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ount Merapi, located in Yogyakarta, emitted high-pressure gas on Sunday afternoon that caused its crater wall to collapse and volcanic ash to fall on its western slope, as smoke billowed up to 1 kilometer into the sky.

As the gas discharge was not followed by other dangerous volcanic activity, the phenomenon was regarded as a small-scale volcanic eruption and the volcano’s alert status remained normal.

“Mount Merapi’s status remains normal because there was no dangerous volcanic activity,” said Volcanic Technology Development and Research Center (BPPTK) head Subandriyo on Monday.

According to Subandriyo, the incident was due to the accumulation of gas produced by the volcano’s magma. As the gas’ exit fumarole was too narrow, the volcano eventually released all the buildup of very high pressure gas by erupting, which caused the crater wall to collapse and this emitted a massive rumble.

“The incident can be called a small-scale ‘volcanic eruption’,” said Subandriyo.

In Bandung, West Java, Geological Disaster Mitigation and Volcanology Center head Surono said the collapse of the crater wall of Mount Merapi on Sunday was a natural process. The collapse was not due to an increase in volcanic activity of the most active volcano in Indonesia.

The lava dome collapsed because rocks and eruption material from the 2010 eruption were still not completely set and stable. “The force of gravity or the weight of the rocks has caused the crater wall to collapse,” Surono said in a text message on Monday.

The lava dome collapsed on Sunday at 6:02 p.m. local time. According to Surono, officers at the Babadan observation post, located more than 5 kilometers from the mountain park, heard a rumbling sound which was a result of the collapse.

They also observed smoke billowing at a height of 1,000 meters above the peak, slanting westward. Surono added that a slight ash cloud occurred, followed by the smell of sulfur. “Based on a report, a rain of ash took place in Jurang Jero and Srumbung. The smoke was not emitted from an eruption, but rather by the collapsed crater dome,” Surono said.

Surono urged residents living around the volcano to remain calm and not panic due to unclear rumors. “Information on volcanic activity can be obtained from the Yogyakarta BPPTK,” added Surono.

Subandriyo said the amount of gas pressure was unclear, as his office was unable to conduct the measurements. The crater emitted thick smoke mixed with gas and ash and rose up to 1 kilometer. Its shape resembled a pyroclastic flow.

According to Subandriyo, the phenomenon was the first after it erupted in 2010. “The volcanic magma is currently rich with gas. This did not occur before the 2010 eruption,” he said. He added that in the past month, the crater dome of the 2,800-meter tall volcano often collapsed because the structure of the dome was not yet stable and due to the drought.

He also called on residents living along the slope of the mountain to remain calm because the status remained remains normal. However, he has advised trekkers not to approach the peak as it was quite dangerous. “Climbers should only hike up to Pasar Bubrah,” said Subandriyo. Pasar Bubrah is located around 400 meters from the peak.

Yoto, a resident in Jengglik hamlet, Ngablak district, Magelang, Central Java, who was at the western slope of Mount Merapi, said he heard a loud rumble coming from the peak of the mountain on Sunday afternoon. “I heard the rumble at around 6 p.m.” he said.

According to him, the gas emission led to ash rain which lightly covered Purwosari hamlet, Ngablak village.

Jamin, head of Kali Tengah Kidul hamlet, Cangkringan, Sleman, Yogyakarta, located around 4 kilometers from the peak, said he also heard the rumble on Sunday afternoon.

However, the incident did not cause residents to evacuate due to the ash rain. “I heard the rumble, but residents remained calm. We are used to hearing these rumbles,” said Jamin.