Today is May 17th 2023, and we are nearly at the anniversary of the Pacific Northwest's most
destructive volcanic eruption in recorded history. As on May 18th of 1980, what was once the almost
perfect symmetrical stratovolcano of Mount Saint Helens that towered to a height of 9,677
feet in height in less than a minute exploded and collapsed in a powerful plinian eruption,
incinerating an area of 230 square miles or 600 square kilometers, depositing 2.5 cubic kilometers
of landslide debris, and dropping to a height of only 8,307 feet, while leaving behind a large c
shaped collapse scar. Normally, this anniversary would cause the volcano to reappear in the news
and it has, but for a completely different reason. This is because around 8:45 PM local time on May
14th of 2023, a fairly sizable landslide occurred on a section of the Mount Saint Helens volcanic
complex. Travelling at a speed of 50 miles per hour, this landslide quickly moved more than
2000 feet to the west, blocking a section of the Spirit Lake Highway and taking out a local
bridge. This trapped a dozen people overnight who were eventually rescued by helicopter in addition
to completely cutting off access to the Johnson Observatory which I last visited in July of 2022.
I have outlined where the landslide and mudslide seemingly originated alongside the section of road
it took out. This landslide carved away what I estimate to be a 392 foot wide, 578 foot long,
and an up to 20 meter or 66 foot deep scar on the mountainside, transporting in mere moments a grand
total of 195,000 cubic meters or 6.89 million cubic feet of rock and debris. As for the elephant
in the room, this landslide was not caused by a change in volcanic activity at Mount Saint Helens.
However, since I drove on this exact road not long ago I remember a few details of the regional
topography that can help explain why the landslide occurred and provide additional insight. The first
thing I want to note is that 1500 feet to the east of where the mudslide originated is a patch
of brightly colored brown, orange, and red ground that stands out from the surrounding white to gray
colored rock. This patch represents significant hydrothermal alteration and staining caused by
what are most likely a series of regional hot springs which are acidic and leave iron staining
compounds behind as it flows downslope. While this is the only easily visible patch, it would not
surprise me if other regional streams had similar alteration occurring within them due to traces of
hot spring water. What I am trying to establish is that this very specific valley was already at a
heightened risk of landslides occurring due to the hydrothermal alteration occurring within sections
of it. Although this is by no means an extreme example of such, when you see highly altered
rock on a volcano, it is almost a guarantee that a landslide or series of landslides will
eventually occur, with for example this slope of the Iliamna volcano in Alaska showing hydrothermal
alteration. Second is the geology of the slope that failed. It was composed of thousands if not
tens of thousands of years worth of some of the most unstable and high risk rock types in terms
of landslide risk. Specifically, the slope that collapsed was composed of more than 50 feet thick
of lahar deposits aka older mudflows caused by volcanic processes, pyroclastic flow deposits,
and ash. All three of these materials were when combined only loosely consolidated. Thus,
since the area was covered in snow from an unusually wet winter, the weather played
the actual trigger for this landslide. Whereas on May 7th the high temperature in the region was 62
degrees Fahrenheit on the day of the landslide the high temperature was 93 degrees Fahrenheit, likely
being an all time high for that day of the year at Mount Saint Helens as the region experienced
a heatwave. This caused an increased amount of snow to melt and soak into the ground as water,
thus further destabilizing the slope until there was no longer enough friction present to maintain
the hillside, causing it to collapse. In terms of the closure of Johnson Ridge, the road leading to
it will likely remain closed in my opinion for weeks at a minimum. Thus I recommend that if
you want to get close to Mount Saint Helens on the day of the eruption anniversary, instead visit the
Windy Ridge site to the northeast of the volcano. Thanks for watching! If you would like to request
a specific topic, please leave a comment below. Additionally,
I would like to thank my new YouTube member David Vollweiler for supporting this channel!