Bali holiday plans have already been thrown into turmoil but Indonesia has warned that the first major eruption of Mount Agung on the tourist island hasn't happened yet and is imminent as a muddy lava spews from the volcano.
"What we are seeing at the moment are small explosions, throwing out hot gases and fragments of molten rock, or ash," said David Pyle, a volcano expert at Oxford University told AFP.
"The probability of a large eruption is high, but this may take some days or weeks to unfold."
Denpasar International Airport was closed on Monday morning, and remains closed today after Mount Agung sent volcanic ash high into the sky. That's what volcanologists call a phreatic eruption, caused by the heating and expansion of groundwater.
Officials now say cancellations could be extended as a sludge-like cold lava, known as lahar, has appeared — often a prelude to the blazing orange lava seen in many volcanic eruptions.
"The potential for a larger eruption is imminent," the Disaster Mitigation Agency said in a statement after raising the alert from three to its highest level of four, referring to the visible glow of magma at Mount Agung's peak.
Images show volcanic mud flows on the mountainside. Lahar carrying mud and large boulders can destroy houses, bridges and roads in its path.
What does this mean for tourists?
Major airlines have cancelled flights to Bali following the warnings that Mount Agung's volcanic activity could escalate further.
Mount Agung volcano continues to spew dangerous volcanic ash and steam more than 9km into the sky above Bali with Denpasar international airport remaining closed on today.
Indonesia's Volcano Observatory Notice for Aviation remains at the highest red rating - indicating a further eruption with significant emission of volcanic ash into the atmosphere is imminent.
Denpasar airport's closure, due to volcanic ash, is to be reassessed at 4pm local time on Tuesday.
The closure of the airport has scrambled summer holiday plans and left thousands of tourists stranded in Bali.
In a statement, the airport said 445 flights — including 196 international flights — were cancelled on Monday, affecting about 59,000 passengers.
Toay's cancellation is expected to impact 30,000 passengers trying to leave Bali.
An estimated 5000 passengers with Jetstar alone have been impacted by Mount Agung's eruption, the airline told AAP.
Last night, Jetstar acknowledged "further disruptions are possible this week depending on weather conditions".
The airline is offering affected customers the option of instead flying to destinations including Phuket, Singapore, Fiji or Tokyo at no additional cost. Other major airlines are all monitoring the situation but are unable to fly until the massive ash cloud dissipates.
The Bureau of Meteorology, which is monitoring the situation from its Volcanic Ash Advisory Centre in Darwin, said height of the eruption was "steadily increasing".
"Ash is currently observed to a height of 30,000 feet (or 9144 metres) and a small amount of ash has fallen at Denpasar Airport and across Bali's south east," the centre said.
"Eruptions and ash falls are likely to continue for at least the next 24 hours."
Geologist Mark Tingay, an associate professor from the University of Adelaide's Australian School of Petroleum, said the eruption had moved on to a more severe phase.
"The volcanic eruption has now moved on to the next, more severe, magmatic eruption phase, where highly viscous lava can trap gasses under pressure, potentially leading to an explosion," he said.
"From my own eyewitness experience, the situation on Bali is very calm, and life is mostly continuing as normal outside of the 8 to 10km exclusion zone. People have been evacuated from the exclusion zone for several months.
"The local authorities are extremely experienced in managing volcanic eruptions, and have the situation extremely well in-hand."
Stranded on holiday
Stranded travellers have made makeshift beds on the terminal's dusty floors. Others were considering a more than 10-hour journey across land to the city of Surabaya to begin a string of flights across Indonesia and eventually home.
But all were frustrated by the lack of information from the airlines.
The first Janeen McKay heard of the cancellations was a text from her brother back in Australia just before she arrived at the airport.
"I had nothing from Jetstar, they had my mobile number," she told AAP. She had been waiting for nearly 12 hours and was told by the airline she couldn't get home until Saturday.
McKay, an office manager, said she needed to get back home to take care of her elderly mother, while her sister, Wendy Lynch, needed to be at work as a nurse on Thursday.
"We had a really nice time in Bali but then we get here and this has just ruined it," McKay said.
"Why does it take five days to get us out of here?
"Not very happy." Veronika Naberezhnova said she was resigned to the waiting game too. "It's a bit annoying," the Department of Human Services employee said. "My family's waiting there (in Sydney) as well, they're all waiting, they're all stressed."
All flights cancelled
Jetstar confirmed due to worsening conditions all yesterday's flights were cancelled.
"While these disruptions are frustrating, we will always put safety before schedule. We appreciate our customers' patience," the airline on Monday morning.
"We are doing everything we can to get customers moving as quickly as possible. Our senior pilots will make an assessment with the latest information and forecasts from the Volcanic Ash Advisory Centre."
Virgin Australia has also confirmed all its flights in and out of Bali have been cancelled.
"Due to the significant volcanic ash and current weather conditions, Denpasar Airport is now closed and we have cancelled today's flights to and from Bali," the airline said.
"The safety of our guests and crew is our highest priority and our team of meteorologists continue to monitor the situation in consultation with the Volcanic Ash Advisory Centre."
AirAsia and AirAsia X flights in and out of Bali have been cancelled, the airline said, as well as flights in and out of Lombok.
Airlines are urging travellers to check their websites for updated flight status information on Monday. Travellers are also advised to check in with their insurance company to see if they will be covered for volcano-related activity.
Authorities have now raised the alert for the erupting volcano to its highest level and are ordering anyone within 10 kilometres of Mount Agung to leave.
An exclusion zone has been in place around the volcano since it started showing signs of a possible eruption in September. More than 140,000 people fled their homes inside the exclusion zone.
Ash from the volcano has covered nearby roads, cars and buildings as activity enters the "magmatic eruption" phrase, said Gede Suantika, an official at the Volcanology and Geological Disaster Mitigation Centre.
"It is still spewing ash at the moment but we need to monitor and be cautious over the possibility of a strong, explosive eruption," he said.
Mount Agung last erupted in 1963, killing more than 1000 people and destroying several villages.
Associate Professor Oliver Nebel from the School of Earth Atmosphere and Environment at Monash University said Mount Agung was part of the part of the Pacific Ring of Fire, made up of active volcanoes and others that have been dormant for decades or centuries that could still be revived.
"[These] volcanoes have a high volatile content (water, CO2, SO2), which, when close to the surface, are released from the melt — pretty much as bubbles out of a champagne bottle," he said.
"This causes molten rock to burst into tiny fragments — so-called defragmentation — which will be carried at high velocity into the sky. We call this a volcanic ash plume.
"Dependent on weather conditions, this ash will remain in the atmosphere and can be carried with winds. If a plane flies through the ash, it will cause abrasive damage to the windscreen and more importantly, when heated up in turbine engines, can be melted again.
"When melt droplets chill against ambient temperature, a fine layer of volcanic glass then stalls the engine. Hence the danger to aviation services."
Prof Nebel said the energy of the eruption could not be accurately predicted, which was why airlines were taking precautions.
"However, once the volcano has erupted, it is unlikely that a more violent eruption is following straight away, so that the local geological survey apparently sees no need to raise the danger level," he said.
- with AAP