By Alexandria San Juan
Taal Volcano continued to spew volcanic ash and lava fountains days after it erupted on Sunday, causing thousands of residents to evacuate their homes as precaution for an imminent “hazardous eruption.”
(Mark Balmores / MANILA BULLETIN)
Cloud of ash measuring at least 50,000 feet high blanketed provinces surrounding the volcano including parts of Calabarzon (Cavite, Laguna, Batangas, Rizal, and Quezon) and Metro Manila, and reached as far as Central Luzon, forcing the suspension of flights in the country’s main airports, classes in schools, and work in government offices.
The Philippine Institute of Volcanology and Seismology (Phivolcs) said Alert Level 4 remains in effect over Taal Volcano, meaning “hazardous eruption is possible within hours or days,” prompting residents, especially those in the 14 kilometer radius danger zone, to evacuate.
Eruptions
While Taal Volcano is known as a destination for both local and foreign tourists for its picturesque view, state seismologists describe it as a “very small but dangerous volcano.”
Located in the province of Batangas, Taal is among 24 active volcanoes in the country, and the second most active next to Mayon in Albay in terms of numbers of eruptions.
Although tagged the smallest volcano in the world, with an elevation of only 0.311 kilometers, Phivolcs said that Taal Volcano has erupted 33 times since 1572, with the last one on Oct. 3, 1977.
Phivolcs has classified Taal Volcano’s eruptions into four categories -- phreatic, phreatomagmatic (interaction of both magma and water), Strombolian (lava fountaining and lava flow), and Plinian (violent explosive eruption with tall eruption columns and widespread fallout tephra or rock fragments).
Taal’s eruption last Sunday was categorized by Phivolcs as phreatic, described by the U.S. Geological Survey as a steam-driven explosion that occurs when water beneath the ground or on the surface is heated by magma, lava, hot rocks, or new volcanic deposits.
Alert levels
Taal’s recent eruption caught unaware residents and tourists by surprise, given signs of restiveness in the past months.
But according to Phivolcs, it has been monitoring Taal’s activity since last year. In fact, it raised the volcano’s danger level to 1 in March 2019.
Below are Taal Volcano’s alert signals according to Phivolcs.
Alert Level 0
-The volcano is quiet. There is no volcanic activity.
-No eruption in foreseeable future.
Alert Level 1
(Alert Level 1 was raised over Taal in March 2019)
-There is a low level of seismicity, fumarolic, or other activity.
-There are magmatic, tectonic, or hydrothermal disturbances.
-No eruption imminent.
Alert Level 2
(Alert Level 2 was raised over Taal around 2:30 p.m., Sunday, Jan. 12)
-There is a low to moderate level of seismicity, and persistence of local but unfelt earthquakes.
-Ground deformation measurements are above baseline levels.
-There is an increase in water and/or ground probe hole temperatures, and an increased bubbling at crater lake.
-Magmatic intrusion is possible which could eventually lead to an eruption, but if trend shows further decline, danger level may go down to 1.
Alert Level 3
(Alert Level 3 was raised over Taal around 4 p.m., Sunday, Jan. 12)
-Relatively high unrest manifested by seismic swarms, including increasing occurrence of low frequency earthquakes and/or harmonic tremor.
-There is a sudden or increasing change in temperature or bubbling activity, radon gas emission, or pH levels of crater lake.
-There is a possible bulging of the edifice and fissuring.
-If the trend is an increasing unrest, hazardous eruption is possible within days to weeks.
Alert Level 4
(Alert Level 4 was raised over Taal around 7:30 p.m., Sunday, Jan. 12)
-Intense unrest
-There are continuing seismic swarms, including harmonic tremor and/or “low frequency earthquakes” which are usually felt.
-Profuse steaming along existing, and perhaps new, vents and fissures.
-Hazardous eruption is possible within days.
Alert Level 5
-Hazardous eruption in progress.
-Base surges accompanied by eruption columns or lava fountaining or lava flows.
-Extreme hazards threatening communities west of the volcano and ash fall on downwind sectors.
Volcanic hazards
Amid the ongoing eruption of Taal Volcano, threats of volcanic hazards persist including those that killed thousands of people during its previous eruptions.
These are some of the volcanic hazards associated with Taal’s eruption.
Lava flow
-Lava flows are rivers of incandescent of molten rock or lava moving downslope or away from an eruption vent.
Tephra fall or ash fall
- Tephra (fragmented volcanic particles) or ash (fragmented volcanic particles less than two millimeters in diameter) propelled through the atmosphere in an eruption plume or an eruption column, eventually fall or gravitationally settle over areas downwind of an erupting volcano, forming blankets of tephra fall or ash fall.
-Tephra or ash fall can accumulate as thick blankets of material, causing infrastructural damages, roof collapse, contamination of water resources, and burial.
-Ash fall is a health hazard and a danger to aircraft and other industrial machinery, such as those for transportation and power generation.
Ballistic projectiles
-Rocks blasted into the air during an eruption that usually land within two kilometers of the vent but can travel as far as five kilometers, or even further, if the eruption is very explosive.
Pyroclastic density currents or PDCs
-
PDCs are the most lethal of all volcanic hazards and can cause incineration, asphyxiation, abrasion, dynamic pressure impact, and burial in hot volcanic material.
-
These are mixtures of fragmented volcanic particles (pyroclastics), hot gases, and ash that rush down the volcanic slopes or rapidly outward from a source vent at high speeds.
Base surge
-A special class of PDC which is also hazardous and deadly.
-Turbulent mass of volcanic ash, rocks, and hot gases that laterally flows away from the base of an eruption column at very high speeds or greater than 60 kilometers per hour.
Volcanic tsunami
-Volcanic tsunami usually occurs after a base surge.
-It occurs in caldera lakes when water is displaced by the deformation of the lake floor caused by rising magma or the entry of PDCs or landslides into the lake, or in seas when water is displaced by PDCs or debris avalanches from volcanoes.
-Volcanic tsunamis are unlike those generated by large magnitude offshore earthquakes, which are long-period waves generated by fault displacement or deformation of the seafloor.
Fissuring and ground subsidence
-Ascending magma can cause the volcano edifice to swell before and during an eruption, causing the ground to break up into fissures, typically along weaknesses in the rock such as fractures or faults.
-After magma has been erupted, its removal from the subsurface can cause the ground to sink and subside and further fissuring to occur.
-Ground subsidence and fissuring are typically accompanied by earthquakes, and can cause infrastructural and house or building damages, loss or degradation of land surface, and re-routing of waterways and rivers.
Phivolcs repeatedly explained that volcanic eruptions are unpredictable, and continued to reiterate its call for a total evacuation of Taal Volcano Island and high-risk areas as identified in the hazard maps within the 14 kilometer radius from the main crater.