On May 18, 1980, Mount St Helens erupted in the US state of Washington. The event, which measured 5 on the Volcanic Explosively Index, has been the most disastrous in modern US history. A huge column of volcanic material rose in the atmosphere and deposited in 11 states, killing at least 57 people and causing more than £ 770 million in damage.
The volcano, which is an active stratovolcano, had no posed a threat since its last period of activity in the Forties and Fifties.
However, it was revealed during Amazon Prime's "The Fire Below Us" documentary how a series of small earthquakes indicated that magma had moved below once again
"It shook itself awake with a 4.1 magnitude earthquake.
"The frequency peaked on March 25 with 22 significant earthquakes in one eight-hour period.
"In response to the increasing threat, the forest service began evacuating personnel and established the first closure zones and warned residents of the earthquake-induced avalanches."
The documentary It was added to reveal how the earthquakes became increasingly frequent and opened a second crater.
"A few days later, a second, larger crater formed next to the first.
" It was not until two months later, on May 18, that any significant activity occurred.
At 8:32 am, a magnitude 5.1 earthquake centred directly below the north slope trigger ed that part of the volcano to slide approximately seven seconds after the shock.
The landslide, the largest in recorded history, traveled at 110 to 155 mph and moved across Spirit Lake's west arm.
The landslide exposed the dacite magma in St. Helens' neck to much lower pressure, causing the gas-charged, partly molten rock and high-pressure steam above it to get a few seconds after the landslide started.
The resulting blast laterally directed at the pyroclastic flow of very hot volcanic gases, ash and pumice formed from new lava, while the pulverized old rock hugged the ground – initially moving at 220mph but quickly accelerating to 670mph. passed over the moving avalanche and spread outwards, devastating a total of about 240 square miles of forest.
Helens' northern foot was visible throughout surrounding areas.
The near-supersonic lateral blast, loaded with volcanic debris, caused devastation as far as 19 miles from the volcano.