MAMBAJAO, Camiguin Island – There are more volcanoes than there are towns here.

More than 10 volcanoes, two of them active, share the 291-square-kilometer island province with the towns of Sagay, Catarman, Guinsiliban, Mahinog, and Mambajao, the capital.

It's the largest concentration of volcanoes per square kilometer in the country, and the number has since increased after volcanologists in the 1980s added three more to make it 10 – and counting. They believe there could be more.

The most violent and deadly in Camiguin is Hibok-Hibok on the northwest, with several vents in the crater and on the flanks. What sets it apart was the white-hot and poisonous sulphuric gases that accompanied its 1951 eruption – extremely fluid but dense enough to cascade rapidly down the slope and “mummify” people on its path.

That was what happened during its most recent outburst, in December that year, when glowing ash clouds – packing temperatures of 800 to 1,000 degrees Celcius seen 160 kilometers away – swiftly rolled down to Mambajao town and charred trees and burned houses along its path; 500 people were solidified and covered in white ash looking like mummies.

“Lava flood buries 15 barrios”, “Hunger, disease peril evacuees”, The Evening News bannered. “New blast blocks rescue effort”, it headlined.

The eruption “caught people unprepared for its fury and devastating impact,” volcanologists reported.

Except for its almost forgotten activity in 1902 when it emitted sulphurous odors, there were no signs of activity in the five decades before the series of sporadic eruptions that lasted from 1948 to 1953; the one in 1951 was most violent.

People were not forewarned because there was no government institution concerned with volcanoes then, although in 1948 a seismic monitoring station was already established in Mambajao.

Hibok-Hibok's eruption prompted the government to create in June 1952 the Commission on Volcanology (COMVOL), later the Philippine Institute of Volcanology and Seismology (Phivolcs) of the Department of Science and Technology.

The COMVOL's first order of business was to create five monitoring stations near the country's five most active volcanoes.

Twenty-two of the country's 37 volcanoes are considered active, including those near urban concentrations, such as Banahaw in Laguna and Quezon; Bulusan in Sorsogon; Hibok-hibok in Camiguin; Kanlaon in Negros Oriental; Mayon in Albay; Pinatubo on the boundaries of Pampanga, Tarlac and Zambales; and Taal in Batangas.

Today, Phivolcs is not taking any chances, here and elsewhere. In Camiguin, digital seismic sensors run 24/7 in Mainit, Catarman; at the peak of Mt. Vulcan 571 meters above sea level; and on the upper slope of Hibok-Hibok in Tagdo, Mambajao.

They pick up volcanic quakes all over the island and transmit the data direct to computers at the Phivolcs Observatory in Sitio Quiboro, 4.7 kilometers from the crater.

Phivolcs-DOST's Hibok-hibok Volcano Observatory

No eruption is expected in the “foreseeable future” as indicated by a network of seismometers, including that at the Hibok-Hibok Observatory; another on the upper southeast slope; and those in Napo, Catarman; Mt. Vulcan Peak Observation Point; Mt. Vulcan Peak Repeater Station; Lawigan, Catarman; Mainit, Catarman Observation Point; and Baylao.

Where out-of-direct signals don't work, four repeater stations transmit seismic data to the observatory where all inputs are monitored 24-hours daily in rotation by three staff members: resident analyst and volcanogist Luisito Salugsugan, science research assistant Arturo Jardin and science aide Angelo Abang.

Except for the observatory, monitoring stations are unmanned, automatic and connected to computers by Local Area Network.

They are equipped with solar-powered seismometers (that measure minute seismic or ground movements), data recorders and transceiver units able to store raw information and transmit digital versions to the observatory's Network Management Unit – all in real time.

When necessary, the observatory – which will soon be replaced by a P2.8-million facility under construction – alerts the Phivolcs office in Diliman, Quezon City, which then analyzes the data and makes the final prognosis.

The central office, when appropriate, alerts local governments if an eruption is imminent.

“It doesn't happen suddenly,” said Salugsugan. “There are visual warnings like smoke, rock falls, dry vegetation at the top and unusual animal behavior.”

“If and when Mt. Vulcan erupts, only Catarman will be affected. If Hibok-Hibok erupts, only Mambajao and Catarman towns will be affected as Mt. Mambajao will be one of the barriers against lava flows. People there can be accommodated in nearby communities,” he said.

“If Mt. Mambajao erupts violently, all towns will be affected,” he added, adding civilian evacuation systems are in place.

“We sleep light and check the computers every two hours,” said Abang, “We know where a quake originates and whether it is an earthquake or a volcanic quake. We can pinpoint where a volcanic quake, which can be detected only by sensors on the island, originates.”

Precursors to an eruption are increasing volcanic quakes and tremors; landslides and rockfalls from the summit not caused by rains; increased steam emissions; progressive ground deformations like tilting; the appearance of vents that emit sulphurous odors; and crater glow.

Based on recorded eruptions in 1827, 1862, 1871, 1897-1902, 1948-1953, patterns have been observed in Hibok-Hibok: short periods of stem emissions from the crater and avalanches of volcanic materials; explosions of heavy clouds of steam and fragmented volcanic rocks; eruption of large amounts of incandescent materials and steam; then a decrease in the amount of steam and ejecta.

The pattern observed during the 1948-1952 eruptions showed a short period of emission of considerable amount of steam from the crater and avalanches of volcanic materials. Explosions or steam blast with emission of heavy clouds of steam, ash and other fragmentary volcanic materials followed.

Eruption of incandescent materials, emission of ash and steam in large amounts, formation of flows and occasional minor crateral outburst, then occurred.

Relatively high and increasing unrest, including numerous strong earthquakes, accelerating ground deformation and rockfalls, increased vigor of steam vents and gas emissions increases the “likelihood” of an eruption, possibly within days to a week.

Low frequency earthquakes, quiet lava emissions and/or dome growth and/or small explosions indicate that magma is close to or at earth’s surface. Hazardous explosive eruption, in this case, is likely within hours or days.

Explosive eruption is in progress when volcanic materials are ejected and flows down valleys and an eruption column rises at least 6 kilometers or 20,000 feet above sea level. Hibok-Hibok eruption is described as “pelean”. This is the accumulation of underground viscuous lava that creates a dome, followed by the formation of glowing clouds containing hot gases that rush down the slopes.

“There is no need to evacuate Camiguin if Hibok-Hibok erupts,” said Salugsugan. “Only Mambajao and Catarman towns will be affected.” He said the taller, 1,580-meter Mt. Mambajao will be able to block volcanic debris from the lower, 1,300-meter Hibok-Hibok.

“Old timers remember that in 1951, heavy rains and strong winds blocked the sulphuric gases from reaching Mambajao and thus prevented more casualties,” said Abang. “Many believe that San Nicolas, the town patron saint, blocked the gases.”

Because of potential volcanic hazards like steam blasts, glowing avalanches, lava flows and lahars, the area three kilometers from the summit has been designated a Permanent Danger Zone where human presence and activities are always prohibited.