In historical times, eruptions of the Sinabung volcano on the island of Sumatra were never described, until a change was announced in August 2010. But how did this dormant volcano awake? At first earthquakes struck in the area around the volcano, followed by small ava­lanches and mostly weak eruptions. There was then no further volcanic activity for almost three years.

The precursor to an awakening of the 2,460 m volcano was again measured in September 2013. Several thousand inhabitants of villages in the surrounding region were evacuated.

In February 2014, further eruptions occurred, during which the explosiveness, height and distance of the ejected rock fragments increased. Scree and gas clouds were hurled into the air up to a height of more than 10 kilometres. A glowing stream of gas, dust, rock and lava flowed down the slopes. According to press reports, at least 15 persons were killed during this most recent eruption.

Sinabung is one of 147 active volcanoes in Indonesia. It is a stratovolcano, whose peak is characterised by earlier eruptions and summit craters which run in a north-south direction.
Its lava consists of mostly andesitic but also dacitic rock, i.e. a rock similar to basalt, but with a much higher silicic acid (SiO2) content. Andesite is frequently found in the Andes (mountain range in South America). The temperature of the lava is between 900 and 1,000° C; it is relatively viscous, and therefore tends to flow rather slowly down the volcanic cone. In the process scree avalanches can form and the still hot, viscous lava is fragmented. Volcanoes whose lava has an andesitic composition can therefore also be particularly explosive. Some­times pyroclastic clouds form which rise up high into the atmosphere and thus affect the health of local residents, agriculture and animal husbandry, as well as air traffic.

The activity of Sinabung shows that dormant volcanoes emit characteristic signals when they become active, making it possible to warn the population.

Immediately off the coast of Sumatra the Indo-Australian Plate subducts beneath the Eurasian Plate. The subduction rate in the Sunda Arc is almost 7 cm per year. The result is repeated violent earthquakes, tsunamis and volcanic eruptions.

Text: Dr. Ute Münch, Earth System Knowledge Platform

text, CC BY;photographs and graphics, if there are no conflicts with other licences as well: CC BY