On April 4th, 2009, residents of Homer in Alaska awoke to find the ground covered with a layer of
ash. At the same time, 60 miles away, a deadly lahar raced down a river on the western side of
the cook inlet, leading to damage at a local oil storage facility. What had just taken place was
only the latest eruption in the long eruptive history of Mount Redoubt. This volcano is
considered to be the most dangerous of the 86 volcanoes in Alaska, and will certainly erupt
again. This video will discuss this volcano, and mention what its future hazards are.
The Redoubt Volcano can be found in south central Alaska,
where it is 110 miles southwest of Anchorage. Among volcanoes in the region, Mount Redoubt is
relatively old. This volcano began forming 890,000 years ago, when highly siliceous lavas reached
the surface. These early eruptions were highly explosive, and created numerous overlapping layers
of lava domes and ash. If you ever visit the volcano, look for the rock which I am showing on
screen. It is known as dacite, which is typically only found at the most dangerous stratovolcanoes.
Dacite lavas continued to build Mount Redoubt into a tall stratovolcano until about 340,000
years ago. Then, for an unknown reason, the lavas which fed the volcano became far less siliceous.
Instead of erupting dacite, the volcano now erupted basalt and basaltic andesite.
Since eruptions were now less explosive, the volcano began an early cone building phase.
Numerous eruptions deposited lava and ash on top of each other, adding to the height of Redoubt.
By 184,000 years ago, the composition of the lava changed once again,
as it became more siliceous. The resulting eruptions were more explosive,
as the erupted lava was now andesite instead of basaltic andesite. Over the next 170,000 years,
numerous pyroclastic flows were generated, which deposited tuff up to 15 miles from this volcano.
If you find a welded rock with several inch wide fragments surrounded by finely grained material,
you are likely looking at an area that experienced a pyroclastic flow.
At the end of the last ice age, Mount Redoubt was likely 400 feet taller than it is today. What
caused the volcano to lose height? The answer is a mount saint helens style eruption which
occurred 12,000 years ago. This eruption caused the north section of the volcano to slide off
the mountain in an explosive manner. Beginning around 10,000 years ago,
lavas at Redoubt once again became slightly more siliceous, thus further increasing the likelihood
of summit collapses. Two additional smaller summit collapses occurred 3700 and 800 years ago,
one of which was in a southward direction. In the last 1,000 years, Redoubt has erupted 17
times. Each of these eruptions were approximately the same size, rating as
a 3 on the volcanic explosivity index scale. In December of 1989, the volcano erupted for
the first time in 22 years. During the course of the eruption, numerous pyroclastic flows and
explosions occurred due to continued collapses of a growing dome of lava at the north summit.
Ash from this eruption reached more than 35,000 feet into the atmosphere, and fell
all across southern Alaska, including anchorage. Due to these plumes of ash, a 747 aircraft flying
overhead experienced complete engine failure in all 4 engines. Here is why this happened. Ash that
enters a hot aircraft engine is quickly melted and resolidified on moving engine parts as glass,
which can cause the engine to flame out or shut down entirely. Luckily, the pilots were able to
restart all of the engines and land safely. A similar eruption occurred in 2009,
but the damage was less significant. So, why is Redoubt considered by the US Geological
Survey to be the most dangerous volcano in Alaska, and the 4th most dangerous volcano in the United
States? The answer, is that a special weight is given in determining the danger of a volcano if
it has caused physical damage. During the last two eruptions, large lahars were generated on the
north side of the mountain as erupting lava and hot ash melted existing snow in the region. These
lahars traveled to the east and heavily damaged a local oil storage facility. This facility is
built on top of numerous existing lahar deposits, and represents an ecological disaster waiting
to happen. It could easily be wiped away by during the next eruption from another lahar.
During the next eruption, you will not want to be within 20 miles of this volcano,
especially in a riverbed downstream from the summit. Two areas are at especially high risk,
which I have marked on this map. I hope that you enjoyed this video.
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