Semeru Eruption Update and Latest News on Volcanic Mud Flood

Semeru is one of the most active volcanoes in Java, Indonesia. This volcano erupted several times in the 2000s. Its most recent eruption was in November. According to the Semeru eruption update, this volcanic activity is still ongoing. This hazard also worsened at the peak of the rainy season.

Heavy rains caused the pyroclastic deposits previously on the higher slopes of Semeru to flow down and inundate residential areas. The locals call this mudflow a ‘Lahar flood'. The mudflow can be as dangerous as the eruption.

Semeru Eruption Update: December 2025

The 2025 Semeru eruption in Indonesia started on November 19. This volcanic activity was followed by a large ash column and a pyroclastic flow that went as far as 8.5 km from the summit. Authorities did not report any casualties or fatalities following the eruption. 

However, it affected communities near the Semeru summit and damaged the early warning system equipment.  Moreover, this eruption was not the last from the Semeru volcano. Several eruptions followed the November explosion. 

1. Late November

About a week after its first massive explosion, Mount Semeru erupted again in late November. On November 25, authorities recorded 45 eruptions from this volcano. Moreover, on November 27, Semeru erupted 54 times.

This high volcanic activity led authorities to increase Semeru’s alert level to level IV. During this period, the volcano emitted ash columns up to 800 m and pyroclastic flows up to 7 km. These volcanic activities caused the displacement of over 500 people. 

Moreover, this eruption forced authorities to advise the locals and visitors to avoid activities on the southeastern slope of the volcano or along the Besuk Kobokan River within a radius of 20 km from the peak. Authorities also established a general exclusion zone within a radius of 8 km. 

2. Early December

In December 2025,  the alert level of Semeru decreased to level III. However, from December 1 to 4, this volcano frequently erupted. The highest recorded volcanic activity was on December 4. That day, this volcano erupted 32 times.

However, there were no reports of ash plumes. The authorities only recorded a volcanic earthquake and a rockfall earthquake. During this period, authorities set a 13-km exclusion zone on the southeastern slope and an 8-km exclusion zone in general.

3. End of December

The eruption of earthquakes in early December was not the Semeru last eruption. According to the report of the Semeru eruption update, this volcano experienced increased activities starting from December 20 to the end of the month.

On December 20 and 21, Semeru emitted ash plumes several times. The height of these columns was between 1,100 and 1,200 m. On December 26, an ash column 900 m tall was spotted on Semeru’s peak.

Four days after Christmas, authorities recorded a few large explosions from Semeru. The next day, this volcano experienced 24 earthquakes. Moreover, the highest ash column recorded on December 29 and 30 was 1,200 m tall.

Semeru Lahar Floods

Semeru volcano eruption update reported that this volcano has erupted over 3,000 times this year. While erupting, this volcano emitted millions of cubic meters of pyroclastic deposits. These materials will turn into a natural hazard called a Lahar flood or mudflows when mixed with large amounts of rainwater.

In December 2025, some areas on the Semeru slope suffered from lahar floods several times. How did these mudflows affect those regions?

1. The Cause of Lahar Flood on Semeru

Several factors trigger pyroclastic deposits to flood residential areas on Semeru. One of these factors is extreme rainfall that lasts for hours on the peak of the volcano. 

The heavy rains increase the flow rate on the upstream of the aBesuk Kobokan River and other rivers on the Semeru slope. Thus, the rainwater sweeps the pyroclastic deposits on the Semeru Peak.

Moreover, pyroclastic deposits are brittle. Therefore, accumulated rainwater can easily turn these materials into cold lava floods. In addition, these materials from Semeru also lowered the capacity of Besuk Kobokan and other rivers. Thus, the volcanic mud easily overflows villages. 

2. Date of Occurrence and Affected Areas

The first Lahar flood was on December 5, 2025. Since then, cold lava floods have occurred several times, including on December 12 and 2. The flood hit the Regoyo river basin and overflowed Sumberlangsep village in Lumajang.

3. Impact of Lahar  Flood

The Lahar flood submerged up to 17 houses and a mosque in Sumberlangsep village. The height of this mud reached 4 meters in several areas. Moreover, the pyroclastic flood inundated roads and bridges. Thus, Sumberlangsep village was isolated.

During the flood, villagers evacuated their houses and sought shelter in the three nearest hillsides. According to the report, approximately 500 people are affected by this disaster.

According to the Semeru eruption update, this volcano is in level III status alert because it is continuously erupting. Authorities also reported that the eruption damaged early warning system equipment. However, they can continue alerting people if they have cell broadcast technology from smsbroadcaster.com. This technology is highly efficient and effective for disseminating eruption alerts.