Some 100,000 people in Bali are being evacuated from a danger zone around a volcano that has been spewing ash thousands of feet into the air since last week, forcing the closure on Monday of the airport on the Indonesian resort island.

The eruption, which is sending white and gray ash streaking off the top of the cone, began on Tuesday and was a first for the volcano in 50 years. Loud explosions from Mount Agung could be heard miles away. Officials say lava is welling up in its crater — a sign that a larger eruption is possible.

"The volcano's alert level has been raised to the highest level," senior state volcanologist Gede Suantika was quoted by The Jakarta Post as saying. "Constant tremors can be felt."

That means a larger eruption is possible. As of Monday, the Indonesian government has designated a 7.5-mile radius around the volcano that it considers a danger zone. That zone was expanded from just 4 miles over the weekend in an apparent sign of increasing concern over the volcano's activity.

Government spokesman Sutopo Purwo Nugroho told reporters in Jakarta that the danger zone encompasses 22 villages and about 90,000 to 100,000 people. According to The Associated Press, he said that about 40,000 people had heeded the evacuation order so far, but that the rest say they feel safe or don't want to abandon their livestock.

Meanwhile, an estimated 59,000 passengers have been affected by the cancellation of 445 international flights to the island since Bali's Denpasar Airport closed on Monday.

According to the AP, "Videos released by the National Disaster Mitigation Agency showed a mudflow of volcanic debris and water known as a lahar moving down the volcano's slopes. It said lahars could increase because it's rainy season and warned people to stay away from rivers."

The BBC adds that Tuesday's event was a "phreatic eruption" which causes "the expulsion of [pressurized] steam from inside the volcano because the magma within is heating up water. This can lead to a build-up of pressure which causes an explosion blasting rock and bits of the crater into tiny pieces of ash."

Indonesia is located on the western rim of a seismically and volcanically active "Ring of Fire" that traces an outline around the Pacific Basin from New Zealand to the tip of South America.

It's the second time in as many months that the people around Mount Agung have been advised to leave. In September, some 144,000 people heeded the government's warning, only to be told they could return a few days later. Also in September, thousands were evacuated from the South Pacific island of Ambae, belonging to Vanuatu when a volcano there showed increased activity.

Mount Agung last erupted in 1963, killing 1,100 people near the volcano's base.

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