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| A lava dome of mount Merapi from Kaliadem, Agustus 23, 2010 (Reynold Sumayku/NGI) |
Tri Mujianto, from the Merapi mountain observatory in Jrakah, in the Selo subdistrict of Boyolali, said the lava dome had disappeared but he could not say precisely when.
"The dome is now no longer there but we were not able to monitor when it collapsed. Some [of the material] may have fallen inside [the crater] while some may have flowed into the channel of Apu River," he said.
They have not been able to determine the cause of the collapse, as there has been no rain in the crater area for days. They also haven't been able to estimate the volume of cold lava in the collapsed dome.
Tri said the alert status for Merapi remained at the normal level but warned that should rains fall over the crater, cold lava stream may flow down through natural river channels. A cold lava stream is congealed lava and other volcanic mud and debris flushed down the slopes of a volcano by heavy rains.
"Entering the rainy season, the frequency of cold lava stream is rising. We have checked the conditions at the craters several times and it appears to still be very much unstable. People on the slopes of Merapi, especially those living on the banks of rivers originating from the peak, should remain alert," he said.
Meanwhile, Subandriyo, the head of the Volcanology office in Yogyakarta, said that parts of the lava dome facing Boyolali district had collapsed, and ventured that it was due to its fragile condition.
"The collapse was not directly recorded because there were so many small deflagrations. On the scale, they did not even reach one kilometer down the slope," Subandriyo said.
He warned that rains with intensity of more than 20 millimeters and lasting more than two hours were enough to trigger flash floods of cold lava down the mountain's slope.
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