What exactly is a volcanic tsunami—and has it happened before? 2
PHIVOLCS say that there is a threat of a volcano tsunami after yesterday's eruption in Taal. Photograph from ABS-CBN News

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

After yesterday’s eruption at Taal, PHIVOLCS gave out a high alert warning of the possibility of a volcanic tsunami. What kind of devastation can we expect?
Jacs T. Sampayan | Jan 13 2020

Yesterday, Taal Volcano erupted after more than 40 years of being dormant. Residents and visitors to the area stood alarmed but in awe at the sight of a 10 to 15-kilometer column of steam and ash emanating from Tagaytay’s famous caldera. As evening struck, PHIVOLCS raised an alert level 4 over the tourist city, sending people scrambling to leave the ridge that straddles Cavite and Batangas, causing a traffic gridlock.

While the southern provinces experienced the brunt of the catastrophe, Metro Manila felt its effects. Not only did it wreak havoc on flight schedules, the eruption caused a blanket of ash to fall throughout NCR reaching as far as Quezon City. As air safety became a concern, people went on a fit of panic buying, queueing up at stores to stock up on N95 face masks and food.

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Apart from the threat of pulmonary problems caused by the ashfall, PHIVOLCS also warned of the possibility of a volcanic tsunami, which may occur in caldera lakes such as the one in Taal Lake.


Water hazard

According to PHILVOCS senior science research specialist Isabel Abigania, volcanic tsunamis, which used be known as seiche, are displacements or disturbances of the water column. This is caused when there is a large underwater volcanic eruption, or if large volcanic deposits from the eruptions are discharged into the sea or nearby body of water. Simply speaking, the force produced from big enough chunks or masses being thrown wildly into the water may lead to big enough waves.

These tsunamis can also be generated from caldera collapse or flank failure. “Since Taal Volcano Island is surrounded by Taal Lake, a body of water, its explosive or violent eruption would generate a volcanic tsunami that would affect nearby shores,” she says. “Similar volcanoes surrounded by water would have the same hazard when they erupt.”

Volcanic tsunamis, according to the International Tsunami Information Center, may happen rarely, but they are usually accompanied by particularly violent eruptions. The displaced volume of water can generate destructive waves within the vicinity of the eruption. Apart from the sudden displacement of water, tsunamis like these can be caused by “a volcano’s slope failure, or more likely by a phreatomagmatic explosion and collapse/engulfment of the volcanic magmatic chambers.”

There have been instances of these tsunamis occurring in history, and arguably the largest and most destructive of which is the explosion and collapse of Krakatoa in Indonesia in August 26, 1883. Waves that reached 135 feet destroyed coastal towns, and villages in both Java and Sumatra, killing almost 40,000 people.

In 1792, the eruption of Japan’s Mount Unzen in Japan also produced a destructive landslide generating a 165-foot tsunami. An earthquake triggered a landslide from its Mayuyama peak, a 4,000-year-old lava dome rising above the city of Shimabara. The landslide swept away the city and reached the Ariake Sea, setting off a tsunami. The devastation led to around 15,000 deaths.

There have been many casualties in past eruptions of Taal, with its destructive and violent explosion in 1911 tallying the most, with a death toll of 1,335. With the threat of the tsunami, authorities are swiftly attempting to bring residents to safer ground.