JAKARTA (THE JAKARTA POST/ASIA NEWS NETWORK) - Another phreatic eruption occurred at Mount Merapi in Magelang, Central Java, at 1.25am on Monday (May 21).

The eruption, which lasted for only 19 minutes, spewed volcanic ash as high as 700 metres into the sky.

It was smaller than the first eruption that occurred on Friday, during which Merapi ejected volcanic ash 5,500 metres from its crater.

A phreatic eruption is caused by magma, which heats up water beneath the ground or on the surface.

Although the Volcanology and Geological Disaster Mitigation Centre (PVMBG) has asked people around the area to remain calm and go about their daily activities, it has called on mountaineers to avoid hiking beyond Pasar Bubrah or the Post II area, which is only 2 kilometres from the top of the mountain.

"Merapi is currently prone to landslides, so it is very dangerous for mountaineers," said PVMBG official Sunarta in a press release on Monday.

He also asked residents in the area to stay alert, especially during rain, because of a possible cold lava flood from the volcano.