Manila, Philippines (CNN) Philippine authorities have urged a "total evacuation" of nearly a million people near the capital Manila, after a volcano spewed ash up to nine miles (14 kilometers) into the air Sunday prompting warnings of a possible "explosive eruption."

The Philippine Institute of Volcanology and Seismology (PHIVOLCS) has raised the alert level to four, meaning an "explosive eruption" could happen in the coming hours or days. Its highest alert level is five, indicating an eruption is taking place.

People watch plumes of smoke and ash rise from Taal Volcano on January 12, 2020, in Tagaytay outside Manila, Philippines.

In explosive eruptions, magma is fragmented and violently expelled from the volcano -- think of a soda can after being shaken -- as opposed to thick lava oozing out.

Taal Volcano isn't actually very big -- but it's considered among the world's most dangerous, owing to the number of people that live in its immediate vicinity, said Erik Klemetti, a volcanologist at Denison University.

There are two zones of concern around the volcano, according to the United Nations Office for the Coordination of Humanitarian Affairs (OCHA). Around 459,000 people reside within a danger zone with a 14-kilometer (8.7 miles) radius around the volcano, according to a map by the agency , while more than 930,000 people live in a wider 17-kilometer (10.5 miles) danger zone.

PHIVOLCS has urged a "total evacuation" of everyone within the 17-kilometer radius around the volcano.

As of Monday, more than 25,000 people had sought shelter in temporary evacuation centers set up by the authorities. The total number of evacuees is likely to be higher, however, with many people choosing to stay with family members and relatives in other parts of the country.

Federal authorities are helping the response and evacuation operation. The army sent 20 military vehicles and 120 personnel to help affected residents, and the secretary of national defense said helicopters were on standby to evacuate people.

The defense secretary also urged residents near the eruption to evacuate, and not to hesitate in leaving their homes. Aid organizations like the Red Cross are assisting on the ground by sending rescue vehicles and supplies.

Photos: Taal volcano erupts in Philippines The landscape of Buso Buso is covered in ash on Sunday, January 19. Hide Caption 1 of 33 Photos: Taal volcano erupts in Philippines Clouds rise over the Taal volcano on January 19. Hide Caption 2 of 33 Photos: Taal volcano erupts in Philippines Evacuees rest at a government complex used as an evacuation center in Tagaytay on January 18. Hide Caption 3 of 33 Photos: Taal volcano erupts in Philippines A man clears ash from the roof of his truck at a fishing harbor in Laurel on January 17. Hide Caption 4 of 33 Photos: Taal volcano erupts in Philippines The Philippine province of Batangas is one of the areas affected by the eruption of the Taal volcano. Many animals had to be left behind as residents evacuated the danger zone. Hide Caption 5 of 33 Photos: Taal volcano erupts in Philippines Evacuees rest at an evacuation center in Batangas on Wednesday, January 15. Hide Caption 6 of 33 Photos: Taal volcano erupts in Philippines Volcano-affected residents ride along an ash-covered road on January 15. They were returning to evacuation centers after inspecting their homes in Agoncillo. Hide Caption 7 of 33 Photos: Taal volcano erupts in Philippines A volunteer carries a dog he rescued from deserted homes in Talisay. Hide Caption 8 of 33 Photos: Taal volcano erupts in Philippines Vegetation is burnt near the site of the volcano. Hide Caption 9 of 33 Photos: Taal volcano erupts in Philippines A family crosses a road that was damaged by volcanic tremors in Lemery. Hide Caption 10 of 33 Photos: Taal volcano erupts in Philippines A resident cleans ash from his roof in Agoncillo on Tuesday, January 14. Hide Caption 11 of 33 Photos: Taal volcano erupts in Philippines Volunteers save animals that were left behind in Batangas. Hide Caption 12 of 33 Photos: Taal volcano erupts in Philippines People watch the erupting volcano from Tagaytay on January 14. Hide Caption 13 of 33 Photos: Taal volcano erupts in Philippines Leila de Castro carries a religious statue recovered from a family member's house as she walks along an ash-covered road in Boso-Boso on January 14. Hide Caption 14 of 33 Photos: Taal volcano erupts in Philippines Men carry piglets that were rescued in Talisay on January 14. Hide Caption 15 of 33 Photos: Taal volcano erupts in Philippines Ash coats the roofs of homes in Tagaytay on January 14. Hide Caption 16 of 33 Photos: Taal volcano erupts in Philippines A resident cleans the roof of his house in Tagaytay. Hide Caption 17 of 33 Photos: Taal volcano erupts in Philippines Evacuees line up to have their children checked by medical personnel at an evacuation center in Tanauan. Hide Caption 18 of 33 Photos: Taal volcano erupts in Philippines A family flees to safer ground as the volcano produces ash in Tagaytay on Monday, January 13. Hide Caption 19 of 33 Photos: Taal volcano erupts in Philippines Erman Batan reacts over her missing husband, Roberto, whom she had not seen since they evacuated their home in Tagaytay on January 13. Hide Caption 20 of 33 Photos: Taal volcano erupts in Philippines Residents prepare their boats to evacuate Talisay on January 13. Hide Caption 21 of 33 Photos: Taal volcano erupts in Philippines Villagers rest on stairs in Batangas. Hide Caption 22 of 33 Photos: Taal volcano erupts in Philippines A youth rides an outrigger canoe while the volcano spews ash on January 13. Hide Caption 23 of 33 Photos: Taal volcano erupts in Philippines A Tanauan resident uses a shovel to clear mud and ash. Hide Caption 24 of 33 Photos: Taal volcano erupts in Philippines Lightning strikes as a column of ash surrounds the crater of the Taal volcano on Sunday, January 12. Hide Caption 25 of 33 Photos: Taal volcano erupts in Philippines People in Tagaytay watch plumes of smoke and ash rise from the volcano on January 12. Hide Caption 26 of 33 Photos: Taal volcano erupts in Philippines Villagers take shelter inside a church in Talisay. Hide Caption 27 of 33 Photos: Taal volcano erupts in Philippines Residents evacuate Tagaytay on January 12. Hide Caption 28 of 33 Photos: Taal volcano erupts in Philippines A column of ash surrounds the crater of the Taal volcano. Hide Caption 29 of 33 Photos: Taal volcano erupts in Philippines An ash column looms over Tagaytay. Hide Caption 30 of 33 Photos: Taal volcano erupts in Philippines Talisay residents use plastic bags to shield themselves from ash mixed with rainwater. Hide Caption 31 of 33 Photos: Taal volcano erupts in Philippines A Talisay resident splashes water on a vehicle covered in ash. Hide Caption 32 of 33 Photos: Taal volcano erupts in Philippines People take photos of the eruption on January 12. Hide Caption 33 of 33

Residents within the immediate area are most at risk from the effects of a potential eruption, including a possible volcanic tsunami in the lake surrounding the volcano, according to PHIVOLCS.

The lava now beginning to erupt is dangerous, Klemetti said, as it's creating "a big lava fountain" that could then spill into nearby towns. But volcanic ash is the biggest danger, said Joseph Michalski, director of the Earth and Planetary Science division at the University of Hong Kong.

"The ash is what will kill you, not the lava," Michalski told CNN. "The ash flow from an exploding volcano can travel hundreds of kilometers an hour."

Other threats include deadly toxic gases emitted from the eruption, and mud flows caused by ash mixing with water vapor in the atmosphere, Michalski added.

A resident clears mud and ash in Tanauan town on January 13, 2020.

If it erupts again, the ash -- which carries microscopic shards of glass -- could potentially be carried 100 kilometers (62 miles) or more, contaminating the air and water supplies in distant locations. More than 25 million people live within 100 kilometers of the volcano.

"(The shards of glass) are hazardous to lungs," Michalski said. "You don't want stuff like that in your lungs. It can get lodged in there and make you quite ill."

Michalski said it's too early to tell just how big the impending eruption could be -- Taal Volcano has a long history of frequent, small eruptions. However, it's also potentially more dangerous than other volcanoes because of its location on a lake -- the interaction of magma with water can make a volcanic eruption much more explosive.

On Sunday, the volcanic ash spread as far as Quezon City north of Manila, prompting the suspension of all flights at the capital's international airport.

A vehicle covered in ash mixed with rainwater after Taal Volcano erupted on January 12, 2020 in Talisay, Philippines.

Photos from the aftermath on Sunday show ash mixing with rain, creating a thick black sludge that blanketed cars, streets, and homes in some towns. Ash is even heavier than snow, meaning excessive pile-ups, especially when mixed with rain, can cause roofs to collapse.

Apart from immediate health hazards and structural damage, a potential eruption could also bring long-term consequences for the area's economy.

The volcano is surrounded by a lake, which is a popular attraction -- meaning many of the towns in the vicinity are tourism hot spots. There are several amusement parks, lakeside resorts and yacht clubs within 17-kilometer zone.

The holiday town of Tagaytay, which lies close to the water's edge, is a popular getaway for Manila residents who often take boats onto the lake and hike up the volcano.

Lightning strikes as a column of ash surrounds the crater of Taal Volcano as it erupts on January 12, 2020.

The future of this tourism economy could depend on how destructive the potential second eruption is -- if it's big enough, it could wipe out the entire volcanic island in the middle of the lake, said Michalski.

It could also serve a severe blow to the many farmers and fishermen who live in the area and rely on the lake and its surrounding land for their livelihoods.

The volcano has seen powerful eruptions before -- one eruption in 1754 lasted six months, and its deadliest eruption took 1,335 lives in 1911. It erupted again in 1965, killing 190 people, and continued to have four more minor eruptions in the years since.

Mariton Bornas, chief of volcano monitoring at PHIVOLCs, said that the agency had monitored tremors at the volcano as early as March 2019 -- but they were surprised by the rapid speed of the eruption on Sunday.