On May 18th, 1980, the largest explosive eruption of the 20th century within the lower 48 US
States began.
As a towering plume of ash rose to a height of 24 kilometers or 79,000 feet, part of Mount
Saint Helens collapsed to the north in a massive lateral blast and landslide.
Over the next several minutes, this would go on to burn all vegetation up to 19 miles
or 31 kilometers from its source.
What had just occurred was a massive volcanic eruption, rating on the volcanic explosivity
index as a 5, representing an unusually large event.
Generally speaking, volcanoes which have produced eruptions of this magnitude since the last
ice age are well known.
These include volcanoes such as Katla, Taupo, Mount Etna, and El Chichon.
Yet, within Alaska, a surprisingly high number of volcanoes have produced eruptions of this
magnitude within the last 10,000 years, but few people have heard of most of them.
One of these lesser known volcanoes which produced an eruption essentially equal in
size to the 1980 eruption of Mount Saint Helens was the Dana volcano far southwest of Anchorage.
The Dana volcano is located on the far southern end of the Aleutian peninsula where it is
35 miles northwest of the city of Sand Point.
This volcano has a medium sized edifice whose summit stands at 1,354 meters or 4,442 feet
above sea level.
This volcano contains a 1 mile wide and 1.25 mile wide depression in its center, but surprisingly
enough this is not a caldera.
Instead, it represents an unusually large volcanic crater which formed through repeated
highly explosive eruptions that did not involve enough magma to cause caldera collapse.
Although it is difficult to tell due to the low satellite resolution of the area, this
massive crater contains a crater lake.
Like the other volcanoes on the Aleutian Peninsula, Dana owes its existence to the same geologic
setting.
To the east of the island, the Pacific Plate is colliding with and subducting underneath
the North American Plate.
The melted material from this collision then migrates upwards, until it erupts onto the
surface, forming a chain of volcanoes.
Although the exact age of the Dana volcano is unclear, it is safe to assume that it has
been erupting intermittently for many tens of thousands of years.
This has constructed a series of now highly eroded overlapping lava domes and volcanic
cones.
These were constructed out of layers of ash and basaltic andesite, andesite, and dacite
composition lavas.
Many of these eruptions produced viscous lobes of lava which travelled up to several miles
beyond the crater rim.
However, based on the level of erosion at Dana, it appears that either this volcano
is nearing the end of its lifespan or in recent times it has erupted quite infrequently.
The only documented eruption from this volcano occurred in 1890 BCE, when a plinian eruption
column likely shot over 20 kilometers into the sky.
Part of the eruption column then collapsed downwards, resulting in highly energetic pyroclastic
flows which were largely funneled to the south, southwest, and west, likely travelling beyond
5 miles distant.
These pyroclastic flows partially filled adjacent valleys with several feet of material, potentially
representing what may have even been a lateral eruption.
In many ways, the products produced by this eruption look somewhat similar to the thick
layers of ash produced by the 1980 eruption of Mount Saint Helens.
In total, the eruption from Dana produced 1 cubic kilometer of tephra and was assigned
a volcanic explosivity index of a 5.
Although this volcano appears to have recently had a very low eruptive frequency, it still
poses a threat to both nearby settlements and air traffic.
Any form of a future explosive eruption could ground aircraft over a wide radius on or near
the peninsula, and particulates of falling ash from an eruption could temporarily contaminate
regional drinking water.
As a result of this and other hazards, the US Geological Survey designated Dana as a
moderate threat volcano.
For those of you in my audience who wish to learn about a very similar volcano within
Alaska that also produced a recent VEI 5 eruption, check out my video on the Yantarni volcano.
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