A chronological account of the ongoing eruption under the Vatnajökull ice cap.

SEISMOLOGY: Páll Einarsson, Bryndís Brandsdóttir
GLACIOLOGY: Magnus T. Guðmundsson, Helgi Björnsson

The Vatnajökull Eruption

An eruption started beneath the Vatnajökull glacier in Central Iceland in the late evening of September 30. The eruption was preceeded by an unusual sequence of earthquakes, beginning on September 29 at 10:48 with a magnitude 5.4 (MS) event at the northern rim of the Bárðarbunga caldera. Similar earthquakes have occurred beneath the Bárðarbunga volcano many times during the last 22 years. None of the previous large earthquakes had significant aftershocks, or were followed by magmatic activity such as this last earthquake.

Numerous earthquakes, including 5 with magnitude over 3, were recorded in two hours following the M 5.4 earthquake by the two analog seismic stations situated in Vonarskarð, just northwest of Bárðarbunga, and Grímsfjall, at the southern rim of the Grímsvötn caldera. Shortly after 1300 hours Science Institute seismologists informed the Civil Defense authorities as well as the scientific community about this unusual seismic activity and the possibility of an impending eruptive activity.

The seismic swarm continued throughout Sunday (Sept. 29) and Monday (Sept. 30), with increasing intensity. Hundreds of earthquakes were recorded each day including over 10 events larger than magnitude 3. The earthquakes were located in the northern part of Bárðarbunga and southwards, towards Grímsvötn. They were accompanied by high frequency (>3Hz) continuous tremor of the same type as was frequently observed during intrusive activity within the Krafla volcanic system 1975 -1984.

Following a meeting of the Science Advisory Board of the Civil Defense Council a public warning of a possible eruption in NW-Vatnajökull was issued on September 30 at 19 h. Later that evening the earthquake activity near Grímsvötn decreased markedly, while that of Bárðarbunga continued. At about 22 h the seismograph at Grímsvötn began recording continuous eruption tremor with small amplitude. The sudden decrease of the earthquake activity and the beginning of the eruption tremor may be taken as evidence that an eruption broke out between 22 and 23 h on September 30. The tremor amplitude increased very slowly during the next hours and reached a maximum at about 6 h in the morning of October 1.

The eruption site was discovered early Tuesday morning (Oct. 1) from an aircraft. By that time three elongate, 1-2 km wide and N23E trending subsidence cauldrons had formed on the ice surface SSE of Bárðarbunga, on the northern flank of the neigbouring Grímsvötn volcano. The cauldron formation indicated that the glacier was 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 was located within the water divide of the Grímsvötn central volcano at approximately 64o 30'N and 17o 22'W so the meltwater from the eruption site drained into the Grímsvötn caldera, raising the ice shelf on the Grímsvötn caldera lake. The cauldrons widened and deepened during the day, and it is estimated that 0.3 km3 of water were added to the Grímsvötn lake in less than 24 h.

On Wednesday (Oct. 2) morning one of the active craters had melted its way through the glacier and a massive steam column rose from the cauldron up to an elevation of 30.000 feet.

1 October

Eruption entirely subglacial. The erupting fissure is 4-5 km long NNE. Two main ice cauldrons have formed above the fissure. Eruption most powerful under northernmost cauldron as it subided some 50 m over a 4 hour period. A shallow linear subsidence structure extends from the eruption site to the subglacial Grimsvotn lake, the surface manifestation of the subglacial pathway for water drained into Grimsvotn. The level of Grimsvotn has risen 10-15 m. Lake level 1410 m.

2 October

Eruption broke through the ice surface at 447 GMT at the northern end of the fissure. At 0800 hours vigorous explosive activity was observed in the crater with the eruption column rising 4-5 km above sea level. In the afternoon the cater in the ice was several hundred meters wide. The eruptive fissure extended some 3 km further to the north as witnessed by the formation af a new elongated ice cauldron trending north. Ash dispersed towards north.

3 October

Opening in glacier at subaerial eruption site growing larger. Water level observed about 200 m below original ice surface. Ice cauldron over northernmost part of eruptive fissure has grown since yesterday. Area of glacier where subsidence has occurred 9 km long and 2-3 km wide. Subaerial eruption pulsating with quiet periods alternating with explosive acitivity. Ash dispershed mainly towards N but also towards SSW. Lake level in Grimsvotn 1460 m. The eruption is now (9th October) taking place on a 9 km long fissure trending NNA to SSW and volcanic products pile up above the fissure forming a mountain ridge which in places is expected to be 200 m high. The total amount of volcanic products was estimated as 0.2-0.3 km3 on Oct. 9, i.e. a daily lava flux of 0.2-0.3 km3 in nine days (To be verified). and only a minor part of this has been spread into the atmosphere. About half of the area of Vatnajokull has been covered by a thin layer of ash.

About the Eruption Mechanism

Iceland is a hot spot, but contains in addition an actively spreading ridge system, in which crust is being formed at the rate of 2 cm/year. Volcanism in Iceland is concentrated on central volcanoes and their associated fissure swarms. Each central volcano has episodes of unrest separated by longer periods of relative quiescence. The general style of magmatism during unrest episodes is characterized by the accumulation of magma into small crustal magma chambers, followed by episodic lateral migration of this magma away from the magma chamber into dikes along the fissure swarm, as observed in Krafla 1975 - 1984.

The Bárðarbunga and Grímsvötn volcanoes are located in the central area of the Iceland hot spot, right above the center of the causative mantle plume. They are among the largest and most productive volcanoes in Iceland. The seismic activity at the beginning of the present activity strongly suggest that the eruption is triggered and most likely fed by an intrusion from a magma chamber underneath the Bárðarbunga volcano. The meltwater from the eruption flows into the caldera lake of the neighbooring volcano Grímsvötn, which is filling to the level where the ice dam will be lifted.

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

July 1995:
A glacier flood from a subglacial geothermal area on Loki Ridge (Eastern Cauldron) NW of Grimsvötn. 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. The swarm lasted a week.
 
August 1996:
A glacier flood from a subglacial geothermal area on Loki Ridge (Western Cauldron) NW of Grímsvötn. 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 at Bárðarbunga, intense earthquake swarm and an eruption north of Grímsvötn.

How the Eruption Affects the Glacier

The Bárðarbunga subglacial caldera has been mapped using radio-echo sounding techniques [Björnsson and Einarsson, 1991]. The glacier rises to an elevation of 1500-2000~m covering a 80~km$^2$ and 700~m deep caldera.

The eruption melts the ice above the fissure, and a depression is created in the glacier surface, up to 2-300 m deep and 2 km wide. This depression was during the first 30 hours of the eruption the only visible sign of the volcanic activity, but then the eruption made its way through the ice cover and volcanic ash has since then been ejected during explosive activity through the water into the atmosphere.

Production of meltwater.

During the first week, the eruption melted ice at the rate of 5000 m3/s but this production of meltwater is now expected to have slowed down. Meltwater drains from the eruption site down to a caldera lake (called Grimsvotn) where it accumulates and causes the lake level to rise. The lake is located beneath a 250 m thick ice cover and held in place by an ice dam. Altogether about 2 km3 of meltwater have now drained into the lake and the lake level has now risen higher than ever observed and we expect the lake water soon to start to drain out of the lake benath the ice dam, flow beneath the ice cap 50 km down to a sandur plain south of Vatnajokull and cause catastrophic flooding damaging the road, the bridges and powerlines. The Road Authority and the Civil Defense are preparing actions there to minimize the damage to the road and to protect people in the flood plain.

VOLCANIC HAZARD FROM BÁRÐARBUNGA

The most recent known eruptions in the Bárðarbunga system occurred in 1766, 1769, and 1862-1864 [Björnsson and Einarsson, 1991]. Volcanic activity in Bárðarbunga could certainly cause catastrophic jökulhlaups (glacier water-bursts), with high potential energy and great erosional power. Prehistoric, catastrophic jökulhlaups (about 7,100 B.P., 4,600 B.P., 3,000 B.P., and before 2,000 B.P.) thought to be responsible for deep glacier-river canyons in northern and northeastern Iceland may have issued from the Bárðarbunga caldera. Meltwater from volcanic activity in the Bárðarbunga system can cause large floods in rivers flowing in all directions, depending on the eruption site [Björnsson and Einarsson, 1991].

REFERENCES

1. VATNAJÖKULL VOLCANOES

Björnsson, H. Hydrology of ice caps in volcanic regions. Vísindafélag Ísl. (Societas Scientarum Islandica, Univ. of Iceland) 45, 139pp and 21 map, 1988.

Björnsson, H. Jökulhlaups in Iceland: prediction, characteristics and simulation. Annals of Glaciology 16, 95-106, 1992.

Björnsson, H., S. Björnsson and Th. Sigurgeirsson. Penetration of water into hot rock boundaries of magma at Grímsvötn. Nature, 195, 580-581, 1982.

Björnsson, H., and H. Kristmannsdóttir. The Grímsvötn geothermal area, Vatnajökull, Iceland. Jökull, 34, 25-50, 1984.

Björnsson, H., and P. Einarsson. Volcanoes beneath Vatnajökull, Iceland: Evidence from radio-echo sounding, earthquakes and jökulhlaups. Jökull, 40, 147-168, 1991.

Björnsson, H., and M.T. Gudmundsson. Variations in the thermal output of the subglacial Grímsvötn caldera, Iceland. Geophys. Res. Lett., 20, 2127-2130, 1993.

P. Einarsson and B. Brandsdóttir. Seismic Activity preceding and during the 1983 volcanic eruption in Grímsvötn, Iceland. Jökull, 34, 13-23, 1984.

Grönvold, K., and H. Jóhannesson. Eruption in Grímsvötn 1983; course of events and chemical studies of the tephra. Jökull, 34, 1-11, 1984.

Gudmundsson, M.T. The Grímsvötn Caldera, Vatnajökull: Subglacial topography and structure of caldera infill. Jökull, 39, 1-20, 1989.

Gudmundsson, M.T., H. Björnsson, and F. Pálsson. Changes in jökulhlaup sizes in Grímsvötn, Vatnajökull, Iceland, 1934-91, deduced from in-situ measurements of subglacial lake volume. J. of Glaciology, 41, 263-272, 1995.

2. OTHER ICELANDIC VOLCANOES