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Eruption of Mt St Helens 1980

20th March, 1980
Moderate earthquake under volcano signified rising magma.

25th March
Mountain closed to climbers.

27th March
First eruption in 123 years. Ash emitted to 3 km into the air and a small crater formed on the summit.

30th March
Sightseers flocked to area.

3rd April
State of emergency declared.

17th April
Risk of landslide recognised on the volcano north flank.

22nd April - 7th May
Volcano stopped erupting but bulging continued. By 27th April the bulge measured 2.5 km across and protruded 80 m.

7th May
Large earthquakes under volcano. Ash and steam emissions resumed after two weeks of quiet.

12th May
A larger than normal earthquake caused a one kilometer long avalanche down the north slope.

14th May.
Only small eruptions.

15th May
No eruptions.

17th May.
30 car loads of residents enter the restricted zone to gather possessions.

18th May 1980
The large eruption began at 8:32 am.
An earthquake shook loose the upper northern flank of the volcano. About 3 cubic km of of the mountain slid down in a massive avalanche at 250 km/hr.
The avalanche released pressure on the volcano and unleashed a huge explosion. A 300-500 km/hr blast of hot gases and fractured rock covered 600 sq km in minutes. 30 seconds after the initial blast the volcano released a Plinian eruption column of ash which rose to a height of 25 km in 15 minutes. The ash reaches Spokane 430 km away in 3½ hours. From noon until 5:30 pm nuee ardentes swept 8 km down the northern slopes of the volcano at 300 km/hr. Mudflows raged down the side of the volcan and were caused by melting of the snow on the mountain.

Effects of the eruption.
57 people were killed.
400 m lost from the height of the mountain.
Total damage bill one billion dollars.