Iceland - Subglacial debacle in Vatnajökull .

A subglacial debacle marked the river Skaftá this January 20.

              The debacle in Skaftá River, 01/20/2014 - photo Reynir Ragnasson - IMO

" Jökulhlaup " is the Icelandic term for a sub-glacial meltdown, literally " ice race."

Several types of jökulhlaups therefore exist depending on the feeding mechanism ( fusion of meteorological origin, hydrographic interglacial network modification, subglacial volcanic eruption ) and the position of the tank ( englacial lake, juxtaglaciaire lake or supraglacial lake).

In this case, the melting of the glacier under the heat of the volcano, forms a lake; he can become unstable enough to pour out suddenly through channels at the base of the glacier overcoming the volcano, and create a major flood .

          Skaftá river bellow the Skaftárdalur valley . - Photo 20.01.2014 - Reynir Ragnasson - IMO

This Jökulhlaup in Skaftá river has its source in two subglacial caldrons, formed by geothermal activity under the Vatnajökull, a huge icecap  overcoming several volcanoes .
They drain an average of every two years , producing a debacle which the flow reaches 1,500 cubic meters per second . And this is the East cauldron which is responsible for the largest floods. His last drainage of September 2012.
 In winter, the flow rate may be higher because the banks are icy .
When the interval between two jökulhlaups is shorter, floods are less important.

Various dangers accompany these floods, and it is advised not to approach the glaciers during these alerts.
Hydrogen sulphide is released by the water coming out of the cap of Vatnajökull. The concentrations of this gas are particularly important at the edge of the glacier, and it can reach lethal levels .
Cracks can develop rapidly near ice cauldrons and where the waters pierce the surface.

Crevasses in the glacier during the eruption of the Grímsvötn 01.10.1996 - photo Magnús Tumi Guðmundsson - Icelandic Inst. of Earth Sciences.

Localisation of the two cauldrons under the Vatnajökull and the way out to the Skaftá river, SW of the glacier.
Map © Isor , LMI , NOAA, VI / via Icelandic Met Office .

Some important jökulhlaups :
- In 1996, Grímsvötn , it reached a peak of 45,000 m³/ s. ( greater than the flow of the Mississippi ) over a width of fifty kilometers and a water depth of three to five meters to 23 h., time of maximum flood.

- The eruption of Eyjafjallajökull in 2010 caused a significant melting of ice which causes jökulhlaups with a maximum flow rate of between 2,000 and 3,000 m³/ s.

Jökulhlaup consecutive of the subglacial eruption of Eyjafjallajökull , 14.10.2010 - Reuters photo archive

The debacle in Skaftá river is currently declining , and has a comparable flux in size with a summer type discharge . This episode did not cause damage.

Sources: Icelandic Met Office .