What is a jökulhlaup?
At the moment (December 30, 2012), water height and conductivity are again on the rise in Múlakvísl, an outlet river of Mýrdalsjökull glacier, the one covering the famous Katla volcano in the south of Iceland.
Plots from Icelandic Metorological Office (vatnafar)
This could be a sign for again another jökulhlaup in this region, Mýrdalssandur, where the Ringroad bridge was destroyed last year – or maybe not.
But what is a jökulhlaup?
Jökulhlaup is an Icelandic word which comes from „hlaup(ið)“ which means „(the) run“ and „jökull“ meaning „glacier“ – one of many Icelandic composita, as in Icelandic language words are set together like in puzzles. English words describing the phenomenon are “glacier run” or “glacial outlet flood”. (There is also a German expression for it: “Gletscherlauf”.)
For a long time, it has been sort of a puzzle, what is happening to glaciers and their lakes as well as under the many Icelandic glacier caps where volcanoes lie in wait for their next awakening. Because jökulhlaup often happen without any (observable) eruption.
Glacier outlet floods can have a lot of different origins.
There can be some which have nothing to do with volcanic activity, like the famous Lituya Bay run. http://en.wikipedia.org/wiki/Lituya_Bay In this case, a landslide in 1958 caused an enormous tsunami. Dam failures by earthquake or material break down can also be behind this, as was the case sometimes in the Alps.
Some floods can also be initiated by surging glaciers (http://en.wikipedia.org/wiki/Surge_%28glacier%29 ), which may or may not stand in a complicated and not yet clear relationship to volcanic activity.
And now we are approaching the really interesting cases.
There are also some different kinds of jökulhlaup, the real ones, which are in some way or other caused be volcanic activity.
Some of you were a bit disappointed by the last glacier run from a region with two famous ice cauldrons in the southwest of Vatnajökull, which is called the Skaftárkatlar (Skaftá cauldrons) in August 2012 http://www.vedur.is/um-vi/frettir/nr/2520 . It was a really small one, of about 185 m3/sec.
As is to be seen in this film https://contour.com/stories/skaftarkatlar-30-08-2012 the glacier runs normally come from sort of an indentation in the big ice caps that cover in the whole 11% of Iceland. And the runs from this region are produced very regularly, in a rhythm of 1-2 years, mostly in the spring or summer, so that the people down in Kirkjubaejarklaustur almost set their clock in connection to them.
They are caused by a high temperature area, and intrusion under the ice of Vatnajökull, which melts more or less the same amount of ice in the same time span. The water is collected in a bowl shaped lake under the ice and when there is enough of it, breakes the ice barrier in front and runs down through a system of tubes in the ice, which is built up and widened by the water pressure and heat of the water. These are – as we have seen this summer again – small glacier runs, which build up rather fast to a certain maximum and then slowly fade out again, “(…) there are, however, other types of Skaftá floods which have no distinct discharge peak but maintain a strong, stable current for up to 2 weeks before terminating.” (H. Björnsson, 2010, 2).
A similar kind of smaller, but volcanically induced hlaups is also found at the Grímsvötn volcanic system. We saw two of them in the last months ( see: http://www.vedur.is/um-vi/frettir/nr/2583 and http://icelandreview.com/icelandreview/daily_news/?cat_id=28304&ew_0_a_id=369682 .
Björnsson, 2002, p.4: 2 drawings, fig.3
In former times, travellers were always very fearful about traversing the sander plains in front of the big glacier caps in Iceland. And the guides, local men, farmers and eg. Postmen, praised like heroes. The most famous one of these was Hannes Jónsson of Núpsstaðir (1880 – 1968) whose life could be subject of a post of its own.
But, if these floods were so small and predictable, why would that be so?
There is still another kind of glacier run to be reckoned with, and that is the one normally connected with the notion: It is the cataclysmic outlet flood, having its origin in a big volcanic eruption under one of the icecaps, by which an enormous quantity of melting water is produced and after a longer or shorter period of time depending on intensity of eruption and last but not least on water pressure and decline of slope rushes down to the sander planes below taking with it a mixture of sediments, gases and icebergs.
In case of an eruption in the Grímsvötn region – Gjálp 1996 was a fissure system which is part of it – , the subglacial lake in the caldera below the mountain Grímsfjall fills, and in filling takes on the form of a sort of subglacial balloon. When a certain limit is reached, the water pressure is high enough and the ice barrier in front of it is broken, and the water rushes downhill. At the beginning through tunnels in the ice, but if the water discharge surmounts a certain limit, the water lifts first the barrier and then the whole glacier and rushes downhill in sort of a big subglacial wave with the glacier riding onto it. And down in the valley, the water spills out in a lot of bigger and smaller outlets at the snout of the glacier and more or less covers the plain in front. (Björnsson, 2010) The jökulhlaup connected to the Gjálp eruption in 1996 produced no less than 55.000 m3/sec at its peak covering all of Skeiðarársandur. (M.T. Gudmundsson, etal. (2004).
Drangajökull, Langjökull, Hofsjökull, Vatnajökull, Eyjafjallajökull and Mýrdalsjökull
When I started researching this, I was surprised to find out that not only had the “usual suspects” Vatnajökull and Mýrdalsjökull produced bigger and smaller jökulhlaup running in all directions, but that smaller ones were known from virtually all 5 big Icelandic ice caps, i.e. also Drangajökull (most probably not volcanic, because it is situated outside of the active rift zone), Hofsjökull and Langjökull. (O. Sigurdsson, 2005) And two years ago, we could observe how the outlet glacier Gígjökull from Eyjafjallajökull covered a small glacier lake with debris during some jökulhlaup. http://en.vedur.is/hydrology/articles/nr/2097
The phenomenon is of course not limited to Iceland, but is possible in all parts of the world with glacier covered volcanoes like Alaska or the Andes in South America.
And there have also been famous floods in the distant past, i.e. the Pleistocene, like the Lake Missoula floods which carved out the Columbia River Gorge in the USA, the Altai floods in Siberia, even one in Germany (Münsterland) and then there is the case of Jökulsárgljúfur in Iceland. Some of them are thought to have involved such an enormous quantity of water that the one of 1996 in Iceland is really dwarfed by comparison. But we’ll speak more about these in later posts.
Disclaimer: I am no specialist, just an interested layman.
⁃ Subglacial lakes and jökulhlaups in Iceland, by H. Björnsson (2002)
⁃ Understanding jökulhlaup, from tale to theory, by H. Björnsson (2010)
⁃ Jökulhlaupannáll 1989-2004, by O. Sigurdsson, etal. (2005) (in Icelandic)
⁃ The 1996 eruption at Gjálp, Vatnajökull ice cap, Iceland: efficiency
of heat transfer, ice deformation and subglacial water pressure, by Magnús T. Gudmundsson, Freysteinn Sigmundsson, etal. (2004)