5 comments on “TUFF.. TUFA?.. IGNIMBRITE – Pyroclastic Deposits

  1. Nice exposition Granyia! There is a lot of confusion arising from the terminology. I guess words develop their own associations over time and usage.. makes things kind of inexact! Glad for your clarifications!

    I still remain kind of concerned about Taal, not least because we have a dyke forming in an extensional setting under a known caldera volcano. At an extremely rough estimate, the dyke has a volume of about 0.3 cu km (1.20 meter in width, 20 km depth and 10 km length) though this could easily be out by an entire order of magnitude (i.e. anywhere up to 3 cu km).
    What is fascinating is how this now pans out. The way I see it, the following scenarios are possible:
    1. The intrusion stalls before it reaches the surface and the volcano goes back to sleep.
    2. The intrusion hits the existing plumbing of the volcano and is suppressed by more buoyant felsic material (crystal mush). This results in a recharged magma chamber that could erupt soon or at a later date (or not at all).
    3. The mafic intrusion could be rich enough in volatiles (given the subduction setting) to broach the surface under its own buoyancy (Mt. Tarawera type eruption), possibly outside of the caldera rim.

    The 64 million dollar questions is how big the existing body (bodies) of crystal mush under Taal volcano are and how close they are to being tipped into an eruptible state by such a strong intrusion of hot mafic material high in volatiles from the mantle boundary, presuming that the intrusion has already fed the plumbing of the central vent (evidenced by the initial eruption).

    The question I still have not managed to find an answer to in my years of reading is what kind of signal does a recharged felsic magma chamber make by itself, quite distinct from the noise of the mafic intrusion? Considering that all a rhyolite body needs to be tipped into an eruptible state is an injection of heat and volatiles, does this have any surface expression or does it occur silently? With regard to Santorini I once read that a phase change of a magma chamber (mush to melt) might even lead to deflation. Do you guys know any more about this?

    Liked by 1 person

  2. On the other hand, the frequency at which Taal has erupted recently suggests that any bodies of crystal mush that exist are probably low in volatiles. Moreover, they may also not be that felsic. In this case, I imagine that an eruption would be more like an Etna type of event with lava fountaining and possible lava flows. The dyke is also probably due to regional extension (tectonic movements) initiating decompression melt that has already reached neutral buoyancy by ascending into the crust. So many scenarios!! It will be interesting to see how it develops from here.

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