It must be the only volcano named after a British ambassador to Spain. Mount St Helens was also known as the Mount Fuji of America: a perfect cone standing above the country side. The 1980 eruption destroyed much and had a significant human cost. It also damaged the mountain badly: the perfect cone gone, and replaced by a diminished, broken, lopsided structure. Photogenicity lost.
The eruption of Mount St Helens was the best predicted of the 20th century. It was no Pinatubo, which had stayed below the radar until a few months before the eruption: this one had been predicted as long as five years before the event. A publication by Dwight Crandell et al. in 1975 stated that Mount St. Helens was the one volcano in the United States most likely to reawaken: an eruption is likely within the next hundred years, possibly before the end of this century; It .. would affect a broad area beyond the volcano. In a follow-up publication, they predicted the damage such an eruption could do. It was an accurate forecast.
There are a number of major volcanoes in the US Cascades region; the domes clearly stand out above the rest. The region has been fairly quiet since the time of Lewis and Clarke. But that is deceptive. Just like the last major earthquake here happened just before our records begin, so also its volcanoes. The wake-up call was LOUD, and it was destructive.
The main event started with an earthquake, on March 20, 1980. This seems not uncommon in major eruptions: an earthquake gets things going. It happened in Krakatoa, three years before the eruption, and in Vesuvius, 17 years before. In St Helens activity ramped up rather more quickly than that. Five days after the first earthquake the mountain first erupted with a (minor) summit explosion. For a month and a half, the summit explosions were phreatic with ash and steam but no magma. A bulge began to grow on the north side, where the surface was moving outward at a staggering speed of 1.5 meter per day; the summit behind it began to sink. On May 18, 1980, the northern flank could no longer contain the pressure. The final collapse was triggered by another earthquake but would have happened without it as well.
The M5 earthquake hit a bit after 8 am on this Sunday in May. Within seconds the north side of the mountain, weakened by the bulging, cracked into several blocks and began to slide down, first into the crater but seconds later also down the outer slope. The rock slide reached speeds well over 100 miles per hour. The devastating avalanche picked up water from Spirit Lake on the way which provided further lubrication. The avalanche deposited a volume of 2.5 km3, covering 60 km2. The Trout river channeled much of the flow and was hit with 50-m deep deposits.
The removal of the side of the mountain opened the way for the main explosion, both through the reduced pressure and because one of the cracks split open the magma chamber which had caused the bulging. This brought up the first magma of the eruption. The blast came up through what was the central crater, but was funneled northward, following the avalanche. Moving at speeds up to 900 km/h, the blast overtook the avalanche, and mowed down the forest like blades of grass. Nearby, nothing was left standing. Further away, trees in the shelter of hills survived. Even further away, trees still stood but all the green was burned off by the heat of the blast wave. The total affected area covered 600 km2.
Although the eruption would continue with further explosions for some 9 hours, the main event was now over, barely 30 seconds since the earthquake that triggered it. The after effects took longer. The avalanche ran for over 10 minutes. The blast wave was faster, taking only a few minutes before weakening to a survivable level. The mushroom cloud was still rising, reaching almost 20 km in 10 minutes. Winds moved it to the east, and ash began to fall, thickly up to 300 km away in some places, thinly much more distant, with the furthest fall close to Santa Fe in New Mexico. The ash amounted to 0.2 km3 DRE. The gaseous, frothy magma coming out of the crater caused a number of pumice-loaded pyroclastic flows throughout the day, but these stayed close to the crater. Steam explosion caused by the heat of these flows continued for weeks, and one even happened a year later.
The pyroclastics melted snow and ice on the mountain, and lahars or mudflows began to flow, on all sides of the mountain. These were damaging, amongst others because of the sediment they dumped in various rivers and reservoirs. Even shipping in the Colombia river was badly affected. The mudflows reached 20 meters in depth along the Trout river. Mudflows can be life threatening and, as people often live near water and rivers, can affect large number of people. Luckily, St Helens erupted into a sparsely populated area.
The sound of the explosion was heard near and far – but not in the middle. 300 km away, the boom broke windows. The sound was heard as far as Canada. But people nearby (where perhaps they shouldn’t have been) heard nothing. One reported comment from a person near (or in) the exclusion zone: Something didn’t feel right, though I couldn’t quite put my finger on what it was. Then it dawned on me. It was absolutely quiet. There was no twittering of birds or scurrying of chipmunks or other soft sounds that are usually in the background. I heard Pam gasp, then cry out, ‘My God, the mountain has blown!’ The mountain had exploded close to them and they heard nothing!
This zone of silence is not unusual. Krakatoa was also deafening nearby, but not further away, although it was heard again thousands of kilometers away (reported even from the Caribbean, on the opposite side of the Earth). Thinking about it, lighting nearby is deafening but distant lightning is seen but not heard. How is this possible?
Sound waves do not quite travel on straight lines in the atmosphere. The reason is that the sound speed changes with temperature: sounds travels slower when it is colder. Higher up in the atmosphere, it is colder and sound slows slows. Imagine a sound wave which travels from the top of Mount St Helens to the ground. The wave travels faster at ground level than higher up. This make the wave bend upward. It reaches the ground further than you would expect from a straight line of sight. There is a distance beyond which the sound no longer reaches the ground, but bends away up in the atmosphere. How far this is depends on how high above the ground the explosion happened. (If there is an inversion layer in the atmosphere, sound waves can get trapped underneath and travel long distances on the ground.) Next time you fly in a hot air balloon, try to distinguish which sounds you can hear: it depends on the temperature gradient in the air.
So some distance from St Helens, the sound wave bend away from the ground. Further away, the explosion could no longer be heard – at least on the ground. But why did the booms reach the ground again much further away? This is because there is a second inversion layer in the atmosphere, at the top of the stratosphere. Here the temperatures increase rapidly with height. When the sound wave reaches here, 25 km up, it now bends the other way, sharply downward – effectively, it reflects off this layer. Now traveling down again, the sound, rather weaker and without the high frequencies, can reach the ground again – 200 km from the source.
The eruption killed life throughout the blast wave region. The avalanche and mudflows were unsurvivable for any life. The blast wave killed animals above ground, including birds. But animals below ground or under water survived: this included amphibians, not normally the most robust of animals. All large animals died. There may be a lesson here for the impact which killed the dinosaurs, when nothing larger than a small dog survived (bad news for the dinosaurs, not known for being small). Why did mammals and crocodiles survive this extinction event? At this time, many mammals occupied an ecological niche for small ground feeders, a niche often used by nocturnal animals. Many will have spend much of their time underground. Crocodiles live mostly under water. This suggests much of the extinction came from the blast wave. (But to muddy the waters, see global warming and dinosaurs)
(Why did birds survive? They are very fragile and not known for living underground. The most likely explanation is that there were places which were not as badly affected, perhaps on the other side of the Earth, but isolated by water. Birds can fly – and thus get off an island. Larger animals could not. Which island? Hard to tell – islands do not survive well. But it may be worthwhile looking for post-extinction fossils underneath the ice of Antarctica.)
Repairing the damage
Intermittent explosive activity at St Helens continued for several years, from the summer of 1980 to 1986. At the same time, dome building started inside the excavated crater. By 1986, the new dome had reached 400 meter above the crater floor. This is less impressive than it sounds: the dome lacked volume. At this rate, it would take 200 year to rebuild St Helens. The speed of rebuilding also raises questions on how old the mountain really was. What is the history of St Helens?
The early dome building was aided by magma from the 1980 eruption, now gas-less and squeezed out in effusive eruptions. As this magma source was depleted, by 1986 activity declined and apart from some explosive eruptions in late 1990 and early 1991, the mountain became largely silent. Activity resumed in 2004, with dome building and minor eruptions continuing for four years. This activity was different: the magma had solidified underground and was pushed up in solid form as ‘Spines’ and smooth ‘whalebacks’, eventually reaching 400 meter high. It pushed out 95 million cubic meter, doubling the volume of the new dome, but the overall rate of dome building had actually declined. Now it would take 400 year to rebuild the mountain. There were two explosions, in 2005, but otherwise this episode was effusive only. From early 2008, silence reigned and nothing much happened. Between March and May 2016, earthquake swarms indicated a slip on a small fault and these may indicate a bit of magma is on the move again. Earthquake activity has since reduced to background levels. However, it is likely that dome building will continue in the next years to decades, through minor eruptions.
Prehistory ended late in the Americas. Written records go back only a brief time. Still, they give a picture of a frequently erupting volcano.
There was a major explosive eruption around 1800. Eruptions were frequent but small between 1831 and 1857. The painting depicts one of these eruptions, notably off-centre. In 1857, the final eruption of this sequence formed a dome on the northern flank, later called Goats Rock lava dome. Between 1857 and 1980 St Helens was quiet. This time span exceeds a living memory, and the population had largely forgotten St Helens was an active volcano. The 1980 flank bulge occurred in the location of the Goats Rock, following the old but forgotten weakness. Eruptions relive their past.
Older eruptions are known from their deposits, and especially the mudflows can be dated through their organic content. These records are good for large eruptions such as 1980. but will have missed many small eruptions (such as the activity since 1980). The records show many eruptive layers. The oldest ones date from about 40,000 yr ago. This means St Helens is actually a very young volcano: other Cascades volcanoes are much older. Activity was not excessive, with perhaps even a 10,000 yr period of nothing. Over the past 4,000 yr, eruptions have been frequent. A number of (pre)-historical eruptions have caused mudflows reaching tens of kilometer, pyroclastics, and deposits reaching neighbouring states. There have been quite a few eruptions larger than that of 1980, most recently in 1480.
Century-long periods of dormancy, such as prior to 1980, appear to be the norm for St Helens. Between 1400 and 1800, there were several eruptive periods separated by hundred-year periods of nothingness. There were longer dormancies from 400-1300, 800BC-400BC, 1600BC-1200BC. The 4000-year record of frequent eruptions and brief dormancy periods was the reason Crandell in 1975 stated that an eruption was likely soon.
With so many eruptions, why was the mountain so symmetric and photogenic? The dome was destroyed multiple times over the past 4000 year, and probably earlier as well: there is evidence for blocks similar to those in the 1980 flank collapse in the old deposits. The steep sides indicate a tendency for explosive eruptions: the combination of collapse and explosion would have been as devastating in the past as it was in 1980. In fact, the beautiful dome before 1980 was a recent formation. The oldest summit rocks were less than 1000 year old, and most of the dome probably formed between 1600 and 1700. The eruption of 1480-1482 may have destroyed the previous dome. But there is every reason to believe that previous domes would have looked similar to the one we lost in 1980. Here is a volcano that keeps rebuilding itself, in a volcanic minecraft. By 2400, it should look as good as new again.
The smoothness of the dome in fact was because of this very youth. Erosion hadn’t had time to act its ravages. As in humanity, volcanic beauty is transitory: try to hold on and find it is slowly lost, but give it up and re-invent yourself can make it last. Volcanic beauty comes from the inside.
Where does the magma come from? The main source is the subduction of the Juan de Fuca plate. The heat, together with the water, creates melt and the melt percolates upward. It collects in magma chambers: each of the Cascade volcanoes is supposed to have such a magma chamber underneath. Recent studies indicate this picture is too simple. Extensive seismic mapping found the expected magma chamber underneath St Helens, 2-10 km deep, but a much larger, deeper chamber is offset and seems to feed both St Helens and the Indian Heaven volcanic field. The 1980 eruption was preceded by a chain of earthquakes on a line linking the deep magma chamber to St Helens; it seems an injection from the deep into the shallow chamber eventually caused the eruption.
4000 years ago, St Helens changed mode from infrequent to frequent eruptions. What happened? Perhaps the connection between the two magma chambers opened up. It turned St Helens into one of the most dangerous volcanoes in the US. Since this time, the Indian Heaven volcanic field has not erupted, although that is in itself not conclusive as apart from activity about 6000BC it has not been very active. Maybe one day the magma will redirect itself and this field will become the next Cascade volcanoes, leaving St Helens to magma-starve.
Let’s come back to the link between earthquakes and volcanic eruptions. The destabilizing effect of a good shake can trigger an eruption that was imminent anyway. The shaking and shifting of the load above changes the pressure, and eruptions are very sensitive to pressure. Darwin writes about how the major Concepcion earthquake which he experiened (M8+) triggered eruptions in four distant Chilean volcanoes in the days after. Earthquakes within the volcanic system open up underground pathways for the magma. They can themselves be caused by increased pressure from the magma, as in the numerous quakes that opened the Holuhraun dike system. This may also be what happened in the Vesuvius quake of 5 Feb 62 AD, which started the sequence leading up to the eruption of 79, although we don’t know that for certain. In subduction zones, the region of the severe earthquakes is offset from that of the volcanic activity, but there can be many fault lines closer to the volcano. That is the case in St Helens where there are regular tectonic earthquakes from local faults. Lacking those, volcanoes have to generate their own earthquakes and they often do.
The Cascadia region is subject to major earthquakes, in fact scary ones. The last one hit on Jan 26, 1700. Prior to that, big shakes are dated around 900 (that one is not well dated and may have been later), 700, 300, 500 BC, 900 BC, etc. Looking at the timeline for St Helens eruptions, these quakes tend to fall in, or coincide with the onset of, a period of dormancy. For instance, the 1700 quake was at the start of a 100 year sleep; no doubt the explosive awakening 100 year later, in 1800, was caused by the kiss of a prince (think before you kiss!). The evidence is far from conclusive, but a working hypothesis could be that the major Cascadia earthquakes suppress activity at St Helens – at least for a while.
Lessons from St Helens
The message from St Helens is that historical activity matters. A frequently erupting volcano is likely to erupt again. The 100-yr dormancy periods are right in the ‘awkward’ range: long enough to outlast living memory, and short enough that the chance of an eruption is rather high. From one side, the number of casualties was not high. From the other side, perhaps it should have been zero. This was not an unexpected eruption, warnings were given five years before and the mountain announced its intentions two months in advance. That is a discussion that can rumble on for a long time. The next large eruption in Cascadia probably won’t be from St Helens. Which ever it is, the challenge will be to provide warning with enough certainty to get people to move, and enough lead time to let them get out. But not so much they will try to go back. Good luck.
Science, 187, 438. http://science.sciencemag.org/content/sci/187/4175/438.full.pdf (paywalled)