Tuesday, April 13, 2010

Volcano Vocab #2: Jökulhlaup

Today's obscure volcanologically-related word is jökulhlaup ("yer-kul-hloyp", "YO-kel-yawp" and "yo-kul-h-loip" in varying pronunciations), which is an Icelandic word for glacial outburst floods, both of water and lahars, formed when a subglacial eruption occurs. The water for these floods is formed when heat from those eruptions melts glacial ice, forming lakes that eventually become unstable enough to break through channels in the base of the glacier and flow out from underneath it. (Apparently the word can also refer to flooding caused by geothermal heat rather than a subglacial eruption, but since it's hard to see what's going on under a glacier in the first place, I wouldn't be too picky about the generation mechanism; suffice to say that some sort of volcanic activity is involved.) To give you an idea of what an unstable subglacial lake would look like, here's a diagram from an excellent overview paper:

Figure 3 from Björnsson (2002), showing a stable sub-glacial lake (a) and (b) an unstable lake likely to form jökulhlaups.

How big are these floods? Here's a quote from the same paper, talking about jokulhlaups from formed by the Grímsvötn volcano under the Vatnajökull glacier:
Jökulhlaups from Grímsvötn have occurred at 1– to 10–year intervals, with peak discharges of 600 to 4–5×104 m3s−1 at the glacier margin, a duration of 2 days to 4 weeks and a total volume of 0.5–4.0 km3.
Obviously, this is not a good thing to be in the way of. (By way of comparison, the mean discharge at Niagara Falls is about 1770 m3s−1 , or about a quarter one-thirtieth of the peak discharge during one of those floods.) I don't have any personal or public domain photos of a jökulhlaup, but the Global Volcanism program has some excellent photos from a 1998 event during an eruption of Grímsvötn.

This topic is quite relevant at the moment because of the recent volcanic activity in Iceland. While the fissure that's erupting at Eyjafjallajökull isn't in danger of melting much ice, there are several other volcanoes that are, such as Katla volcano under the Mýrdalsjökull glacier. Since roughly 10% of Iceland is covered in glacial ice, and the country has more than 30 volcanoes that have been active in the last 10,000 years, this is a major concern (see Ole Nielsen's post on jökulhlaups here).

If you're interested in more Icelandic geologic vocab, the USGS has an English-Icelandic glossary here. And here is the full citation for the Björnsson paper:

Björnsson, H., (2002), Subglacial lakes and jökulhlaups in Iceland. Global and Planetary Change, v. 35, p. 255–271. http://dx.doi.org/10.1016/S0921-8181(02)00130-3

UPDATE: Whoops! Totally forgot about this page over at Andrew Alden's About.com Geology. Lots more detail there!


Matt Kuchta said...

Another great word, Jess.

Jökulhlaups (pronounced YO-kel-yawps, I think), are also invoked to explain certain erosional landforms along the Laurentide Ice margin. Although not volcanic in origin, I think the usage of the term is fitting.

Callan Bentley said...

I like this series... how about a pronunciation guide for each word? For instance, I've always pronounced "guyot" as "gee-oh" -- and I have no idea about "Jökulhlaup!"

Jessica Ball said...

Excellent idea - I've gone back and added pronunciations to this and the previous. Jökulhlaup is pretty tough, though - I'm sure I don't say it correctly, but I'll have to wait until one of the other grads comes back from Iceland and get him to say it for me before I can really get it down.

Unknown said...

And now there is jokulhlaup at eyjafjallajokull,due to the volcano that started today under the glacier.
I am blogging about it and adding videos,on my blogspot.


Jessica Ball said...

Corrected - thanks.

Unknown said...

One of the truly fascinating stories of observational fact in geology defeating consensus is that of the formation of the Channeled Scablands near Spokane, Wa. Jokulhlaups are part of the story. Nova has a splendid documentary http://www.pbs.org/wgbh/nova/megaflood/about.html
J Harlan Bretz is now considered a national hero whose ideas trumped establishment inertia and scholarly disbelief of none other than one of the world's great geomorphologists, RF Flint.

C W Magee said...

Jess, you can leave that last comment in moderation limbo- wrong link. This is the Jokulhlaup:

Sorry about that.

snaefell said...

If you translate Jökulshlaup literally from icelandic it basically mean "glacier run" (Jökull=glacier, hlaup=run). As far as I know (although I am only an amateur) there is no discrimination made in Iceland what the source of a jökulhlaup is. Most of them are caused by geothermal heat under the glaciers - so we already had a minor jökulhlaup in the Skeidara river do to overflooding of a lake under the glacier earlier this year. Not to mention the jökulhlaup caused by the second eruption of Eyjafjallajökull.

Jessica Ball said...


Cool! Working from a volcanologist's perspective, I tend to pay more attention to the geothermal stuff, but it's neat to know that the term is so versatile.