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

the scientific community of this unusual seis­ Center of the Iceland Hotspot mic activity and the possibility of eruptive ac­ tivity. The seismic swarm continued throughout September 29 and 30, with in­ Experiences Volcanic Unrest creasing intensity. Hundreds of earthquakes PAGES 369, 374-375 were recorded each day, including more than 10 events larger than 3 in magnitude. The earthquakes were initially located below Pall Einarsson, Bryndis Brandsdottir, Magnus Tumi Gudmundsson, the northwestern rim of the Bardarbunga cal­ Helgi Bjornsson, Karl Gronvold and Freysteinn Sigmundsson dera (Figure 2) and then, over 24 hours, mi­ grated 20 km southward toward Grimsvotn. The earthquakes were accompanied by high- A volcanic eruption beneath the Vatna­ larly in Iceland, where almost one-fifth of the frequency (3 Hz) continuous tremor, which jokull ice cap in central Iceland (Figure 1) be­ population perished in the resulting famine, indicated that magma from a magma cham­ gan on September 30,1996, along a but also in Europe and North America. The ber in the Bardarbunga region was being in­ 7-km-long fissure between the volcanoes Bar- 1996 eruption offers a rare opportunity to jected to the south, feeding a dyke. darbunga and Grimsvotn. The eruption con­ study an eruption beneath an ice sheet, but Following a meeting of the Science Advi­ 3 tinued for 13 days and produced -0.5 km of such eruptions were common during the sory Board of the Civil Defense Council, a basaltic andesite. Meltwater from the erup­ Pleistocene; that is, prior to 11,000 years B.P. public warning of a possible eruption in tion site flowed into the caldera lake of the northwest Vatnajokull was issued on Septem­ Premonitory Activity and Warning Grimsvotn volcano, where it accumulated be­ ber 30. In the evening the earthquake activity neath a floating ice shelf. The lake's ice dam The eruption was preceded by an unusual near Grimsvotn decreased markedly, while was lifted off the glacier bed on November 4, activity at Bardarbunga continued. The seis­ 3 sequence of earthquakes, beginning on Sep­ and in the next two days more than 3 km of tember 29 at 1048 LT with a magnitude 5.4 mograph at Grimsvotn began recording con­ water drained out beneath the glacier and (Ms) event at the northern rim of the Bardar­ tinuous, low-amplitude eruption tremor. The flushed down to the south coast's alluvial bunga caldera. Many similar earthquakes sudden decrease of the earthquake activity plain, causing extensive flooding and dam­ have occurred beneath the Bardarbunga vol­ and the onset of the eruption tremor may be age to transportation and communication sys­ cano during the last 22 years, but none had taken as evidence that the predicted erup­ tems. significant aftershocks, nor were they fol­ tion had begun. The major eruption occurred in the most lowed by magmatic activity. This time the productive area of Iceland's hotspot. It was earthquake was followed by an intense earth­ The Eruption preceded by several years of unrest, includ­ quake swarm, including five events larger ing both earthquakes and small eruptions, than 3 in magnitude within two hours of the The tremor amplitude increased slowly which may signal the onset of a period of main quake. and reached a maximum on the morning of high volcanic activity in the area. The Ice­ Scientists at the University of Iceland noti­ October 1. The eruption site was discovered land hotspot is centered on Central Iceland, fied the Civil Defense authorities as well as early that morning by an observer in an air- 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. Fig. 1. Tectonic This area is characterized by large volca­ map of Iceland show­ noes, Bardarbunga, Grimsvotn, Hamarinn, ing fissure swarms Kverkfjoll, and Tungnafellsjokull [Bjornsson along the plate and Einarsson, 1990], and a large part of it is boundaries and vol­ covered by the Vatnajokull ice cap (Figure canoes and calderas 1). Mapping of the subglacial topography by in the Eastern Vol­ radio echo sounding has revealed large cal- canic Zone (EVZ). deras in the Bardarbunga, Grimsvotn, and Letters mark the vol­ Kverkfjoll volcanoes [Bjornsson, 1988]. A canoes Krafla (K), hint of a yet larger circular structure can also Tungnafellsjokull be seen in the subglacial topography. It ex­ (T), Bardarbunga tends from the southern flank of Bardar­ (B), Kverkfjoll (Kv), bunga and encloses Grimsvotn. Hamarinn is Hamarinn (H), situated on its western rim (Figure 2). Grimsvotn (G), and the Skeidardrsandur The Bardarbunga and Grimsvotn volcanic alluvial plain (S). systems are among the most productive sys­ The central area of tems in Iceland and have fed some of its larg­ the Iceland hotspot is est fissure eruptions; for example, the Laki defined by the volca­ eruption in 1783. This eruption produced 12- 3 noes Bardarbunga, 14 km of basalt and was the largest lava erup­ Grimsvotn, Kverk­ tion ever witnessed by man. It had a fjoll, and Tung­ pronounced effect on the climate, particu- nafellsjokull. Glaciers are shaded Pall Einarsson, Bryndis Brandsdottir, Magnus Tumi Gudmundsson, and Helgi Bjornsson, Sci­ in gray. The box ence Institute, University of Iceland, Dunhaga shows the area de­ 3, 107 Reykjavik, Iceland; and Karl Grinvold, picted in Figure 2. Freysteinn Sigmundsson Nordic Volcanological Institute, University of Iceland, Grensasvegi 50, 108 Reykjavik, Iceland

This page may be freely copied. Eos, Vol. 78, No. 35, Spetember 2, 1997 craft. By that time two elongate 1-2-km-wide, northeast trending subsidence cauldrons had formed on the ice surface south-south­ east of Bardarbunga (Figure 3a) on the north­ ern flank of the neighboring Grimsvotn volcano. The cauldron formation showed that the glacier was being melted by an erup­ tion 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 caus­ ing the meltwater from the eruption to drain into the caldera lake. A shallow linear subsi­ dence structure was visible at the glacier sur­ face, marking the subglacial pathway of the meltwater draining into the Grimsvotn cal­ dera. The cauldrons widened and deepened during the day, and the level of the Grimsvotn lake rose by 10-15 m. About 0.3 km3 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 0 20 km the volcanic tremor. 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 The eruption was most powerful during 1996 (thick bent line near center). The calderas of the Bardarbunga and Grimsvotn volcanoes are the first 4 days. Most of the activity was hid­ clearly seen in the topography, as well as a hint of a larger circular structure (dashed line). The den below the glacier, but in the morning of seismic stations of Vonarskard and Grfmsfjall, shown with stars, provided invaluable data on the October 2 an opening formed in the glacier seismicity accompanying the eruption and the ensuing flood. The thin wavy line in the northwest surface, through which an eruptive column corner is the glacier margin. 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 The ice thickness reaches 800-900 m in lease when the water load was removed from colored the glacier surface (Figure 3b). The places, but the average thickness is 400 m. the caldera floor. The latest event of this type opening in the glacier grew larger in the fol­ The glacier, which has a surface area of occurred in 1934. lowing days and the subsidence area grew to 8200 km , covers some of Iceland's highest Jokulhlaups can also occur as the result 9 km long and 3-4 km wide. An ice canyon and most active volcanoes. These volcanoes of eruptions. An eruption in 1938 on the melted along the central axis of the depres­ erupted frequently throughout historic time, northern flank of Grimsvotn formed a sub­ sion (Figure 3c). Water flowed southward but few direct observations have been made. glacial ridge, which coincides with the south­ along the canyon toward the Grimsvotn cal­ ern part of the 1996 eruptive fissure dera. The volcanic tremor stopped on Octo­ The Grimsvotn caldera has an active geo- [Bjornsson, 1988; Gudmundsson and Bjorns­ ber 13, indicating that magma transport to thermal system, which melts ice at the rate of 3 son, 1991]. The 1938 eruption was almost en­ the eruption site had ceased. 0.2-0.5 km /yr during normal times [Bjorns­ son and Gudmundsson, 1993; Gudmundsson tirely subglacial, breaching the glacier A schematic section of the subglacial et al., 1995]. The water is contained by an ice surface shortly at the end of the eruption. The eruption and the path of the meltwater is dam that closes an outlet in the eastern cal­ meltwater drained into the Grimsvotn cal­ shown in Figure 4. The length of the main 3 dera wall, so the 250-m-thick ice shelf that dera, causing a large flood of -4.7 km , eruptive fissure was 7 km, but in addition a floats on the caldera lake rises -10-15 m per [Bjornsson, 1992, Gudmundsson etal, 1995]. minor subglacial eruption occurred on the 3 year. Eventually, the water penetrates the ice The peak discharge of some 30,000 m /s was southeast rim of the Bardarbunga caldera, 6- dam, drains through a tunnel in the ice that reached in about 3 days, and the flood re­ 7 km to the north. Two small depressions expands as it melts and flows beneath the ice ceded slowly over the next 2 weeks. The formed in the ice surface there. toward the coast in a catastrophic flood rate of meltwater flow rose 15 - 20 m per day. [Bjornsson, 1988,1992]. No observations are The usual lake level that triggers a jokulhlaup Vatnajokull Ice Cap and Floods From available to put constraints on the timescale was reached in 4 days, but the uplift contin­ the Grimsvotn Caldera Lake of this penetration process. ued beyond this critical level. The rate of up­ The resulting floods, called jokulhlaups in lift decreased, mainly for two reasons: Vatnajokull, the largest ice cap in the the geological literature, normally last 2-3 melting slowed as the vigor of the eruption di­ world outside the Arctic and Antarctic, is a weeks. Jokulhlaups from Grimsvotn have oc­ minished and the surface area of the lake in­ temperate glacier, which means that all but a curred every 4-6 years during the last five creased. The rate of uplift decreased to 1 m 15-m-thick surface layer is at the melting decades; each releases 1-2 km3 of water from per day when the eruption stopped, corre­ point during the winter. The ice and bedrock the caldera lake. In earlier decades the sponding to an inflow of 400-500 m3/s. By the topography of Vatnajokull were mapped by floods were larger and sometimes followed end of October the lake level was approach­ radio echo sounding to trace the path of melt­ by eruptions in the Grimsvotn caldera, which ing the point where ice closing the outlet of water at the base of the ice [Bjornsson, 1988]. were presumably triggered by the pressure re­ the lake would be lifted off the glacier bed. A

This page may be freely copied. Eos, Vol. 78, No. 35, September 2, 1997

Fig. 3. Photographs of the eruption, a) Subsidence cauldrons form­ ing 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 caul­ drons 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.

was cut when one ucts are different from those of Grimsvotn, as major bridge was defined by the eruptions of 1922,1934, and washed away and 1983. They are also different from typical Bar- two others were se­ darbunga products, which generally contain jokulhlaup was anticipated and was ex­ verely damaged. A 10-km segment of the MgO in the range 7.2-8.8%. pected to release more than 3 km3 of water road disappeared, the main high tension Since the 1996 eruption does not have the over a few days. power line was severed, and the fiber optics chemical characteristics of the main magma cable of the telephone system broke in the systems of Grimsvotn or Bardarbunga , we flood. suggest it was fed by a subsidiary system pro­ The November 1996 Jokulhlaup ducing relatively evolved magma. The migra­ tion of seismic activity from Bardarbunga to On November 4 the sudden appearance The Magma and the Eruption the eruption site and the intrusion tremor at of a continuous, high-frequency (> 3 Hz) Mechanism tremor on the Grimsfjall seismograph indi­ the beginning of the current activity strongly cated that the ice barrier was failing. The am­ Volcanic activity in Iceland is generally suggest that the eruption was fed by an intru­ plitude grew steadily for the next several confined to large central volcanoes and their sion from a magma chamber underneath the hours as the water front migrated 50 km associated fissure swarms. Each central vol­ Bardarbunga volcano. Most likely this was a downstream beneath the glacier. The water cano has episodes of unrest separated by small chamber, not the main magma cham­ emerged at the glacier edge as a flood wave longer periods of relative quiescence. During ber of Bardarbunga. the following morning, 10 1/2 hours after it times of unrest magma accumulates in crus- lifted the barrier at Grimsvotn. The flow rate tal magma chambers, often followed by epi­ Previous Activity of Bardarbunga increased steadily during the day. sodic lateral migration of the magma away The flood began in the easternmost river, from the chamber into dikes along the fissure The Bardarbunga volcanic system Skeidara, and several hours later the rivers swarm, as observed at Krafla in the northern erupted from 1697 to 1720, in 1766,1769, and farther west on the Skedararsandur alluvial rift zone from 1975 to 1984 [Einarsson, 1991]. from 1862 to 1864 [Bjornsson and Einarsson, plain were also flooding. In the afternoon the The ash produced in the current eruption 1990]. The large fissure eruptions of Vat- flood had reached all the rivers on Skeidarar- is almost aphyric glass with minute amounts naoldur in 871 and Veidivotn in 1480 that oc­ sandur. The maximum discharge rate was of crystals, plagioclase, augite, olivine, and curred up to 100 km southwest of the highest ever recorded, -45 000 m3/s on magnetite. Microprobe analysis of the chemi­ Bardarbunga were fed by the Bardarbunga November 5. The flood ended on the morn­ cal composition shows the glass is basaltic volcanic system. The Bardarbunga caldera ing of November 7. This is the most rapid andesite. The composition varies signifi­ and its surroundings are covered by glacial course of events ever recorded for a cantly; Si02 is in the range 51.8-54.2% and ice that is 900 m thick in places. Melting of jokulhlaup from Grimsvotn. The total volume MgO is in the range 2.5-3.7%. Samples that this ice during eruptions could cause cata­ of water released from the glacier is esti­ were richest in MgO erupted earliest. The strophic floods much larger than those of his­ mated at 3.5 km3. lack of suitable samples from previous sub- toric times. The caldera is thus a likely source The flood caused $15 million in damage glacial eruptions makes it difficult to charac­ of prehistoric catastrophic jokulhlaups at to the transportation and communication sys­ terize the different subglacial volcanoes. It is about 7,100 B.P., 4,600 B.P., 3,000 B.P., and tems of southeast Iceland. A road connection clear, however, that the 1996 eruptive prod­ before 2,000 B.P., that cut up to 100 m deep

This page may be freely copied. Eos, Vol. 78, No. 35, Spetember 2, 1997

September 30 1996 in Vatnajokull, the unusually long recent qui­ S N escence of the area, and the escalating trend Grimsvotn 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 mag­ matic activity in these areas in the coming months and years is likely. The activity in this

0 2 4 6 8 10 12 14 16 18 20 22 part of the volcanic zones in historical time is characterized by small but frequent erup­ October 15 1996 tions. Keeping in mind, however, the poten­ tial 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 rea­ son to worry when they become restless. E

Acknowledgments We thank M. Garcia and S. Self for their constructive criticism of this article. Finnur 0 2 4 6 8 10 12 14 16 18 20 22 km Palsson gave invaluable help with figures Fig. 4. Vertical section along the eruptive fissure and through the Grimsvotn caldera, showing and data. 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 References of the glacier and crosses a ridge on its way to the Grimsvotn caldera lake. It is driven by a gradi­ ent in a potential that is mainly determined by the surface topography of the ice. The bedrock to­ Bjornsson, H., Hydrology of ice caps in vol­ pography has only a small effect on the flow of meltwater. canic regions, Soc. Sci. Islandica, 45, Reykjavik, 139 pp., 1988. Bjornsson, H., Jokulhlaups in Iceland: Predic­ glacier-river canyons in northern and north­ the caidera as well. A jokulhlaup in Novem­ tion, characteristics and simulation, Ann. eastern Iceland [Bjornsson, 1988]. The erup­ ber 1986 from a subglacial geothermal area Glaciol, 16, 95-106, 1992. tion of 1938 on the north flank of Grimsvotn east of Hamarinn and northwest of Bjornsson, H., and P. Einarsson, Volcanoes be­ neath Vatnajokull, Iceland: Evidence from ra­ was followed by an unusually long, relatively Grimsvotn was followed by a distinct tremor dio-echo sounding, earthquakes and quiet period of the Vatnajokull volcanoes. episode, presumably a small eruption, trig­ jokulhlaups, Jokull, 40, 147-168, 1990. The eruption of September-October 1996 gered by the pressure release [Bjornsson and Bjornsson, H., and M. T. Gudmundsson, Vari­ marks the end of the quiet period. On a Einarsson, 1990]. ations in the thermal output of the sub­ glacial Grimsvotn caldera, Iceland, shorter timescale the eruption was preceded Jokulhlaups from the same geothermal Geophys. Res. Lett., 20, 2127-2130, 1993. by a series of seismic and magmatic events in area in August 1991 and July 1995 were also Einarsson, P., Earthquakes and present-day tec- the Vatnajokull area that possibly began as followed by presumed eruptions. An intense tonism in Iceland, Tectonophysics, 189, 261— early as 1974 and escalated in the year before earthquake swarm occurred in February 279, 1991. Gudmundsson, M. T., and H. Bjornsson, Erup­ the eruption. The events began in June 1974 1996 near the Hamarinn Volcano. A tions in Grimsvotn, Vatnajokull, Iceland, with a large earthquake (mb = 5.1) at Bardar­ jokulhlaup from a second subglacial geother­ 1934-1991, Jokull, 41, 21-45, 1991. bunga, the first in a series of such events that mal area northwest of Grimsvotn in August Gudmundsson, M. T., H. Bjornsson, and F. still continues. An eruption occurred within 1996 was followed by a tremor episode Palsson, Changes in jokulhlaup sizes in the caldera lake of the Grimsvotn volcano in thought to indicate an eruption, and seis- Grimsvotn, Vatnajokull, Iceland, 1934-91, de­ duced from in-situ measurements of sub­ May 1983, which may have been followed by micity increased during the next few weeks. glacial lake volume, J. Glaciology, 41, a small subglacial eruption in August 1984 in The historical record of high volcanic activity 263-272, 1995.

Navy to Release of a Navy nuclear sub, thought that if the est-ever quantity of Arctic Ocean bathymetry Navy already had declassified these tracks in data. Storehouse of the 1980s, maybe it would release additional This data will help scientists understand data that could help fill enormous gaps in the topography and water circulation of the Arctic Ocean Data knowledge about the environment of the Arc­ 8,850,000 square kilometer (3.42 million tic Ocean. square-mile) Arctic Ocean, which could pro­ USARC held a flurry of discussions with a PAGES 369-370 vide important clues about global warming, number of agencies, including the State De­ the spread of radioactive contaminants, and George Newton, Jr. was reading an article partment and the Navy's submarine service, potential mining of natural resources. last winter about the navigation of Arctic po­ to convince them that the information repre­ The data, which cover the period of 1957 lar submarines when he noticed an accompa­ sented a significant scientific storehouse that to 1982, will include water depth, latitude, nying map that indicated about 193,000 could be released without compromising na­ longitude, and general timeframes of when kilometers (120,000 miles) of U.S. military tional security. Newton's realization of what the information was recorded. The Navy an­ submarine tracks. Newton, the chairman of other information that map could reveal cata­ ticipates that the labor-intensive process of the U.S. Arctic Research Commission lyzed the announcement last week that the sanitizing boxloads of paper tape echogram (USARC) and a former second in command Navy soon will release to the public the larg- notations could take 6 months. The Navy will

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