The geography of Indonesia is dominated by volcanoes that are formed due to subduction zones between the Eurasian plate and the Indo-Australian plate. Some of the volcanoes are notable for their eruptions, for instance, Krakatau for its global effects in 1883, Lake Toba for its supervolcanic eruption estimated to have occurred 74,000 Before Present which was responsible for six years of volcanic winter, and Mount Tambora for the most violent eruption in recorded history in 1815.
Mount Sinabung (Gunung Sinabung) is a Pleistocene-to-Holocene stratovolcano of andesite and dacite in the Karo plateau of Karo Regency, North Sumatra, Indonesia, 25 miles from Lake Toba supervolcano. Many old lava flows are on its flanks and the last known eruption, before recent times, occurred in the year 1600.
Solfataric activities (cracks where steam, gas, and lava are emitted) were last observed at the summit in 1912, but no other documented events had taken place until an eruption in the early hours of 29 August 2010. With the 2010 eruption, Sinabung joins other, long inactive volcanoes such as Fourpeaked Mountain in Alaska which have erupted in recent years.
Most of Indonesian volcanism stems from the Sunda Arc, created by the subduction of the Indo-Australian Plate under the Eurasian Plate. This arc is bounded on the north-northwest by the Andaman Islands, a chain of basaltic volcanoes, and on the East by the Banda Arc, also created by subduction. Sinabung is an andesitic-dacitic stratovolcano with a total of four volcanic craters, only one being active.
On 29 August 2010 (local time), the volcano experienced a minor eruption after several days of rumbling. Ash spewed into the atmosphere up to 1.5 kilometres (0.93 mi) and lava was seen overflowing the crater. The volcano had been inactive for four centuries with the most recent eruption occurring in 1600.
On 31 August, 6,000 of the 30,000 villagers who have been evacuated returned to their homes. In Indonesia the volcano was assigned category “B”, because it was not active for more than 400 years, which means it is not necessary for it to be monitored intensively. (Other volcanoes, in category “A”, must be monitored frequently).
On Friday 3 September, two more eruptions were noted. The first happened at 04:45 am in the early morning, forcing more villagers to leave their houses - some of them had just returned the day before. This eruption was the most intense so far, with ash spewed up into the atmosphere about three kilometres high.
Some hours before the eruption a warning had been issued through the volcanology agency, and most villagers were prepared to leave quickly. A second eruption occurred the same evening, around 18:00 pm. The eruption came with earthquakes which could be noticed in a 25 kilometres distance around the volcano.