On January 31st, 2022, the US Geological Survey issued a special statement regarding a well-known
volcano in Oregon.
They reported that the Three Sisters volcano was showing signs of unrest, and that magma
was on the move at depth.
Specifically, a magmatic intrusion was detected underneath the volcanic complex at a depth
of approximately 6 kilometers or 4 miles.
Since this magma is slowly creeping upwards towards the surface, it is pushing up the
overlying crust thus causing ground deformation.
This uplift is occurring in a similar manner to a mole digging a tunnel in a grass yard.
Alongside this uplift, swarms of minor earthquakes due to the ongoing intrusion of magma have
also been detected.
However, these earthquakes are all relatively minor, ranging from magnitude -0.6 to 0.7.
These earthquakes are centered around the area of where the uplift is occurring, which
happens to be approximately 3 miles west of the south sister volcano.
So, does this mean that this volcanic complex will erupt?
To this question I must say most likely no, but with an asterisk.
Whenever an intrusion of magma occurs, there is always a possibility that a new eruption
will occur.
However, 9 times out of 10 an intrusion of magma does not reach the surface and solidifies
at depth.
The shallower an intrusion of magma is, the more likely an eventual eruption is.
For context, small amounts of uplift have been ongoing at the Three Sisters Volcano
since 1995, although in all likelihood had been ongoing for a few decades beforehand.
Since then, the crust overlying the intrusion has been uplifted by approximately 12 inches
or 30 centimeters.
This uplift has not occurred all at once but rather through periods of more significant
uplift followed by longer phases of minor uplift.
One such period of more rapid uplift is ongoing, but only totals a fifth of an inch since August
of 2021.
However, in October and December of 2021, and then again in January of 2022, swarms
of earthquakes with a magmatic signature were detected in the vicinity of the ongoing uplift.
These closely spaced earthquake swarms are the justification behind why a special notice
was issued as they signal above background levels of activity.
While the ongoing volcanic activity is still above normal levels, it is not yet sufficient
to raise the volcano's alert level, so at the moment it remains at green.
The magma at the root cause of this uplift does not appear to have originated from the
mantle, but rather from an underlying magma chamber which has been present at least since
the Three Sister's last eruption in 440.
As a whole, this volcanic complex consists of three separate stratovolcanoes which were
active between 120,000 and 2,000 years ago.
Recent eruptions have however largely occurred away from their summits, creating either cinder
cones near the North Sister, or a chain of rhyolite composition lava domes near the South
Sister.
Because of the proximity of the ongoing uplift to a chain of rhyolite lava domes, it can
be assumed that if this intrusion were to reach the surface, it would also form a lava
dome.
However, the underground pathways which magma takes before it erupts are not always so simplistic.
An intrusion which begins near South Sister could result in an eruption near the North
Sister.
With this being said, there is also a slight possibility that this intrusion could eventually
feed an eruption at another nearby volcano such as the Belknap Crater shield volcano
to the north or Mount Bachelor to the south.
Assuming that a lava dome forming eruption was to occur if the intrusion reached the
surface, two hazards would be ashfall alongside superheated rock and gas currents called pyroclastic
flows.
These would be released at the onset of any eruption, but would also occur whenever a
lava dome partially collapses, and travel up to 9 miles from an erupting vent.
However, since very few people live within this radius of the volcano, the most significant
danger posed by a future eruption would be lahars.
Lahars are a type of dangerous mudflow with the viscosity of concrete, which sweep away
anything in their path, and are associated with 20% of volcanic eruption casualties.
The towns of Belknap Springs, Sisters, McKenzie Bridge, Blue River, Vida, and to a lesser
extent, Springfield are all located on previous lahar deposits.
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