How Mount St. Helens rebuilt its lava dome after the 1980 eruption

The May 18 collapse created the space later filled by lava

A magnitude 5.1 earthquake on the morning of May 18 accompanied the collapse of the volcano’s north flank. The debris avalanche and lateral blast reshaped the summit and left a deep crater. The later episodes in the table begin one week after the climactic eruption. Source 2 Source 1

Five later episodes in 1980 produced explosive activity

The USGS identifies five explosive episodes between May and October 1980. Their columns rose eight to nine miles above sea level and pyroclastic flows descended the north flank. The June episode registered 54.5 million cubic yards, the largest value in the post-May 18 table. Domes formed in June and August were destroyed by later explosive episodes. Source 1

The October 1980 episode began a longer phase of growth

The October 16 eruption began a series of 17 episodes that built a new dome. The USGS measured the dome at 876 feet above the 1980 crater floor after that period. Most dome growth took place as viscous lava accumulated in short, thick flows on its surface. Source 1

The 1983 entry covers an extended episode

The 29.3 million cubic yard entry beginning February 7, 1983 represents activity over nearly a year. Magma moved primarily inside the dome during that episode, pushing its eastern side outward by at least 250 feet. Comparing volumes shows material produced over each episode, which can last hours, days or much longer. Source 1

Sources

• USGS fact sheet and magmatic episode table, 1980 to 2000
• USGS account of the climactic May 18 eruption