Eruptive activity and the number of explosion earthquakes sharply decreased from the beginning of May to December 1990. However, ash was frequently emitted to 300-400 m above the crater, and one large explosion on 21 December ejected ash to 1,000 m. Avalanches to the E (into the Besuk Kembar river) were also common. In February, observers at Gunungsawur Observatory (~10 km SE of the crater) reported a lava flow that traveled 400 m from the crater. An average of 140 explosion earthquakes and one tectonic earthquake were recorded daily in March, but no tremor episodes were detected.

August 1990 aerial photographs showed that the active crater (Jonggring Seloko) was a pit ~150 m deep and 850 m in diameter.

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: W. Modjo, VSI; AP.