Mount Sinabung is a 4,260m tall volcano in the North Sumatran region of Indonesia. It formed over centuries, 15,00,000 years ago during the last Ice Age with layers upon layers of thick magma oozing out slowly from the top and building up along the sides, raising its height. This kind of volcano is called a ‘stratovolcano.’ It is a part of a chain of volcanoes around the Indonesian islands of Java and Sumatra that were created because the Indo-Australian tectonic plate and the Eurasian plate collided with each other, the former gradually sliding underneath the latter. As the plate gets pushed underneath the weight of the Eurasian plate, the friction and pressure causes its rock to melt into magma, which rises to the top to form a volcano.
Sinabung has been active since 2010 when it reawakened from a 400-year-old slumber from its last recorded eruption in 1600, and rumbled for a month. In September 2010, it erupted at least thrice, spewing ash and dust up to 3km in the air. Rain mixed with this ash, covering everything with a thick layer of mud for kilometres in the vicinity. The government had already begun monitoring the volcano intensively and evacuated several thousands.
In the second half of 2013, it erupted again and kept continuously belching out small quantities of ash and dust into the air. In 24 hours, between January 4 and 5, 2014, the volcano erupted over a 100 times. In February 2015, Sinabung released a heavy screaming jet of gas and dust into the air, killing 16 people. In May 2016, it again killed seven people who thought it had been safe to move back to evacuated regions.
The area around the volcano has been since evacuated properly and Sinabung erupted several times more between 2016 and 2018.
In February 2018, Sinabung gave a shudder and explosively erupted, releasing more material into the sky than ever before. It shot dust and ash clouds over 5km into the sky, becoming news all over the world. Thanks to social media, images and videos of this eruption spread widely. There have been no casualties or injuries as citizens had evacuated to a safe distance away well beforehand.
But why does this volcano release ash and dust only instead of the lava that we associate with typical volcanoes? The explanation lies in the type of volcano that Sinabung is. A stratovolcano (commonly known as composite volcano) has only a single vent at the top. Because it is disturbed by the subducting plates below it, the thick magma doesn’t flow quickly. But the pressure builds up to such high levels that when an explosion finally occurs, all the gases and ash burst forth, hiding from view this thick lava (magma that is released in a volcano is called lava).
Such an explosion and subsequent flow of material is called a pyroclastic flow. It’s deadly; the rocks, ash, and gases can race down the mountain and cover entire cities in a matter of minutes. They reach speeds of up to 700 km/hr and the emitted gases can be as hot as a 1000 degrees Celsius. The ejecta will first completely cover the sky and cut off sunlight, and then fall down on the surrounding land, covering everything. They cause massive destruction to plant and animal life, and can even hurt oceans. Indeed, a pyroclastic eruption of Mt. Vesuvius is what wiped out the civilisation of Pompeii.
Indonesia has over 120 active volcanoes, and most of the deadly eruptions through history have been recorded here; all of them pyroclastic flows. A famous example is the Krakatoa eruption of 1884, which was the loudest sound ever produced on earth, heard all the way on the other side of the globe. Mt Tambora erupted in 1815 and is considered the deadliest volcanic eruption in history. It killed over 70,000 people. It’s pyroclastic material blocked out sunlight and travelled such large distances that it brought about a “volcanic winter” where ash fell all over the world, crops and animals were killed everywhere, the global weather changed, and humanity had to live through a famine.