On July 9th, 1953, a volcano which was thought to be dormant violently erupted to the west of
Anchorage. As a towering plume of ash reached high into the atmosphere, deadly pyroclastic flows
raced to the southeast of the mountain. Luckily, despite the volcanoes proximity to Anchorage,
no one lived in the path of this destruction. The volcano I am referring to is Mount Spurr,
which produced another large eruption in 1992. This video will discuss this volcano,
and mention what its future hazards are. Mount Spurr can be found in south central Alaska,
where it is 80 miles west of Anchorage. The reason Spurr is of particular interest is that
its the closest active volcano to Anchorage. Mount Spurr began forming 255,000 years ago,
when magmas related to plate subduction off the southern coast of Alaska traveled upwards
through the crust. This lava consolidated a few miles northeast of Chakachamna Lake,
and erupted onto the surface through a central cone. At the time, this section of Alaska was
covered with a thick glacier, as it was a glacial maximum during the planet's most
recent ice age. When lava interacts with snow, water, or ice, it often causes an explosion.
This was no different, and as a result a series of highly explosive eruptions marked the beginning
of Mount Spurr. After the initial explosive phase subsided, intermittent mildly explosive
eruptions started to build this stratovolcano. Over the next two hundred thousand years,
overlapping layers of lava flows AND ash added to the height of this volcano. The lava which erupted
during this period formed a rock type known as andesite, which is the typical light gray rock
associated with stratovolcanoes around the globe. By the end of the last ice age 12,000 years ago,
Mount Spurr stood at about 13,000 feet tall. However, the volcano began to show signs of
structural instability in its southern flank. Then, 9,000 years ago, a large section bulged
outwards as a small eruption began near the summit. Eventually, this bulge burst in a massive
explosion. Much like what occurred at Mount Saint Helens in 1980, the southern half of the mountain
slid away in a large landslide. Debris from this landslide traveled more than 15 miles,
and in some cases deposited 300 foot wide sections of rock at even further distances.
Mount Spurr now had a 3 mile wide caldera and a 2500 foot deep scar from this landslide.
What had just occurred was likely 10 times larger than the 1980 eruption of Mount Saint Helens.
In the aftermath of this eruption, a new vent formed at approximately the same site
of the original summit. From this vent erupted both andesite and dacite lavas,
which indicated an increase in silica. Over the next several thousand years,
these lavas built a series of overlapping lava domes, creating the modern peak of Mount Spurr.
Then, 7000 years ago, a new vent began forming 2.5 miles south of the main summit. This vent,
known as "Crater Peak", represented a satellite cone of Mount Spurr. Interestingly,
the lavas which erupted from this cone were far less siliceous,
and produced basaltic andesite. Despite the low silica content from the eruptive lavas,
eruptions produced by this cone were still highly explosive. Since about 5,000 years ago,
Crater Peak has remained the only active vent on the entire volcano. Then, in 1953, a sub-plinian
style eruption occurred from this vent. This sent a plume of ash high into the atmosphere,
and vaporized the crater lake in the middle of the cone. A similarly large eruption took
place in 1992, which caused about a quarter of an inch of ash to fall on Anchorage. On the volcanic
explosivity index, both of these eruptions were rated as a 4 on the scale, making them an order
of magnitude larger than the most recent eruption from Lassen Peak. Eruptions this size occur
about once every 250 years from Mount Spurr. Luckily, future eruptions from Mount Spurr
will not necessarily be deadly, since no one lives in direct harm's way from this volcano.
Regardless, during the next eruption, you would not want to be within 20 miles of the eruption's
center. In addition to significant ashfall and pyroclastic flows, Mount Spurr's eruptions have
also created long distance lahars which travel along the Beluga and Chakachatna Rivers. If
you are in one of those rivers at the time of an eruption, get to high ground immediately. Overall,
Mount Spurr represents a highly explosive volcano which shouldn't cause much damage to any nearby
cities other than the occasional inch of ashfall. I hope that you enjoyed this video. If you would
like to request a specific topic, please leave a comment below!