Grimsvotn volcano in Iceland remains silent despite flood
Posted on06 November 2010. Tags: Eyjafjallajokull, Grimsvotn Volcano, Iceland Eruption, Iceland Volcano, Volcano Eruption
Despite the flooding in the Gigjukvisl river in south Iceland this week, all appears calm under the Grimsvotn volcano at the moment. One or two small
tremors have been measured there over the past couple of days, however, there has been no further signs of a possible eruption, according to
Thorunn Skaftadottir, a geologist working for The Icelandic Met Office (Veðurstofa Íslands), told mbl.is yesterday that a number of small
earthquakes have been recorded close to Krysuvik on the Reykjanes peninsula. The source of the earthquakes was about four kilometers deep; but
activity is not uncommon in this area so these events could be unrelated.
The flooding came from an increase in run-off meltwater from the Vatnajokull ice cap, which suggested the possibility of an eruption. Due to the major
disruptions caused to European air traffic in spring 2010 by the Eyjafjallajokull volcano eruption, this week’s flooding and talk of another
possible eruption has made world news.
Grimsvotn last erupted in 2004 creating an ash cloud that caused some disruption to air travel, although not on the same scale as the eruption of
Eyjafjallajokull. If Grimsvotn was to erupt, any further disruption to air traffic would depend on the size of the eruption, the amount of ash
generated and also weather conditions.
However, after the experience and knowledge gained from the Eyjafjallajokull eruption, airlines and aviation authorities have had to become better
equipped to deal with such situations and any future volcano eruptions in Iceland.
Jökulhlaup from Grímsvötn subsides
Figure 1. The jökulhlaup from Grímsvötn (click to enlarge).
The jökulhlaup (glacial outburst flood) from Grímsvötn that began near the end of October is now coming to an end.
The flood reached a maximum level shortly after noon on November 3, and scientists from IMO visited the site on that day to study the effects of the
flood on the region adjacent to the ice margin. Two IMO technicians have performed regular discharge measurements on the bridge over the river
Gígjukvísl throughout this week (Figure 1) and the results from their measurements are shown in Figure 2.
Figure 2. Discharge (m3/s) measurements at Gígjukvísl bridge: From the curve the total amount of floodwater is estimated 0.45 km3.
The discharge curve is typical for jökulhlaups from Grímsvötn that do not result from volcanic activity: Over the course of several days, the
amount of water flowing through an ice tunnel at the glacier bed steadily increases. Loss of frictional heat from the floodwater causes melting of the
tunnel walls, thereby increasing the flow capacity of the tunnel.
From conductivity measurements and water level recordings it seems likely that the jökulhlaup started already on October 28. The total volume of
water in the subglacial lake beneath the ice cover in Grímsvötn, located 50 km up-glacier, was estimated to be 0.7 km3 at the start of the
jökulhlaup. The maximum discharge of the flood was slightly above 2600 m3/s and the total volume is estimated to have been 0.45 km3, making this
jökulhlaup rather small in comparison with many earlier floods from Grímsvötn.
In past centuries, most jökulhlaups from Grímsvötn have entered the course of the river Skeiðará. This time, however, floodwater that emerged
from beneath the eastern part of the glacier went westwards along the glacier margin and then entered the river Gígjukvísl. Skeiðará has deposited
very large amounts of sediment on the eastern part of Skeiðarársandur plains over the centuries, increasing the elevation of the sandur area there
relative to the central part. In addition, the glacier has carved a trench during times of advance. Thus, it was clear that retreat of the glacier
over the past 15 years would sooner or later lead to a drastic shift in the direction of meltwater flow from this part of the glacier. In the summer
of 2009, this shift occurred and water has ceased to enter the course of Skeiðará.
Although this was the first jökulhlaup to take the westward course along the glacier margin, inspection on the ground and from the air on November 3
did not reveal drastic signs of water erosion or ice breakup. The water flowed swiftly along the glacier margin (Figures 3 and 4), descending under
tongues of ice at two locations and emerging again on the downstream side. There were, however, clear signs of slumping on sediment banks at the
southern side of the water flow, and thus great care must be taken during visits to the area.
Meltwater along the margin
Figure 3. The jökulhlaup flowing westwards in its course along the margin of the Skeiðarárjökull outlet glacier. Photo: Þorsteinn Þorsteinsson.
The new pathway
Figure 4. View towards the Skaftafell mountains, east of Skeiðarárjökull. A sediment bank is visible to the left and a continuation of this feature
is seen on the right hand side of the photo. The water that previously flowed into the Skeiðará river breached this sediment bank in 2009, leading
to the start of meltwater flow along the margin and into Gígjukvísl. Photo: Þorsteinn Þorsteinsson.
Figure 5 shows the location where the greatest amount of upwelling of water was observed and Figure 6 shows a scientist measuring the water
temperature at this site. Earlier data from the subglacial lake in Grímsvötn have indicated that the water temperature there is close to the melting
point and the value obtained at the site shown in Figure 6 was -0,025°C ± 0,006°C (i.e. very close to the melting point). Loss of potential energy
as the water descends 50 km downslope through the ice tunnel generates frictional heat in the water, but this heat seems to be entirely lost through
the process of melting the tunnel walls.
Samples were taken on site and the composition of the floodwaters will be analysed by geochemists at the Earth Science Institute, University of
The main upwelling
Figure 5. The main location of upwelling on the easterly part of the ice margin. Water was also seen to emerge at other locations. Photo taken at
15:40 PM on November 3. The mark of a higher water level 2-3 hours earlier (during peak flow) is seen on the debris-laden ice walls. Two people can be
seen on the ice. Photo: Þorsteinn Þorsteinsson.
Measurements at the upwelling
Figure 6. Temperature measurement at the upwelling site. The scientist doing the measurement is connected to a safety line. A temperature recorder
lowered into the water stores data in its memory, which later are downloaded to a computer. Samples were taken to investigate whether frazil ice was
forming in the water; such ice can form as water flows quickly uphill beneath a glacier and supercools due to rapid lowering of pressure. No frazil
ice was found at this location. Photo: Þorsteinn Þorsteinsson.