The Eruption of Mount St. Helens

On May 18th, 1980 the eruption of Mount St. Helens in southwest Washington disrupted the lives of thousands and changed more than 200 square miles of rich forest into a grey, lifeless landscape. Here are some facts about this historic eruption with the vocabulary we learned this past unit:

Height:MSH was 9,677 feet before the eruption and 8,363 feet after. 1,314 feet was removed by the May 18th eruption.

Debris Avalanche: The largest landslide in recorded history swept down the mountain at speeds of 70 to 150 miles per hour and buried the North Fork of the Toutle River under an average of 150 feet of debris. Some areas are covered by as much as 600 feet. In all, approximately 23 square miles of material was removed from the mountain.

Lateral Blast: The lateral blast swept out of the north side of MSH at 300 miles per hour creating a 230 square mile fan shaped area of devastation reaching a distance of 17 miles from the crater. With temperatures as high as 660 degrees F and the power of 24 megatons of thermal energy, it snapped 100 year old trees like toothpicks and stripped them of their bark.

Lahars: The snow on MSH that was not instantly flashed to steam by the heat, melted and formed large mudflows that destroyed 27 bridges, 200 homes, 185 miles of roadway, and 15 miles of railway.

Pyroclastic Flows: Pyroclastic flows rolled out of the crater for hours after the eruption. Covering 6 square miles, they sterilized the remaining soil with temperatures nearing 1,300 degrees F.

Ash: The massive ash cloud grew to 80,000 feet (18 kilometers) in 15 minutes and reached the east coast in 3 days. Although most of the ash fell within 300 miles of the mountain, finer ash circled the earth in 15 days and may continue to stay in the atmosphere for many years.

Deaths: 57 people were killed as a result of the eruption. Of these, 21 bodies were never recovered from the blast zone.

Lava Domes: The old lava dome rises 876 feet above the crater floor and is about 3,500 feet in diameter. there have been no dome building eruptions on this dome since October 1986. If the dome were to re-establish the growth pattern it had in the 1980's, it would take 200 years to rebuild MSH to its pre-1980 size. The new lava dome (as of 2/1/2005) rises 1363 feet above the 1980 crater floor and is approximately 1500 feel long and 500 feet wide. It has been nicknamed the "whaleback" because of it's distinctive shape.

Damage Estimates: $1.1 billion for timber, civil works and agricultural losses. This does not include money for personal property losses, the cost of ash clean-up, or the loss of tourism in the area immediately after the eruption.

Current Monitoring: MSH can't even twitch without scientist knowing about it. Seismic disturbances, gas emissions, temperature, elevation changes (deformation), water levels, sediment flow rates, and even magma movement are all carefully monitored.

Future Behavior: MSH is expected to continue erupting but no one knows for how long. Pyroclastic flows, lahars, ejection of ash and pumice, and even the possibility of lava flows may all lie somewhere in MSH's future.