Volcano Tales: Shiveluch

Jun 09, 2017 No Comments by 574 views

At any given day, there are about 20 volcanoes erupting around the globe. Some of them are notorious for frequently showering nearby cities with ash, others wake up and blow without any warning – but all of them have their own intriguing story. For this blog, I will regularly pick one of the active volcanoes reported by the Smithsonian/USGS Weekly Volcanic Acitvity Report, and take a closer look at their history and their peculiarities.

In this blogpost, we will travel about 2000 km to the West of Bogoslof volcano, which I featured in March. But before we do that, we should spend a minute with an update on Bogoslof. Since March, the dust ash seemed to have settled around our Aleautian island volcano, and little explosive activity had been reported  – until 28 May, when a major eruption produced an ash column that reached an altitude of more than 10 km, prompting authorities to raise the Aviation Color Code to red:

The fact that we didn’t hear much in the media about Bogoslof since March doesn’t mean there was nothing happening, of course: A new lava dome has been growing and impressively towers the steaming central lake, as captured by aerial photographs from 16 May:

So Bogoslof is still very much alive and keeps scientists at the Alaska Volcano Observatory busy, as you can read about in this interesting interview with one of its researchers:

But there is so much more going on in the world of volcanoes, so let’s move on now to a volcano that made it into the Smithsonian Weekly Volcanic Activity Report almost every week since 1999:

Shiveluch, Central Kamchatka

Located in the volcanic wonderland that is Kamchatka in Eastern Russia, Shiveluch is remarkable even from a non-volcanologist perspective. Due to its isolated location in the Central Kamchatka Depression, Shiveluch is a stand-alone mountain that can be easily spotted even from space:

With an altitude of 3307 m, it towers more than 3 km over the surrounding plains, which makes it a so-called ultra-prominent peak – in fact, it is ranked number 73 in the list of most prominent peaks on Earth.

But also in terms of its volcanic vigour, Shiveluch is rather exciting: Not only has it been constantly active since 1999, but there are more than 60 Holocene tephra fall deposits described, and the Global Volcanism program lists 105 eruptive periods in the Holocene, with Volcanic Explosivity Indices up to VEI = 5 (think Mount St. Helens). The modern cone associated to this activity, ‘Young Shiveluch’, has nested in a 9 km wide breached caldera that formed in a late Pleistocene / early Holocene eruption of ‘Old Shiveluch’, a massive stratovolcano with a diameter of about 50 km, remnants of which form the present-day summit of the massif.

On this picture, you can also see the impressive marks left by the most recent major explosive event in November 1964, when a dome collapse created a 1.5 x 3 km wide breached crater. The edifice collapse was followed by a short phreatic phase and a Plinian eruption that produced ~0.8 km³ tephra, about 3 times as much as Eyjafjallajökull in 2010. The related debris avalanche and juvenile pyroclastic deposits cover more than 100 km² to the South of the volcano, shaping its modern-day appearance. And all of this happened in just one hour – that’s what I call a proper paroxysm!

But tephrochronologists at the Institute of Volcanology and Seismology in Petropavlovsk-Kamchatsky think this was a rather small (and common!) event: They reconstructed at least seven events in the last 10,000 years that were bigger (with 1-3 km³ tephra volume produced), for example in 1000 AD and 1650 AD. They also find that Shiveluch has been much more active in the last 2000 years than previously, with a magma discharge rate 10 times greater than that of usual arc volcanoes!

And Shiveluch has certainly lived up to this assertion since the 1964 eruption. After a well-deserved break, a dome started to form in the 1964 crater in 1980, and since then it has stepped up his game further. In April 1993, after a rapid increase in seismicity, an explosive eruption reached column heights of 17 km, and since a short repose period 1995-1997, Shiveluch hasn’t come to a rest, with frequent pyroclastic flows, ashfall and lahars, reaching up to 30 km from the crater. A glance into the image archive of the Kamchatka Volcanic Eruption Response Team (KVERT) or the webcam will quickly show you – Shiveluch is always busy!

So Shiveluch is more than your average arc volcano, but its southbound neighbours are good company and read like a who-is-who of subduction zone volcanism: Tolbachik, Bezymianni, Klyuchevskoy, the latter being even more productive than Shiveluch. In short, this area plays in the Volcano Champions League, which is great for volcano enthusiasts, but concerning for the busy air traffic in the region. So how come there is so much volcanic activity here?

It may help to scroll up to the overview map again, and appreciate the rather complex tectonic arrangement we find ourselves in at Shiveluch: The Pacific Plate is subducting beneath the Okhotsk Plate towards the West, forming the Kuril-Kamchatka trench, and causing modern-day volcanism in Kamchatka. But the Pacific Plate is also subducting beneath the Bering Plate in the North. Two subduction zones converging at an angle to each other will inevitably create havoc, and take a guess where the junction of the two trenches is located – off the coast next to Shiveluch, which also coincides to be the northernmost major active volcano in Kamchatka. A popular theory explains both the absence of volcanism further North and the high productivity of Shiveluch and its explosive friends with a slab detachment North of Shiveluch, originating at the AKJ.

A popular model of the geodanymics of Kamchatka: subduction of the Pacific Plate beneath the Okhotsk Plate causes arc volcanism in South and Central Kamchatka; Northern Kamchatka is dominated by mantle upwelling after a slab break-off with no active subduction; Shiveluch is sitting on the subducting Pacific Plate edge, with input of both subudction fluids (and melts?) and hot fertile mantle from the North, accounting for the high magmatic productivity of the region. From Portnyagin et al. (2005)

In this scenario, there is no oceanic plate subducting beneath the Northern sector of Kamchatka since 10 Million years ago, and the small amount of melt being generated is due to decompression of hot mantle rather than subduction related fluid-fluxing. On the other hand, Shiveluch sits on top of the Pacific Plate edge, with subduction-typical fluid-triggered melting being enhanced by influx of hot fertile mantle from the North. To make the picture even more difficult, the Emperor Seamount Chain – a hot spot chain whose youngest tail is Hawaii – is being subducted roughly beneath the area where Shiveluch and its vigorous neighbours are located. It has been argued that the subducting seamounts could contribute even more fluids and thus further boost melt generation.

But what are we actually melting at Shiveluch? At first glance, we observe a typical subduction-related mineralogy and geochemistry of the erupted tephra and dome rocks dominated by fluid-fluxed mantle melting: Medium-K, calc-alkaline andesites whose hornblende and plagioclase phenocrysts record a history dominated by magma mixing events at multiple storage depths, spiced up with some fractional crystallisation, and a hearty mafic injection triggering the eruption in a matter of days to weeks.

So far, so straightforward. But upon a closer look, Shiveluch shows some peculiar features that have triggered the ‘Adakite-Alarm’ with some researchers: high Mg and low Y concentrations, as well as elevated Sr/Y and La/Yb ratios, have made scientists postulate that Shiveluch melts contain a component of slab melt. The adakite-supporters argue that the Pacific Plate is melted by the hot mantle flow around its edge after the slab break-off in the North, whereas the adakite-deniers suggest amphibole-fractionation as a mechanism to produce the observed signatures.

To sum up, Shiveluch is a true allrounder, offering some exciting challenges to scientists and enthusiasts of all trades: Mountaineers, physical volcanologists, pilots, geodynamicists and petrologists…Wait, is someone missing here? Right, the seismologists claim their share as well: They have set out recently to explore the possibility of Shiveluch sharing a giant, interconnected magma reservoir with its Southern neighbours.

And thus I will leave you here, with pictures of the active dome ‘quietly puffing’, and with confidence that Shiveluch will continue to grace the Smithsonian Weekly Volcanic Activity Report with its presence for a good while longer.

 

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About the author

I'm a PhD student of volcanology at the Natural History Museum London / Imperial College London. My research is focussed on pre-eruptive processes at Popocatépetl volcano, Mexico. I have a weakness for molten rocks and the smell of sulphur.
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