The following additional information is from a report by scientists from the Volcano Observation Section, Geological Survey of Indonesia.

The eruption was preceded by an earthquake swarm that began on 4 September. The number of felt earthquakes reached a maximum of 120/day early in the swarm, then gradually declined. At 0700 on 15 September, a minor pyroclastic eruption began from a new vent (A in figure 1) at about 1100 m elevation on the S flank, producing a thick ash cloud. This activity was succeeded by lava effusion from vent A. Another pyroclastic eruption occurred on 17 September from a second new vent (B in figure 1), about 300 m S of vent A. Lava effusion began shortly afterwards from B. The two flows coalesced near vent B and moved downslope 200-300 m/day. The rate of movement gradually decreased to 10 m/day as the flow front reached more level terrain. By 21 October, the flow was ~ 50 m thick near its source, ~6 km long, and was only 400 m from Ulu City, but lava extrusion had apparently ended. The flow front continued to advance 5-10 m/day. Thick "smoke" was still being emitted from the summit crater Karangetang. Lava volume was estimated at 2 x l0⁷ m³.

Figure 1. Map of Api Siau, showing the 1974 and 1976 lava flows and the 1976 nuée ardente deposits. Courtesy of VSI.

Geological Summary. Karangetang (Api Siau) volcano lies at the northern end of the island of Siau, about 125 km NNE of the NE-most point of Sulawesi. The stratovolcano contains five summit craters along a N-S line. It is one of Indonesia's most active volcanoes, with more than 40 eruptions recorded since 1675 and many additional small eruptions that were not documented (Neumann van Padang, 1951). Twentieth-century eruptions have included frequent explosive activity sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters; collapse of lava flow fronts have produced pyroclastic flows.

Information Contacts: D. Hadikusumo, VSI.