There were no indications that a major eruption is imminent despite the phreatic eruption that occurred on Mt. Pinatubo on Tuesday, Nov. 30, said Science and Technology Undersecretary and Philippine Institute of Volcanology and Seismology (Phivolcs) Director Renato Solidum Jr.

"Kung titignan natin yung history, hindi natin inaasahan na magkakaroon ng kahalintulad ng 1991 eruption kaagad-agad pero syempre hindi natin pwedeng tanggalin yung posibilidad na magkaroon ng mga maliliit na explosions kaya patuloy nating binabantayan ito nang mabuti (If we look at Pinatubo's eruption history, we do not expect that there will be a similar 1991 eruption immediately but of course we cannot rule out the possibility of small explosions so we will continue to monitor it closely," Solidum said in a DZBB interview.

Solidum explained that magma rose to the surface of Mt. Pinatubo that led to its eruption in 1991, unlike the Nov. 30 event where no magma ascent has been observed.

"Nung 1991 at 1992 may umakyat talagang magma na umabot sa ibabaw at nung 1991, isinabog ng masyadong malakas. Dito sa nangyayari, wala pang umaakyat na magma tayong naitatala sa kadahilanang ang mga lindol ay masyadong malalim pa mga 10 kilometers or below (In 1991 and 1992, magma rose to the surface, and in 1991, it exploded very strongly. In this case, we have not recorded any rising magma because the earthquakes are very deep, about 10 kilometers or below)," he said.

Phivolcs on Tuesday said a phreatic eruption occurred at Pinatubo Crater between 12:09 p.m. and 12:13 p.m.

It defines phreatic eruption as "steam-driven explosions that occur when water beneath the ground or on the surface is directly heated by hot rocks or new volcanic deposits, e.g. PDC deposits, lava, or indirectly by magma or magmatic gas."

"Considering that there has been very low seismic activity in the volcano in the past days and low diffuse volcanic CO2 (carbon dioxide) flux measured at Pinatubo Crater Lake, and high infrasound over seismic energy released by the eruption, the event was likely driven by shallow hydrothermal processes beneath the edifice," it said in its advisory issued Tuesday.

Solidum reminded the public that Mt. Pinatubo's alert status remains "normal."

"The prevailing Alert Level 0 status of the volcano is currently under consideration pending the results of ongoing GPS (global positioning system) and InSAR (Interferometric Synthetic Aperture Radar) data processing," Phivolcs pointed out.

The public, however, should not enter the Pinatubo crater as shallow phreatic or hydrothermal explosions, such as the recent event, could occur without a warning, it added.