Global Volcanism Program Volcanoes of the World Krakatau Volcanic Activity Reports
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The renowned Krakatau volcano lies in the Sunda Strait between Java and Sumatra. Caldera collapse, perhaps in 416 AD, destroyed the ancestral Krakatau edifice, forming a 7-km-wide caldera. Remnants of this volcano formed Verlaten and Lang Islands; subsequently Rakata, Danan and Perbuwatan volcanoes were formed, coalescing to create the pre-1883 Krakatau Island. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan volcanoes, and left only a remnant of Rakata volcano. The post-collapse cone of Anak Krakatau (Child of Krakatau), constructed within the 1883 caldera at a point between the former cones of Danan and Perbuwatan, has been the site of frequent eruptions since 1927.

During six lava-producing eruptions between 1958 and 1980, flows moved S and SW from the SW crater. Observations are frequently made from Carita Beach on the coast of Java, ~40 km E. The local VSI volcano observatory is located at Pasuaran, ~42 km E.

Reference: Simkin, T., and Fiske, R.S., 1983, Krakatau 1883: The Volcanic Eruption and its Effects; Smithsonian Institution Press, Washington, 464 pp.

Index of Volcanic Activity Reports

Reports are organized chronologically and indexed below by Month/Year (Publication Volume:Number), and include a one-line summary. Click on the index link or scroll down to read the reports.

Complete Contents of Volcanic Activity Reports

All information contained in these reports is preliminary and subject to change.

09/72 (CSLP 61-72a) Large eruption plume on 26 June

Card #1449 (26 September 1972) Large eruption plume on 26 June

"Max Sons . . . was a member of a group in Indonesia. Shortly before noon local time on 26 June 1972, the group was returning from Sumatra to Djakarta in a DC-3 aircraft and circled Krakatoa to take pictures. As they were pulling away heading toward Djakarta the volcano came alive, with a classic mushroom cloud quickly forming above the island. In 10 minutes the column rose to perhaps 10,000 feet, and by the next day it could be seen 50 miles away, as the group flew from Djakarta to Singapore."

Information Contact: Max Sons, Standard Oil Company.

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10/72 (CSLP 61-72b) Underwater eruption in April; continued activity through September

Card #1466 (16 October 1972) Underwater eruption seen along S coast on 11 April

[Krakatau] apparently exhibited activity prior to the reported 26 June 1972 eruption.

"11 April 1972 at 0330 GMT the vessel was passing 9 miles S of Anak Krakatau (say 6°18'S, 105°26'E [from Durban towards Djakarta]) when we observed a disturbance on the south side of the island which at first looked like breakers. On closer observation it was seen that an underwater eruption was taking place all along the south side of the coast and gases were also seen to be given off from this disturbance. Smoke was also observed from fissures on the peak of the island. The smoke started at about 100 feet from the summit up to about 400 feet, and a considerable amount of white ash was seen on the southwest side of the island." (Captain Rodger, W. Sinclair, 2nd Officer, C. McConachie, 3rd Officer, Cadets R. Lawrie and L. Macintosh.)

Information Contacts: C.W.A. Browitt, Institute of Geological Sciences, Geophysical Laboratory, Edinburgh, Scotland; Captain J.R. Rodger, S.S. Benvannoch.

Card #1467 (17 October 1972) Review of activity during June-September

"From the second week of June 1972, ash eruptions from the active cinder cone Anak Krakatoa in the center of the island group took place with time intervals of 15 minutes, 30 minutes to one hour. Ash clouds, white to grey colored, reached heights of 50-100 m causing ash showers depositing thin layers of whitish ash in the vicinity, while bigger eruption material fell back into the crater. Weak rumblings accompanied the eruptions, and could be heard from time by people along the coast of west Banten. In the night fire could be observed in the eruption clouds. The activity . . . lasted during the months of June, July, August and September. According to inhabitants of Carita village (west coast of Banten), the activity diminished in September and stopped completely at the end of the month. According to a report by a tourist who visited the volcano on 1 October from 1000 to 1200, no activity was observed. Only fire could be seen in a small crater within the central crater.

"This recent eruption . . . was limited to small ash eruptions accompanied by weak rumblings, which lasted for almost four months. This kind of rather quiet activity is very common for this volcano . . . which came into existence in 1929 and grew bigger by the day: recent height over 170 m. To people living on the coastal areas of western Java and southern Sumatra the activities are more or less routine and do not cause fear at all. As the activity of the volcano is considered harmless (at least for the present time), the Volcanology Division currently does not carry out continuous observations, but makes only incidental investigations on the spot to record remarkable changes of the active cinder cone, topographical changes of the island, changes of chemical composition of eruption products, etc."

Information Contact: Djajadi Hadikusumo, Geological Survey of Indonesia.

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12/72 (CSLP 61-72c) Eruptive activity observed on 21 July

Card #1512 (18 December 1972) Eruptive activity observed on 21 July

The following extract is from the meteorological logbook of MV Dardanus, Captain D.M. Belk, on passage Durban towards Djakarta.

"21 July 1972 between 1345 and 1530 GMT. The vessel was approaching the Sunda Strait and the sky in the northeast had a reddish tinge, varying in intensity. Soon it was observed through binoculars that it was an island throwing hot ashes and lava into the sky. At first the island was thought to be Rakata . . . but it was difficult to ascertain as it was still 25 miles off. When the vessel was 5 miles off Rakata it was observed that the island erupting was . . . a small island about 2.5 miles north of Rakata and about 510 feet high.

"The eruptions lasted for periods of 20-30 seconds at intervals of 2-7 minutes. When see through binoculars, the whole island seemed to be ablaze for 40-50 seconds after each eruption. The estimated height of the ashes and lava being thrown into the air was about 500 feet, estimated by the height of the island itself. When to leeward . . . the smell of burning was strong but not much smoke was encountered. Wind SSE force 2." (J. Dixon, 3rd Officer.)

Information Contacts: C.W.A. Browitt, Institute of Geological Sciences, Geophysical Laboratory, Edinburgh, Scotland; Captain D.M. Belk, MV Dardanus.

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07/78 (SEAN 03:07) Incandescent material ejected

Activity began in mid-July and continued through early August. On 2 August, the volcano ejected a "huge" column of incandescent material, visible from the W Java coast . . . .

Information Contacts: Reuters; D. Shackelford, Villa Park, CA.

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10/78 (SEAN 03:10) Tephra eruption since July from Anak Krakatau

An eruption from a summit crater of Anak Krakatau (figure 1) began on 10 July and was continuing in October. Lightning over the summit was seen from a nearby village on 10 July and small amounts of basaltic ash were ejected. Other explosions occurred on 14, 18, 21-23, and 30 July, and on 3 August. During the largest explosions (on 14 and 22 July) tephra clouds, including some bombs, rose 400-500 m above the crater. Activity was confined to vapor emission 11-13, 15-17, and 26-29 July.

Figure 1. Map of Anak Krakatau, 1978. Courtesy of VSI.

VSI scientists visited the volcano 21-23 July. Tephra emission occurred at intervals of 15 minutes to 6 hours. Ash was always ejected by the explosions but larger tephra was only occasionally present in the eruption clouds.

The eruption had declined by the third week of August but had returned to July levels when the volcano was revisited 2-5 October. Activity was Strombolian, consisting of discrete groups of explosions. Each explosion group lasted an average of 9 minutes, with an observed time interval between first explosions of successive groups ranging from 5 to 27 minutes. The first explosion of each group was always the largest, typically ejecting bombs 75-100 m above the crater. Some bombs fell back into the crater and others described parabolic arcs, falling 300-400 m away and forming impact craters averaging 40 cm in diameter and 10-15 cm deep. Dark gray, ash-laden, cauliflower-shaped clouds ejected with the bombs rose 200-400 m at an average velocity of 5 m/second. Lightning was visible in the ash clouds. Coarse ash fell 500 m from the crater and finer material was blown into Sunda Strait. Water vapor was emitted from cracks and fissures formed along the inner wall of the active crater. Vapor emission appeared to increase 3-5 seconds before the first explosion of each group.

A single component (vertical) Hosaka electromagnet seismograph recorded 554 explosion earthquakes during 77 hours of observation, using a 0.3 second transducer 1 km from the crater. Using the minimum amplifier magnification (about 2,000x) a maximum double amplitude of 15 mm was recorded.

Information Contacts: R. Hadisantono and Suratman (July-Aug activity), L. Pardyanto (October activity), VSI.

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07/79 (SEAN 04:07) Tephra and lava flow from 1978 crater

Activity from Anak Krakatau's 100-m-diameter 1978 crater resumed in mid-July. Bombs (average diameter 1 m), lapilli, and ash were ejected every 5-15 minutes, rising 200 m and covering the area within about 700 m of the crater (figure 2). Lava flowed W, reaching the coast about 450 m away. A danger zone has been delineated within 3 km of the crater.

Figure 2. Oblique airphoto of Anak Krakatau, 12 September 1979, looking approximately NW. Verlaten Island (caldera rim) is in the background. An eruption column rises from the 1978 crater. Photo taken by Maurice Krafft. [Originally in 4:10 with a brief sumary of previous reports.]

The 1979 eruption is stronger than that of 1978, when ash and lapilli were ejected, but no bombs or lava flows.

Information Contacts: A. Sudradjat, VSI.

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09/79 (SEAN 04:09) Tephra emission continues, but lava extrusion has ended

Lava extrusion had ended by early September, but tephra emission continued. Activity fluctuated during ten days of observations in early and mid September, but usually consisted of discrete explosions at intervals ranging from 20 seconds to 40 minutes. Ash clouds rose to as much as 2 km above sea level and incandescent tephra formed fountains that reached several hundred meters height. Some of the explosions were audible up to 50 km away. Activity continued at the end of September, with ejection of 1-2 m bombs and finer pyroclastics taking place about every 2.5 minutes.

Maurice Krafft visited Anak Krakatau 5-8 and 13-15 September and flew over the volcano for 3 hours on 12 September. A 3-man team (Rudy Hadisantono, Stephen Self, and Michael Rampino) investigating the products of the 1883 eruption observed the volcano 10-12 September.

The following is from Maurice Krafft.

5 September: three vents, aligned NE-SW, were active in the 1978 crater. The SW vent emitted clouds that rose 200 m; an ash cloud rose 300 m from the middle vent; and incandescent bombs (average diameter about 0.5 m) from the NE vent reached 400-500 m in height, covering the area within about 1,000 m of the vent. Ash and gases from the NE vent rose 1,000 m. Explosion frequency averaged one every 3-4 minutes.

6 September: The SW vent had become quiescent, but explosions from the other two vents occurred every 5-10 minutes. Bombs reached 400-500 m above the crater and fell as much as 1,200 m away, on the E end of the island. At about 2100, activity began to weaken, and continued to decline during the night. Explosions on 5 and 6 September were heard within 50 km of Krakatau, on Java and Sumatra.

7 September: Only the NE vent remained active. An eruption cloud containing considerable ash but very few bombs was ejected every 20-30 minutes, rising about 500 m. Most of the explosions were not audible, but noisy explosions ejected bombs at about 2-hour intervals.

8 September: Weaker activity; explosion frequency declined to every 30-40 minutes and audible events that ejected bombs were less common than the day before.

The following is from Stephen Self.

"During the night of 9-10 September, the explosions could be heard, accompanied by volcanic tremor, on mainland W Java, 45 km from Krakatau.

"The activity observed on 10 and 11 September consisted of periodic explosive ejection of juvenile bombs, non-juvenile lithic blocks and large amounts of fine ash. The interval between explosions varied from 20 seconds to 20 minutes with no obvious pattern of periodicity. The explosions were often frequent enough to maintain an eruption column of fine ash and gases to a maximum of 2,000 m above sea level; winds blew the column WNW.

"The estimated initial volocity of the most powerful explosions was 150-170 m/second based on timed plume-rise during the gas thrust phase. Convective rise velocities varied from 10 to 20 m/second; large blocks (1-2 m diameter) were ejected into the sea up to about 1 km from the active crater.

"At night, the ejecta were incandescent, forming spectacular lava fountains up to 200 m above the vent. The activity, therefore, has the characteristics of Strombolian explosions, but produces much more fine ash than in more basic Strombolian activity.

"A new cone has been built around the 1978 crater and reaches about 150 m above sea level. At times, deposition on the cone was so heavy that it was 50% coated by glowing bombs at night. Fine gray deposits were accumulating on the older islands of the Krakatau group, with a total of 3 cm on the N and central parts of Sertung Island (about 5-6 km NNW of the crater).

"On the afternoon of 11 September, the activity had dwindled to occasional weak convective plumes and the team landed on Anak Krakatau. They ascended the 1930-40 crater rim on the E side of the island and collected fresh bombs ejected from the active vent (on the W side of Anak Krakatau). The bombs were andesitic and varied from massive and glassy to poorly vesicular lava with a plagioclase phenocryst content of 15-20%. The phenocrysts were up to 3-5 mm in length.

"The coarse ejecta were purely magmatic and it appeared that there was no contact between sea water and the rising magma. However, the large quantity of fine ash may suggest some phreatomagmatic mechanism.

"The team last observed the volcano late on 12 September, when the activity was slightly less than on 10 and 11 September."

The following is from Maurice Krafft:

12 September: A 3-hour morning overflight revealed explosions every 4-5 minutes, producing 800-m-high ash clouds. After some explosions, bombs fell into the sea at the W coast of the island.

13 September: Explosions occurred at about 20-minute intervals. Ash clouds were voluminous and rose about 1,500 m, but few bombs were ejected.

14 September: Activity was similar to the previous day. In addition, a considerable amount of lightning was observed in the ash clouds.

15 September: Explosion frequency dropped to one each 30-40 minutes, but ash clouds continued to rise 1,000-1,500 m.

The following is from Adjat Sudradjat. At the end of September bombs 1-2 m in diameter fell as much as 400 m from the crater, and finer pyroclastics fell as much as 700 m away. Two eruption columns were visible, indicating that there were two active vents. Quiet intervals between explosions were about 2.5 minutes long.

Information Contacts: M. Krafft, Cernay; R. Hadisantono and A. Sudradjat, VSI; S. Self and M. Rampino, NASA, New York.

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12/79 (SEAN 04:12) Eruption ends

The average number of eruption tremors/day recorded at Pasauran, 30 km from the volcano, was 70 in July, 50 in August, and 100 in September, then dropped sharply to two in October and November. No tremors were recorded in December, and detonations and ejection of incandescent materials were no longer observed.

Information Contacts: A. Sudradjat, VSI; Suparto S., Java and Sumatra Observatory, VSI.

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04/80 (SEAN 05:04) Explosions eject incandescent tephra

Activity began to increase at the end of March. Detonations from explosions were heard 50 km away and window glass trembled on the W coast of Java. Incandescent material rose 200 m above the vent, which approximately coincided with the 1975 eruption center. About 65 explosion events were recorded on 13 April, and 290 on 16 April. The strongest activity occurred during a 5-hour period on 19 April, when 200 explosions were recorded. There were 335 explosions on 20 April, but activity was declining the next day. Rough seas prevented a landing on the island.

Information Contacts: A. Sudradjat, L. Pardyanto, and Suparto S., VSI.

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09/80 (SEAN 05:09) Explosions continue

Increased activity from Anak Krakatau began in March, when detonations were heard from Pasauran, 40 km away on the W coast of Java. Incandescent material was thrown to 200 m height during a period between 19 April at 2300 and 20 April at 0415, when 200 explosions were recorded. Activity declined after 20 April but continued intermittently through September.

A stronger explosion occurred on 9 September at 0039. It rattled windows and shook houses in Pasauran, and a 3-4 cm-amplitude explosion event was recorded on the seismograph there. Ash clouds reached about 1.5 km altitude in September and explosions continued at the end of the month. The source of the 1980 activity is a new vent that approximately coincides with the 1975 crater and is about 250 m NW of the 1978-79 eruption center.

A research group consisting of a biologist, oceanographer, environmentalist, and volcanologist are studying Krakatau under the auspices of the centennial commemoration of the 1883 eruption. The Centennial Committee invites scientists worldwide to participate in a 3-year period of research at Krakatau.

Information Contacts: A. Sudradjat and L. Pardyanto, VSI; M. Krafft, Cernay; Kompas, Jakarta.

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10/81 (SEAN 06:10) Small ash clouds

Explosions resumed 20 October after several months of fumarolic activity. Guy Camus and Pierre Vincent visited the volcano for four hours during the afternoon of 19 October, but noticed no premonitory activity. Explosions began between 0300 and 0400 the next morning. From Rakata Island (about 3 km SE of Anak Krakatau), Camus and Vincent noted 19 explosions in the two hours just after sunrise, before leaving the island. They had seen several others by mid-afternoon during discontinuous observations from a boat. Most were initiated by a "cannon-like" explosion from the main cone, followed by convective growth of an eruption column (typically to 400-600 m, but occasionally to [2] km in height). No noise could be heard on Rakata Island. The explosions usually lasted one to several minutes, but the last one observed by Camus and Vincent as they left the area began at 1511 and continued until 1525. Most of the eruption columns were dark, containing abundant ash but few blocks and no incandescent material. Water vapor could be seen condensing at the top of several eruption columns and lightning was occasionally observed. Ash fell on Sertung Island, about 2 km W of Anak Krakatau.

Information Contacts: G. Camus and P. Vincent, Univ. de Clermont-Ferrand.

Further References: Camus, G., Gourgaud, A., and Vincent, P.M., 1987, Petrologic evolution of Krakatau (Indonesia): implications for a future activity: JVGR, v. 33, p. 299-316.

Siswowidjoyo, S., 1985, The renewed activity of Krakatau volcano after its catastrophic eruption in 1883: Proceedings of the Symposium on 100 Years Development of Krakatau and its Surroundings, Indonesian Institute of Sciences, Jakarta, p. 192-198.

Sudradjat, A., 1985, The morphological development of Krakatau volcano, Sunda Strait, Indonesia: Proceedings of the Symposium on 100 Years Development of Krakatau and its Surroundings, Indonesian Institute of Sciences, Jakarta, p. 141-146.

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02/88 (SEAN 13:02) Ash emission; glow; felt earthquakes

An increase in the volume of white fume from the summit crater was observed beginning 11 February. Similar activity, stronger than it had been for several years, continued intermittently until 28 February, when plume color darkened and emissions became more frequent. Plumes apparently rose a few hundred meters above the crater. On 1 March, glow was visible from the Java coast, . . . suggesting the presence of new lava. Earthquakes were felt 5-6 March from Anyer, 55 km E of the volcano.

Information Contacts: VSI; A. Ritter, Carita Beach Hotel, Java.

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03/88 (SEAN 13:03) Two lava flows from new crater; glowing blocks

On 16 March, a group from VSI visited Anak Krakatau. The eruption . . . had created a new crater on the SSE flank of the 1960-81 Vulcanian cone. The new crater was ~80 m wide and 150-200 m in length, with the long axis oriented approximately NNW-SSE. Its low point was on the SSE side at an altitude of 140 m. Lava emerging from a vent at that point fed one small flow moving SSW, and a second, longer flow to the SE. The second flow had descended from the 1960-81 cone onto the old crater floor (between the pre-1960 crater rim and the younger inner cone).

A single vigorous fumarole, on the SSW slope of the 1960-81 cone ~50-80 m W of the new 1988 crater, was active about every 5 minutes . . . . Reports from the forestry observer on Sertung Island (4 km W) indicated that incandescent blocks were ejected from this vent. Some of the blocks were thrown into the sea outside the new crater, ~500 m from the vent. During the 16 March visit, this vent was only mildly active, so geologists were able to enter it and collect samples from the current activity. Hand-specimen petrography indicated that the 1988 lava resembles the 1980 plagioclase andesite (SiO₂ 53-54%) and is clearly not like the dacite bombs of the 1981 Vulcanian eruption (SiO₂ 62-63%).

Information Contacts: VSI.

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04/88 (SEAN 13:04) Frequent explosions eject small plumes

The eruption continued through April. During the first half of April, explosions from the 1988 crater occurred at the rate of 105/day; during the second half of April, the frequency decreased to ~45/day. Plume heights ranged from 100 to 600 m. A seismometer 800 m from the vent recorded neither deep nor shallow earthquakes during April; explosions and rockfalls may have saturated the instrument. Geologists have not visited the island since 16 March.

Information Contacts: VSI.

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10/92 (BGVN 17:10) Lava flows and incandescent tephra

An eruption that began at 1802 on 7 November ejected lava fragments to 150 m height, followed by an ash explosion to 800 m. Increased seismicity during the first week in November preceded the eruption. A VSI team climbed the volcano on 12 November and reported that the eruption was from Anak Krakatau's NE crater. Lava flows extended 300 m NE and 100 m SE, filling a valley. Approximately 36,000 m² of the island has been covered by an estimated 178,000 m³ of lava, mostly basaltic andesite with porphyritic to vitrophyric texture. Degassing and ejection of lava fragments was continuing on 12 November at about 3-minute intervals, to heights of 100-200 m. The number of explosion earthquakes decreased from 499 on 11 November to 406 on the 12th, and 296 on the 13th (figure 3). Volcanic tremor with a maximum amplitude of 30.5 mm and a frequency of 0.4 Hz was recorded from 1905 on 12 November until 0800 the next day.

Figure 3. Number of earthquakes recorded at Anak Krakatau, 1-13 Nov 1992. Courtesy of VSI.

Eruptive and seismic activity was continuing on 14 November. Based on the number of explosion earthquakes and the characteristics of volcanic tremor and occasional A-type events, VSI believes that the eruption may continue for several months at the current level of activity. VSI is discouraging visits to the island until further notice.

Information Contacts: W. Modjo, VSI.

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11/92 (BGVN 17:11) Incandescent tephra ejection; lava reaches sea

The eruption . . . was continuing in early December. Incandescent lava was ejected to 100-150 m height, with ash from intermittent explosions (at intervals of 3-32 seconds) rising 400-500 m. Explosion earthquakes remained frequent at 1,000-4,000/day, but no A- or B-type earthquakes have been recorded since 11 November. Lava flowed SE, and down the NE flank to the sea. Lava volume was ~5.5 x 10⁶ m³, covering an area of ~2 x 10⁶ m². Tourists were advised not to visit the island until further notice.

Information Contacts: W. Modjo, VSI.

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01/93 (BGVN 18:01) Lava flows continue; Strombolian explosions; ash columns to 400 m

The eruption . . . continued in 1993. The strongest explosive activity occurred on 12 November 1992. Bombs fell to several hundred meters N of the vent and smaller tephra reached the N coast. Lava flowed 1 km to the N coast and entered the sea, extending >100 m beyond the shore. Lava continued to advance in January, but feeding of the flow from the vent may have stopped by mid-February. Strombolian explosions ejected lava fragments, visibly incandescent at night, in early February and ash columns rose 100-400 m. The number of explosion earthquakes varied from 500-2,000/day (figure 4), at intervals of 5 seconds to 5 minutes. Explosions can sometimes be observed from the volcano observatory . . . . Tourists have been advised to remain at least 3 km from the island until further notice.

Figure 4. Number of daily explosion earthquakes, 10 November 1992 to 7 February 1993. Courtesy of VSI.

Information Contacts: W. Tjetjep, VSI.

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05/93 (BGVN 18:05) New lava flows; one person killed

Activity has continued . . . since . . . 7 November 1992. A mid-November lava flow reached the NW coast and entered the ocean (figure 5). This lava flow continued to advance until early February. Beginning in February, a new lava flow traveled SSE, destroying seismic stations maintained by VSI and Gadjah Mada Univ (GMU). Another lava flow descended to the N in April, overflowed the old crater rim in May, and burned a forested area near the coast. Analyses of the April 1993 lavas showed 53.5% SiO₂, an increase from the November 1992 lavas (51.3%).

Figure 5. Sketch map of Anak Krakatau showing recent lava flows (November 1992-April 1993), VSI seismometer locations, and ballistic bomb area. Courtesy of VSI.

Scientists from GMU reported degassing events and lava-block ejections on 6 May. Sharp thunder-claps were also heard, and ash fell heavily during most of their 8-hour visit. As of 13 June, the most recent activity observed by VSI geologists was a vertical Vulcanian explosion with a column ~150 m in diameter and 200-600 m high. There was no evidence of pyroclastic flows. Tephra deposits have built a new cone to at least 280 m asl, much higher than the previous peak (199 m elevation). A moderate explosion on 13 June killed one tourist and injured five others climbing on the old crater rim. Since the eruption began, tourists have been advised by VSI and the local government to remain at least 3 km from the island.

A new vertical seismometer was installed by VSI on 17 April ~10 m from the coast and 15 m asl on the SE side of the island (figure 5). Data are telemetered to the volcano observatory in Pasauran . . . . From 18 April to 7 May there were 4 days with >1,500 explosions and another 7 days with >1,000 (figure 6). Seismically detected explosions then gradually declined into early June. The number of explosion earthquakes/day was generally lower during this period than in late 1992 and early 1993.

Figure 6. Number of daily explosion earthquakes at Anak Krakatau, 18 April to 13 June 1993. Courtesy of VSI.

GMU placed a new seismic station on the E side of the outer crater rim on 6 May. Long-period and short-period 3-component measurements were also carried out during the visit to the island. Approximately events were detected during 6 hours of long-period seismic measurements (cutoff frequency 0.2 Hz). About 80% of these events could be correlated with visual observations of activity. Felt or observed earthquakes occurred an average of every 4 minutes, with stronger events every 7-8 minutes. Analysis indicates that the non-correlated seismic events had deeper hypocenters.

Information Contacts: W. Tjetjep, VSI; K. Brotopuspito, GMU.

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07/93 (BGVN 18:07) Explosions continue; bombs destroy another seismometer

Anak Krakatau has now been in continuous eruption for nine months . . . . Continued explosions (140-400/day) have built a new cone to ~280-300 m elev, which is 80-100 m above the old peak (figure 7). A seismometer installed by VSI was destroyed by a ballistic bomb on 16 July; lava flows have previously destroyed two other seismometers. Earlier this year, lava flows filled the valley between the cone and the old rim, and overflowed onto the outer slope of the old rim. The April 1993 lava flow descended the N flank to the coast, where it burned trees. The recommended off-limits zone, 3 km from the island, remains in effect.

Figure 7. Sketch of Anak Krakatau showing topographical changes, as of July 1993, since the eruption began in Nov 1992. View is looking SE; scale is approximate. Drawn by Igan S. Sutawidjaja; courtesy of VSI.

Information Contacts: W. Tjetjep, VSI.

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10/93 (BGVN 18:10) Details of seismicity in mid-August

In mid-August GMU scientists repaired damaged seismic equipment and conducted a seismic study on Anak Krakatau. GMU's seismic station was damaged by volcanic bombs on 18 May, only 12 days after installation. The new station consists of a 1-Hz vertical-component seismometer on the E flank of the island, closer to the coast and farther from the source vent than the damaged station (figure 5).

On 14 August the GMU team also deployed a 3-component seismograph with a 0.2 Hz cutoff frequency and collected data from 0900 to 1700. During this 8-hour interval >100 events were registered, with >90% correlated with minor explosions seen at the surface. In contrast to the bulk of the events, which had shallow sources, the events without visual correlation caused particle motions implying generation at greater depth . . . . Volcanically quiet intervals indicated little seismic contribution from ocean waves; such waves were chiefly of low amplitude and confined to the 0.5-3 Hz frequency range.

About four hours of the vertical-component seismic record are shown on figure 8. Many events appeared at 3-minute intervals. Longer intervals of quiet also occurred, and typically terminated in strong seismic shocks and eruptions. A more detailed 3-component record of a comparatively large event on 14 August (figure 9) shows relative quiet prior to the event, and near 0.2, 0.5, and 1.1 minutes, peaks in the seismic signal. The lower portion of figure 9 shows the computed smoothed spectra (from maximum entropy spectral analysis) for the three components.

Figure 8. Anak Krakatau vertical-component seismic records for two 2-hour intervals on 14 August 1993: beginning at 0859 (top), and beginning at 1105 (bottom). Courtesy of A. Brodscholl and K. Brotopuspito.

Figure 9. Anak Krakatau 3-component seismic record (top), and associated smoothed spectra for the event marked on figure 8 (bottom). Courtesy of A. Brodscholl and K. Brotopuspito.

During installation of the new system, scientists witnessed degassing and ejection of silica-rich volcanic bombs. They found no pumice. Observers on [Carita Beach] noted that lava glowed strongly in early May but had stopped by mid-June. As of 14 August glowing had not reappeared. [News reports during the 1994 eruption indicated that activity ceased in October 1993.]

Information Contacts: A. Brodscholl and K. Brotopuspito, GMU.

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04/94 (BGVN 19:04) Activity resumes in March after 5 months of quiet; ash clouds and tephra ejection

Press reports have described renewed activity . . . from late March through early May. Activity apparently began again on 19 March after about five months of quiet. At the end of March, thick black ash plumes rose 300 m while "red flames and glowing lava" were observed at night to rise 200 m. Eruption noises like thunder could be heard at Carita Beach . . . . Scientists from GMU stated in the newspaper Suara Pembaruan that their seismograph was not operational at the time of the eruption, but began functioning again on 26 March.

Wimpy Tjetjep, Director of VSI, reported to the same newspaper on 6 April that volcanic materials had been thrown within a 3,000-m-radius from the crater. Ash and tephra emissions were continuing, and 160-170 weak volcanic earthquakes had been recorded in the previous week. Officials at the local observatory reportedly described the activity as "spouts of fire" and thick black ash emissions alternating with explosions. Ejection of blocks as large as 1-m-diameter were also reported. Fishermen at Carita Beach described the April activity as "gigantic fireworks floating in the middle of the sea at night."

The most recent report of activity was on 5 May, when VSI volcanologists at the local observatory told the Antara News Agency that an eruption ejected lava 200 m into the air. They also noted that as many as 222 "eruptions" had been recorded on 3 May. All of the reports emphasized that the island remains off-limits to visitors.

Information Contacts: Suara Pembaruan News; Antara News Service; UPI.

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07/94 (BGVN 19:07) Frequent ash explosions (300-450/day) reach heights up to 500 m

Ash explosions continued at a rate of 300-450/day in early August. The height of the ash columns, measured from the [Pasuaran Observatory] during clear weather, ranged from 150 to 500 m above the summit, with incandescent projections evident at night. The sporadic eruptions have deposited ash over almost the entire island. During the second week of August, explosion earthquakes averaged 460 events/day. Occasionally, explosion sounds were heard and vibrations felt at the observatory.

Information Contacts: VSI.

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12/94 (BGVN 19:12) Ash eruptions in October-December seen by pilots

A pilot on a 16 October Qantas Airlines flight enroute from Perth to Singapore reported a light-gray, moderately dense, mushroom-shaped ash cloud rising to ~2 km altitude and drifting ~55 km W from the summit. Another pilot report on 28 November noted ash from the volcano rising to ~3 km altitude. A similar report on 18 December stated that an ash cloud was as high as 3 km and drifting E.

On 26 July 1994, J. Sesiano made observations from the sea, from the summit of the 1930 peak, and from the slopes of the active cone. Activity consisted of violent explosions (averaging one every 15 minutes) with ejecta (small tephra and bombs) erupted vertically to heights of 100-200 m that fell back into the crater. Finer particles were blown NW by winds. Longer intervals between eruptions (30-45 minutes) were followed by a single or series of very strong explosions that generated a dark plume 1,000-1,500 m above the crater. Ejecta from these stronger explosions, including some bombs >1 m³ in size, fell over the entire cone. Boulders were seen rolling down the slopes, reaching and slowly burying the November 1992-Apr 1993 lava flow (18:07), which had a surface temperature of ~30°C and was still degassing.

A rough examination was done on two thin sections made from fresh lava samples (collected while still hot). Phenocrysts were mostly zoned plagioclase, some augite, and rare olivine with reaction rims. The groundmass consisted of fine-grained opaque grains and scattered plagioclase, but no glass. Plagioclase composition, determined from very few properly oriented crystals, was An$45-55. Based on these observations, the samples were identified as andesite to basaltic andesite.

Information Contacts: J. Sesiano and J. Bertrand, Univ de Genéve; BOM Darwin, Australia; T. Fox, ICAO.

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03/95 (BGVN 20:03) Explosions continue, sending ash plumes daily up to 500 m above the summit

Volcanic activity continued through January-March 1995, sending grayish white plumes 150-500 m above the summit. Sounds like thunder were sometimes heard at the VSI observatory . . . and glow was visible at night as high as 50 m above the summit. The daily number of explosions in January and early February fluctuated between 50 and 150 events. From mid-February to mid-March the average number of explosions increased to 150-200 events/day (figure 10).

Figure 10. Daily number of explosion earthquakes (bars) and height of the ash plume (line) at Krakatau, January-March 1995. Courtesy of VSI.

Information Contacts: W. Tjetjep, VSI.

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06/95 (BGVN 20:06) Frequent explosions send ash 400 m high

According to news reports at the end of May 1995, authorities closed the volcano to tourists, permitting them to come no closer than 3 km. A VSI official told UPI that ~7,200 explosions were recorded during May; during the second week in June, ~2,000 explosions were recorded. Occurring every 3 minutes, the explosions shot ash ~150-400 m high.

The following supplements reports in 19:4, and adds information about April-June 1994 (VSI, 1994a). During March 1994 Strombolian eruptions had plumes that rose 50-400 m. These eruptions spewed incandescent ejecta every 5-10 minutes and were accompanied by sounds like "thunder-claps." From 26 March to the end of the month, 109-230 earthquakes were recorded each day. Similar Strombolian eruptions continued from April through June 1994, with the plume rising 50-300 m above the crater (VSI, 1994b). Incandescent volcanic materials were ejected to heights of 50-150 m above crater rim. Between 1 April and 17 May 1994, 50-450 earthquakes occurred each day. Following 30 days with an inoperable seismograph (16-30 June 1994), 10-600 earthquakes were recorded/day.

References: Volcanological Survey of Indonesia, 1994a, Krakatau Volcano: Journal of Volcanic Activity in Indonesia, v. 2:1, p. 2.

Volcanological Survey of Indonesia, 1994b, Krakatau Volcano: Journal of Volcanic Activity in Indonesia, v. 2:2, p. 1-2.

Information Contacts: VSI; AP; UPI.

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07/95 (BGVN 20:07) Unusually loud sounds shown on seismic records

During June there were unofficial reports of unusually loud noises heard on the W coast of Java . . . . On 27 June, GMU scientists visited islands around Anak Krakatau and heard some very loud sounds; only some of which correlated to visual activity at Anak Krakatau. The observers compared their observations to a 1993 visit, when the volcano emitted steam-bearing discharges accompanied by lightning. The eruptions on 27 June 1995 appeared dissimilar because they were ash-rich and without visible steam. In addition, the 27 June eruptions produced string-shaped columns with mushroom-shaped tops; lightning was absent.

The group deployed two seismometers for five hours of observation. A vertical-component long-period seismometer (0.2 Hz cutoff) was put on Panjang Island, 3.6 km W of Anak Krakatau's summit, and a 3-component broadband seismometer was put on Sertung Island, 3.2 km WNW.

Typical seismograms, showing two of the three components recorded on Sertung Island, appear on figure 11. In one case, a low-frequency seismic signal arrived ~8 seconds prior to a sharp onset, reaching amplitudes of 0.6 mm/sec in the vertical component (figure 11). The second seismometer also recorded ~8 seconds of seismic signal before the onset of the air wave. Other events also showed the same 8-second delay between the seismic signal and these air waves. The case shown was correlated with a small eruption that generated a loud sound and ultimately spawned an ash cloud of undisclosed dimension. Assuming a shallow source for the eruption, the travel times for first arrivals of the strong impulsive signals across the 3.2 km source-to-receiver distance were on the order of 10 seconds, roughly the velocity of sound in air (0.33 km/sec). Thus, the strong impulsive signals were probably due to pressure waves transmitted through the air.

Figure 11. Seismic record from Anak Krakatau showing the vertical component for a typical event received on nearby Sertung Island. The data were collected on 27 June 1995 (t = 0.0 sec corresponds to 1049 local time). Approximate arrivals of the seismic and air-wave signals are indicated. Courtesy of Wahyudi and A. Brodscholl.

Information Contacts: Wahyudi and A. Brodscholl, GMU.

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01/96 (BGVN 21:01) Steaming and fumarolic activity; cone description

During an approved visit on 6 November, the volcano was steaming but not erupting. A large sulfur-stained plug of lava, ~20% the diameter of the summit cone, was bulging out of a black cinder cone just below and W of the summit; it appeared to be inside the SW margin of a broad depression. A smaller sulfur-stained plug was farther S in another depression. The landing site on the SE shore was a black-sand beach with tiny dunes of white pumice. While climbing the SE slope of the older cone, the party crossed water-eroded fields of pyroclastic material dotted with volcanic bombs. The ascent to the summit went through deep cinder deposits covered with a blanket of loose breadloaf-sized stones. From the summit complicated internal crater structures could be seen. One symmetrical cone (~100 m across) had a sulfur-lined inner rim that was fuming. The largest and most active fumaroles were inside this cone's S rim. A smaller cone within the larger one was almost horseshoe-shaped and steeper to the S. Bombs on the summit cone were as large as 1-2 m in diameter.

The current activity began in March 1994 (Bulletin v. 19, no. 4) after almost continuous activity from November 1992-October 1993. Activity was intermittent during 1995 (Bulletin v. 20, nos. 3, 6, & 7). This renowned volcano lies in the Sunda Strait between Java and Sumatra. Caldera collapse, perhaps in 416 A.D., formed a 7-km-wide caldera. Three subsequent volcanoes coalesced into pre-1883 Krakatau Island. Caldera collapse during the 1883 eruption destroyed all but a remnant of one volcano. The post-collapse cone of Anak Krakatau (Child of Krakatau), located within the 1883 caldera, has had frequent eruptions since 1927.

Information Contact: Steve O'Meara, P.O. Box 218, Volcano, Hawaii 96785 USA (Email: 102022.111@compuserve.com).

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09/96 (BGVN 21:09) Thick plume to an altitude of 3.7 km on 29 September

At about 1140 on 29 September, a Qantas Airlines pilot reported a thick plume near Krakatau that rose to an altitude of 3,700 m and drifted NW at low levels and E at high levels. There was no definite signature on GMS satellite images.

The current activity began in March 1994 (Bulletin v. 19, no. 4) after almost continuous activity from November 1992-October 1993. Activity was intermittent during 1995 (Bulletin v. 20, nos. 3, 6, & 7). This renowned volcano lies in the Sunda Strait between Java and Sumatra. Caldera collapse, perhaps in 416 A.D., formed a 7-km-wide caldera. Three subsequent volcanoes coalesced into pre-1883 Krakatau Island. Caldera collapse during the 1883 eruption destroyed all but a remnant of one volcano. The post-collapse cone of Anak Krakatau (Child of Krakatau), located within the 1883 caldera, has had frequent eruptions since 1927.

Information Contacts: Bureau of Meteorology, Northern Territory Regional Office, P.O. Box 735, Darwin NT 0801, Australia; NOAA/NESDIS Satellite Analysis Branch, Room 401, 5200 Auth Road, Camp Springs, MD 20746 USA.

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11/96 (BGVN 21:11) July and August lava flows; September and October ash explosions

Although a pilot report described a 3.7 km tall eruption column from Anak Krakatau on 29 September (Bulletin v. 21, no. 9), ash columns during that same month more typically reached only 800 m above the summit. During the bulk of September explosions took place at 5-minute to two-hour intervals; bombs up to 20 cm in diameter reached the N and NE coastlines, areas lying ~1-1.5 km from the vent. Lava flows during July-August reached the island's W coast and added to its size. Two vents emitted lava and Strombolian eruptions in the N part of the main crater.

During October, ash explosions occurred every minute, followed by rumbling sounds and lava fountains as high as 600 m above the crater. The main crater produced all the activity during October with the other two craters remaining quiet. There were no lava flows released to the surface during October. However, weak red glow was occasionally observed at night (from the Pasuaran observatory).

Krakatau is famous for its devastating 1883 eruption, which killed at least 36,417 people, chiefly due to tsunamis that reached 40 m heights. Resulting explosions from that eruption were heard 4,650 km distant and atmospheric effects were recognized around the world. Anak Krakatau, or "Child of Krakatau" is a young cone that emerged as an island in the center of the collapsed caldera.

Information Contacts: Wimpy S. Tjetjep, Volcanological Survey of Indonesia, Jalan Diponegoro No. 57, Bandung 40122, Indonesia (Email: vsimvo@ibm.net).

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07/97 (BGVN 22:07) Activity increases in May

The following describes the volcanism during March-May based on reports by the NOAA Satellite Analysis Branch (SAB), a team of the Société Volcanologique Européenne (SVE), and Mike Lyvers. Lyvers noted that the Indonesian government's 5-km exclusion zone around the island has not deterred local boat operators from anchoring offshore or even landing tourists on Anak Krakatau.

SAB reported that on 6 March at 0442 an unidentified aviator saw a significant eruption with ash reaching an altitude of ~7 km. This cloud, however, was not seen in GMS satellite imagery.

Members of the SVE visited the island twice in April. They learned that during March at Carita, a beach resort on the W coast of Java 40 km from the volcano, there were ashfalls and explosions from the volcano were heard. During April, emissions became less prominent and more irregular. During their first visit on 9-10 April they did not observe any plumes. After landing they ascended to the first crest line where the group encountered several bread-crust bombs and their substantial impact craters. As they were ascending the cone of the volcano the visitors felt the heated ground through their hiking boots. There were fumaroles on both the flank and the summit. The crater, 150-200 m in diameter, was breached to the W; the crater floor was occupied by large blocks, and it was possible to distinguish two vents aligned on a fissure trending SE-NW.

The group returned on 17-18 April, after another eruptive episode. This time they observed enormous new blocks at the summit. The S vent continuously emitted white steam; the N vent sporadically discharged brown-black ash that rose up to 500 m above the vent. The SVE team watched from a spot in front of the cone, ~400 m from the summit, when at 1820 the S vent exploded generating an ash plume and throwing incandescent projectiles ~200 m above the crater. One projectile landed very close to the observation point. The next morning, ash on the tents suggested that the volcano had another explosion. The group witnessed another eruption as they were leaving the island by boat at 1000.

SVE members learned that after spending 21-22 April on the island, Guy de St. Cyr (a French tourist-guide) saw plumes accompanied by projectiles. He described the ash as an unusual pink color. During the night, incandescent explosions were took place about every 30 minutes; several incandescent blocks fell over the dome's N side. The next morning, during a boat tour around the island, some blue smoke rose from mid-way up the W-SW flanks of the dome, conceivably a sign of minor lava flows.

During the afternoon and evening of 17 May, Mike Lyvers visited the island by boat. The previous few days, when observed from Carita Beach, the volcano had been quiet. In contrast, on 17 May it erupted almost continuously, issuing minor amounts of ash and sometimes a few bombs. Occasionally, larger explosions sent incandescent ash high into the sky, generating a spectacular display of volcanic lightning and covering the cone with glowing bombs. The volcano seemed to show no obvious pattern to its activity, with random fluctuations in the intensity of eruption.

Krakatau is famous for its devastating 1883 eruption, which killed at least 36,417 people, chiefly due to tsunamis that reached 40-m heights. Resulting explosions from that eruption were heard 4,650 km distant and atmospheric effects were recognized around the world. Recent activity included lava flows that were reported in July and August 1996 and ash explosions during September and October (Bulletin v. 21 no. 11).

Information Contacts: NOAA/NESDIS Satellite Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Spring, MD 20746 USA; Société Volcanologique Européenne, C.P. 1, 1211 Genève 17, Switzerland (Email: 101626.3303@compuserve.com; URL: http://ourworld.compuserve.com/homepages/HGaudruSVE/); Mike Lyvers, School of Humanities and Social Sciences, Bond University Gold Coast, Qld. 4229 Australia.

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02/99 (BGVN 24:02) Sporadic ash eruptions in February and March 1999

Krakatau erupted on 5 February 1999 accompanied by thunderclaps and an ash plume that reached a height of ~1,000 m above the summit. The activity continued until 10 February with ash plumes reaching ~100-300 m above the summit. The continuing sporadic eruptions deposited small amounts of ash over most of the island; a deposit of ~0.3 mm was measured near the observatory. On 11 February, the glow of ejecta was observed reaching ~25 m above the summit and continued during the night.

Activity decreased early during the week of 9-15 March. Weak booming noises were heard twice on 9 and 10 March, but plumes were not observed. At the end of the week booming noises were rare, and a white-gray ash plume was seen on 14 March that rose 100-300 m above the summit. The current activity is a continuation of eruptions that began in 1992.

Krakatau is famous for its devastating 1883 eruption, which killed at least 36,417 people, chiefly due to tsunamis that reached 40 m heights. Resulting explosions from that eruption were heard 4,650 km away, and atmospheric effects were observed around the world. Recent activity has included lava flows and ash explosions from the post-caldera cone of Anak Krakatau reported in 1996 and 1997 (Bulletin v. 21, no. 11, and v. 22, no. 7).

Information Contacts: R. Sukhyar and Dali Ahmad, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.dpe.go.id/; Email: sukhyar@vsi.dpe.go.id, dali@vsi.dpe.go.id).

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04/99 (BGVN 24:04) Explosive eruptions continue in April

After a repose of twenty months Anak Krakatau erupted again at 1615 on 5 February (Bulletin v. 24, no. 2). Several scientists, including some from the Volcanological Survey of Indonesia (VSI) and from Unocal Geothermal of Indonesia, visited Krakatau from 28 March to 6 April. This report combines their observations.

Seismic activity preceding and coincident with the eruption went undetected because of ballistic bomb damage to seismometers. Until 3 April, activity typically involved 5-10 explosions per day. Beginning at about 1500 on 3 April ash explosions became almost continuous. During the interval 0955-1230 on 4 April, the volcano erupted every 1-3 minutes from a new crater a few hundreds of meters S of the summit crater that formed during 1992-97. Accidental blocks, lava bombs, and ash reached heights of 250-300 m above the crater rim. About a third of the eruptions were Strombolian, with showers of lava and bombs (occasionally 1 m across) ejected 50-100 m above the vent and falling onto the upper flanks. Some ballistic fragments 20-30 cm in diameter rose above the associated ash cloud and landed 800 m from the vent on the upper flanks before rolling down to the shore. Eruptions were often accompanied by thunderous blasts and rumbling sounds heard several kilometers from the crater, including at Pasauran and Kalianda observatories 42 km from Krakatau.

Figure x4. Lava and bombs exploded from the summit of Anak Krakatau on 4 April. This view was from the sea looking toward the E. Courtesy of VSI; photo by Karsten Moran, Jakarta International School.

Figure x5. Ballistic fragments flew above and to the side of rising ash clouds during the eruption at Krakatau. The view is toward the N. Courtesy of VSI; photo by Karsten Moran, Jakarta International School.

A wedge-shaped deposit of fresh ash and bombs was visible on the crater rim (the rim is higher on the SE due to prevailing northwesterly winds that blow ash and other ejecta in that direction). Ash clouds were light gray. Observers noticed fine black ash that fell on their boat as they passed under the plumes ~500 m downwind from the crater. The ash was crystal-poor and frothy, suggesting that it was mostly juvenile material.

A solfataric plume originating at ~200 m elevation on the N flank discharged steam and bluish gas. Nearly a dozen other solfataras discharged steam and non-condensible gas and deposited bright yellow native sulfur around vents near the summit (figure 9). Another fumarolic area was centered at 140 m elevation on the W flank below the active crater.

Figure 9. An ash plume rises from the summit crater above a fumarolic area on Krakatau's W flank, seen here looking toward the NE. The light-colored patches are mostly native sulfur. Courtesy of VSI; photo by Karsten Moran, Jakarta International School.

Scientists observed several boatloads of tourists who had landed on the accessible SE beach. Officials had closed an area of 3 km radius around the vent, but many tourists defied the prohibition and climbed to the ridge 400 m from the summit vents. Escaping gases continued to pose a very serious hazard.

The renowned Krakatau volcano lies in the Sunda Strait between Java and Sumatra. Caldera collapse, perhaps in 416 AD, destroyed the ancestral Krakatau edifice, forming a 7-km-wide caldera. Remnants of this volcano formed Verlaten and Lang Islands; subsequently Rakata, Danan and Perbuwatan volcanoes were formed, coalescing to create the pre-1883 Krakatau Island. Caldera collapse during the catastrophic 1883 eruption destroyed Danan, and Perbuwatan volcanoes, and left only a remnant of Rakata volcano. The post-collapse cone of Anak Krakatau (Child of Krakatau), constructed within the 1883 caldera at a point between the former cones of Danan and Perbuwatan, has been the site of frequent eruptions since 1927.

During six lava-producing eruptions between 1958 and 1980, flows moved S and SW from the SW crater. Observations are frequently made from Carita Beach on the coast of Java, ~40 km E. The local VSI volcano observatory is at Pasuaran, ~42 km E.

Information Contacts: Igan S. Sutawidjaja, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi. dpe.go.id; Email: igan@vsi.dpe.go.id); David Sussman, Unocal Geothermal of Indonesia, Sentral Senayan-1 Office Tower, 11th Floor, Jalan Asia Afrika No. 8, Jakarta 10270, Indonesia (Email: dsussman@unocal.com); John Moran, c/o USAID, Jalan Medan Merdeka Selatan No. 5, Jakarta 10110, Indonesia; Rene Wassill, Wisma Met. I, 5th floor, Jalan Sudirman Kav 26, Jakarta, Indonesia.

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05/99 (BGVN 24:05) Occasional explosions producing ash columns

Following several months of intense activity that began on 5 February (Bulletin, v. 24, no. 4), Anak Krakatau became relatively quiet in late April. From the end of April until the end of May, only several explosions were heard. On 26 April a weak explosion sent a white-gray ash plume 200-500 m high. Between 4 and 17 May there were two blasts per week, each accompanied by a glow and white-gray ash reaching between 100 and 400 m high. In the week from 18 to 24 May, in addition to two explosions, a shock on the morning of 20 May registered at 2 on the MMI scale.

Anak Krakatau was very active from 1992 to 1997, depositing 6.8 x 10⁶ m³ of lava flows. The island was enlarged by 378,527 m² and the height of the cone increased from 159 to ~300 m.

The renowned Krakatau volcano lies in the Sunda Strait between Java and Sumatra. Caldera collapse, perhaps in 416 AD, destroyed the ancestral Krakatau edifice, forming a 7-km-wide caldera. Remnants of this volcano formed Verlaten and Lang Islands; subsequently Rakata, Danan and Perbuwatan cones were formed, coalescing to create the pre-1883 Krakatau Island. Caldera collapse during the 1883 eruption left only a remnant of Rakata. The post-collapse cone of Anak Krakatau (Child of Krakatau), constructed between the former Danan and Perbuwatan cones, has been the site of frequent eruptions since 1927.

Information Contacts: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.dpe.go.id/).

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08/99 (BGVN 24:08) Strombolian eruption continues; new seismograph 27 May

The Strombolian eruption of Anak Krakatau continued during May-August. White-gray ash was emitted in plumes up to 600 m above the summit. Detonations were accompanied by the ejection of incandescent material. A new seismograph was installed on 27 May, replacing one destroyed by falling lava bombs in April (Bulletin v. 24, no. 5). During many weeks of May and June thousands of explosions were recorded on seismographs. An earthquake of intensity MM II was recorded on 10 June and another of the same intensity occurred during the week ending 16 August. On several days during the reporting period the volcano could not be seen because of thick haze.

Anak Krakatau was very active from 1992 to 1997, depositing 6.8 x 10⁶ m³ of lava flows. The island was enlarged by 378,527 m² and the height of the cone increased from 159 to ~300 m.

Information Contact: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.dpe.go.id).

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05/00 (BGVN 25:05) Elevated May-June seismicity associated with small ash plumes

This report covers the period of 29 May-26 June 2000. Krakatau volcano was last reported on in Bulletin v. 24, no. 8 (August 1999) and had remained at a stable level of activity up until 0600 on 29 May, a time when seismometers began registering ~150 earthquakes attributed to the beginning of an eruption. Cloud cover made visual inspection impossible on 29 May but inspection on 30 May revealed a dark gray ash plume rising 500 m into the air. Further seismic data from 30 May registered an increase in the number of both deep and shallow volcanic earthquakes; eruption and emission earthquakes increased to ~330 events daily before dropping back down to ~200 events on 31 May. During 3-5 June seismicity remained elevated. A white plume reached ~20 m on 3 June, a very dark plume rose to ~50-500 m on 4 June, and a gray plume ascended to ~50-300 m on June 5. Bad weather inhibited visual observation throughout the rest of the reporting period.

Three booming explosions on 17 June coincided with a decrease in the number of volcanic earthquakes, as well as a marked increase in small explosion earthquakes. Seismic data for the week of 20-26 June contained several large and small explosion earthquakes; a thick gray ash plume was emitted on 25 June, rising to ~250 m. Krakatau remained at alert level 2 (on a scale of 4).

Background. Krakatau lies in the Sunda Strait between Java and Sumatra. Caldera collapse destroyed the ancestral Krakatau edifice, forming a 7-km-wide caldera. Remnants of this volcano formed Verlaten and Lang Islands; subsequently Rakata, Danan, and Perbuwatan volcanoes were formed, coalescing to create the Krakatau island extant prior to 1883. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan volcanoes, and only left a remnant of Rakata volcano. The post-collapse cone of Anak Krakatau (Child of Krakatau) grew within the 1883 caldera at a point between the former cones of Danan and Perbuwatan The subject of this report, Anak Krakatau, has been the site of frequent eruptions since 1927.

Information Contact: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.dpe.go.id).

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