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Waiting for Redoubt's big boom

By eklemetti on May 5, 2009.

The clock is ticking for the new dome growing at Redoubt to collapse. What will happen if/when it does collapse? Good question!


The new dome at Redoubt. Image courtesy of AVO/USGS, taken by Game McGimsey, April 30, 2009.

Most likely, the following will occur:

  • First, extrusion of lava will cause the dome to over-steepen, precipitating the collapse (although a large seismic event could also trigger collapse as well). An excellent example of this is the spine that grew at Mont Pelee on Martinique in 1902 before the famous eruption that destroyed St. Pierre and killed >30,000 people.
  • When it cannot hold itself up anymore, the dome will crumble gravitationally.
  • As the rock breaks away and moves downslope, the pressure on the underlying hot magma will lessen and cause an explosive component to the eruption.
  • If directed upward, the explosive component will produce a subplinian-to-plinian ash column (up to 10 km / 30,000 feet according to AVO).
  • The hot material of the dome that moves downslope will produce a pyroclastic flow, specifically what is known as a "block & ash flow". Here is a brief video of one from the dome collapse at Soufriere Hills, Montserrat.
  • This hot pyroclastic flow (or nuee ardente) will melt snow/ice on Redoubt and mix with runoff to form lahars that will travel down the drainages on the edifice, possibly all the way out to the inlet.


Generation of pyroclastic flows from dome collapse. Figure courtesy of the USGS.

Impressive, eh? Of course, it might not happen exactly like this. There are many variable (extent of collapse, pressure from underlying magma, magnitude of pyroclastic flows generated and many others), but more than likely, block & ash flows and lahars will be generated when the dome does collapse.

The latest from AVO suggests they think it will happen soon and without much warning, so keep an eye on the webcams (DFR and Hut).

Tags
Alaska
Ash fall
explosive eruption
lahar
Pelee
pyroclastic flow
Redoubt
Soufriere Hills
united states
volcanic hazards
volcano monitoring
volcanology basics

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Can I correct you on one small point? The famous spine of Pelee in 1902 appeared some months AFTER the catastrophic blast which destroyed St. Pierre, not before it

Can I correct you on one small point? The famous spine of Pelee in 1902 appeared some months AFTER the catastrophic blast which destroyed St. Pierre, not before it

Apologies, Erik; a computer glitch meant my post was sent twice, sorry :o(

Mike Don - I think this specific spine was, in fact, formed after the deadly eruption that took out St. Pierre, but many of the pyroclastic flows from the 1902-03 eruption of Pelee were formed by the collapse of domes/spines like this.

In fact the situation at Redoubt is strongly similar to that at Unzen in Japan, in 1991, where a lava dome grew perched in a small crater just above a steep slope, and when the dome began to grow over the crater rim onto the steep slope, it began to collapse. A similar scenario must be imagined for Montagne Pelée in Martinique in 1902. As pointed out correctly by Mike Don, the huge famous spine grew many months after the disaster of 8 May 1902. Generally the pyroclastic flows generated by the collapse of lava domes growing (and collapsing) on steep volcano flanks are called "Merapi-type pyroclastic flows", after Merapi volcano in Java (Indonesia), which does this sort of thing quite frequently. Differently from Merapi and Unzen, though, collapse of Redoubt's lava domes is always accompanied by stronger explosive activity, at least this was the case on 4 April this year, and possibly the imminent dome collapse will have a strong explosive component as well. Maybe this is because Redoubt's magma is more gas-rich, as is evident from the highly explosive start of its eruptive sequences, whereas Unzen, for example, started off with a few minor phreatic or phreatomagmatic bursts and then dome growth began virtually without any magmatic explosive activity.

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