Semeru Eruption: The Chronology of Eruption

Saturday (4/12/21), the tallest mountain in Java island (Semeru) in Lumajang, East Java has erupted. As of Monday night (6/12/21), the Head of the Operations Control Center of the National Disaster Management Agency (BNPB), Abdul Muhari, through a press statement that the victims of the eruption are 22 people died, 56 people suffered burns or injuries, and 22 were reported missing. Meanwhile, the number of people affected by the volcanic ash was 5,205, and 2,004 people were displaced.

Chronology of Semeru Eruption

The National Disaster Management Agency explained the eruption of Mount Semeru in Lumajang, East Java, on Saturday afternoon, December 4, 2021. The Mount Semeru Volcano Observation Post at Gunung Sawur Post, Candipuro, detected vibrations of lava floods or hot clouds since 14:47 WIB. “The maximum amplitude of vibration is 20 millimeters,” said the Head of the BNPB Disaster Information and Communication Data Center, Abdul Muhari, on December 4, 2021. At 15.10 WIB, PPGA Gunung Sawur reported volcanic ash from hot clouds sliding to Besuk Kobokan. The hot cloud smelled of sulfur. “In addition, visual reports from several locations also darkness due to fog from volcanic ash,” said Muhari. The Center for Volcanology and Geological Hazard Mitigation (PVMBG) stated that incandescent lava avalanches were observed at a sliding distance of 500 to 800 meters from the avalanche center 500 meters below the crater. BPBD Lumajang then warns the resident not to go near the Mujur River Basin and Curah Bobokan.

How Semeru Eruption Occurred

The former head of the Center for Volcanology and Geological Hazard Mitigation (PVMBG), Surono, explained the eruption of Mount Semeru are because of the release of the big mound that created by a minor eruption in the long term that mixed with rain and turned some of the lava domes into stone, some of which are still liquid by avalanches. In contrast with Merapi 2010, the crater collapses into an eruption and hot clouds. Surono also explained that the mound produces steam or gas mixed with fine dust, gravel material to chunks that form hot clouds of avalanches. Because it was created from an earlier fallen lava dome and not a hot cloud of eruption, he said the eruption that occurred is possible. The following eruptions will occur when the dome is still there. If the dome is no longer there, no more hot clouds will fall because nothing was aborted.
Professor of the Faculty Geological Engineering of Padjadjaran University, Prof. Nana Sulaksana, said that the eruption did not occur quickly on Saturday afternoon. Still, indeed there has been an eruption of magmatism activities long before. Just yesterday during the big eruption, coincidentally with heavy rainfall. Nana explained that the eruption of Mount Semeru was caused by two working forces, namely endogenous and exogenous. Endogenous forces occur from magma activity that pushes volcanic material up to the surface, while exogenous are caused by heavy rain. Volcanic material that accumulates in the dome is in contact with the water. The accumulated material is then drained by water and drifts down through valleys and rivers. As a result, the lava flood could sweep the area in the Semeru valley. “If there is no rain, then all the material that comes out has not immediately turned into lava. This is because of the rainy season, it happened to be a heavy rain, the material piled up above was exposed to water and drifted into the river,” said Nana. Semeru eruption has its own character. This is because every volcanic in Indonesia has its magma chamber. Judging from the type of eruption, Mount Semeru has a large eruption explicitly based on research and historical results. After that, it will fall asleep. This character is different from other mountains such as Merapi or Sinabung.

The current condition

On Tuesday (7/12/2021) morning, volcanic material was still coming out from the Semeru crater. Meanwhile, the weather around the mountain is currently sunshiny. Some residents have been returned to their houses. This morning, the President of Republic Indonesia, Joko Widodo, came to the disaster area to visit the refugee and assure relocation for 2000 residents.

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