On May 22nd, 1915 the Lassen Peak volcano is California produced a powerful explosion.
As a mushroom shaped eruption plume rose more than 30,000 feet into the atmosphere, pyroclastic
flows raced across the nearby landscape, destroying 3 square miles of forest.
What had just occurred was the largest single explosion of Lassen Peak which erupted continuously
from May of 1914 to June of 1917.
Although the destruction caused by the eruption that day would go on to cause Lassen Volcanic
National Park to be created to protect the area, the eruption was rather small.
Going back further in the recent history of Lassen Peak, you can find far more powerful
eruptions.
One of these older eruptions was so powerful that it coated the nearby landscape in layers
of ash taller than a 3 story building and caused several inches of ash to fall as far
away as Yellowstone National Park.
So, when and where exactly did this eruption occur?
Just how powerful was it?
This video will answer these two questions and discuss the geologically recent megaeruption
of the Lassen Peak volcanic complex.
To understand just how powerful this megaeruption was, visualize this.
The town of Manton is 17 miles to the west of Lassen Peak, which is generally upwind
away from where ash would fall during volcanic eruptions.
Yet, during a span of less than 24 hours, it was covered in 197 feet or 60 meters of
ash.
Closer to the vent which produced this eruption, the ash piled up to more 260 feet thick.
However, at the site of the eruption, what used to be a set of jagged peaks was instead
a massive several mile wide crater in the ground called a caldera which was more than
1000 feet deep.
If you were to have viewed western North America from space after the eruption occurred, you
would have seen the states of California, Oregon, Idaho, Nevada and Utah all covered
in a gray layer of ash.
Everything just described was caused by an eruption which occurred 609,000 years ago
at a vent southwest of the modern Lassen Peak at the modern site of Mount Tehama.
When the eruption occurred, Mount Tehama had yet to form and instead there was a complex
of large lava domes in its place, which looked quite similar to the modern summit of Lassen
Peak and its associated secondary peak called the Chaos Crags.
These groups of lava domes were closely spaced along a line of activity, representing a massive
underlying magma chamber.
They were built by 200,000 years of activity, during which the underlying magma chamber
leached material from the surrounding crust, slowly becoming more and more silica rich.
Eventually, its composition changed from dacite to rhyolite.
This change was quite significant, as it made the magma more viscous.
The more viscous magma is, the less easily gasses in it can escape to the surface such
as sulfur dioxide and carbon dioxide.
Thus, with more gas present, the magma chamber became overpressurized, leading to an unusually
powerful explosive eruption.
When this volcano finally erupted, all of this gas was released in a short timespan,
sending a plume of ash more than 45 kilometers or 147,000 feet into the atmosphere.
As millions of tons of volcanic rock were ejected per second, the column soon collapsed
downwards, producing a highly energetic pyroclastic flow.
This flow likely incinerated everything in a 20 mile radius, reaching up to 40 miles
distant in some locations.
Then, due to the large volume of material which had erupted, the center of the volcano
collapsed downwards, forming a large caldera.
The reason why this caldera is no longer visible is it was quickly covered in lava from subsequent
eruptions.
One of these long eruptive sequences built a now destroyed 11,000 foot tall volcano on
top of the caldera, so any resemblance of the original caldera rim is long gone.
While we don't know how large the caldera was, it likely was at least 3 miles wide,
and may have been as much as 6 miles long.
In total, the caldera forming eruption which formed the so called Rockland Caldera ejected
50 cubic kilometers of volcanic rock, which was 50 times larger than the 1980 eruption
of Mount Saint Helens.
This was the 2nd largest volcanic eruption to occur in the entire Cascade mountain range
in the last 1 million years.
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