wunder beta🌋 Mount St Helens 1980: hour by hour
At 8:32 a.m. on 18 May 1980 an earthquake struck, and twenty seconds later the north flank of the mountain began to slide. Follow the day on its own clocks — a record landslide in seconds, a sideways blast at 300 mph, a column in the strato
What you’ll learn
- Before the slideExplain what was happening inside Mount St. Helens before 18 May 1980, and why the morning itself gave so little warning.A cryptodome of magma pushed the north flank out about 140 metres. The instruments showed no unusual change that Sunday morning — and then a magnitude 5.1 earthquake struck at 8:32 a.m.
- The first minutesTrace how one earthquake became a record landslide and a sideways blast within minutes.The flank detached as the largest debris avalanche ever recorded, uncorking the pressurised interior. The blast crossed 600 square kilometres at up to 480 km/h.
- Hours of ash and mudFollow the eruption outward and downstream, through the ash column, the lahars and the ashfall.The column passed 24 km in under fifteen minutes and ran for nine hours, while the largest lahar drained from the landslide deposit and peaked at midnight, 80 km downstream.
- What the day left behindRelate the reshaped summit to the human toll, and to what survived the blast and why.The mountain lost 400 metres and gained a horseshoe crater, and is still losing height. Fifty-seven people died. Recovery ran on the organisms that survived in place — above all the ones the May snowpack protected.
Questions this course answers
What actually set the collapse of the north flank in motion?
At 8:32 a.m. PDT on 18 May 1980 a magnitude 5.1 earthquake struck roughly a mile below the volcano. About twenty seconds later the bulged north flank began to slide.
What was the bulge on the north flank actually made of?
Beneath the bulge sat a cryptodome — magma intruded into the mountain that never erupted. It pushed the flank outward about 2 metres a day, roughly 140 metres (450 feet) in all by 17 May.
Why was the first blast so destructive to the north rather than overhead?
Losing the flank let superheated water flash to steam and blow out through the scar. The blast reached at least 480 km/h (300 mph) and devastated about 600 square kilometres — 230 square miles.
How does the debris avalanche of 18 May 1980 rank?
The USGS describes it as the largest debris avalanche on Earth in recorded history — about 2.5 cubic kilometres of mountain, which ran 23 km down the North Fork Toutle River valley.
How high did the eruption column climb, and how quickly?
The column passed 24 km — 80,000 feet — in under fifteen minutes. The Plinian phase then continued for nine hours, peaking between 3 and 5 p.m.
Where did the largest lahar get its water?
The biggest lahar formed as water escaped from inside the debris-avalanche deposit through most of the day. It grew as it travelled and peaked at about midnight in the Cowlitz River, 80 km (50 miles) downstream.
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