Indonesia's Mount Slamet Hazard Radius Cut to 2 Km, Alert Status Unchanged

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A 100-200 meter white smoke emitted from the peak of Mount Slamet was visible from the Slamet Volcano Observation Post, Gambuhan Village, Pulosari District, Pemalang Regency, Central Java, on Friday, February 14, 2020. Volcanology experts from UGM revealed the discovery of hot cloud deposits marking that the mountain had erupted seven times. (ANTARA/HO-Pos PGA Slamet)

TEMPO.CO, Jakarta - Indonesia's Center for Volcanology and Geological Hazard Mitigation (PVMBG) has reduced the recommendation zone radius around the peak crater of Mount Slamet from 3 to 2 kilometers. However, the activity status of the highest mountain in Central Java remains unchanged, still at Level II (Caution) until today, Wednesday, June 3, 2026.

The acting Head of the Geological Agency, Lana Saria, stated that the decision was made after visual and instrumental monitoring showed relatively high volcanic activity, but not significantly increased. This condition is compared to when the recommendation radius was expanded on April 4.

"Intensive observations continue to be carried out to anticipate changes in activity and potential hazards that may occur," Lana said in her statement on Wednesday, June 3, 2026.

Mount Slamet is a stratovolcano with a cone shape and a height of around 3,432 meters above sea level. Administratively, the mountain is located in the areas of Pemalang, Banyumas, Brebes, Tegal, and Purbalingga.

Previously, the recommendation radius was increased from two to three kilometers based on the analysis of thermal crater images showing a temperature increase. The spread of heat anomalies around the crater also expanded. "The temperature rise indicates an increase in thermal activity around the crater," said Lana. (Read: Waspada Kemungkinan Gunung Slamet Akan Meletus Freatik / Beware of the Possibility of Mount Slamet Erupting Phreatically)

As a comparison, heat anomalies were still localized in the central part of the crater in 2024. But this year, their distribution has developed more widely and formed a circular pattern around the crater walls. Lana explained that this condition indicates the development of fracture systems and an increase in the intensity of volcanic gas release (degassing) in the crater area.

During the period from April 4 to June 3, 2026, no further increase in thermal activity was observed. Thermal drone measurements showed fluctuating temperatures of the crater's solfatara ranging from 407.2 to 478.7 degrees Celsius.

These measurements indicate that thermal activity is still relatively high but tends to be stable. There is no escalation compared to the conditions that formed the basis for increasing the recommended distance on April 4, 2026.

In addition to thermal parameters, several other monitoring indicators also show fluctuating activities but generally tend to decrease. Although Mount Slamet's activity has not returned to normal or background conditions.

Lana urged the public, hikers, and tourists to comply with the established recommendation radius. Currently, she added, potential hazards that still need to be aware of include eruptions that can produce volcanic ash, mud rains, and the potential projection of incandescent materials that may reach areas within a 2-kilometer radius from the peak.

Additionally, there is the potential for high-concentration volcanic gas emissions limited to the crater area. PVMBG also reminded that ash rain can occur around the crater or in other areas along the direction and speed of the wind.

Read: Indonesia's Mount Slamet Sees Spike in Volcanic Earthquakes

Indonesia's Mount Slamet Sees Spike in Volcanic Earthquakes

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