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A review of ash cloud classification

By eklemetti on February 19, 2009.

Jonathan Castro has reminded me that I need a refresher on the proper terminology for ash columns related to volcanic eruptions. It is very easy to start mincing words and using them inappropriately - and that is the sort of sloppy reporting and discussion I am trying to avoid.

So, to refresh my (and our) memory on ash clouds and how to classify them, we can go back to one of the indispensable textbooks on volcanology, Cas and Wright's Volcanic Successions. The classification scheme they provide is summarized in the figure above. It shows the heights of various eruption columns and the relationship between types of eruptions and their "explosiveness". Jonathan is likely correct in classifying this eruption in the vulcanian range (<20 km ash column), especially if it was driven by explosions in the dome. If anything, Chaiten's activity today could be sub-plinian, but a true plinian eruption requires a taller ash column height and higher levels of explosivity.

UPDATE 2/20/2009: Here is a link to an article that talks about the processes that go along with each eruptive style. Remember, the ash column nomenclature is not solely dependent on height, but also process and composition. {Thanks to Boris for reminding me to emphasize this.}

Tags
Ash fall
Chaiten
Chile
explosive eruption
speculation
volcanic hazards
volcanology basics

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What I have seen today Chaitén Volcano qualified for a "Strombolian" category column just after the increase of the eruption, returning to "Hawaiian" for the remainder of the day.

By Ron de Haan (not verified) on 19 Feb 2009 #permalink

now, now ... the term Strombolian is applied to strictly to a type of mildly explosive activity that produces discrete ejections of fragments of molten lava, and in the Hawaiian case we're dealing with still more fluid lava producing fountains, lava lakes, and very mobile lava flows. Such terms do not at all apply to silicic volcanoes such as Chaitén where you will never see any liquid lava to be thrown out. In fact, the use of Strombolian and Hawaiian in a scheme denoting the size of tephra plumes can lead to misinterpretations.
The case of the Chaitén activity yesterday is one of partly explosive, partly gravitational disruption of a gas-rich lava dome. Normally the explosive part is called Vulcanian, while there is no generally accepted term for plumes rising from dome collapse, although their generation from pyroclastic flows makes them most similar to "co-ignimbrite" clouds. In such complex cases rather than trying to forcefully apply a technical term from the existing terminology (with its obvious advantages but also limits), it would be appropriate to remain largely descriptive, and speak of - as said above - partly explosive, partly gravitational disruption of an active lava dome. How much gravitational versus explosive, will remain to be seen from analysis by the volcanologists on site.
Greetings from a volcano that indeed is often producing Strombolian activity, Mount Etna

That classic photo of Redoubt from April 21, 1990 (the atomic like mushroom cloud one) for instance was apparently also produced more by dome collapse rather than eruptive (i.e. explosive) activity, which in my book would exclude it from being termed Plinian.. pity nobody has come up with a name for dome-collapse- ash-columns that sticks... Unzenian..Redoubtful..Montserratian.. hmm.

Boris - Thanks for pointing out the fact that I didn't emphasize the connection between the ash columns and the eruptive style. The ash column nomenclature isn't solely dependent on the height of the column, but also the physical processes going on during the eruption and, to an extent, the composition and volatile content of the magma involved. I've posted a link (yes, to Wikipedia, but its a good summary) as an update to this post that details the types of eruption styles that go with the ash column nomenclature. Hope that clears things up a bit.

What is the composition of the gas carrying the ash? Water vapour?

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