In a mobile phone interview with the Manila Bulletin, Solidum said, "The activities of the volcanoes are dependent on its heat or magma sources. It just so happened that the (four) volcanoes are active all at the same time."

According to Solidum, the period of unrest shown by the four volcanoes currently on heightened alert — Mayon in Albay, Bulusan in Sorsogon, Kanlaon in Negros Oriental, and Taal in Batangas — is just coincidental.

"Physically, each volcano have their own magma chamber. These (magma chambers) are not connected physically," he explained.

Solidum, however, said that aside from the four volcanoes on heightened alert status, other active volcanoes in the country which are continuously being monitored by the institute include Hibok-Hibok in Camiguin and Pinatubo located on the boundaries of Pampanga, Tarlac, and Zambales.

Meanwhile, the Phivolcs director said Bulusan Volcano’s alert level might be upgraded, depending on its "level of intensity."

"We do not know if we will (raise the alert level of Bulusan volcano). It will be dependent on the signs the volcano will show (and on the) level of intensity of the volcano," said Solidum.

Based on the institute’s Bulusan Volcano Update at 8 a.m. yesterday, small volcanic earthquakes had occurred in Bulusan Volcano after the steam and ash explosion at 7:04 p.m. last June 13.

"Release of sulfur dioxide contained in the volcanic plume was 423 tonnes (or metric tons) per day (t/d), a slight increase from previous day’s 295 t/d," said Phivolcs.

Meanwhile, another minor steam-ash explosion was recorded early morning yesterday in Negros Oriental’s Kanlaon volcano, its seventh in a series of steam and ash explosions for this month.

Based on Phivolcs’ 8 a.m. update yesterday, the minor steam-ash explosion occurred from 5:09 to 5:30 a.m.

"The grayish steam clouds rose to about 900 meters above the active crater and drifted to the northeast and northwest," said Phivolcs.