When most people think of the most dangerous volcano in the lower 48 US states, they generally
think of a single system; Mount Saint Helens.
Yet, despite having produced 44 separate volcanic eruptions in the last 5,000, including 8 equal
in size or larger than its infamous 1980 eruption, it is not the most dangerous volcano.
Rather, listed as a highly dangerous so called decade volcano among the likes of Vesuvius
in Italy and Merapi in Indonesia is a different volcano; Mount Rainier.
The reason?
An unusual layer of white to yellowish brown rock found downstream of Mount Rainier.
This specific rock unit can be found over a wide area in the ground including an area
northeast of Tacoma, in Puyallup, Greenwater, Auburn, and Buckley.
It does not represent a layer of ash but rather something far more destructive; one of the
world's largest recent lahars, the Osceola Lahar.
Approximately 5,600 years ago, a large section of Mount Rainier collapsed, creating a debris
flow which mixed with volcanic products from an ongoing eruption.
This mass of material was subsequently funneled into a series of river channels, overtopping
their sides with a wall of mud and superheated rock up to 200 feet in height.
This lahar would go on to cover 220 square miles or 550 square kilometers of land in
material, traveling all the way to Puget Sound and wiping out all vegetation in this large
scale natural disaster.
Today, more than 100,000 people live on top of this recent lahar deposit, and all would
be placed at risk if a similar event was to occur again.
Since Mount Rainier is covered by a thick series of glaciers, any future eruption would
be guaranteed to melt large sections of overlying ice, once again placing tens if not hundreds
of thousands of people at risk.
So, why did this lahar occur, and how often do events of this magnitude occur?
This video will answer these 2 questions and discuss why Mount Rainier truly is a volcano
some scientists do not want to witness another eruption from.
Today, Mount Rainier is quite an impressive stratovolcano as it stands at 14,411 feet
tall.
Yet, in 3,600 BCE, the volcano was even taller, being the tallest point in the lower 48 US
states.
At the time, Mount Rainier was somewhere between 15,000 and 16,000 feet in height.
At the time, the volcano was undergoing a small scale series of phreatic eruptions,
which marked the beginning of a longer VEI 3 explosive eruption sequence.
Large swaths of the northern flank of Mount Rainier had turned shades of white, yellow,
and red due to significant hydrothermal alteration.
This replaced significant quantities of harder volcanic rock with clay which is incredibly
prone to erosion.
As the phreatic phase of the eruption ended, magma interacted with the volcano's water
table, creating a series of powerful explosions.
This sent particles of ash and lava down Rainier's slopes, resulting in 3 small scale lahars.
As the eruption progressed, it soon changed to purely magmatic activity.
Then, due in part to the ongoing eruption, a water rich avalanche began on the volcano.
This avalanche soon cascaded to include boulders of rock, and soon an entire slope of the hydrothermally
altered Mount Rainier.
As 3.8 cubic kilometers of rock collapsed to the north and northeast, the volcano lost
more than 1,000 feet in height.
This material was largely funneled into a river channel where it overtopped many natural
barriers and carrying with it large logs from ripped out trees.
With a high density this lahar moved at 42 miles per hour, destroying everything in its
path.
Although much of the lahar's material was deposited along the way, by 3 hours later,
it reached Puget Sound.
What had just occurred was Mount Rainier's largest known lahar.
Luckily, events of similar magnitude are incredibly rare, repeating at a rate of less than once
every 10,000 years.
However, lahars which are 10 to 20 times smaller occur at Mount Rainier about once every 1,000
to 2,500 years, having a similarly destructive effect over a lesser area.
Here is a quick USGS map of areas covered by recent lahars from Mount Rainier.
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