The glacier covered Mount Baker is one of the most active volcanoes in the lower 48
US states, as in the last 1,000 years it has erupted 18 times.
With this being said all of its recent eruptions, the last of which occurred in 1884 were relatively
small, largely consisting of small phreatic explosions at its summit.
However, the eruptions from this volcano were not always so small.
In fact, this volcanic field once produced a catastrophic eruption which buried the distant
towns of Seattle and Wenatchee in a foot of ash.
Closer to the eruption site, the eruption produced even more drastic results.
Some areas were buried in more than 3,300 feet or 1 kilometer of ash, which had a distinctive
white color still quite prominent today.
However, at the source of the eruption was a 1 mile deep hole in the ground.
All of this occurred during the formation of the Kulshan Caldera, which originated in
a megaeruption 1.15 million years ago.
The Kulshan Caldera is located is far northwestern Washington State, where it is located in close
proximity to the border with Canada.
The site of this massive eruption can be found on the highly eroded northeast flank of Mount
Baker, where its distinctive white ash can be even seen from satellite.
Although the caldera rim has largely eroded away, the original caldera measured 5.8 miles
wide and 3.2 miles long.
Long before Mount Baker began to form, the volcanic field which it is a part of was active.
Offshore of the state, the Juan de Fuca tectonic plate collided with and subducted underneath
the North American plate.
The melted crust from this collision then migrated upwards, erupting on the surface
and forming a chain of volcanoes.
One of these intrusions of magma resulted in an eruption at the site of the eventual
Kulshan Caldera approximately 1.3 million years ago.
This eruption began with an explosion as the magma interacted with nearby groundwater.
From the newly created crater emerged a volume of highly viscous dacite magma, which slowly
began building into a gray colored lava dome.
Over time, more intrusions occurred, depositing dikes of magma at depth, while also forming
a series of small volcanic cones and lava domes.
Over time, the composition of the magma in these eruptions became more silica rich, signaling
a warning sign of events to come.
These eruptions continued over the next 150,000 years across more than a dozen separate vents,
which never solidified into a single large volcanic edifice.
Rather, if you were to have looked at the landscape pre-caldera collapse it would have
been coated with occasional large lava domes, explosion craters, and layers of ash from
associated eruptions.
However, the rate of eruptions was insufficient to relieve the pressure building in the underlying
massive magma chamber.
By 1.15 million years ago, this magma chamber was 11 kilometer wide and contained more than
100 cubic kilometers of molten rock.
Finally, an intrusion of dacite magma occurred, causing the two magmas to mix, resulting in
a huge volume of molten rock shooting towards the surface.
After a series of small phreatic eruptions, the eruption became increasingly large.
Then, the top of an existing lava dome was destroyed in a powerful explosion.
Millions of tons of rock subsequently shot high into the atmosphere, reaching a height
of 44 kilometers or 144,000 feet.
As a massive mushroom cloud grew over the volcano, part of the eruption column collapsed,
releasing highly energetic pyroclastic flows.
Although abundant glaciers were present at the time, these flows raced across the landscape
in all directions, burning all vegetation in a 30 mile radius.
To the southeast where the ash cloud drifted, the day seemingly turned to night due to the
sheer amount of falling material.
Then, as the eruption had partially emptied the underlying magma chamber, the overlying
rock became unstable and collapsed downwards, forming a large caldera.
In total, 75 cubic kilometers of volcanic rock was erupted in a less than a week.
This ranks the Kulshan caldera forming eruption as the second largest to occur in the Cascade
Mountain range in the last 2 million years.
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