MERAPI Central Java Lat. 07.54S, Lon. 110.44E; summit elevation 2911 m.
This strato volcano with an active summit lava dome has a grim history. It is located 70 km SE of Dieng and immediately N of Yogyakarta, a city of half a million persons. 32 of its 67 historic eruptions are known to have been accompanied by nuees ardentes, more than any other volcano in the world, and 11 of them have caused fatalities. The volcano is carefully watched by several VSI observatories and monitored by at least 6 telemetering seismometers.
3/76-Activity increased in Feb compared to the previous months. The number of volcanic earthquakes increased, the lava dome grew higher, and red glares were observed at 2 points on the dome. The number of glowing avalanches decreased from Jan, however, and there was only one nuee ardente, on 4 Feb, that traveled about 750 m SW into the Batang River.
On 5 March at 1503, reddish white smoke emitted above the summit. The volcano was hidden behind clouds. On 6 March, small avalanches took place continuously from 0135 to 0348, then the volcano was covered by clouds. At 1702, three avalanches were observed. From 1733 on, bigger nuees ardentes took place, interspersed with minor ones, until 13 March.
From the evening of 8 March until the evening of 9 March, when the volcano became visible from the Ngepos Volcano Observatory 11 km SW of the summit, it could be observed that the lower part of the lava dome, estimated to be about 400,000 cubic meters, or almost one third of its volume (1.4 million cubic meters), had slid away. During the events on 6 and 7 March nuees ardentes moved SW, entering a tributary of the Batang River and continuing much farther downstream into the rivers Sat, Blongkeng, and Bebeng. The prevailing wind blew E, depositing finer distance of 37.5 km. Thickest ash falls were on the ESE measured 5 mm and temporarily panicked the villagers.
Nuees ardentes caused forest fires on the upper SW flank near the tributaries of the rivers Sat, Blongkeng, and Bebeng. Roughly 580,000 cubic meters of avalanche material were deposited on the upper SW flank.
11/76-Merapi was unusually active on 6 and 7 Nov, emitting 3000-m ash clouds, and nuees ardentes that moved 2.5 km down the SW flank. No casualties or serious property damage were reported. The new dome, which began growing after the March activity, had an estimated volume of 20,000 cubic meters in June.
3/77-The new lava dome had increased in volume from about 2,000 cubic meters in June, 1976 to about 2.5 million cubic meters in late Feb, 1977. Avalanches continued to occur at intervals of 15 minutes or more; sometimes, but not every day, nuees ardentes accompanied the avalanches, especially after a heavy rainfall.
7/79-Newspaper reports state that hot avalanches from the lava dome decreased in frequency during May, after as many as 40/day had occurred in April. The May avalanches traveled as much as 1.75 km. Growth continued at the new dome, which first appeared in Jan 1979 and now covers remnants of the 1973 and 1978 domes.
1/80-Lava dome extrusion continued through 1979 from a SW flank vent about 200 m below the summit. A lobe of lava extending a short distance down the upper SW flank gave the dome an asymmetrical form and occasionally spawned nuees ardentes d'avalanche. The last significant nuee ardente traveled about 6 km in Aug 1979, but remained within the forbidden zone, where human access is prohibited.
2/81-The lava dome that began to emerge at the summit in 1979 was still growing in Feb and had reached an altitude of 2947 m. Lava fragments from the E and central part of the cone had moved 2.0 km toward the Batang River, and 250-500m farther in Dec. Personnel at the Ngepos Observatory have counted 34 larger and 468 smaller lava avalanches in recent months; the time interval was not reported. Nuees ardentes d'avalanche occurred in Nov and Dec but were confined to the summit area. Two Minakami A-type earthquakes, the first in several months, were recently recorded by the seismograph at the Babadan Observatory (4 km from the summit). No important lahars have occurred along the Putih, Bebeng, and Krasak Rivers since the beginning of this year's rainy season.
12/81-Landsliding on the summit lava dome 29 Nov was followed by a nuee ardente d'avalanche that flowed 3-4 km down the Batang River valley. Ash fell 15-20 km to the NW. An avalanche of incandescent lava from the dome set fire to 15 hectares of tropical mixed forest. On 1 Dec approximately 3/5 of the lava dome slid down the upper flank, flooding the Krasak and Batang Rivers and adding 1.2 million cubic meters to the deposits from previous debris flows. As of early Dec, there were about 6.9 million cubic meters of unstable material in the summit area. Authorities feared that monsoon rains could cause large cold lahars, and officials of Magelang District, W of Merapi, remained on constant alert in early Dec. AFP reported that the volcano continued to emit ash in mid-Dec.
A tripartite electro-seismometer at Babadan Observatory and 2 single-detector seismographs near the volcano recorded continuous trains of tremors and very shallow earthquakes that may have been landslide events. Continuous records of magnetic values showed no significant changes. No sharp variations in CO2 and H2S emissions were detected. Tilt measurements showed deformation of 10-20 urad, within the normal variation.
1/82-Heavy ashfall that began 6 Feb damaged crops and halted traffic in the nearby Boyolai region, according to a press report dated 8 Feb.
6/84-The quoted material is a report from Adjat Sudradjat.
"Merapi erupted 15 June between 0215 and 0600, accompanied by nuees ardentes that extended 7 km down rivers (the Batang, Bebeng, and Krasak) on the SW side of the volcano. An eruption plume rose to 6 km height and caused ashfall in Muntilan, Ambarawa, and Semarang, approximately 60 km N of the volcano. The eruption was accompanied by detonations. The first explosion was followed by a milder eruption producing a plume to 2 km height and a nuee ardente to 6 km distance at noon. The frequency of nuees ardentes progressively decreased until the morning of 16 June. No eruptions were observed the following day. Lahar material estimated to exceed 4 million cubic meters along the Bedeng, Krasak, and Putih Rivers, may threaten Magelang city (population 125,000).
An image from the NOAA 7 polar orbiting satellite on 15 June at 1441 showed a hazy area that did not seem to be weather-related extending from 7 degrees S to 5 degrees S and from 110 degrees E to the edge of the image at 106 degrees E. The hazy emission apparent at the volcano. No evidence of activity could be seen on the image returned the next day at about the same time.
Newspapers reported ashfalls at Magelang (30 km NW of Merapi) and Salatiga (35 km NE of the volcano). Visibility near Salatiga was limited to 10 m and more than a cm of ash covered roads, slowing traffic. More than 2 cm of ash fell at Solo (45 km E of the volcano) and ashfall was reported at Cilacap, on the coast 160 km SW of Merapi.
"Seismographs detected a progressive increase in seismicity from 4 counts per day on 8 June to 59 on 12 June. A warning was issued 13 June, and the evacuation of 1000 persons from forbidden zone section VI (Kemiren village) was immediately implemented. The tong-tong warning system was tested again to be sure that it was operational. The eruption was preceded by an intense lava avalanche on 13 June that caused a nuee ardente d'avalanche (nuee ardente of Merapi type).
"As of 22 June, no volcanic A-type earthquakes had been recorded. The dominant seismicity has been continuous tremors, very shallow volcanic earthquakes, and avalanche events with multiphase signatures. Judging from this evidence, 2 possibilities are: 1) the 15 June eruption is the final phase of the 1969-1984 lava dome growth (effusive stage); or 2) the 15 June eruption is evidence of the has phase of a new cycle. Both possibilities would show quiescence of A-type earthquakes. The rate of dome growth, which was reported to have sharply increased 20-21 June, may tend to support the latter hypothesis if it continues.
"After the 1969 gas explosion, Merapi continued to build a lava dome in its summit crater. Since the orifice is not symmetrical, the lava dome becomes unstable as it grows, and portions slide away, producing avalanches and nuees ardentes d'avalanche. the maximum growth of the dome was 3.6 million cubic meters and the rate of growth was seemingly constant at about .1 million cubic meters per month. Intensive sliding usually occurs in rainy seasons, in Nov annually, and removes 20-30% of the dome's total volume. The lava blocks that slide away from the dome may originate lahars, affecting a large area of the SW sector of the volcano."