The volcano is part of a chain of volcanic mountains stretching from northern Sumatra to the Lesser Sunda Islands.

[12] Semeru's deadliest eruption occurred on 29 August 1909 when pyroclastic flows and lava destroyed 38 settlements and 600-800 hectares of farmland.

[13] In May 1981, heavy rains caused the crater lake at the summit of the volcano to overflow, triggering a flash flood.

[14] The eruption was thought to have begun when a lava dome at the summit crater collapsed due to intense precipitation.

Initial reports confirmed three people, a store owner and two miners, were missing when the Gladak Perak Bridge in Lumajang collapsed due to a lahar.

More than 40 people were seriously wounded by pyroclastic flows,[36] with four burn victims taken to the intensive care unit for their severe injuries.

[39] However, the Indonesian National Board for Disaster Management (BNPB) said that ten miners thought to be missing at a sand mine in Lumajang were rescued, with some others still unaccounted for.

[40][45] The BNPB released an update stating that 15 people have died from the eruption while the number of missing individuals have risen to 27.

[7] Local news sources in the evening reported at least 22 fatalities, adding that only five bodies remained unidentified.

[52] At the Dr Haryoto Hospital, a cooler container was sent to store additional bodies due to overcapacity.

[55] Recovery teams found the body of a headless person at a sand mine in Sumberwuluh, Lumajang.

[56] The Deputy Regent of Lumajang said the identification of bodies through fingerprints were impossible due to the severe injuries endured.

[63] Officials updated the death toll to 57; with 48 of the victims found at the eruption site while another nine people died during hospitalization.

[67] The destruction of the Gladak Perak Bridge blocked vehicle access to villages in the area.

[70] Many village homes and vehicles in East Java were covered by heavy volcanic ash.

[71] On 5 December, volcanic ash fell in Pronojiwo District of Lumajang, forcing many evacuees, residents and aid officials to flee.

[81] According to the Amil Zakat National Agency, the eruption caused 310 billion Rupiah worth of damage to homes, public infrastructures, local businesses, and basic services.

[86] Local authorities urged residents to keep a distance of at least 5 km away from the main summit crater as there were concerns of large pyroclastic flows.

[87] The head of the BNPB warned of increased volcanic activity and the presence of pyroclastic flows in the volcano.

[88] The Meteorology, Climatology, and Geophysical Agency (BMKG) issued a two-day weather forecast on 6 December warning residents of heavy rain that could trigger more lahars.

The agency added that East Java had entered the rainy season where the threat of lahars are higher.

[89] The BNPB provided meals ready-to-eat packs, blankets, mattresses, masks and refugee tents to assist the displaced residents.

[91] Indonesian President Joko Widodo would visit the Lumajang Regency to lead the BNPB in the post-eruption recovery efforts on 5 December.

[90] Meanwhile, Khofifah Indar Parawansa, the Governor of East Java, relocated her office to the Lumajang Regency to aid in the evacuation process of residents in the danger zone.

[88] In an attempt to prevent further casualties, the Ministry of Energy and Mineral Resources suspended all mining-related operations in the immediate danger zone.

[94] A rescue operation to locate several miners unaccounted for on the afternoon of the eruption was planned to begin once activity on the volcano has decreased.

The Deputy Regent of Lumajang said that rescue and recovery efforts could not be carried out the night before due to difficult access to the affected areas.

[95] Recovery efforts to find victims were hampered after rescuers encountered soil that was still too hot to enter.

[98] He added that the relocation would take place after recovery efforts were complete in order to prioritize searching for missing people and treating the injured.

[99] Separately, Muhadjir Effendy, the Coordinating Minister for Human Development and Cultural Affairs of Indonesia said that he was searching for possibilities to connect Lumajang and Malang after the eruption severed the Gladak Perak Bridge.