Although the mainstream media seems to have lost interest in the Chaiten eruption beyond the “human interest” (or should I say “salmon interest”) aspect of the eruption, there is still a lot going on at the volcano. The SERNAGEOMIN recently released this excellent photo of the erupting caldera:

Chaiten Dome - SERNAGEOMIN

What is pretty clear here is thank a new rhyolite dome is erupting on top of the old dome. (On a side note, when I was visiting the Smithsonian last week, we were all lamenting the fact that no one has come out and said what composition this lava is? I’m 99% convinced it is rhyolite, e.g., high silica, lava, but I haven’t seen an official determination so far EDIT 5/27: Thanks to one of my readers, Hawkeye, for pointing out that the SERNA report I link to above lists the composition as 73-75 wt% silica, a rhyolite). From the looks of the picture, there isn’t a lot of new lava dome on the surface, so much of the volume of erupted material came out in the form of ash and pyroclastic material.

The most recent update from the SERNAGEOMIN show the waning eruption and the new dome:

Chaiten 2008 Dome

The eruption is now classified as “sub-plinian”, so a much smaller volume and dispersal area (~5-500 square km). Some of the rivers are choked with volcanic material, mostly ash and pyroclastic material again, but they are creating a lot of lahars.

Chaiten 2008 Lahars

The seismicity at Chaiten has been gradually going down for the last few days, so this might mean that the eruption is finally winding down – here, a full 3.5 weeks since it started. What is fascinating about this eruption is the idea that what we’re seeing might just be the emplacement of a new dome inside the caldera itself, what is referred to a “resurgent dome”, where eruptions come up through the middle of the caldera after the cataclysmic eruption rather than along the ring fractures at the edge of the caldera. What is yet to be seen is how long is magma was sitting underneath the Chaiten caldera before it erupted and what might triggered such a rapid evacuation of the magma chamber. 



  1. #1 Hawkeye
    May 27, 2008

    I read last week from some official source (don’t remember the link) that the first ash samples from the current eruption consisted of 73-75% silica. That’s quite high… much higher than St Helens.

  2. #2 ewklemetti
    May 27, 2008

    Thanks for that info! I’ll have to try tracking down that data … but 73-75 wt% SiO2 makes it a good, healthy rhyolite!

  3. #3 Hawkeye
    May 27, 2008

    I found the silica % data. It is in the May 23 SERNAGEOMIN update linked to at the top of this blog post.

  4. #4 ewklemetti
    May 27, 2008

    Wow, I missed that (oops!) Thanks for pointing it out!

  5. #5 Alan Sullivan
    May 28, 2008

    Plume is playing at 21,000′ again today, with code red from Buenos Aires VAAC. Don’t be too sure this is winding down. It may be far from done. We are seeing a very unusual eruptive pattern — nearly unique. You are more conversant with the record than I. Can you think of any well-described volcanic event that compares with this one, to date?

  6. #6 ewklemetti
    May 29, 2008

    I don’t think it is out of the question that the eruption will wax and wane over the next few weeks, depending on what is going on down under the volcano. As we saw with the 2004-08 eruption at St. Helens – albeit at a much smaller scale and much less explosive – eruptions of viscous silicic lavas like rhyolite (or dacite in the case of MSH) might go on for years. I imagine we’ll see the power of the eruption fluctuate as the supply rates of magma and the volatile content of the erupting magmas change.

  7. #7 Werner Luis
    May 30, 2008

    Here is the analysis of Chaiten ash fallen in 4 cities of Rio Negro, Argenina

  8. #8 ewklemetti
    May 30, 2008

    Oh that is great! Thanks! It looks like most of the ash that has been analysed is between 73-75 wt% silica, which is what was reported early. However, the first two analyses are down in the mid 60 wt% silica, which is much lower, closer to a dacite. I wonder if there was some change in the composition during the eruption.

  9. #9 Werner Luis
    May 30, 2008

    I suppose you saw this article in the “Revista geológica de Chile”:

  10. #10 Werner Luis
    May 30, 2008

    60 wt% silica:
    I think those values are wrong

  11. #11 Erik Klemetti
    May 30, 2008

    Thanks for that additional paper … I think I saw it once in passing, but it does have older Chaiten data that suggests that those <73 wt% silica analyses are either misidentified or incorrect, but time will tell once the data is more disseminated.

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