The picturesque and tourist favorite Taal volcano is now a source of fear, as state seismologists warn of an imminent hazardous eruption.

While Taal Volcano has shown relative calm on Tuesday, Alert level 4 remains on the island, with forced evacuation enforced within 14-kilometer radius from the craters.

As experts look out for signs of a bigger eruption, here are other hazards the public must watch out due to Taal Volcano's unrest.

1. PYROCLASTIC FLOW

Pyroclastic flows refer to the downslope lethal flow of ash and rocks mixed with hot gases during a volcanic eruption. At a speed of more than 60 kilometers per hour, the turbulent mass of volcanic material and gas is enough to burn everything in its path. Such hot flows are responsible for the iconic and horrific statue-like remains of the people of Pompeii after the eruption of Mount Vesuvius in AD 79.

In the 2019 scientific journal article “A synthesis and review of historical eruptions at Taal Volcano,” written by Perla Delos Reyes and Philippine Institute of Volcanology and Seismology’s Ma. Antonia Bornas, Dir. Renato Solidum and others, noted the devastating impacts of “pyroclastic density currents” from Taal volcano during its eruption in 1749, 1754, 1911 and 1965. They wrote that it is “the biggest threat in the case of renewed volcanic activity.”

Not often observed, base surges is a kind of pyroclastic flow that are fast expanding rings of turbulent mixture of fragments and gas at the base of ash plumes.

Its appearance is comparable to an atomic bomb explosion. While it is not as strong as a bomb explosion, Phivolcs’ Volcano Monitoring and Eruption Prediction Division chief Ma. Antonia Bornas said it is considered the “worst-case scenario for Taal.”

“It will devastate the lakeshore since it is able to cross from the main crater and over water,” she told ABS-CBN.

• READ: Taal 'hazardous eruption' like 'atomic bomb' but won't carry same energy: Phivolcs

Base surges were actually first identified by geologists during the 1965 Taal volcano eruption. But it has happened before in Taal’s previous eruptions.

In their journal article last year, Delos Reyes and Bornas estimate that the base surge from the 1754 eruption, which is considered the worst so far, reached 16 kilometers or more.

2. LAVA FLOW

Phivolcs defines lava flow as the “stream-like flow of incandescent, molten rock material” from an erupted volcano. Any area buried in lava flow will be unusable for a long time because it hardens into rock.

The institute has monitored lava flow from the Taal volcano by Monday morning.

• READ: Taal spews lava fountains

UP National Institute for Geological Sciences professor Carlos Arcilla said the lava flow is actually a “good sign” for Taal volcano since the release of magma means limited potential damage.

• READ: A silver lining: Taal lava fountain shows 'limited' potential damage - UP geologist

The current magmatic eruption of Taal is a "quiet type," but a possible explosive eruption remains if the volcano spews more magma, UP Resilience Institute Executive Director Mahar Lagmay said.

3. ASH FALL

Hours after the Sunday eruption of Taal Volcano, its vicinity and as far as parts of Metro Manila saw ashfall, affecting road visibility and slowing down vehicles as Filipinos scampered for safety.

The ash fall or the showering of fine-to-coarse grained volcanic material alarmed many, prompting panic buying of face masks all over the region.

• READ: Mercury Drug replenishing face mask supply, says to keep price unchanged
• READ:Muddy roads hamper Batangas town evacuation from Taal wrath

The ash fall also grounded planes in Metro Manila.

• READ: NAIA flights canceled due to Taal ash explosion, Clark to be told to shut down too

“Silica (in the ash fall) melts when it is heated. It can stick to a jet engine and cause the stalling of an airplane,” Lagmay said.

4. SHOCKWAVE

Shockwaves are created by explosive eruptions. It has been observed in Talisay and San Nicolas when Taal volcano erupted in 1911. The shockwave, which is a change in atmospheric pressure was described then as a strong wind “as if shot from the mouth of a gigantic cannon.”

Bornas said there were accounts of people being swept away by the acoustic gravity waves.

5. LAHAR

Lahar is defined by Phivolcs as the “rapidly flowing thick mixture of volcanic material and water usually generated along river channels by extreme rainfall.”

Those who witnessed images from the Mt. Pinatubo eruption in almost three decades ago would remember the devastating effects of lahar on Pampanga. In large amounts, it can bury infrastructure and kill people.

6. VOLCANIC GASES

Volcanoes can emit various toxic gases, releasing it into the atmosphere in the form of water vapor, hydrogen sulfide, sulfur dioxide, carbon monoxide, hydrogen chloride and hydrogen fluoride. These are gases trapped in the cavities of volcanic rocks or emanating directly from lava. It can also come indirectly from ground water heated by magma.

Lagmay said inhaling these gases in large amounts can be fatal.

7. SECONDARY EXPLOSIONS

Phivolcs calls it “secondary explosions” when hot pyroclastic flow deposits at the base of the volcano come into contact with ground water.

8. LANDSLIDES

Landslides or debris avalanches occur when a large portion of the volcano collapses. This is usually triggered by the eruption or by an earthquake.

Lagmay said this can also trigger a tsunami.

9. VOLCANIC TSUNAMI

Unlike tsunamis caused by earthquakes, a volcanic tsunami or seiche is caused by the sudden displacement of water during a volcanic eruption or landslide. Such big waves can affect communities beside the shore.

The most destructive of recorded volcanic tsunamis was the 1883 explosion of Krataktau volcano in Indonesia, which killed almost 40,000 people because of the 135-feet waves generated by the explosive eruption.

• READ: What exactly is a volcanic tsunami—and has it happened before?

10. FISSURE

In the journal article of Delos Reyes and Bornas, historical accounts recorded instances of “fissure eruptions,” especially during the initial stage of the 1965 eruption of Tall.

In 1911, historical accounts mentioned how “fissures opened in the ground amid horrifying roars, said fissures extending from the northern and northeastern beach of the lake as far as the neighborhood of the town of Calamba.”

Bornas said such fissures or the opening up of the ground is caused by the magma pushing its way out of the Taal volcano. Fissures often appear before an explosive eruption and then subside after it is over.

“The volcano is spewing material so there is empty space underground,” Bornas explained.

She said it has happened before in Lemery, Taal, San Nicolas, Talisay, and Tanauan in Batangas.

Bornas warned that houses can definitely be destroyed by such ground movements.

On Tuesday, a road crack was discovered in Lemery, Batangas.

• READ: LOOK: 3-km crack opens in Batangas as Taal rumbles

11. EARTHQUAKES

Two other phenomenon already experienced during the current Taal volcano explosion are earthquakes and lightning.

Earthquake swarms are often detected before a volcano erupts. These are often low in magnitude and are not as destructive as earthquakes that are tectonic in origin.

12. LIGHTNING

Lightning was also seen around the ash plumes of Taal volcano on Sunday afternoon. Lagmay said these are caused by the expelling of ash.

“Those particles are very dense. It will go up and will rub against each other, resulting in an electric charge,” he said. The electrically charged particles are the ones that create lightning.

USGS said in its website that increased lightning activity during eruptions can pose hazards to power generation sites, substations, and transmission and distribution lines.