Outside of the west coast states in the lower 48 U.S. states, Idaho statistically has the
highest chance of producing the next volcanic eruption.
This could occur at a number of large volume lava fields that have a dark black color visible
from satellite, with the largest of these, the Craters of the Moon volcanic field, spread
across an area nearly the size of Rhode Island.
Yet, if you were to look on the U.S. Geological Survey webpage of various active volcanoes
entirely within the state of Idaho; you would only find 4 listed.
These are from west to east; the Black Butte Crater Lava Field, Craters of the Moon, Wapi
lava field, and Hell's Half Acre.
Yet inbetween these dark colored lava fields you will find another area covered with a
fairly large amount of cooled basaltic lava which isn't labeled.
And no, this feature is not part of Hell's Half Acre.
Rather, this is the completely separate Cerro Grande volcanic field that just barely missed
the 11,650 year cutoff used for designating a volcano as active, as it last erupted 11,960
years ago.
Thus, this often-forgotten lava field could best be classified as dormant, meaning it
could theoretically reactivate and erupt again at some point in the future, but is far less
likely to do so than an active volcano.
The Cerro Grande volcanic complex consists of three lava flows spread out over an area
of approximately 77 square miles.
From north to south there is the North Robbers lava field, the South Robbers lava field,
and the Cerro Grande lava field.
The North and South Robbers lava fields might look distinct, but they both formed in a single
eruption 11,960 years ago.
The eastern Snake River Plain represents a weak area of crust where the Yellowstone Hotspot
once progressed through over millions of years, producing voluminous explosive eruptions.
This hotspot like many others does not only produce volcanism where it is centered but
also at parts of the plume trail albeit in a weaker manner when compared to the plume
center in the direction of tectonic plate movement.
This caused basalt magma to occasionally intrude into the crust, which then used crustal weak
points which in this case were rift zones to travel to the surface and erupt.
Whereas Craters of the Moon and Wapi lava field eruptions occurred along the Great Rift,
at Cerro Grande they occurred along the Arco-Big Southern Butte Volcanic Rift Zone.
Evidence of this rift zone can be found in part at the oldest lava flow in the complex,
Cerro Grande.
There, two more than 4,000-foot-long solidified fissures exist that parallel the broader rift,
being the site where lava once erupted along the entirety of these two lines, spattering
molten rock onto the surface in an effusive manner.
Additionally, the other two smaller lava fields each contain a fissure that stands at between
10 and 35 feet taller than the surrounding landscape which also mostly parallels the
direction of the overall rift zone.
The Cerro Grande lava field formed during an eruption in 11,380 BCE which began when
an intrusion of magma into the underlying crust reached the surface and divided into
three lengthy fissures.
These at first all simultaneously erupted along their entire lengths, initially erupting
at a rate of around 50 cubic meters per second.
Tall lava fountains were generated, but, as lava piled up around and above the eruptive
fissures, sections of them became clogged, causing small vents to become dominant.
This led to two of the three fissures ceasing their eruption a mere month after the eruption
began.
Eventually material at the third fissure developed into a spatter cone, which over time grew
in height to be approximately 200 feet higher than the surrounding landscape.
A 1600-foot-wide pit crater formed in its center, and lava tubes carried molten rock
downslope to more distant locations.
This eruption probably lasted for around 8 years before coming to a close, depositing
significant amounts of ropey pahoehoe lava.
After 2,000 years of no eruptions occurring, the field erupted again over a timespan of
approximately a week in the year 9,960 BCE.
A small spatter cone at the South Robbers field erupted first before being followed
by the initiation of a new vent 2 miles to the northwest 2 days later.
Across the timespan of about a week before the eruption ended, these vents erupted around
12 million cubic meters of lava.
As a final note, the odds of the Cerro Grande volcanic field erupting again in the next
1,000 years are incredibly low, but not zero.
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