Eos, Vol. 78, No. 35, September 2, 1997

Center of the Iceland Hotspot Experiences Volcanic Unrest PAGES 369, 374-375

Pall Einarsson, Bryndis Brandsdottir, Magnus Tumi Gudmundsson, Helgi Bjornsson, Karl Gronvold and Freysteinn Sigmundsson

A volcanic eruption beneath the Vatna­jokull ice cap in central Iceland (Figure 1) be­gan on September 30,1996, along a 7-km-long fissure between the volcanoes Bar-darbunga and Grimsvotn. The eruption con­tinued for 13 days and produced -0.5 km 3 of basaltic andesite. Meltwater from the erup­tion site flowed into the caldera lake of the Grimsvotn volcano, where it accumulated be­neath a floating ice shelf. The lake's ice dam was lifted off the glacier bed on November 4, and in the next two days more than 3 km 3 of water drained out beneath the glacier and flushed down to the south coast's alluvial plain, causing extensive flooding and dam­age to transportation and communication sys­tems.

The major eruption occurred in the most productive area of Iceland's hotspot. It was preceded by several years of unrest, includ­ing both earthquakes and small eruptions, which may signal the onset of a period of high volcanic activity in the area. The Ice­land hotspot is centered on Central Iceland, where it overlaps with the Eastern Volcanic Zone (EVZ, Figure 1), which represents the mid-Atlantic plate boundary. The plates are separating at ~2 cm per year.

This area is characterized by large volca­noes, Bardarbunga, Grimsvotn, Hamarinn, Kverkfjoll, and Tungnafellsjokull [Bjornsson and Einarsson, 1990], and a large part of it is covered by the Vatnajokull ice cap (Figure 1). Mapping of the subglacial topography by radio echo sounding has revealed large cal-deras in the Bardarbunga, Grimsvotn, and Kverkfjoll volcanoes [Bjornsson, 1988]. A hint of a yet larger circular structure can also be seen in the subglacial topography. It ex­tends from the southern flank of Bardar­bunga and encloses Grimsvotn. Hamarinn is situated on its western rim (Figure 2).

The Bardarbunga and Grimsvotn volcanic systems are among the most productive sys­tems in Iceland and have fed some of its larg­est fissure eruptions; for example, the Laki eruption in 1783. This eruption produced 12-14 km 3 of basalt and was the largest lava erup­tion ever witnessed by man. It had a pronounced effect on the climate, particu-

Pall Einarsson, Bryndis Brandsdottir, Magnus Tumi Gudmundsson, and Helgi Bjornsson, Sci­e n c e Institute, University of Iceland, Dunhaga 3, 107 Reykjavik, Iceland; and Karl Grinvold, Freysteinn Sigmundsson Nordic Volcanological Institute, University of Iceland, Grensasvegi 50, 108 Reykjavik, Iceland

larly in Iceland, where almost one-fifth of the population perished in the resulting famine, but also in Europe and North America. The 1996 eruption offers a rare opportunity to study an eruption beneath an ice sheet, but such eruptions were common during the Pleistocene; that is, prior to 11,000 years B.P.

Premonitory Activity and Warning

The eruption was preceded by an unusual sequence of earthquakes, beginning on Sep­tember 29 at 1048 LT with a magnitude 5.4 (Ms) event at the northern rim of the Bardar­bunga caldera. Many similar earthquakes have occurred beneath the Bardarbunga vol­cano during the last 22 years, but none had significant aftershocks, nor were they fol­lowed by magmatic activity. This time the earthquake was followed by an intense earth­quake swarm, including five events larger than 3 in magnitude within two hours of the main quake.

Scientists at the University of Iceland noti­fied the Civil Defense authorities as well as

the scientific community of this unusual seis­mic activity and the possibility of eruptive ac­tivity. The seismic swarm continued throughout September 29 and 30, with in­creasing intensity. Hundreds of earthquakes were recorded each day, including more than 10 events larger than 3 in magnitude. The earthquakes were initially located below the northwestern rim of the Bardarbunga cal­dera (Figure 2) and then, over 24 hours, mi­grated 20 km southward toward Grimsvotn. The earthquakes were accompanied by high-frequency (3 Hz) continuous tremor, which indicated that magma from a magma cham­ber in the Bardarbunga region was being in­jected to the south, feeding a dyke.

Following a meeting of the Science Advi­sory Board of the Civil Defense Council, a public warning of a possible eruption in northwest Vatnajokull was issued on Septem­ber 30. In the evening the earthquake activity near Grimsvotn decreased markedly, while activity at Bardarbunga continued. The seis­mograph at Grimsvotn began recording con­tinuous, low-amplitude eruption tremor. The sudden decrease of the earthquake activity and the onset of the eruption tremor may be taken as evidence that the predicted erup­tion had begun.

The Eruption

The tremor amplitude increased slowly and reached a maximum on the morning of October 1. The eruption site was discovered early that morning by an observer in an air-

Fig. 1. Tectonic map of Iceland show­ing fissure swarms along the plate boundaries and vol­canoes and calderas in the Eastern Vol­canic Zone (EVZ). Letters mark the vol­canoes Krafla (K), Tungnafellsjokull (T), Bardarbunga (B), Kverkfjoll (Kv), Hamarinn (H), Grimsvotn (G), and the Skeidardrsandur alluvial plain (S). The central area of the Iceland hotspot is defined by the volca­noes Bardarbunga, Grimsvotn, Kverk­fjoll, and Tung­nafellsjokull. Glaciers are shaded in gray. The box shows the area de­picted in Figure 2 .

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Eos, Vol. 78, No. 35, Spetember 2, 1997

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Fig. 2. Map of the northwest Vatnajokull area, showing bedrock topography mapped by radio echo sounding, epicenters of earthquakes September 29-30 (dots), and the eruption fissure of 1996 (thick bent line near center). The calderas of the Bardarbunga and Grimsvotn volcanoes are clearly seen in the topography, as well as a hint of a larger circular structure (dashed line). The seismic stations of Vonarskard and Grfmsfjall, shown with stars, provided invaluable data on the seismicity accompanying the eruption and the ensuing flood. The thin wavy line in the northwest corner is the glacier margin.

craft. By that time two elongate 1-2-km-wide, northeast trending subsidence cauldrons had formed on the ice surface south-south­east of Bardarbunga (Figure 3a) on the north­ern flank of the neighboring Grimsvotn volcano. The cauldron formation showed that the glacier was being melted by an erup­tion on a 4-km-long fissure at the base of the glacier, which was 400-600 m thick. The northern cauldron deepened some 50 m in 4 hours. The fissure was located within the drainage basin of the Grimsvotn caldera caus­ing the meltwater from the eruption to drain into the caldera lake. A shallow linear subsi­dence structure was visible at the glacier sur­face, marking the subglacial pathway of the meltwater draining into the Grimsvotn cal­dera.

The cauldrons widened and deepened during the day, and the level of the Grimsvotn lake rose by 10-15 m. About 0.3 km 3 of water was added to the lake during the first 15 hours of the eruption. The vigor of the eruption could be monitored in three different ways; that is, by the volume of the depressions in the ice caused by the melting, by the volume of meltwater accumulating in the Grimsvotn caldera, and by the intensity of the volcanic tremor.

The eruption was most powerful during the first 4 days. Most of the activity was hid­den below the glacier, but in the morning of October 2 an opening formed in the glacier surface, through which an eruptive column rose to 4-5 km altitude. Later that day the eruptive fissure extended some 3 km farther to the north. Ash dispersed to the north and colored the glacier surface (Figure 3b). The opening in the glacier grew larger in the fol­lowing days and the subsidence area grew to 9 km long and 3-4 km wide. An ice canyon melted along the central axis of the depres­sion (Figure 3c). Water flowed southward along the canyon toward the Grimsvotn cal­dera. The volcanic tremor stopped on Octo­ber 13, indicating that magma transport to the eruption site had ceased.

A schematic section of the subglacial eruption and the path of the meltwater is shown in Figure 4. The length of the main eruptive fissure was 7 km, but in addition a minor subglacial eruption occurred on the southeast rim of the Bardarbunga caldera, 6 -7 km to the north. Two small depressions formed in the ice surface there.

Vatnajokull Ice Cap and Floods From the Grimsvotn Caldera Lake

Vatnajokull, the largest ice cap in the world outside the Arctic and Antarctic, is a temperate glacier, which means that all but a 15-m-thick surface layer is at the melting point during the winter. The ice and bedrock topography of Vatnajokull were mapped by radio echo sounding to trace the path of melt­water at the base of the ice [Bjornsson, 1988].

The ice thickness reaches 800-900 m in places, but the average thickness is 400 m.

The glacier, which has a surface area of 8200 km , covers some of Iceland's highest and most active volcanoes. These volcanoes erupted frequently throughout historic time, but few direct observations have been made.

The Grimsvotn caldera has an active geo-thermal system, which melts ice at the rate of 0.2-0.5 km3/yr during normal times [Bjorns­son and Gudmundsson, 1993; Gudmundsson et al., 1995]. The water is contained by an ice dam that closes an outlet in the eastern cal­dera wall, so the 250-m-thick ice shelf that floats on the caldera lake rises -10-15 m per year. Eventually, the water penetrates the ice dam, drains through a tunnel in the ice that expands as it melts and flows beneath the ice toward the coast in a catastrophic flood [Bjornsson, 1988,1992]. No observations are available to put constraints on the timescale of this penetration process.

The resulting floods, called jokulhlaups in the geological literature, normally last 2-3 weeks. Jokulhlaups from Grimsvotn have oc­curred every 4-6 years during the last five decades; each releases 1-2 km 3 of water from the caldera lake. In earlier decades the floods were larger and sometimes followed by eruptions in the Grimsvotn caldera, which were presumably triggered by the pressure re­

lease when the water load was removed from the caldera floor. The latest event of this type occurred in 1934.

Jokulhlaups can also occur as the result of eruptions. An eruption in 1938 on the northern flank of Grimsvotn formed a sub­glacial ridge, which coincides with the south­ern part of the 1996 eruptive fissure [Bjornsson, 1988; Gudmundsson and Bjorns­son, 1991]. The 1938 eruption was almost en­tirely subglacial, breaching the glacier surface shortly at the end of the eruption. The meltwater drained into the Grimsvotn cal­dera, causing a large flood of -4.7 km3, [Bjornsson, 1992, Gudmundsson etal, 1995]. The peak discharge of some 30,000 m3/s was reached in about 3 days, and the flood re­ceded slowly over the next 2 weeks. The rate of meltwater flow rose 15 - 20 m per day. The usual lake level that triggers a jokulhlaup was reached in 4 days, but the uplift contin­ued beyond this critical level. The rate of up­lift decreased, mainly for two reasons: melting slowed as the vigor of the eruption di­minished and the surface area of the lake in­creased. The rate of uplift decreased to 1 m per day when the eruption stopped, corre­sponding to an inflow of 400-500 m3/s. By the end of October the lake level was approach­ing the point where ice closing the outlet of the lake would be lifted off the glacier bed. A

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Eos, Vol. 78, No. 35, September 2, 1997

Fig. 3. Photographs of the eruption, a) Subsidence cauldrons form­ing above the subglacial erupting fissure 14 hours after the start of the eruption. The diameter of the cauldrons is ~2 km and the depth is -100 m. b) Aerial view from the north showing the subsidence caul­drons above the eruptive fissure on the third day of the eruption. Most of the 7-km-long fissure is erupting under the ice, but one crater near its center is erupting through the ice. The Grimsvotn caldera is seen in the background, and the Oraefajokull intraplate volcano (-2000 m asl) is in the distance. The glacier surface in the foreground is colored by light ash fall, c) The ice canyon on the 12th day, with crater rim emerging from the meltwater river.

jokulhlaup was anticipated and was ex­pected to release more than 3 km 3 of water over a few days.

The November 1996 Jokulhlaup

On November 4 the sudden appearance of a continuous, high-frequency (> 3 Hz) tremor on the Grimsfjall seismograph indi­cated that the ice barrier was failing. The am­plitude grew steadily for the next several hours as the water front migrated 50 km downstream beneath the glacier. The water emerged at the glacier edge as a flood wave the following morning, 10 1/2 hours after it lifted the barrier at Grimsvotn. The flow rate increased steadily during the day.

The flood began in the easternmost river, Skeidara, and several hours later the rivers farther west on the Skedararsandur alluvial plain were also flooding. In the afternoon the flood had reached all the rivers on Skeidarar-sandur. The maximum discharge rate was the highest ever recorded, -45 000 m3/s on November 5. The flood ended on the morn­ing of November 7. This is the most rapid course of events ever recorded for a jokulhlaup from Grimsvotn. The total volume of water released from the glacier is esti­mated at 3.5 km3.

The flood caused $15 million in damage to the transportation and communication sys­tems of southeast Iceland. A road connection

was cut when one major bridge was washed away and two others were se­

verely damaged. A 10-km segment of the road disappeared, the main high tension power line was severed, and the fiber optics cable of the telephone system broke in the flood.

The Magma and the Eruption Mechanism

Volcanic activity in Iceland is generally confined to large central volcanoes and their associated fissure swarms. Each central vol­cano has episodes of unrest separated by longer periods of relative quiescence. During times of unrest magma accumulates in crus-tal magma chambers, often followed by epi­sodic lateral migration of the magma away from the chamber into dikes along the fissure swarm, as observed at Krafla in the northern rift zone from 1975 to 1984 [Einarsson, 1991].

The ash produced in the current eruption is almost aphyric glass with minute amounts of crystals, plagioclase, augite, olivine, and magnetite. Microprobe analysis of the chemi­cal composition shows the glass is basaltic andesite. The composition varies signifi­cantly; S i 0 2 is in the range 51.8-54.2% and MgO is in the range 2.5-3.7%. Samples that were richest in MgO erupted earliest. The lack of suitable samples from previous sub-glacial eruptions makes it difficult to charac­terize the different subglacial volcanoes. It is clear, however, that the 1996 eruptive prod­

ucts are different from those of Grimsvotn, as defined by the eruptions of 1922,1934, and 1983. They are also different from typical Bar-darbunga products, which generally contain MgO in the range 7.2-8.8%.

Since the 1996 eruption does not have the chemical characteristics of the main magma systems of Grimsvotn or Bardarbunga , we suggest it was fed by a subsidiary system pro­ducing relatively evolved magma. The migra­tion of seismic activity from Bardarbunga to the eruption site and the intrusion tremor at the beginning of the current activity strongly suggest that the eruption was fed by an intru­sion from a magma chamber underneath the Bardarbunga volcano. Most likely this was a small chamber, not the main magma cham­ber of Bardarbunga.

Previous Activity of Bardarbunga

The Bardarbunga volcanic system erupted from 1697 to 1720, in 1766,1769, and from 1862 to 1864 [Bjornsson and Einarsson, 1990]. The large fissure eruptions of Vat-naoldur in 871 and Veidivotn in 1480 that oc­curred up to 100 km southwest of Bardarbunga were fed by the Bardarbunga volcanic system. The Bardarbunga caldera and its surroundings are covered by glacial ice that is 900 m thick in places. Melting of this ice during eruptions could cause cata­strophic floods much larger than those of his­toric times. The caldera is thus a likely source of prehistoric catastrophic jokulhlaups at about 7,100 B.P., 4,600 B.P., 3,000 B.P., and before 2,000 B.P., that cut up to 100 m deep

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Eos, Vol. 78, No. 35, Spetember 2, 1997

S e p t e m b e r 3 0 1 9 9 6

S N

Grimsvotn

0 2 4 6 8 10 12 14 16 18 20 22

O c t o b e r 1 5 1 9 9 6

E

0 2 4 6 8 10 12 14 16 18 20 22

km Fig. 4. Vertical section along the eruptive fissure and through the Grimsvotn caldera, showing conditions at the beginning and end of the eruption. The ridge that formed in 1938 is shown, and a schematic representation shows the new volcanic construct. The meltwater flows at the bottom of the glacier and crosses a ridge on its way to the Grimsvotn caldera lake. It is driven by a gradi­ent in a potential that is mainly determined by the surface topography of the ice. The bedrock to­pography has only a small effect on the flow of meltwater.

glacier-river canyons in northern and north­eastern Iceland [Bjornsson, 1988]. The erup­tion of 1938 on the north flank of Grimsvotn was followed by an unusually long, relatively quiet period of the Vatnajokull volcanoes.

The eruption of September-October 1996 marks the end of the quiet period. On a shorter timescale the eruption was preceded by a series of seismic and magmatic events in the Vatnajokull area that possibly began as early as 1974 and escalated in the year before the eruption. The events began in June 1974 with a large earthquake (mb = 5.1) at Bardar­bunga, the first in a series of such events that still continues. An eruption occurred within the caldera lake of the Grimsvotn volcano in May 1983, which may have been followed by a small subglacial eruption in August 1984 in

Navy to Release Storehouse of Arctic Ocean Data PAGES 369-370

George Newton, Jr. was reading an article last winter about the navigation of Arctic po­lar submarines when he noticed an accompa­nying map that indicated about 193,000 kilometers (120,000 miles) of U.S. military submarine tracks. Newton, the chairman of the U.S. Arctic Research Commission (USARC) and a former second in command

the caidera as well. A jokulhlaup in Novem­ber 1986 from a subglacial geothermal area east of Hamarinn and northwest of Grimsvotn was followed by a distinct tremor episode, presumably a small eruption, trig­gered by the pressure release [Bjornsson and Einarsson, 1990].

Jokulhlaups from the same geothermal area in August 1991 and July 1995 were also followed by presumed eruptions. An intense earthquake swarm occurred in February 1996 near the Hamarinn Volcano. A jokulhlaup from a second subglacial geother­mal area northwest of Grimsvotn in August 1996 was followed by a tremor episode thought to indicate an eruption, and seis-micity increased during the next few weeks. The historical record of high volcanic activity

of a Navy nuclear sub, thought that if the Navy already had declassified these tracks in the 1980s, maybe it would release additional data that could help fill enormous gaps in knowledge about the environment of the Arc­tic Ocean.

USARC held a flurry of discussions with a number of agencies, including the State De­partment and the Navy's submarine service, to convince them that the information repre­sented a significant scientific storehouse that could be released without compromising na­tional security. Newton's realization of what other information that map could reveal cata­lyzed the announcement last week that the Navy soon will release to the public the larg-

in Vatnajokull, the unusually long recent qui­escence of the area, and the escalating trend of the recent events suggest that the central area of the Iceland hotspot has entered a new period of unrest.

The unrest is not limited to one volcanic system. Both Bardarbunga and Grimsvotn are involved, as well as the area between Grimsvotn and Hamarinn. Continued mag­matic activity in these areas in the coming months and years is likely. The activity in this part of the volcanic zones in historical time is characterized by small but frequent erup­tions. Keeping in mind, however, the poten­tial of the Vatnajokull volcanoes to produce catastrophic events of global importance, such as the Laki eruption of 1783 from the Grimsvotn volcanic system, there is every rea­son to worry when they become restless.

Acknowledgments

We thank M. Garcia and S. Self for their constructive criticism of this article. Finnur Palsson gave invaluable help with figures and data.

References

Bjornsson, H., Hydrology of i ce caps in vol­can i c regions, Soc. Sci. Islandica, 45, Reykjavik, 139 pp., 1988.

Bjornsson, H., Jokulhlaups in Iceland: Predic­tion, character is t ics and simulation, Ann. Glaciol, 16, 9 5 - 1 0 6 , 1992.

Bjornsson, H., and P. Einarsson, Volcanoes be­neath Vatnajokull, Iceland: Evidence from ra­dio-echo sounding, earthquakes and jokulhlaups, Jokull, 40, 147-168 , 1990.

Bjornsson, H., and M. T. Gudmundsson, Vari­ations in the thermal output of the sub­glacial Grimsvotn caldera, Iceland, Geophys. Res. Lett., 20, 2 1 2 7 - 2 1 3 0 , 1993.

Einarsson, P., Earthquakes and present-day tec-tonism in Iceland, Tectonophysics, 189, 261— 279, 1991.

Gudmundsson, M. T., and H. Bjornsson, Erup­tions in Grimsvotn, Vatnajokull, Iceland, 1 9 3 4 - 1 9 9 1 , Jokull, 41, 2 1 - 4 5 , 1991.

Gudmundsson, M. T., H. Bjornsson, and F. Palsson, Changes in jokulhlaup sizes in Grimsvotn, Vatnajokull, Iceland, 1934-91, de­duced from in-situ measurements of sub­glacial lake volume, J . Glaciology, 41, 2 6 3 - 2 7 2 , 1995.

est-ever quantity of Arctic Ocean bathymetry data.

This data will help scientists understand the topography and water circulation of the 8,850,000 square kilometer (3.42 million square-mile) Arctic Ocean, which could pro­vide important clues about global warming, the spread of radioactive contaminants, and potential mining of natural resources.

The data, which cover the period of 1957 to 1982, will include water depth, latitude, longitude, and general timeframes of when the information was recorded. The Navy an­ticipates that the labor-intensive process of sanitizing boxloads of paper tape echogram notations could take 6 months. The Navy will

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