma
intruded into the crust.
This magma formed a dike of material and soon caused a series of increasingly shallow earthquakes.
Eventually, the magma burst through the surface, causing a fountain of molten rock to shoot
several hundred feet into the sky.
Eruptive activity later shifted to a moderately explosive one, causing intermittent ejections
of ash in addition to the thrown lava bombs.
Much of this ash fell back to the ground, building a 500 foot tall cinder cone.
In the more than 2 million years since, much of this volcanic cone has eroded away, leaving
behind a raised mound of material.
Over the following 1.8 million years, a similar eruptive sequence occurred about once every
4,400 years.
97% of the vents formed were cinder cones, built to a height of between 200 and 1,000
feet.
Much of the early eruptive activity occurred in the western part of the field near the
town of Show Low.
Around 1.6 million years ago, volcanic activity increased, leading to the creation of some
unusual features.
Due to the mixing of some underlying magma with rock in the crust, select eruptions produced
unusually viscous basalt lava which instead of moving downslope quickly like typical lava
flows built up into a rounded lava dome.
One of these lava domes formed a feature known as Wolf Mountain 1.56 million years ago which
measured 3800 feet wide and 500 feet tall.
By 1.1 million years ago, eruptive vents originated in what is now the central part of the volcanic
field.
Due to abundant groundwater in this area, some of this material caused an explosive
flareup to occur.
Underlying intrusions of magma heated adjacent groundwater, causing it to flash to steam.
Eventually, the pressure became too great, causing the ground to explode upwards in what
is known as a phreatic eruption.
This formed a series of several thousand foot wide craters in the ground which represented
this field's most explosive eruptions.
As cinder cones began erupting on the plains of the Colorado Plateau, eruptive frequency
slowly decreased.
Several new cinder cones had a complex and elongated profile, and in one case a double
cinder cone formed on top of an earlier vent.
This vent called twin Knolls would produce the field's last eruption 308,000 years ago,
producing a black lava flow which is still visible today.
In all likelihood, this magnificent volcanic field will never erupt again as the magma
which caused its volcanism is no longer present.
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