CASE STUDY - Mt. St Helens

Location, Cause, Effects, Responses, Management

Where is Mt. St Helens located?

Where is Mt. St Helens located?

Northwest North America, USA, state of Washington

When did the eruption happen?

May 18th 1980

• Which plates were involved?

• What type of crusts?

• What type of plate boundary?

• Juan de Fuca plate (oceanic) and the North America plate (continental)

• Destructive plate boundary

Describe what caused the eruption

• The Juan de Fuca plate gets subducted underneath the North American plate

• The subducted Juan de Fuca plate melts in the Benioff zone, creating more magma

• The trigger stimulus was a magnitude 5 earthquake underneath the volcano

• This caused a bulge to form on the north flank of the volcano (cryptodome)

• An avalanche occured and the volcano went on to erupt in a lateral blast producing pyroclastic flows

How did the volcano change in the months leading up to the eruption?

• 20th March, first earthquake happened under the volcano

• Cryptodome formed (bubble of magma that couldn’t escape the volcano), bulge on the northern side of the volcano, grew 6ft a day

• 10,000 earthquakes in 2 months

What hazards were produced from the eruption?

• Avalanche / landslide

• Lahars (boiling mudflow)

• Pyroclastic flows (currents of hot gas and ash)

• Tephra

• Ash fallout

• Lava flows

List as many primary effects as you can

• 57 deaths - most outside the evacuation zone

• Volcano devastated 370 sq miles

• 400m was blown off the top of the volcano

• Approx. 7000 animals died

• Every living thing within a 27km radius were killed in the blast

• The water produced from melted ice and snow created lahars, which choked all rivers, killing all fish and aquatic life

• 12 million salmon died

• Telephone lines and electricity supplies were knocked out

• 250 homes, 47 bridges, 15 miles of railway and 185 miles of roads destroyed

List as many secondary effects as you can

• Tourists no longer visited the area, causing a reduction in the local economy

• The timber industry in the area was unable to continue with production because tens of millions of trees were flattened

• 15cm of ash fell causing traffic chaos and airline flights were cancelled

• Ash clogged up car engines and farm machinery. The cost of ash damaged to farmers crops and machinery totalled to £100 million

List as many immediate responses as you can

• 31st March, declared a state of emergency, set up a 20 miles “exclusion zone”

• There was an immediate search and rescue operation to rescue stranded or injured people

• Shelter was provided to those who lost their homes

• Medical supplies were given to those who were injured

• Some helicopter rescues took place taking casualties to hospital

• The aid operation rescued 198 people

List as many long term responses as you can

• Money was given to restock the salmon hatcheries

• Millions of trees were replanted, costing $300 million

• A million tonnes of ash was removed. This cost $1 million in the town of Yakima and took 10 weeks

• Compensation was given to farmers, costing $70 million

• New tourist facilities were built to attract people back, creating jobs and injecting money back into the economy

What methods are being used to manage Mt. St Helens now?

• Seismic station installation

• Tilt-leg spider on top of the north-face of the new dome

• Seismic monitoring station (SWFL)

• GPS station

• Gas sampling station

• Gas sampling

• Electronic tiltmeter

How is Mt. St Helens being managed now?

Seismic station installation

USGS, in conjunction with the University of Washington maintain seismic stations at Mt. St Helens.

An increase in seismicity (eathquakes) is often the first precursor to an approaching eruption

How is Mt. St Helens being managed now?

Tilt-leg spider on top of north-face of the dome

This station houses a GPS and an accelerometer which measures the vibration, or acceleration of motion of the flank of the volcano

How is Mt. St Helens being managed now?

Seismic monitoring station (SWFL)

Solar panels are used to recharge the batteries and keep the station running

How is Mt. St Helens being managed now?

GPS station

Tripod built in the 1980s serves as a modern GPS station at Studebaker Ridge, on the western flank of Mt. St Helens

View is to the north

How is Mt. St Helens being managed now?

Gas sampling station

Monitoring equipment installation on Mt. St Helens’ dome

Camera / gas sampling station is to the right, underneath the helicopter

How is Mt. St Helens being managed now?

Gas sampling

USGC vulcanologists gather gas samples bu hand from vents on the dome and crater floor

They measure SO₂ before, during and after eruptions to determine “emission rates”

During eruptions, emission rates typically increase by 5 to 10 times their pre-eruptive value

How is Mt. St Helens being managed now?

Electronic tiltmeter

An electronic tiltmeter uses a small container filled with a conducting fluid and a ‘bubble’ to measure a change in slope

Electrodes placed in the fluid and into the bubble determine the bubbles’ position - as the bubble moves, voltage output from the electrode changes in a way that correlates to the amount of tilt that caused the bubble to move