Phreatic eruption is steam-driven explosions that occur when water beneath the ground or on the surface is heated by magma, lava, hot rocks, or new volcanic deposits.
The intense heat of such material may cause water to boil and flash to steam, thereby generating an explosion of Mt Merapi that occurred around 7.40 a.m. and caused ash rain in the surrounding of the mountains.
"Based on the information from the residents, there was a rumble followed by ash rain. But we encouraged them to remain calm because the condition has returned to normal," the agency`s Emergency Division head Didik Wahyo Nugroho told Antara in Jakarta on Friday.
The agency has sent teams to three districts located in the danger zone of Merapi namely Srumbung, Dukun and Sawangan to help calm the residents.
Official of the Merapi observation post Yulianto confirmed the phreatic eruption occurred at around 07:40 pm.
The eruption, according to him, was the remains of the eruption of 2010. The same thing also happened in 2013.
Meanwhile, the residents living on the slopes of Merapi in Donokerto Village, Sleman Regency of Yogyakarta, were panicked and scattered outside their houses during the phreatic eruption.
"At first people heard a roar, some of them thought it was the sound of a helicopter, but it lasted for about five minutes and followed by a thick and large white smoke rising up from the Merapi crater," said Donokerto resident Rony Arya.
"There was thin ash rain and grains of sand were carried over here, as the clouds became dark like cloudy rain," he added.
According to him, the panicked residents were immediately evacuated to the nearest refugee camp to anticipate the worst possible outcome.
"But after waited for an hour and there was no subsequent eruptions, residents finally returned to their homes," he said.
However, the village officials called on the residents to keep their distance at three kilometers aways from the peak of Merapi.
"Some residents were back to their activities, but some others remain alert," Arya noted.