Posted by: Anne Jefferson | April 19, 2010

A selected few Eyjafjallajokull links

The eruption of Eyjafjallajokull (which means Island Mountain Glacier in Icelandic) started out in March as a relatively quiet and tourist-friendly Hawaiian style eruption. That petered out and then a few days later, the magma reemerged subglacially, producing the spectacular ash-producing phreato-magmatic eruption that has transfixed the world and stranded all would-be European air passengers. The Boston Globe’s Big Picture coverage has been as-usual spectacular. Check out these two photo sets (15 April, 19 April). NASA’s Earth Observatory has also been producing some nice images of the ash plume. Follow them here.

And few days into the eruption, the magmatic heat produced enough glacier melt not just to fuel the ash production but also to generate an outburst flood, as captured on video below:

The video above includes a rather spectacular train of standing waves, which led to some debate amongst some friends and I over whether they represented sub- or super-critical flow. Fortunately, I have access to an expert on the subject, Gordon Grant, who weighed in thusly:

The standing waves represent what is best referred to as “trans-critical flow”, that is flow that oscillates around Froude No. = 1. We have real-time (though very small scale compared to what you’re seeing in the video) measurements from Grant (1997; available on the WPG website). Basically flow on the downstream portion of the wave is accelerating (Fr >1), while flow on the upstream portion is decelerating. The flow then oscillates between Fr > 1 and Fr < 1, maintaining overall flow at close to Fr ~ 1. Cross-sectionally averaged Froude Number tends to be slightly less, due to drag at the boundary. See paper for details.

The paper to which he refers is Grant, G.E. 1997. Critical flow constrains flow hydraulics in mobile-bed streams: a new hypothesis. Water Resources Research. 33: 349-358. PDF available here: http://www.fsl.orst.edu/wpg/pubs/criticalflow.pdf

For the best scientific coverage of the on-going Icelandic eruption, you absolutely can’t miss Erik Klemetti’s Eruptions blog. Erik is a volcanologist and has been doing an astounding job of keeping up with this (and all other volcanic activity). He also benefits from an active, engaged, and informed community of commenters to keep the rest of us up-to-the-minute on volcanic activity around the world.

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Responses

  1. When discussing the volcanic event today with my Geomorph prof, he mentioned how the volcano itself and its eruption by itself is nothing to write home about, but that the interaction with the glacier is what is driving the massive tephra fallout.

    How does the melting of the glacial ice and firn and whathaveyou exacerbate the ash cloud?

  2. The presence of water (in the form of glacier ice) adds significantly to the explosivity of the eruption. If you want to know more, you should be reading the posts by John Stevenson at http://johnalexanderstevenson.blogspot.com/. He’s an expert on glacio-volcanic interactions and has been writing the most scholarly posts around this eruption.


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