Ash emissions and pyroclastic flows continued at Semeru during June-September. In early June emissions of "white colored ash" rose up to 700 m above the summit. Lava avalanches flowed along the Besuk Kembar river valley to distances of up to 750 m from the crater. A pilot reported seeing the top of an ash plume drifting northeast at 6,100 m altitude at 0944 on 13 June. Explosions and lava avalanches occurred frequently during the first week of July, although thick haze often precluded visual observation. On 16 July a pyroclastic flow along the Besuk Kembar river reached a distance of 2 km from the summit.

During the week of 3-9 August white-brown ash emissions rose 600 m above the summit and explosions and avalanches increased in number. At 0410 on 5 August a pilot reported a volcanic ash plume at 6,100 m altitude moving west. During 10-16 August there were eight ash explosions producing columns reaching 500 m above the summit. Five lava avalanches flowed down the Besuk Kembar river valley. Pyroclastic flows and rockfalls moving 2 km down Besuk Kembar river originated from Jonggring Seloko Crater during 18-23 August. Scientists at Sawur Observatory could see lava rockfalls running 400-700 m down Besuk Kembar.

J. Bardintzeff reported that on 15-16 August activity seemed to alternate from rest to explosiveness over periods of several hours. Explosions followed each other at intervals of 15 minutes, but later this interval increased to 1 hour. The first explosion of the period was the strongest and generated a plume 1,000 m high. Explosions were observed at 0732, 0812, 0840, 0905, and 0950 on 16 August.

Pyroclastic flows and rockfalls, with moderate ash emissions, continued in September. An ash explosion during the week 14-20 September produced thick dull-white to dark-colored ash that reached 500 m above the summit.

Geological Summary. Semeru, the highest volcano on Java, and one of its most active, lies at the southern end of a volcanic massif extending north to the Tengger caldera. The steep-sided volcano, also referred to as Mahameru (Great Mountain), rises above coastal plains to the south. Gunung Semeru was constructed south of the overlapping Ajek-ajek and Jambangan calderas. A line of lake-filled maars was constructed along a N-S trend cutting through the summit, and cinder cones and lava domes occupy the eastern and NE flanks. Summit topography is complicated by the shifting of craters from NW to SE. Frequent 19th and 20th century eruptions were dominated by small-to-moderate explosions from the summit crater, with occasional lava flows and larger explosive eruptions accompanied by pyroclastic flows that have reached the lower flanks of the volcano.

Information Contacts: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Jacques-Marie Bardintzeff, Laboratoire de Petrographie-Volcanologie, bat 504, Universite Paris-Sud, 91405, Orsay, France.