When people think of volcanoes in the the state of Oregon, they tend to think of Mount Hood which
last erupted about 150 years ago. However, there are about a dozen other active volcanoes within
the state. One of these active volcanoes is referred to as the three sisters. This group of
volcanoes has erupted 6 times in the last 10,000 years, and almost certainly will erupt again. This
video will discuss this group of active volcanoes, and mention what their future hazards are.
For this video, we are defining an active volcano as any volcano or volcanic field which
has erupted in the last 10,000 years. Although 10,000 years may seem like a long timeframe,
this is the definition used by the global volcanism program.
The Three Sisters are a group of three volcanoes which can be found in the central west portion
of Oregon, about 40 miles northwest of the Newberry Volcano. They are all classified as
stratovolcanoes, and are considered part of the cascade mountain range. Stratovolcanoes contain
alternating layers of both cooled lava and ash, and tend to have relatively steep slopes.
The Three Sisters formed at different periods over the last 120,000 years. The North Sister
was the first to begin forming. From 120,000 to 40,000 years ago, this volcano grew after
numerous explosive and effusive eruptions to a height of 11,000 ft or 3,400 meters. However,
due to erosion over the last 40,000 years, this volcano has lost about 1000 ft in elevation.
The middle sister formed more recently. This volcano formed in a series of eruptions between
40,000 and 14,000 years ago. One of these eruptions 38,000 years ago produced the
Obsidian Cliffs lava flow. As the name implies, it produced a large amount of the volcanic
glass known as obsidian, and was an important source of obsidian for nearby Native Americans.
The tallest of the three Sisters volcanoes is the Southern Sister. Although it was the second of
the volcanoes to form, it has remained the most active in recent times. It began forming 50,000
years ago, and continued to build through frequent eruptions until about 22,000 years ago. After this
period, the volcano became dormant for 20,000 years. This dormancy ended about 2300 years ago,
when the volcano produced a series of explosive eruptions created the rock mesa lava flow.
300 years later, the area erupted again, and formed a series of rhyolite lava domes
in a 3 mile or 5 kilometer north to south chain. In the last 10,000 years, the south sister was not
the only volcano which erupted. In fact, the most recent eruption took place northwest of the north
sister. Although the north sister is extinct, the region around the volcano can sometimes produce
cinder cone eruptions. The most recent of these occurred in 440 AD, and created the Collier cinder
cone. Four of the last 6 eruptions in this region were quite similar and also produced cinder cones.
The other two eruptions were highly explosive. These eruptions contained
lava with a much higher percent of silica. Due to this, the lava did move very quickly,
and it was easier for pressure to build. So, what is the hazard posed by these volcanoes?
Any future eruption could pose a variety of hazards, although a non cinder cone forming
eruption is inherently more dangerous. The two greatest hazards the volcano could pose
are pyroclastic flows and lahars. Pyroclastic flows would be created by any explosive eruption,
and could travel about 10 miles from their source. If there was a large amount of snow
on the ground at the time of an eruption, it could generate a deadly lahar. Lahars
are a type of deadly mudflow which in this case could travel up to 50 miles away along
riverbeds. The map shown on screen shows several towns which could be effected by a future lahar.
Due to these hazards, in the most recent volcano hazard assessment from the US Geological Survey,
the Three Sisters were rated as a very high threat volcano. Due to recent ground deformation detected
west of the south sister, the odds of a future eruption remain elevated but are still quite low.
I hope that you enjoyed this video. If you would like to request a specific topic, please leave
a comment below. After all, a video on this volcano was requested by a number of comments.