Mt St Helens Volcanic Eruption 1980
Volcano Learning Zone > Volcanic eruption Casestudies > Mt St Helens
Where is the volcano?
Mt St Helens (8365 ft , 22,550m) is in the Cascade Volcanic Range in Washington State, USA. The Cascade Range has 160 volcanoes and Mt St Helens is considered the most active, and lies 100 miles south of Seattle and 50 miles from the coast at Portland.
Why is there a volcano here?
Mt St Helens is a stratovolcano within the Cascade Volcanic range in Washington State and had been dormant for 100 years prior to 1980. As an Eastern remant of the Pacific Plate, the Juan da fuca plate,has subducted beneath continental North America the Cascade volcanic chain has formed. The Mt.St.Helens volcano has been active for around 40,000 years but the now damaged cone we see today has formed over the last 2,200 years
Eruption Style
The volcano is highly explosive and erupts in the Plinian style like Vesuvius. Sudden violent explosions are accompanied by towering clouds of hot ash, dust and rocks thrown high into atmosphere. Pyroclastic flows and volcanic mud flows called Lahars flow down the mountain at speed and consume the surrounding landscape. lavas from basalt to andesites have been erupted forming layers of ash and lava.
In many stratovolcanoes a central summit crater has dome of dacitic lava which is squeezed out of the main vent , similar to Montserrrat. In 1980 this did not occur and instead a crypto dome formed (crypto means hidden) causing the Northern slope of the volcano to bulge and grow. From mapping the deposits around the volcano it seemes that lateral blasts and side domes were not unprecedented.
Sequence of Events
On May 18 1980 Mt.St.Helens produced an enormous lateral blast which obliterated everything for 20 miles north of the volcano. The summit decreased from 2930m to 2550m and a gaping crater 1.5km wide was created.
The sequence of events that occured had not been witnessed before as until then geologists had always seen volcanoes erupt upwards but this was different this was sideways!
In March 1980 earthquakes and a phreatic(steam) 2km eruption column alerted the USGS that the volcano was certainly waking up. Throughout the spring the mountain continued to splutter and steam the northern side of the mountain was bulging out by 1.5m/day. A 5 miles exclusion zone was placed around the volcano and famously Harry Truman owner, of the Spirit lake lodge resort, refused to leave.
On the morning of May 18 Geologist David Johnson was on a ridge 10km north of the volcano when a 5.1 earthquake struck. Suddenly the bulge on the north side of the volcano slid downwards in a massive landslide at 300km/hour and crashed into Spirit lake causing an enormous wave of water the rest of the landslide ploughed down the Tootle river valley and mixed with vast quantities of lake water and ice . Davids last message was to his colleagues in the nearby Vancouver USGS base ( not Canada) "Vancouver,Vancouver .. this is it!"
The landslide uncorked the magma in the crypto dome and released the pressure. The gas in the magma instantly expanded and shattered the rocks at supersonic speed creating a lateral of blast ash and hot rocks which overtook the landslide at reached speeds of 1,000km/hour (670 miles/hour) . The blasts power has been estimated at 500 times that of Hiroshima. 600km2 of forest was blown away and trees combed down like hair, sap exploded in trees in the intense heat. Trees up to 20 miles away were engulfed in the blast 4 times the distance of the exclusion zone.
The third string to Mt.St.Helens bow was a 19km (12miles) high eruption cloud which powered vertically out of the volcano in a matter of minutes. The cloud turned day to night and ash fall was heavy. Lightning crackled around the cloud.
Mudflows or Lahars formed within minutes of the eruption. lake water,melted glacier ice, hot ash and debris all combined to send hot ( 29-33C) cement like flows down the Toutle river. Pyroclastic flows started to form from the crater just after the blast as fresh magma came into contact with the air. Material appeared to overflow the craters edge and flow down the Toutle valley as a grey turbulent cloud.
Harry Truman and David Johnson along with 55 other people did not survive. Both have had ridges named after then within the Mt.St.Helens national monument. The area within the monument is being left to recover naturally. Within 8km of the volcano all trees were obliterated, from 8km to 30km flattened and outside this dead but left standing. Outside the boundary logging companies have replanted trees.
Mount St. Helens showed significant activity on March 8, 2005, when a 36,000-foot (11,000 m) plume of steam and ash emerged. A new dome is growing and will eventually fill the 1980 crater.
Effects and Aftermath
Rescue helicopters looking for survivors were grounded or could not land due to ash. Ash removal from roofs nad roads was costly and lengthy costing 2 million dollars just in one town(Yakima)
Hazard Management
All volcanoes in the Cascades are monitored by the Cascades Volcanic Observatory CVO part of the United States Geological Survey USGS from their base in Vancouver,not Canada but Washington State USA. On March 27th 1980 a phreatic eruption signalled the begnning of an active phase. The CVO moved in a team of Geologists to monitor the volcano.Small eruptions of steam and ash blasted from the old summit dome continued and attracted attention from the press and tourists. Evacuation plans were prepared and roads closed leading to the volcano. As the mountain swelled with magma, and bulged outwards at a rate of 1.5m/day,it was obvious that the Spirit Lake area to the north of the volcano needed to be evacuated. The Washington State Governor placed a 5 mile red zone around the summit only Harry Truman an 83 year old resort owner refused to move. When the volcano erupted on May 18th the lateral blast extended 20 miles 4 x the exclusion zone.
Try our other Mt St Helens entry , World volcano section or Volcanic Hazards page.