On March 27th, 1980, the snow covered peak of Mount Saint Helens began producing a series
of small volcanic explosions.
A new eruption had begun, which remained at a relatively low intensity for the next month
and a half.
During this time, a large body of shallow magma intruded underneath the volcano's northern
slope, causing a section of it to bulge upwards by as much as 450 feet.
This bulge eventually burst, leading to one of the largest volcanic eruptions of the 20th
century on May 18th of 1980.
Areas up to 20 miles distant from the volcano were scorched by a lateral blast and pyroclastic
flows, and 230 square miles of forest was destroyed.
Although the destruction caused by that day would result in the volcano becoming protected
as a national monument, that eruption did not remotely compare to other far larger eruptions
which occurred within the state in the last several million years.
Two of these older eruptions occurred in rapid succession, and were so powerful that it coated
the nearby landscape in layers of white ash taller than a 10 story building.
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 twin megaeruptions
of the Hannegan Caldera.
To understand just how powerful these megaeruptions were, visualize this.
Picture a similar volcano such as Mount Baker with its snow and glacier covered peak.
Now imagine this peak completely destroyed, leaving behind a massive several mile wide
and 1 mile deep caldera in the ground.
Surrounding and also filling this caldera is a layer of ash which is slightly more than
900 meters or 3,000 feet thick.
This is exactly what occurred at Hennegan Peak during two eruptions approximately 3.7
million years ago.
The Hannegan Peak caldera is located in far northern Washington State near the border
of Canada, where it is only 15 miles east northeast of Mount Baker.
Although the caldera is no longer quite prominent, its outline is shown on screen.
It measures 3.5 kilometers wide and 8 kilometer long.
The construction of the Hannegan Peak stratovolcano began approximately 4 million years ago, when
a large intrusion of magma reached the surface.
As this magma interacted with nearby groundwater, it generated a series of explosions.
From the newly created crater emerged a volume of highly viscous andesite lava, which slowly
began building into a gray colored lava dome.
Over the subsequent tens of thousands of years, large lobes of andesite lava flowed down the
volcano, forming a several thousand foot tall edifice.
During this period, the composition of magma during these eruptions became more silica
rich, signaling a warning sign of events to come.
As a large rhyolite rich magma chamber continued to build, the continuing low rate of eruptions
was unable to relieve the still building pressure.
Then, 3.72 million years ago, a section of the northern magma chamber intruded into a
large fault, resulting in a powerful eruption on the surface.
As millions of tons of volcanic rock were ejected each second, a plume of ash shot more
than 44 kilometers or 144,000 feet into the atmosphere.
Part of the eruption column subsequently collapsed downwards, generating energetic pyroclastic
flows.
These pyroclastic flows were largely funneled into existing low points and valleys, and
in addition to superheated ash, wiped out all vegetation in a 30 mile radius.
As white rhyolitic ash and pumice continued to pile up, the underlying magma chamber was
showing signs of structural instability.
Then, since the northern section of this chamber had been sufficiently drained, a large section
of overlying land collapsed downwards, forming a tilted block of material.
In total, 60 cubic kilometers of ash and lava had been ejected in an eruption assigned a
volcanic explosivity index of a 6.
Over the next 22,000 years, a crater lake developed on top of the collapse scar, depositing
a 100 foot thick layer of sediment.
Then, 3.7 million years ago, a similarly powerful explosive eruption began several miles to
the south at Ruth Mountain.
As more thick white ash was emplaced and vegetation was wiped out in a widespread area, a similar
process occurred.
The higher southern caldera block soon collapsed downwards, forming the caldera's modern appearance.
In total, both megaeruptions produced a combined 120 cubic kilometers of tephra.
This caldera was later partially filled in by basaltic andesite to rhyolite composition
lava domes, which continued to intermittently erupt for the next 800,000 years.
Since then, the top several hundred feet of the volcano has eroded away, creating its
modern appearance.
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