The United States contains 162 active volcanoes.
Of these, 85 are located in Alaska.
The vast majority of volcanoes within the state are towering stratovolcanoes which have
steep slopes and are built out of overlapping layers of lava and ash.
These stratovolcanoes, such as Mount Redoubt, are generally built out of of a gray rock
type called andesite.
When lava from these volcanoes erupt, it does not travel very fast or far, thus allowing
the creation of a steep slope.
In contrast, shield volcanoes such as Mauna Loa in Hawaii are created by fast moving lava
which has a low viscosity due to its low silica content.
This lava typically cools as a black volcanic rock called basalt.
Thus, 99% of shield volcanoes are built out of this rock type.
However, 200 miles east northeast of Anchorage within Alaska is a shield volcano which breaks
this rule, as it does not contain any basalt.
The volcano I am referring to is Mount Wrangell, which erupts an unusually fluid type of andesite.
As such, Wrangell should technically be a towering stratovolcano, which is why the overall
mountain is so weird.
Mount Wrangell last erupted in 1930 and will certainly erupt again.
This video will discuss this volcano and mention what its future hazards are.
I want to note that frequent reports of tall white plumes rising from this volcano are
made from concerned citizens.
However, these harmless white plumes are merely gas being vented from fumaroles on the volcano's
summit.
If this color was to change, then it would suggest an eruption.
The Wrangell shield volcano began forming 600,000 years ago, when magma rose towards
the surface and erupted several miles east of Mount Drum.
At the time, the Mount Drum stratovolcano and Sanford shield volcano were both active,
suggesting that Wrangell began forming due to the same at depth magma reservoir.
These early eruptions produced an unusually high volume of lava, potentially thousands
of cubic meters per second.
As such, gas which would normally get trapped underneath the lava easily escaped into the
atmosphere, causing a lack of highly explosive eruptions.
Additionally, due to the rapid rate which the gray lava erupted at, it was quickly pushed
downslope by the mass of newly erupted lava.
Over the next 400,000 years, the vast majority of Mount Wrangell was built.
Wrangell eventually grew in size to a 20 mile wide towering shield volcano.
Eventually, 200,000 years ago, the rate of eruptions at Wrangell decreased.
Then, 50,000 years ago, a large flank eruption occurred.
As a vast volume of lava emerged to the southwest of its summit, it began traveling a long distance.
Eventually, this lava had traveled an impressive 36 miles or 58 kilometers away to the southwest
before it stopped.
Then, due to the large volume of magma erupted, a 4 x 6 kilometer section of the volcano collapsed,
forming a large caldera.
Since then, this caldera has been largely overlaid by other recent eruptions and glacier
ice, although some parts of the crater are several thousand feet deep.
Beginning about 25,000 years ago, a number of vents erupted to the west of the caldera,
forming a group of overlapping cinder cones and spatter cones.
Over the span of several thousand years, this vent erupted a volume of lava equal to what
is contained in the entirety of Mount Saint Helen's edifice.
By the time the eruption had ended, a new secondary peak had been created called Mount
Zenetti.
Since 1760, Wrangell has erupted 12 times.
The vast majority of these eruptions were marked by short term phreatic explosions due
to the interaction of rising magma with snow and glacier ice.
However, a few of these eruptions produced an orange glow within the summit crater, hinting
at a small lava flow.
The last of these eruptions occurred in 1930.
Since then, no additional eruptions have occurred.
Due to the frequent eruptive history of Wrangell, it is only a matter of time before it erupts
again.
The volcano tends to produce an eruption about once every 24 years, although it has been
a long time since the last eruption.
When the next eruption occurs, its main threat would be lahars.
Due to the large volume of snow and ice on the summit, the heat of an eruption would
cause much of this to melt.
This water would then race downhill with fragments of ash and volcanic rock, creating a phenomena
known as a lahar.
On screen, I have labeled a number of rivers which could be effected by a future eruption.
As a result of these hazards, the US Geological Survey designated Wrangell as a moderate threat
volcano.
I hope that you enjoyed this video!
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