1980 Eruption of Mount St. Helens - Later Eruptions

Later Eruptions

Further information: 2004–2008 volcanic activity of Mount St. Helens

St. Helens produced an additional five explosive eruptions between May and October 1980. Through early 1990, a total of at least 21 periods of eruptive activity had occurred. The volcano remains active, with smaller, dome-building eruptions continuing into 2008.

An eruption occurred on May 25, 1980 at 2:30 a.m. that sent an ash column 9 miles (14 km) into the atmosphere. The eruption was preceded by a sudden increase in earthquake activity and occurred during a rain storm. Erratic wind from the storm carried ash from the eruption to the south and west, lightly dusting large parts of western Washington and Oregon. Pyroclastic flows exited the northern breach and covered avalanche debris, lahars and other pyroclastic flows deposited by the May 18 eruption.

At 7:05 p.m. on June 12, a plume of ash billowed 2.5 miles (4.0 km) above the volcano. At 9:09 p.m. a much stronger explosion sent an ash column about 10 miles (16 km) skyward. This event caused the Portland area, previously spared by wind direction, to be thinly coated with ash in the middle of the annual Rose Festival. A dacite dome then oozed into existence on the crater floor, growing to a height of 200 feet (61 m) and a width of 1,200 feet (370 m) within a week.

A series of large explosions on July 22 broke more than a month of relative quiet. The July eruptive episode was preceded by several days of measurable expansion of the summit area, heightened earthquake activity, and changed emission rates of sulfur dioxide and carbon dioxide. The first hit at 5:14 p.m. as an ash column shot 10 miles (16 km) and was followed by a faster blast at 6:25 p.m. that pushed the ash column above its previous maximum height in just 7.5 minutes. The final explosion started at 7:01 p.m. and continued for over two hours. When the relatively small amount of ash settled over eastern Washington, the dome built in June was gone.

Seismic activity and gas emission steadily increased in early August, and on August 7 at 4:26 p.m., an ash cloud slowly expanded 8 miles (13 km) into the sky. Small pyroclastic flows were sent through the northern breach and weaker outpouring of ash rose from the crater. This continued until 10:32 p.m. when a second large blast sent ash high into the air. A second dacite dome filled this vent a few days later.

Two months of repose were ended by an eruption lasting from October 16 to 18. This event obliterated the second dome, sent ash 10 miles (16 km) in the air and created small, red-hot pyroclastic flows. A third dome began to form within 30 minutes after the final explosion on October 18, and within a few days, it was about 900 feet (270 m) wide and 130 feet (40 m) high. In spite of the dome growth next to it, a new glacier formed rapidly inside the crater.

All of the post-1980 eruptions were quiet dome-building events, beginning with the December 27, 1980, to January 3, 1981, episode. By 1987 the third dome had grown to be more than 3,000 feet (910 m) wide and 800 feet (240 m) high.

Further eruptions occurred over a few months during 1989–1991, and the mountain became active again in late 2004 building a new dome. This activity lasted until January 2008.