Every community has one. The ‘bad lad’ from the rough family on the wrong side of the tracks – never quite fits in with the crowd, always getting in trouble. Your folks warn you to keep away from people like that – too violent and unpredictable. As the years go by some grow out of it, but some get worse. A few mentions in the petty crime column of the local paper, and the next thing you know it says ‘Occupation: Gangster’ in their passport.
If there were ever such characters in the volcano world, then Shiveluch certainly fits the bill. Kamchatka is one of the world’s most geologically active regions, and it has more than its fair share of volcanic villains. However, for levels of violence and the extraordinary number of repeat offences, none can touch Shiveluch. And there’s no sign of the bad boy mellowing as it gets older.
Shiveluch (sometimes spelled Sheveluch) is often categorised as being part of the Kliuchevskaya group of volcanoes, and while there may be some good reasons for that, for this writer it just does not fit. Comprising (mostly) andesitic results of Pacific/Okhotsk plate subduction, the Kliuchevskaya volcanoes form a neat, self-contained group that is dominated by the terrifying beauty of Kliuchevskoy and the stark, silent form of Kamen. Nestling alongside is Bezymianny (Bezymianny – Not so anonymous http://volcanocafe.wordpress.com/2012/03/25/bezymianny-not-so-anonymous/), which seems hell-bent on catching up with its giant neighbours. Krestovsky, Uchovsky and the Tolbachik boys (Plosky and Ostry) are also part of the gang.
Meanwhile, Shiveluch stands aloof from this group, separated from Kliuchevskoy by around 80 km. Much of that distance is the forested wetland formed by the Kamchatka River as it drains the peninsula’s central valley, which lies between an older arc of volcanic activity and the currently active arc.
It’s ugly, too. Without doubt I am anthropomorphising volcanoes too much here (for which apologies, but it’s far too late to stop now), but Shiveluch is no ‘looker’ in the way Kliuchevskoy or Kronotsky are. Millenia of explosive eruptions, structural collapses and the belching of material across the surrounding land have left a jagged and complex topography of shattered mountain – a hideous, violent child that only a mother could love.
“His old man was no angel, either…”
What we know today as Shiveluch emerged in the late Pleistocene era some 65,000 years ago, making it much older than the currently active volcanoes of the Kliuchevskaya group. It was a fairly typical stratovolcano, building up layer upon layer into a gigantic edifice of more than 4,000 metres height.
Examination of the rocks from this time reveals a mix of typical andesite, which in some instances produced short viscous lava flows of no greater than 5 km length, and some basaltic andesite with lower silica levels that produced free-flowing lava runs that possibly emanated from sills within the structure and reached distances of up to 15 km from the summit. The most recent of these flows shows no evidence of glacial erosion, suggesting that it was produced at the beginning of the Holocene period, around 10,000 years ago. Still flanking the volcano to the southwest today is the Baidarny ridge, a lava flow dating to around this period.
The boss is dead …
Although it is difficult to ascertain much of Shiveluch’s history before that time, it is known that the giant mountain, as it then was, met a most spectacular fate. A catastrophic event blew away much of the southern side of the volcano, taking the summit with it. As a result a massive horseshoe-shaped caldera was formed of some 7 km width.
The collapsed material travelled up to 35 km and spewed out over an area of 350 km⌃2. Without knowing the shape of Shiveluch prior to the cataclysm it is impossible to define the true extent of the collapse. If the mountain had been a typical stratovolcanic cone then the collapse could have involved a mind-blowing 40 km⌃3 of material, although estimates based on the debris field suggest somewhere more in the region of 25-30 km⌃3. Either way it was an ‘event’ of gargantuan proportions.
In a paper published in 1974 the date of the collapse was established at 30,000 BP, but without any organic material available to conduct Carbon 14 testing this date was derived from indirect geological evidence. Studies in the 1990s of deposits from the huge debris avalanche revealed no evidence of glacial erosion, so it is now assumed that the huge event occurred around 10,000 years ago – just after the last period of glaciation in Kamchatka. Radiocarbon dating of the earliest available organic material that lies in layers above the avalanche debris shows that it is around 8,000-8,500 years old, lending credence to the 10,000 BP theory.
Either way, the boss was dead. Or maybe he just retired after a final, devastating effort – a few small andesitic cones (the Karan domes) on the uncollapsed western flank have been active since the big collapse, and occasionally show weak fumarolic behaviour to remind the world of this beast’s history.
Birth of a wrong ’un
A silent, jagged peak also remains, standing 3283 metres high. Today it is known as Stary (Old) Shiveluch. What is left of the shattered summit watches like an approving parent over a new monster, Molodoy (Young) Shiveluch. This evil child has certainly taken up the baton handed down by Dad, and in the process has become even more volatile. Nevertheless, it took Young Shiveluch around 4,000 years to produce anything of major note, but considering the sheer scale of Dad’s collapse that is perhaps not surprising.
From the floor of the caldera left by the 10,000 BP event arose a series of Peléan lava domes. Whereas Old Shiveluch’s magmas usually had silica contents of between 54.5 and 56.5 percent, those of the Young Shiveluch have, with two exceptions during periods of particularly high Kamchatkan volcanic activity, silica contents of between 59.5 and 62.5 percent. They are therefore more viscous, allowing more rapid dome growth, but in the process they build a weaker edifice. The ascent of highly viscous magma is also more violent than that of free-flowing basalt, in turn producing stronger earthquakes that rock the edifice to a much greater extent.
Between 10,000 BP and 1964 AD there have been numerous eruptions as the Young Shiveluch has grown, collapsed and grown again. Many of them were explosive events that not only built the edifice through extrusion, but resulted in significant tephra ejection.
However, through tephrochronology it has also been established that Shiveluch has experienced at least six major collapse events as well, prior to the most recent in 1964. These events occurred 5,700, 3,700, 2,600, 1,600, 1,000 and 600 years ago. Each of them involved the deposits of avalanche material across a wide area. The deposits, along with other volcanic layers, are graphically exposed in several places, most notably in the deep cuts formed in the Baidarnaya, Kabeku, Kamenskaya and Sukhoi Il’chinets river valleys that run off from Shiveluch’s lower slopes. Until work by Alexander Belousov and colleagues in the mid-1990s showed otherwise, these avalanche deposits were originally thought to have been from lahars, or glacial moraines.
In a number of places burnt larch trees have been found intact in the pyroclastic flow deposits. Although carbonised, the annual growth rings – and the all-important differences in growth rate – are still discernible and allow a more precise dating of more recent eruptions when plotted against the known ring patterns of other trees.
In an area where few people have ever lived there is not the wealth of records to back up the science. Written notes go back only to 1739, and it was not until 1854 that a major eruption at Shiveluch was reliably documented, at least in terms of when it happened. Some volcanologists suggest that this event (along with others) also involved a major dome collapse, although other researchers question these findings, suggesting instead that there was only a minor flank collapse, lahar or other landslide feature rather than a major debris avalanche.
In any case, the 1854 eruption was a big one, classified as a VEI5 and ejecting an estimated 2 km⌃3. It left a crater of around 1.5 km width in the larger caldera, although this crater had almost certainly formed during the previous major collapse event that dated to around 1430 AD (also referred to as the 600 BP event).
In terms of the amount of material ejected, the 600 BP collapse was the largest event since Old Shiveluch had catastrophically failed, and it may have figured in local folklore that was recorded by explorers in the mid-18th Century. The old story goes that it was ground squirrels burrowing in the foot of the volcano that caused it to run away to its present location, leaving behind two giant footprints in the form of lakes. These lakes lie at the outer reaches of the 600 BP avalanche field, and were formed by the event.
Following the 1854 eruption Shiveluch set about rebuilding again. Over the next 100 years a number of lava domes formed in the new crater. A large dome was formed between 1925 and 1930, and from 1946 to 1949 another dome, named Suelich, grew alongside. Together they almost filled the crater formed by the 1854 eruption. From around 1950 activity slowed down, and became fumarolic in nature only.
Maybe steam was all the ‘Bad Boy’ had left ……