Bardarbunga/Grimsvötn Volcanoes

Iceland

Date: Tue, 1 Oct 1996 15:27:59 -0100

A subglacial eruption near the Bardarbunga and Grimsvotn volcanoes in Iceland.

An eruption broke out beneath the Vatnajokull glacier in Central Iceland in the evening of September 30. The eruption was preceded by an unusual sequence of earthquakes, beginning on September 29 with a magnitude 5 event in the Bardarbunga caldera. Similar earthquakes have occurred in Bardarbunga many times during the last 22 years, without eruptive activity and almost without aftershocks. This time the earthquake was followed by an intense swarm of small earthquakes, that spread out to the neighbouring volcanoes Grimsvotn and Hamarinn. A warning of a possible eruption was issued to air traffic in September 30 at 19 h. Later that evening the earthquake activity near Grimsvotn decreased markedly, while that of Bardarbunga continued. At about 22 h the seismograph at Grimsvotn began recording continuous eruption tremor with small amplitude. The amplitude increased very slowly during the next hours and reached a maximum at about 6 h in the morning of October 1. A large depression in the glacier ice of Vatnajokull was discovered from over flying aeroplanes in the early hours.

The glacier is apparently being melted by an eruption on a 4 km long fissure beneath the glacier, which is 400-600 m thick in this location. The fissure has a NNE-SSW strike and is located on the northern flank of the Grimsvotn central volcano. The melt water is draining into the Grimsvotn caldera and raising the ice shelf on the caldera lake. The eruption continues at the time of writing (14 h UTC, October 1).

The present eruption is a continuation of a remarkable series of seismic and magmatic events in the Vatnajokull area that began in 1995: July 1995: A glacier flood from a subglacial geothermal area on Loki Ridge (Eastern Cauldron) NW of Grimsvotn. The draining of the water reservoir was followed by a distinct tremor episode, presumably a small eruption triggered by the pressure release. This event was followed by a general increase in seismicity during the next few months.

February 1996: An intense earthquake swarm centered on the Hamarinn Volcano, lasted a week.

August 1996: A glacier flood from a subglacial geothermal area on Loki Ridge (Western Cauldron) NW of Grimsvotn. The draining of the water reservoir was followed by a tremor episode, presumably a small eruption triggered by the pressure release. This event was followed by increased seismicity during the next few weeks.

September 1996: Present activity: Large earthquake, intense earthquake swarm and an eruption N of Grimsvotn.

September 30 at 19 h. Later that evening the earthquake activity near Grimsvotn decreased markedly, while that of Bardarbunga continued. At about 22 h the seismograph at Grimsvotn began recording continuous eruption tremor with small amplitude. The amplitude increased very slowly during the next hours and reached a maximum at about 6 h in the morning of October 1. A large depression in the glacier ice of Vatnajokull was discovered from over flying aeroplanes in the early hours.

The glacier is apparently being melted by an eruption on a 4 km long fissure beneath the glacier, which is 400-600 m thick in this location. The fissure has a NNE-SSW strike and is located on the northern flank of the Grimsvotn central volcano. The melt water is draining into the Grimsvotn caldera and raising the ice shelf on the caldera lake. The eruption continues at the time of writing (14 h UTC, October 1).

The present eruption is a continuation of a remarkable series of seismic and magmatic events in the Vatnajokull area that began in 1995:

July 1995: A glacier flood from a subglacial geothermal area on Loki Ridge (Eastern Cauldron) NW of Grimsvotn. The draining of the water reservoir was followed by a distinct tremor episode, presumably a small eruption triggered by the pressure release. This event was followed by a general increase in seismicity during the next few months.

February 1996: An intense earthquake swarm centered on the Hamarinn Volcano, lasted a week.

August 1996: A glacier flood from a subglacial geothermal area on Loki Ridge (Western Cauldron) NW of Grimsvotn. The draining of the water reservoir was followed by a tremor episode, presumably a small eruption triggered by the pressure release. This event was followed by increased seismicity during the next few weeks.

September 1996: Present activity: Large earthquake, intense earthquake swarm and an eruption N of Grimsvotn.

600 m thick over the eruption site and the cauldrons indicate a flow of water to the Grimsvotn caldera to SE. In the Grimsvotn caldera is a lake that periodically surges to south, under Skaftarjokull forming "jokulhlaup". It was estimated that the eruption had filled 25% of the volume of the caldera and a surge is evident if the eruption continues. Weather was fine throughout the day.

Today 2nd Oct is SE rain and storm, visibility low and observation from air difficult. From seismometers it is evident that the eruption continues and in 09 news it was reported that the eruption had broken through the glacier and the eruption column is in the order of 5 km. It is not possible to forecast the length of the eruption, several small eruptions have occurred in this area but also some of the largest in Iceland are from this fissure system.

Eruption below Vatnajokull glacier is more concentrated in one place as is usual in such eruptions.

Flight observation before 09 this morning confirmed the ash-column to reach several km. This eruption is at the same site as in 1938. The thickness of the glacier is thinner at this place as the 1938 eruption did build a subglacial ridge. The glacier is around 450 m thick where the eruption did erupt through the glacier. The 1938 eruption is expected to have lasted about 1 week, perhaps longer as it was only observed when it had melted through the glacier.

Water continues to flow to Grimsvotn caldera at a rate about 5000 m3/sek. A surge in Skeidara river is expected to begin tomorrow, after evt. 24 hrs if this inflow rate continues.

Ash is travelling N and NE at the speed of 70 km/h and in the next few hours it is supposed to fall in N and E Iceland. All flight is cancelled in N and E Iceland at the moment.

Eruption column is 15000 feet, and now the eruption fissure is 10 km long and has extended through the glacier at 2 sites, the second site is 10 km to the north of the opening that opened at 05 this morning. Ash has started to fall in N Iceland Hazard zone is now Vatnajokull, Axarfjordur to Hunafloi in N Iceland and as a low pressure is going over Iceland, ash can fall anywhere the next 24 hours.

Date: Wed, 2 Oct 1996, 12:00

Bergur Bergsson, eye witness was 13 km S of Kverkfjoll in the early morning reports: Steam 05:18 steam turned from light to dark. This is believed to indicate time of the eruption to break through the glacier. The fissure is believed to be appr. 10 km at north of Grimsvotn caldera The eruption column is at 12h 2 km over eruption site glacier rising to 6 km further north.

In the eruption 1938 not much tephra came from the eruption, but a lot of water came from Grimsvotn, making the jokulhlaup in Skeidara the largest one in the century

It is evident from eye witness and seismic information that the northern eruption site is near the watershed in Vatnajokull so some of the meltwater could travel north, to Jokulsa a Fjollum river, through Dettifoss waterfall to Axarfjordur Bay.

Wind blows ash from E Asarfjord to Skagafjordur. Later ash is predicted to be blown to N and then to S. Next 24 hours ash can be distributed all around the country if the eruption continues.

At Bardardalur N Iceland, very small tephra was falling at 12:00 and no marked increase at 15:00.

Warning been issued to farmers that fluor can be hazardous to sheep and cattle, in areas where ash is falling.

At 15:00 hrs. the activity seems to have increased in the last hours. This is evident as at At Bardarbunga more earthquakes activity has been for the last hours. It is possible that some eruption might start there, but this is to early to forecast. Bardarbuna is the a high area in W Vatnajokull reaching 2000 m altitude. In early afternoon there is a cloud cover over the eruption site so direct observation is not possible, interpretation is mainly from seismic sources. Ashfall is very small or none yet in inhabited

areas.

Date: Thurs, 3 Oct 1996

The Skeidararsandur area will be closed to night 20-11 because of possible flooding. Activity started in Bardarbunga in Sunday then moved to the area N of Grimsvotn caldera where an eruption started on Monday. To day activity has moved back to Bardarbunga central and large earthquakes up to M4, have occurred in the afternoon.

Bardarbunga and Grimsvotn are two central volcanoes under Vatnajokull, Grimsvotn has been the more active one (last 1983), but eruptions have also happened in Bardarbunga in historical times. Now it seems that activity is in both these centrals which is unusual, there is even thought that an eruption might be already started in Bardarbunga. As there is a dense cloud cover over the eruption sites it is not known what is really happening. The eruption continues N of Grimsvotn caldera. If an eruption starts

in Bardarbunga the meltwater will more likely go towards north.

Eruption continues and the vapour/ash column now extends to 30.000 feet or 10 km altitude and has been seen from aircraft on international routes. It is seen from Hofn village in SE Iceland being white in colour and as it seems little ash. It is now one ash column, tilting to N. Little ashfall is reported from N Iceland. Observations in late afternoon yesterday indicated that the eruption had moved to a new collapse in the glacier 1-1.5 km from where it started. The fissure is believed to be 6-8 km long. The direction seems to be north, but this is not clear yet.

It seems that water flows to south under the glacier, no indications are of meltwater in rivers to the north.

Activity started in Bardarbunga in Sunday then moved to the area N of Grimsvotn caldera where an eruption started on Monday. Wednesday afternoon the activity has moved back to Bardarbunga central and large earthquakes up to M4, have occurred in the afternoon. Seismic activity was less last night and the volcanic tremor less then yesterday regarded due to easier passage for the magma to the surface. It is hoped that earthquakes in Bardarbunga proper do not indicate beginning of an eruption more to the west. At the moment the eruption site is between Bardarbunga and Grimsvotn and hopefully it stays there. Apart from flooding/surging to the south to the Skeidararsandur outwash plain no constructions are in danger and no inhabited places. It is therefor an ideal situation for an eruption, though we geologist would have liked it to be more accessible.

Jokulhlaup (surge) from Grimsvotn caldera has not started, it is feared it might be quite large. At the moment the road management is enforcing flood-levees and preparing to cut through the roads to make easier passages for the water, thus saving the bridges in the flood plain.

Yesterday was cloudy with heavy rain. It cleared up in late afternoon. Daylight is now 08-18. Today is clear weather and good view should be of the eruption from the air. As the glacier has fractured a lot around the eruption site all traffic is banned in the western half of Vatnajokull glacier.

Date: Thur & Fri, 3 & 4 Oct 1996

According to measurements the waterlevel in Grimsvotn caldera has reached the same level as was highest before the surge (jokulhlaup) last summer. As the glacier is fractured and uneven after the great inflow of water to the caldera several m uncertainty exists on when the surge will start, but it is not long time left to wait. It is expected it takes 2-3 days to reach maximum. The water has to lift the Skeidararjokull glacier to make an outlet for the meltwater. Seismometer on Grimsfjall Mt. and it is hoped it will show the beginning of the flood. Direct observation from the air (Magnus Tumi) indicates that the northern end of the eruption fissure is more N-striking than the first subsidences and eruptions in the beginning. It is believed to be related to the last day's earthquakes in Bardarbunga further west. There is still possibility that the eruption moves over the watershed and meltwater could flow north, still water goes to Grimsvotn.

There is not much ash coming from the eruption but has been falling on the glacier and to north. Usually not much ash has come from eruptions in Vatnajokull. Now the ash is distributed to W and SW. There is good and clear weather over the eruption today, the observations are mostly from the air.

Friday 4th October 1996 10:00

The eruption continues. The eruption fracture is now about 8 km long, not much ash is ejected, but flight is not encouraged over Iceland. The Grimsvotn caldera is now full of meltwater but the water has not started to flow under Skeidararjokull to the sandur plain to the south.

The waterlevel in Grimsvotn intra-glacier lake, is higher than before in the century, or since measurements started. As some volcanic material has been gathering in the Grimsvotn caldera since the flood/surge in 1938 it is not expected that the surge will be as great as then. It has been estimated that the flow in 1938 was around 50.000 m3/sec but now it is hoped it will not be more than 10-20.000 m3/sec - these are speculations of course. The bridges S of the glacier are built to withstand 9000 m3/sec. The surge last summer was 3000 m3/sek.

The site of the volcanic fissure is according to the Weather bureau at N 64 30-32' W 17 22'. The Tomso satellite station has been thinking of putting images on the net http://www.tss.no, I have not seen this myself. In morning: N strong wind in Vatnajokull and surroundings At 8:45 some changes in the eruption fissure, large columns rose many km up in a short time, no changes in Skeidara. It is common that the eruption site changes along the fissure, lastly finding a nice spot to be concentrated on.

Date: Fri, 4 Oct 1996, 16:00

This morning it was thought that the eruption was only subglacial but in the afternoon it was observed from the air that it is still going on much on the same scale as yesterday. Eruption continues 6-8 km, no changes from last message. In the morning earthquake activity increased for a while, and supported that activity was still going strong.

No indications are yet that the meltwater has started its travel under the glacier. Beginning of surge will be indicated with sulphurous smell of the Skeidara river.

Weather forecast is:

Weekend W and SW strong wind, cloudy changing sun and overcast.

Monday rainy. Tuesday N and bright.

Date: Mon, 7 Oct 1996 09:00

The eruption is still going strong. Ashfall has not been a problem, mainly it has been falling around the vent. The eruption seems to have concentrated in one vent before the weekend, as is common in fissure eruptions. The site of the volcanic fissure is according to the Weather bureau at N 64 30-32' W 17 22'. The eruption column has been observed this morning. Earthquakes have been less than last week, the activity is now mainly magma flowing up through fractures in the crust.

The jokulhlaup/surge in Skeidararjokull has not started, the water level is 20-25 m higher than last summer when the surge was 3000 m3/sec in the peak. It is still estimated that the coming jokulhlaup will be in the order of 10-15,000 m3/sec. The experts in these surges are astonished that the water has not yet started to flow under the glacier and have stopped forecasting when it will start, - nature is, as so often before, rather behind in solving complicated hydrologic equations!

Bad weather is now in Iceland, roof blowing of houses in N Iceland and roof plates being lifting off here and there. The worst weather is now in N Iceland. Strong wind is in S Iceland but not a lot of rain. It is expected that weather will be rather on the bad side this week, but as meteorologists are better telling us how the weather is than how it will be, we remain optimistic.

Nordisk Vulkanologisk Institut.

Ongoing subglacial eruption in Vatnajökull, Iceland

On September 29, 1996 at 10:48 an earthquake of magnitude 5 on the Richter scale was detected within the Vatnajökull icecap in SE Iceland. This event was followed by an intense earthquake swarm with a large number of small events with intermittent larger quakes of magnitude 3-4 on the Richter scale. The activity continued at similar level until shortly before midnight on September 30th when volcanic tremor was recorded and earthquake shocks became less frequent. In the afternoon of September 29 a warning was issued to national and international aviation authorities that an explosive (phreatic) eruption might be expected within the near future and an ash cloud might interfere with the dense air traffic across the Atlantic. In the morning of October 1st an over flight discovered a subsidence bowl in the glacier surface at a location where an eruption had occurred in 1938. More or less continuous over flights during the day observed continuous increase in the size and depth of the initial subsidence bowl as three additional bowls formed on a line N30E indicating intensive melting at the base of the glacier along a fissure 5-6 km long.

Simultaneously the ice cover on the 10 km diameter Grimsvötn caldera some 15 km to the south of the active subglacial fissure started to rise indicating that melt water from the eruption was flowing into the caldera depression and lifting its ice cover.

On Oct 2nd, in the early morning, an over flight observed that the eruption had broken through the ice. Rhythmic explosions resulted in black ash clouds rising to a height of 500 meters while the buoyant eruption column rose to 3000 meters before being deflected by a southerly wind at a velocity of 30-40 knots. Visibility and flying conditions were extremely poor and allowed observation for only a few minutes.

The Vatnajökull glacier in Europe is a temperate glacier covering about 8300 km2 in the SE part of Iceland. Volcanic fissure systems of the Mid- Atlantic Ridge plate boundary are partly covered by the western part of the ice sheet. Two major volcanic centres lie beneath the ice, the Bardarbunga volcanic centre and the Grimsvötn volcanic centre both with large subglacial caldera depressions. The Bardarbunga centre is a part of a fissure system extending over 100 km to the south and some 50 km to the north of the glacier. The last eruption within the Bardarbunga centre occurred in 1910, but eruptions on the fissure system have occurred in 871 AD, 1477 AD and 1862 AD, all producing substantial amounts of lava.

The Grimsvötn centre is the more active of the two with an eruption frequency during past centuries close to one eruption per decade. The last eruption occurred in 1983. As Bardarbunga the Grimsvötn centre is a part of a fissure system which includes the Laki fissure, which in 1783 produced about 12-14 km3 of basaltic lava. Within the ice filled Grimsvötn caldera intense geothermal activity continuously melts the ice to form a subglacial lake, which at intervals of 5 to 10 years is emptied along subglacial channels to create large floods (jökulhlaup) on the sandur plain, Skeidararsandur, on the Icelandic south coast. The lake was last emptied in 1996 and the water level is presently low.

The present eruption fissure is located between these two volcanic centres with a direction parallel to the regional tectonic lineament. The subglacial topography directs meltwater from the erupting fissure toward the Grimsvötn caldera which is rapidly filling. By the evening of Oct. 1st the ice cover above the subglacial lake had risen 10-15 meters. A rise of the water level by additional 35 meters will trigger a flood.

The river Skeiðará, originating in the subglacial lake of the Grímvötn caldera, has started to increase. At present the frost in Iceland is between -2 to -20 thus glacial rivers should be at minimum. This morning, however, scientist observing that the river Skeiðará was increasing its discharge. The rivers smells strongly of sulphur and the water is loaded with sediments. It is expected that the river increases its discharge rate for the next couple of days, until it reaches its maximum of some 20.000 to 30.000 m3/s.

Jokulhlaup started on the morning of November the 5th. quite abruptly. The growth was fast and many icebergs were floating in the flood water from the glacier reaching all the way to the sea. Grimsvotn tremor started at 21:30 on Monday 4th Nov, the jokulhlaup came out of the glacier 11 hrs later. Distance is about 50 km from Grimsvotn to the Skeidararsandur. The discharge growth was more sudden than was ever expected to be possible. While all was quiet at 0800 the flood had increase its discharge 80-100 times in less than 2 hours.

Núpsvotn flooding starts around 1720 on 5th of Nov.

Gigja, bridge fell at 13:00

Saeluhusakvisl lot of water flowed over the bridge

Skeidara strong current flow, bridge damaged

The estimated peak-flow was around 45.000 m3/s, that is about 10.000 to 15.000 m3/s higher than in the jokulhlaups of 1934 and 1938. Flood peak was reached at about 2300 hrs. The flood has been decreasing rapidly since 2300 hrs on 5th of Nov. The flood channel from Grimsvotn caldera can be clearly seen. It forms a depression like structure on the surface of the glacier dotted with several holes. It is estimated that the Grimsvotn lake has been totally emptied, since the glacier seal has been destroyed due to melting of the water. This has never occurred in Grimsvotn before. Such high melting can only be associated with lake water temperature greater or equal to 10 deg. Celsius.

It is now clear that the total damage are about 10-15 milj. US$, these are the main constructions that were hit:

1) Skeidara bridge not destroyed but badly damaged.

2) Bridge over Gigja has disappeared.

3) Saeluhusakvisl bridge standing but damaged. It was completely submerged during the flood.

4) Roads were damaged and at least 10 km totally washed away.

5) Electric lines and telephone cables damaged, partly washed away

A small eruption started in Bardarbunga around 1300 hrs on 6th of Nov. The eruption lasted for about 20 to 30 min. According to seismograms at the Meteorological office, the eruption was initiated by some intrusive activity. The intrusive activity is based on recorded eruption tremor picked up by on of the seismometers. Eruption column reached about 4 km in to the air. Relation between pressure decrease due to the flooding have been suggested as the main cause of the eruption.