Eruptions

I am beginning to feel like a broken record, but the latest reports from Chile indicate that the ongoing eruption at Chaiten is ramping back up again, almost 3 months after the initial eruption began. As usual, the nitty-gritty details are limited, but reports of increased ash emissions and seismic activity are heralding this increase in activity.

For certain, Chaiten is one of the most important eruptions in any of our lifetimes. This is really not because of the amount of material that has been erupted (although when all is said and done, it will be a significant volume), but rather for the style and longevity of the eruption. A volcanologist friend of mine with more access to details of the Chaiten eruption has said that what we are really seeing is an eruption like might have produced Little Glass Mountain in the Medicine Lake Caldera. However, what is surprising is that the eruption to produce such a feature – a rhyolite dome – has had such longevity and produced so much ash along with the dome. Chaiten had not erupted in ~9,000 years, and more likely than not, a batch of eruptable magma was not sitting just below the surface for that time period – if that was the case, we would have likely seen signs of it such as degassing or seismicity. This means that more likely than not, there was some sort of intrusive event that triggered the eruption, such as the intrusion from depth of a hot basalt magma into a “crystal mush” of rhyolite that tipped the system and caused the eruption. Of course, this is complete conjecture on my part as we really have no samples of the magma itself beyond ash, which can tell us that it is, in fact, the first rhyolite erupted since 1912, but cannot tell us much about the timescales of storage and crystallization. What is really needed as samples of the dome lavas so the crystals (however sparse) can be analysed and dated and any inclusions of basalt(?) in the rhyolite can be sampled. Then we can really start having fun to pick apart the evolution of such a remarkable eruption as the 2008 eruption of Chaiten – remember, even here in the continental US, we have very similar volcanoes (Crater Lake for example) that could see similar eruptions to Chaiten in the future.

Comments

  1. #1 werner luis
    July 30, 2008

    Erik:
    As I posted in your blog some weeks ago, there is an apparent error in the assumed age of the last eruption. The volcano Chaiten is surroundet by rainforest (selva Valdiviana) with 3000 mm rainfall. When you look at this picture: http://www.inglaner.com/images/chaiten/domo/domo_pre-erupcion_02.jpg
    you will observe very little vegetation. The only explanation is some event that changed the surface of the dome. I gather that it was a recent eruption, some 300-600 years ago.
    Regards,
    Werner Luis

  2. #2 Erik Klemetti
    July 30, 2008

    Interesting … although my initial interpretation would be some sort of mass movement (non-volcanic) associated with the copious rainfall and the steep flows of the dome. However, it would also not be entirely surprising that there could have been minor eruptions from the dome since the last major eruption 9,000 years ago. Still lots of stuff to figure out with Chaiten not even counting the current events.

  3. #3 boris behncke
    August 6, 2008

    A crucial question about such silicic (formerly “acidic”) eruptions is how long they go on explosively. Very little is known about rhyolite eruptions, like the one at Chaitén, because none have occurred since volcanology became a modern science. Here’s a bit of a hint, though. On Lipari island in the Aeolian archipelago, north of Sicily (Italy), a rhyolitic eruption occurred in the early Middle Ages (6th to 8th centuries AD), starting with an explosive phase of unknown duration, and then a rhyolitic lava flow (obsidian, volcanic glass, actually) slowly advanced for more than one kilometer in about 200 years (!!!). This might give us an idea of the timescale of such eruptions. I would not be surprised to see Chaiten erupt for quite some time, although explosive phenomena might gradually taper off. Finally, geophysical data would have been necessary to understand where the magma was stored and accumulating before the eruption. Normally magma accumulates at some depth below a quiescent volcano, being fed from greater depth, and undergoing compositional changes in the reservoir. Normally, very gas-rich, silicic magma concentrates in the upper portions of the reservoir, which makes the reawakening of such volcanoes so violently explosive. We do have this problem at Vesuvius volcano in Italy, where no eruption has occurred since 1944 but magma is slowly accumulating, and will be erupted sometime in the future (I would guess in many decades or even centuries). Often the input of hotter, less silicic (or more mafic) magma into the reservoir is the actual trigger of large explosive eruptions, because such intrusions disturb the state of quasi-equilibrium in the reservoir.
    Sound complicated? Still, I have very much simplified things in this brief comment. Volcanoes are at least as complicated as we humans are… and what do we really understand of human beings except that they produce a lot of a mess …?

  4. #4 Erik Klemetti
    August 7, 2008

    Thanks for the comment, Boris. That is a good summary of some of the issues surrounding our understanding of volcanic processes. As much as we think we know, there is still a lot that needs to be examined and answered – which is why it is so exciting. My research focuses on these problems of the timescales, longevity and evolution of magmatic systems (for me, all from an isotopic/geochronology standpoint), and when we can start really combining geophysical information/modeling with petrologic information, then we can really get somewhere!

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