Mount Saint Helens volcano exploded 40 years ago

On May 18, 1980, the eruption of Mount Saint Helens in the United States was one of the most devastating eruptions in modern human history. A colossal power eruption that has streaked 600 km2 from the menu. An event that allowed researchers to acquire a lot of useful knowledge to predict this kind of eruptions.

Relive the cataclysmic eruption of Mount Saint Helens On May 18, 1980, the United States experienced the most destructive volcanic eruption in its history. The explosive cataclysmic eruption of young Mount Saint Helen has erased no less than 600 km2 from the map. Not without having given some warning signs. And to have offered researchers an unprecedented opportunity to study the phenomenon.

It was May 18, 1980, at 8:32 a.m. local time. A earthquake of magnitude 5.1 out ofRichter scale shakes the north face of the mount saint helens, a stratovolcano active located in Washington State in the United States, somewhere between Seattle and Portland. The flank of volcano give in. Nearly 3 km3 of rock stand out in an impressive landslide which progresses at 250 km / h. It covers everything in its path with a layer of rubble going in places up to 150 meters thick.

At the same time, the magma previously imprisoned explodes. The amount ofenergy clear is colossal: the equivalent of 100 atomic bombs ! The blast is heard as far as California, about 300 km away. The breath is devastating. The ground is literally torn off. Millions oftreesand of Pisces , tens of thousands of birds and thousands of large animals are shredded. Over an area of ​​600 km2 !

Ten years ago, NASA published these satellite images that showed how nature was recovering from disaster. In false red color – then in true color, green – areas of vegetation. © Nasa Earth Observatory, Youtube

In less than 15 minutes, ashes and gas rise up to more than 20 km high. An incredible cloud which travels at more than 500 km / h. The heatextreme that emerges melts the snow. Mud then breaks over the surrounding valleys. Roads, bridges, houses are destroyed by what specialists call a lahar looking like tsunami.

During the following hours, the region is plunged into the darkness. And other explosions sound. In total, the energy released is evaluated at the equivalent of 2,500 atomic bombs! Ashes fall over 55,000 km2– or approximately the area of ​​the Grand Est region. In the following two weeks, they will tour the Earth .

The event will have lost 400 meters high at Mount Saint Helens and will have replaced its summit with a crater of 2 km by 3.5. And the toll is heavy: two billion dollars in damage and above all, 57 dead. Among them a photojournalist, Reid Blackburn, who will have left behind, unpublished photos of the cataclysm.

Phenomena predicting the eruption

The story of this eruption had started as early as March. While an earthquake shook Mount Saint Helens. A earthquake magnitude 4.2. Followed by several other, shallow ones. The situation had put the geologists and volcanologists from the U.S. Geological Survey (USGS) on alert. Because this type of event often betrays the push of a magma trying to make its way to the surface. Through a vertical eruption of limited magnitude or a lateral and more devastating eruption? No one was able to say it then.

This was followed by a series of gas and magmato-phreatic eruptions. The result of an encounter between magma and water or snow-covered soils. A state of emergency had then been declared and experts had flocked. The earthquakes had continued. But the broadcasts gas remained constant. Not enough, according to current knowledge, to indicate the imminence of a rash.

However, several weeks later, Mount Saint Helens had deformed. A bump on the north flank. A dome that rose two meters a day. And earthquakes still present, and even more intense. The authorities then decided to evacuate the area. Before the volcano seems to calm down. It was May 14. Only four days beforevolcanic eruption cataclysmic we know him!

A rash useful to researchers

If some imagine that with the current tools and knowledge, volcanologists could have predicted this eruption, the truth is that it is it that allowed this science to greatly progress. For never had an eruption been studied so closely. An accessible volcano. Numerous images. Debris close at hand. And renewed interest in explosive eruptionsof this type in various disciplines such as geology, the seismology, geophysics or evenhydrology.

The eruption of Mount St. Helens also led to a new era in the volcano monitoring. Earthquakes, deformations or gas emissions have revealed their secrets. Models can now help researchers predict outbreaks worldwide. And instruments, both on the side of volcanoes and in space, are on the lookout for the slightest evolution. Instruments that signal, for example, deformations at the centimeter scale, continuously and in real time.

30 years ago, Mount St Helens exploded

In 1980, one of the most violent eruptions of recent millennia had occurred in the western United States. Mount St Helens had been active for a few weeks when, on May 18, 1980 in the morning, the entire summit exploded, changing the appearance of the volcano and the region for years to come …

Cassian Pirard article published on 20/05/2010

Spring 1980. It has been a few weeks since the Mount St Helens , a volcano located 150 kilometers from Seattle, shows signs of deep magmatic activity when the mountain range of the Cascade Range, the result of the collision between the Pacific plate and the North American continent, had been asleep since 1842. The increasing increase of earthquakes shaking this region of Washington State pushes the USGS (American Geological Service) to put the mountain under permanent surveillance pending an eruption, ending a phase of 150 years of inactivity.

On March 27 and then on March 29, a series of phreato-magmatic eruptions (eruptions of steam heated by magma) and gaseous are visible at the top of the crater. Five days later, with increasingly alarming signs of an impending major eruption, the region's governor declared a state of emergency. The area surrounding the volcano will be completely closed to the population a few weeks later.

During the following weeks, despite the absence of eruptions, the shape of the volcano changed and a huge hump of wash solidified grows two meters per day on the north side of the building. A month later, this dome adventitiousrises more than 120 meters in height and fills a good part of an old crater formed 350 years earlier. In early May, minor volcanic activity resumes around the edge of the lava dome, attracting tourists to the region. As this activity ended on May 16, most of these non-scientific visitors left the region.

On May 18 at 7 a.m., geologist David A. Johnston transmits telemetry data on the state of the volcano. This seems to be in normal activity, the emissions of sulfur , the temperature and growth of the dome, which now reaches 150 meters high, does not show anything abnormal.

Rocks thrown at 250 km / h

At 8 hours 32 minutes and 17 seconds, an earthquake of magnitude 5.1 on the Richter scale is triggered under the north slope of the volcano, causing one of the largest landslides ever recorded by Humanity. The mass of rocks movement travels at 250 km / h over distances of more than 20 kilometers, covering entire valleys of more than 200 meters of rubble on average. The event will be photographed by tourists from the region who will be saved by the topography of the place.

The departure of the lava dome that covered the volcano during the landslide produced the same effect as the cork of a bottle of champagne. The magma contained inside the volcano, which gradually pushed the solidified lava outwards, is now free to be entirely expelled. A few seconds aftercollapse from the dome on the sides of the mountain, the entire upper part of the volcano is transformed into a pyroclastic cloud of several hundred degrees which descends the slopes of the volcano to speeds dizzying.

The effect of the volcano explosion is gigantic and the region will be completely devastated within 30 kilometers. David A. Johnston, as well as 57 other people in this area, will not survive the arrival of the pyroclastic cloud, displacing gas and ash at speeds close to that of sound. The forest will also be destroyed, which will lead to landscapes similar to those of Toungouskain Siberia. The 0.2 cubic kilometer explosion of lava will be heard hundreds of kilometers away, in California and British Columbia.

During the next ten hours, a series of minor pyroclastic clouds will accompany the formation of a column of ash which rises in the stratosphere . Over 1.3 cubic kilometers of volcanic dust will be released into theatmosphere and local fallout will affect the entire northwest of the United States. At 5.30 in the afternoon, the paroxysmal eruption of St Helens came to an end, the column starting to collapse on itself. However, it will take several days for the situation to stabilize in the region, affected by major lahars (mudslides), blackouts and the decommissioning of the main means of transport.

In the months and years that followed, Mount St Helens experienced new episodes of dacitic lava dome formations. Fortunately, the pressureaccumulated will never be comparable to that reached during this day of May 18, 1980. The volcano has since caused only minor or intermediate eruptions.