When people think of volcanoes in the the state of Washington, they tend to think of
Mount Saint Helens or Mount Rainier. Both of these volcanoes have erupted since Washington
became a state in 1889. This video will discuss the latter of these two volcanoes.
Mount Rainier has the potential to be a much more dangerous volcano than Mount Saint Helens, as a
future eruption could destroy tens of thousands of structures as far away as Tacoma. This video will
discuss this active volcano, what its future hazards are, and odds of a future eruption.
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 Rainier can be found in the central western
portion of Washington state. It is about 40 miles north of Mount Saint Helens and Mount Adams. Like
the majority of active volcanoes in Washington, Mount Rainier is a large stratovolcano.
Stratovolcanoes contain alternating layers of both cooled lava, and ash and tend to have relatively
steep slopes. Just like Mount Saint Helens, Mount Rainier is part of the cascade mountain range,
and exists due to the juan de fuca plate subducting underneath the north american plate.
Mount Rainier began forming about 500 thousand years ago. Today, it is the tallest point in the
entire cascade mountain range, and climbs to over 14,400 ft or 4390 meters tall.
It is a dominantly andesite volcano that formed over 4 eruptive periods, each of which lasted
a little more than 100,000 years long. During these periods, the volcano erupted hundreds,
if not thousands of times, and added to the height of the mountain. Not all of these
eruptions took place on the volcano's summit. For example, one eruption 40,000 years ago took
place 6 miles to the south at Ricksecker point. In the last 10,000 years, Mount Rainier has had
26 confirmed or suspected volcanic eruptions. One of the most spectacular eruptions during
this period took place about 5600 years ago. This eruption triggered an edifice collapse of weakened
hydrothermally altered rock. This generated an enormous destructive lahar which is today referred
to as the Osceola mudflow. This very destructive mudflow had a volume of 4 cubic kilometers or 1
cubic mile, and reached as far away as Tachoma and the Seattle suburb of Kent. This eruption left a
horseshoe shaped crater which can still be seen today on the northeastern side of the volcano.
Technically speaking, the last verified eruption from Mount Rainier was in 1450,
but there are numerous reports of several volcanic eruptions in the 1800s. It is likely
that as many as 7 eruptions took place during this century, the latest of which was reported in 1894.
In other videos, I would have said a bit more about the geologic history of this mountain.
However, I need to stress that a future eruption from Mount Rainier could pose a huge loss of life.
The volcano is considered so hazardous that it was classified as one of the world's 16
so called "decade volcanoes". Decade volcanoes were chosen from dangerous volcanoes that are
in close proximity to highly populated areas and have a history of large and destructive eruptions.
Three other examples of decade volcanoes include Galeras in Columbia,
Vesuvius in Italy, and Taal in the Phillipines. So, why is Mount Rainier classified as a highly
destructive volcano, and rated as the third most dangerous volcano in the United States?
The answer is lahars, and that 150,000 people currently live on a lahar deposit
that is less than 5,000 years old. Lahars are a type of destructive debris flow that
are similar to flash floods but are typically much larger and more destructive. They form
when a volcanic eruption occurs on a snow topped mountain, which quickly melts much of the snow.
As the water travels down the slope of a mountain, it picks up trees, rocks, and anything else in its
path which creates a lahar. Lahars have a viscocity similar to that of wet concrete,
and can move at up to 120 miles per hour. In the past 5,000 years, several large lahars have
occurred from Mount Rainier. If one was to occur today, it could effect a number of towns which lie
on recent Lahar deposits. Some of the larger towns which could be effected by a future lahar
include Tacoma, Puyallap, Paradise, and Orting. This is not a complete list, however. With this
being said, a lahar is not always generated by an eruption, and the extent of a lahar is largely
determined by the size of an eruption. Luckily, a future eruption is not likely for some time. The
overall chance of an eruption from Mount Rainier each year is about 1 in 350. As a result, there
are lahar sirens and an early warning system in place in much of the lahar hazard zones. I would
not worry about a future eruption, but much like if you live in California, you need to be aware of
the hazards posed by regional geologic features. I hope that you enjoyed this video! If
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