The state of Oregon contains about a dozen active volcanoes. These active
volcanoes include the Newberry volcano, the three sisters, and mount bachelor. However,
only one of these volcanoes has erupted since Oregon became a state (1859). The last volcanic
eruption within the state occurred from Mount Hood in 1866. This video will discuss this active
volcano, and mention what its 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.
Mount hood is a very prominent volcano which can be found in the northwest
portion of the state, and is about 40 miles east of the Portland metropolitan area.
Mount hood is a tall stratovolcano which is part of the cascade mountain range. Stratovolcanoes
contain alternating layers of both cooled lava and ash, and tend to have relatively steep slopes.
Mount hood began forming about 500,000 years ago. This early activity formed much of what is today
the base of the volcano. Relatively frequent geologic activity continued until 100,000
years ago, when a major eruption destroyed the summit and north side of the mountain.
This eruption was most likely much larger than the 1980 eruption of Mount Saint Helens.
Eruptions which occurred after this period generally took place around the large scar which
formed from this major eruption. As a result, the volcano gained back some height which it lost
from the prior eruption. The frequent eruptions of this mountain ended about 12,000 years ago.
After this period, Mount Hood became dormant for 10.5 thousand years. This dormancy ended about
1500 years ago, when the volcano erupted again. One of these eruptions caused a large collapse
on the southwest portion of the mountain, which today can still be seen from satellite.
This large rock is known as the "Crater rock", and is a very steep lava dome. It formed through
several eruptions after this recent landslide, the most recent of which occurred in 1781.
The last two confirmed eruptions from this volcano occurred in 1859 and from 1865 to 1866,
although these were relatively small in size. Although no eruptions are occurring today,
evidence that the volcano is still active can be found near its summit. There, you can find
steaming fumaroles, hot springs, and significant deposits of yellow sulfur mineralization.
So, what is the hazard posed by these volcano? Some of the previous eruptions from Mount Hood
have blanketed large parts of the state with a layer of ash, while areas closer to these large
eruptions experienced pyroclastic flows. However, these two hazards are not the main concern from a
future explosive eruption. The main hazard posed by this volcano is lahars. Lahars are a deadly
type of mudflow which is similar to a flash flood, but much larger, and has a viscosity similar to
that of concrete. They typically form when a large amount of snow near the summit is rapidly melted
by an eruption. However, volcanic eruptions are not the only source of lahars. Lahars can also be
formed by earthquakes, excessive precipitation, and large landslides. (pause 2 seconds). In the
past 2000 years, lahars have traveled more than 50 miles away from Mount Hood along riverbeds.
The map shown on screen shows several towns which could be effected by a future lahar.
Of particular danger in any future lahar is the town of Troutdale, which is part of the
Portland metropolitan area. Any medium sized lahar could destroy a large part of the town,
especially the area around the Troutdale airport. On this map, orange represents
the largest potential lahar, while the color yellow represents a much more likely event.
The good news is that any future lahar would take about 3.5 hours to reach the town.
Due to these hazards, in the most recent volcano hazard assessment from the US Geological Survey,
Mount Hood was rated as a very high threat volcano. Of the 161 volcanoes in the united
states which was given a hazard rating, mount hood was ranked as number 6. This
represents the most hazardous and likely volcano to erupt in the state of Oregon.
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