Mt St Helens Volcano - John Seach


Washington, USA

46.20 N, 122.18 W,
summit elevation 2549 m

Mount St Helens Volcano erupted on 1980 and was one of the best studied eruptions of the 20th century. The Mt St Helens eruption was the first on mainland US (excluding Alaska) since 1916. The eruption of Mount St Helens volcano on 18th May 1980 produced a 19 km (12 mile) high eruption cloud. Tremor, and two small earthquakes occurred at the volcano on 13th January 2008.

2011 Earthquake swarm
A swarm of earthquakes hit Mt St Helens volcano on 14th February 2011. The earthquakes were centered in an area near the Johnston Ridge Observatory, about 7 km north of the crater. The earthquakes had a focus at a depth of about 2-4 km. At least 12 small earthquakes were measured in the area since Monday. The initial earthquake was the largest with a magnitude of 4.3 and hit around 10:35 a.m. local time. The earthquakes are possibly caused by fault movement under the volcano as a result of magma removal during the 1980 eruption. Earthquake swarms occurred in the same area in January 2011 and August 1980. The USGS stated "at present there appears to be no signs of unrest in the volcanic system". The current volcano alert level for Mount St. Helen's remains at normal.

Lava dome growth paused in late January 2008.

1980 Eruption - Sequence of events

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

25th March 1980
Mountain closed to climbers.

27th March 1980
First eruption in 123 years. Ash emitted to 3 km into the air and a small crater formed on the summit.

30th March 1980
Sightseers flocked to area.

3rd April 1980
State of emergency declared.

17th April 1980
Risk of landslide recognised on the volcano north flank.

22nd April - 7th May 1980
Volcano stopped erupting but bulging continued. By 27th April the bulge measured 2.5 km across and protruded 80 m.

7th May 1980
Large earthquakes under volcano. Ash and steam emissions resumed after two weeks of quiet.

12th May 1980
A larger than normal earthquake caused a one kilometer long avalanche down the north slope.

14th May 1980
Only small eruptions.

15th May 1980
No eruptions.

17th May 1980
Amazingly on the day before the eruption 30 car loads of residents were allowed to enter the restricted zone to gather possessions.

18th May 1980
The large eruption at Mt St Helens 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.

Mt St Helens Volcano Eruptions

2004-08, 1990-91, 1989-90, 1980-86, 1921?, 1903?, 1898?, 1857, 1854, 1853, 1850, 1849?, 1848, 1847, 1842-45, 1835, 1831