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Report on Krakatau (Indonesia) — September 1979


Scientific Event Alert Network Bulletin, vol. 4, no. 9 (September 1979)
Managing Editor: David Squires.

Krakatau (Indonesia) Tephra emission continues, but lava extrusion has ended

Please cite this report as:

Global Volcanism Program, 1979. Report on Krakatau (Indonesia) (Squires, D., ed.). Scientific Event Alert Network Bulletin, 4:9. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN197909-262000



6.102°S, 105.423°E; summit elev. 155 m

All times are local (unless otherwise noted)

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.

Geological Summary. The renowned volcano Krakatau (frequently misstated as Krakatoa) lies in the Sunda Strait between Java and Sumatra. Collapse of the ancestral edifice, perhaps in 416 or 535 CE, formed a 7-km-wide caldera. Remnants of that volcano are preserved in 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, and left only a remnant of Rakata. This eruption caused more than 36,000 fatalities, most as a result of tsunamis that swept the adjacent coastlines of Sumatra and Java. Pyroclastic surges traveled 40 km across the Sunda Strait and reached the Sumatra coast. After a quiescence of less than a half century, the post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former cones of Danan and Perbuwatan. Anak Krakatau has been the site of frequent eruptions since 1927.

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