Mount St. Helens Eruptive Activity, 1980-1984
The Two-Month Precursory Period
The Mount St. Helens volcano reawakened in March 1980 after more
than a century of quiet. A magnitude
4.0 earthquake on March 20
was followed by two months of intense earthquake activity, and phreatic
"steam-blast" eruptions which began on March 27. Ejecta from these
phreatic eruptions were composed of fragments of pre-existing rocks; no
magma was tapped during these
eruptions. These events were caused by
the intrusion of viscous magma into the volcano,
shoving the north flank
outward more than 300 feet and creating the famous `bulge.' Repeated
surveys during April and May showed that the bulge was growing northward
at an average rate of about five feet per day.
Continue to The Eruption
Return to:
Eruptive Activity
Living Lab Curriculum
Mount St. Helens