The 2,914-meter (9,616-foot) rumbling volcano has been on standby alert for more than a week, one level below that which would require a mandatory evacuation for more than 29,000 people living around its fertile slopes.

The heat clouds, known locally as "shaggy goats" and scientifically as nuees ardentes, are a typical feature of Merapi eruptions, said Mas Ace Purbawinata, who heads the observation department in Indonesia's main vulcanology office.

"These heat clouds are very characteristic of a Merapi eruption and are usually followed by much slower lava outflows," Purbawinata said.

The heat clouds -- high-density mixtures of hot, dry rock fragments and gases that move away from the vent that spews them at high speeds -- can reach a temperature of up to 600 degrees Celsius and speeds of more than 100 kilometres (62 miles) an hour, he warned.

Depending on the force of the blast -- Merapi does not have an open crater but rather one covered by an unstable lava dome -- the clouds could well exceed the 12 kilometer danger radius around Merapi's centre, he said.

Geologist Syamsul Rizal, who heads the disaster potential evaluation center of the same office, said that Merapi's massive lava dome, with a volume of more than one million metric tonnes, was acting as a "stone cork".

Under the lava dome, tremendous pressure from the magma and gas continuously exerts pressure on the dome and seeks the weakest point to break out, he said.

The dome of Merapi, according to field reports, was growing in various places at a rate of four to six centimeters (one-and-a-half to four inches) per day, causing a deformation in the lava dome, he said.

Two types of eruption could happen at Merapi: either the magma would seek the weakest point in the dome and burst out, sending a burst of heat clouds and lava out in one direction, or the lava dome could disintegrate from the pressure, causing an eruption in all directions.

Merapi's last eruption in 1994 emitted heat clouds that travelled some 7.5 kilometers down the slope of the volcano, killing more than 60 people and forcing the evacuation of 6,000 people.

Meanwhile on Monday, authorities around Merapi were trying to convince the population living in the most dangerous zones to evacuate to safer places.

Only a few hundred of the thousands living in the danger zone have been evacuated so far, mostly the elderly, women and children.

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