a explosive eruption of unexpected magnitude of one of Indonesia’s largest volcanoes ended up killing more than a dozen people after pyroclastic flows tumbled down the mountain’s slopes. Located on the Indonesian island of Java, Semeru has erupted for almost 8 years now, but on December 4 the volcano set off an explosion that reached over 50,000 feet (15 kilometers) when the lava dome at summit probably collapsed due to heavy rains. Some of the pictures from the minutes after the eruption are really surprising because people rushed to escape the hot ash flows.
The unexpected explosion
This rash seems to come out of nowhere according to Indonesian officials. Although the volcano has been erupting since 2014, Semeru had not shown a sharp increase in earthquakes that usually accompany new magma and eruptions. However, heavy rains falling on the precarious lava dome at the top likely caused the dome to collapse. This, in essence, is like uncorking a bottle of champagne, causing bubbles to form in the magma so quickly that an explosion ensues.
Ash and debris covering the landscape near Semeru in Indonesia after the eruption of December 4, 2021. Credit: BNPB Indonesia
Images of the area around Semeru show deep ash deposits, probably due either to the direct fall of ash from the explosion or to the pyroclastic flows that tumbled down the volcano. These streams of ash, debris and hot volcanic gases can quickly bury the landscape, destroying buildings, vegetation and people. (By the way, ignore the reports of “lava flows” – there was probably none involved in this, although the local language translation has confused mudslides and lava flows in the past).
Stretches of forest near Semeru were destroyed by the eruption of December 4, 2021. Credit: BNPB Indonesia.
Over 8,000 people live within 10 kilometers (6 miles) of the volcano and over a million within 30 kilometers (18 miles). Many villages near the volcano have been inundated by pyroclastic flows and lahars (volcanic mud flows) created by the mixture of ash and rain. People near the volcano filmed the chaos as they also tried to flee the area.
There is speculation that the ash plume from this eruption may have been caused by a phenomenon called “phoenix cloud”. This is when the ash and pyroclastic gases begin to rise after depositing all the heavy debris in the stream. This can lead to ash plumes that are very humid and cold compared to those produced by a direct blast in the atmosphere.
Dozens of people are still missing and the rescuers are busy with thick ash and bad weather in their attempts to locate survivors. Thousands of buildings were also damaged by the eruption, mainly from the roofs collapsed by ash and mudslides / pyroclastic flows. More 1,000 people now live in temporary shelters early in recovery from the rash and they may need to stay in shelters for the foreseeable future.