On the morning of March 30th, 1977, residents of Kodiak Island in Alaska woke up to a light
dusting of ash on the ground.
Additionally, a strong sulfur haze had crept over the landscape, indicating the eruption
of a nearby volcano.
What had just occurred was the first documented eruption of a previously unknown volcano within
Alaska.
This ashfall originated from an eruption to the west of the island.
The new volcano would later be named the "Ukinrek Maars", which translates to "two holes in
The Ukinrek Maars can be found in the southwest portion of mainland Alaska where it is located
150 miles west of the city of Kodiak.
This remote volcano consists of four lava domes, two maar explosion craters, and one
cinder cone.
Although its only documented eruption occurred in 1977, this volcanic complex had at least
three other different eruptions spanning a timespan of several tens of thousands of years.
The first eruption from this volcanic complex began 25,000 years ago when a volume of viscous
magma rose towards the surface in this part of Alaska.
When it erupted, it did so initially in a highly explosive manner.
Eventually, a large volume of highly viscous magma erupted from this vent began to take
on the shape of a dome.
As this dome continued to build in size, parts of it intermittently collapsed, generating
a series of pyroclastic flows.
By the time the eruption was over, a 200 meter tall and 800 meter wide lava dome had been
constructed out of a rock type called dacite.
Not long after, two additional smaller lava domes formed just to the west.
What had just formed are called the Gas Rocks lava domes.
Several thousand years later, another eruption occurred 2200 feet to the east.
However, this time the lava which erupted was far less viscous, resulting in a slightly
less explosive eruption.
Over the span of several months, a 120 meter tall cinder cone was built by intermittent
ash explosions along with small flows of lava.
For the next 22,000 years, this area lied dormant.
Then, in 350 BC, a new eruption occurred.
A small phreatic eruption of unknown intensity generated a short distance pyroclastic flow
and ejected boulders of rock in a 1 mile radius.
Even today, moderate amounts of carbon dioxide gas rise to the surface just north of these
domes, hinting that this area could eventually produce another eruption.
On March 30th, 1977 at 4:00 am local time, a volume of magma rose towards the surface
3 kilometers or 2 miles south of the gas rocks lava domes.
This magma eventually interacted with some groundwater, causing it to flash to steam.
This steam then expanded until the pressure was too great and the ground exploded upwards
in what is termed a phreatic eruption.
As a plume of ash rose more than a mile into the sky, a large amount of ash drifted in
an eastward direction, thus causing it to rain down on Kodiak Island.
Over the next 36 hours, intermittent smaller explosions occurred at the new volcanic crater
which had formed, called the west maar.
This maar measured 170 meters wide and 35 meters deep.
Eventually, this maar stopped erupting, and activity shifted 1800 feet to the east.
On April 2nd, a new explosion occurred, creating a separate maar explosion crater.
Over the next two days, more phreatic explosions occurred, thus enlarging the crater.
Starting on April 5th, these explosions began to eject small fragments of lava.
Then, on April 7th, a basaltic lava dome began building in the east maar.
Over the next 47 hours, it grew in size to a height of 49 meters or 160 feet tall.
Afterwards, the lava drained away, thus ending this eruptive episode.
What remained was a 300 meter wide and 70 meter deep crater with a lava dome.
Since then, both of these maars have been overlaid by a crater lake
Although no eruptions have occurred since, this area will produce more eruptions in the
future.
Technically speaking, these future eruptions could occur over an area of approximately
25 square miles adjacent to a regional fault called the Bruin Bay Fault.
When the next eruption occurs, it will generate several hazards.
The main hazard is its tall ash columns, which could disrupt regional air traffic.
Additionally, during eruptions it will produce pyroclastic flows which can travel up to to
5 miles away.
As a final note, I recommend implementing a 2 mile exclusion zone near the gas rocks.
Bubbling carbon dioxide in the water can make vessels suddenly become less buoyant, creating
a sinking hazard.
As a result of these hazards, the US Geological Survey designated the Ukinrek Maars as a moderate
threat volcano.
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