By Juan Escandor Jr.
NAGA CITY---Why do volcanoes thrive in Bicol where the most active ones recorded in the historical period continue to act up and change the landscapes?
Bicol peninsula nestles underneath colliding tectonic plates that plunge down to the earth’s mantle which spawn volcanoes, according to an American volcanologist.
Because of the geological location, volcanoes grow in the region even though others die while some regrow, explains volcanologist Christopher Newhall, in a lecture he shared on January 15 at the Ateneo de Naga University.
Newhall, who studied volcanoes in Bicol, said that
among the volcanoes that grew in the region, four are active that continuously change the landscapes around them.
He identified Mayon, Bulusan, Asog/Iriga and Isarog to be the four active volcanoes that had erupted within the period of 10,000 years, that he said is the standard duration of which volcanoes can be categorized as active.
In the contemporary period, Mayon and Bulusan are the most active volcanoes which periodically act up while Isarog and Asog/Iriga showed activities way back thousands of years ago.
Mayon is located in Albay province and Bulusan grew from an old volcano in Sorsogon province while Isarog and Asog/Iriga are all located in Camarines Sur province.
“The behavior of (the 2,463-meter) Mayon is relatively frequent. By relatively frequent I mean every few years..small to moderate sized eruptions. Some of them are explosives, there are others that are not,” Newhall said.
He said the famous big historical eruption of Mayon was in 1814 when the Cagsawa church was buried but another big eruption in 1897 which was not so well known almost destroyed the whole town of Sto. Domingo.
He said the latest sequence of Mayon eruption began in 1993 with a fresh batch of magma or molten rock that came in and full of gas.
Newhall said in 2000-2001, the eruption of Mayon had still plenty of gas and “quite explosive and it formed a lot of pyroclastic flows” that came down close to villages and towns around the volcano.
“The next several eruptions, 2009, 2013 and 2014 turned out to be not explosive with only lava that came out,” he said.
Newhall said the explosive eruptions of Mayon produce ashfalls, pyroclastic flows, the hot avalanche that came down the volcano with temperature of at least 1,000 degrees centigrade and travelling about 100 km/hr., lahar when it rained on, and lava.
Mayon is a type of volcano with an open system as contrasted to Mt. Pinatubo which is a plug system.
He said in an open system volcano, “the magma in the conduit never solidifies, stays molten and is always releasing gas at the top.”
“In the plug system, the magma in the conduit solidifies in between eruptions, so it’s really a rock plug, a good example is Pinatubo,” Newhall explained.
On Bulusan volcano that towers at 1,565 meters, the American volcanologist describes the present times eruption as frequent and small that produce a little bit of ash.
He said the last magmatic eruption of Bulusan when the magma came out of the surface was in 1918 even though he said that magma cannot be very far along the surface because of the event of steam explosions and the presence of sulfur gas.
Newhall said Bulusan grew out of a much bigger volcano whose crater is now the whole town of Irosin.
He said the old crater that had collapsed which is now known as the Irosin caldera blew up 41,000 years ago with the volume of magma that came out was estimated to reach “at least several hundred cubic kilometers.”
“For comparison, Pinatubo produced five cubic kilometers of magma and a big eruption of Mayon is less than one cubic kilometer,” Newhall said.
Citing the study of volcanologist Hannah Mirabueno, he said the gigantic eruption of Bulusan during that time, the biggest known eruption in Bicol, covered the whole province of Sorsogon in very thick pyroclastic deposit.
“The general phenomenon of calderas is that you got material coming out of one or more vents, and it comes out so fast, so much magma that is erupted, that literally the top collapses down into the magma reservoir,” Newhall explains.
The 1,196-meter Mt. Asog/Iriga has a different story which parallels to the Mt. Helens in Washington State with its side collapses when the eruption occurred and triggered a huge landslide.
Newhall said the eruption that happened 1,800 years ago was not a slow geologic process but rapid and catastrophic process.
He said when it erupted, one side of the mountain collapsed and cascaded down with each landslide event an explosion behind it.
“The lake was formed because the landslide blocked the river. The water couldn’t get out anymore,” Newhall said.
He said the last eruption of Asog did not only form the lake but also produced small hills that are not dead volcanoes with big volcanic rocks on them.
“We (geologists) call them hammocks---pieces of the old volcano that slid down. it’s the landslide deposit,” Newhall said.
He debunked the popular belief that Asog with its eruption creating the Lake Buhi happened in the 1600s.
The eruption left a horseshoe crater facing the town of Buhi and livelihood for fisherfolks today who benefit from the lake, Newhall said.
The highest peak in Camarines Sur province, Mt. Isarog is 2,000 meters high.
According to Newhall, Isarog, based on carbon dating, erupted 5,600 years ago.
He said the eruption of Isarog had triggered gigantic landslide as shown by the study of “turbulent” deposits in a quarry in the village Hiwacloy in Goa town where charcoal from woods in the forest charred by pyroclastic flow.
Newhall wondered why one of the towns surrounding Isarog is named Tinambac which means “pile up.”
He said Isarog is a multi-generation volcano in which new volcano grew out of the old volcano as exhibited by its irregular shape.
Showing a map, Newhall pointed to an area between the towns of Tigaon and Goa where the Ranggas River is that he said is a very deep canyon that looks like a bottleneck.
He said its geology shows of a breakout of the crater lake at the top of the volcano that eroded to its side, broke out, and curved a very deep canyon.
Newhall surmised that the breakout event of the crater lake might have led to the belief that Isarog is filled with water.
(Note: The source, Christopher Newhall, considers himself a semi-retired volcanologist, married to a Bicolana, and now settles with wife in Sto. Domingo, Albay at the foot of Mayon Volcano. He delivered the lecture as part of the Ateneo de Naga University’s series in honor of Jesuit priest and anthropologist Fr. Frank Lynch. Newhall studied historical and modern eruption precursors, and reconstruction of eruptive behavior from “stratigraphic, petrologic and historical data.”
Newhall’s studies include volcanoes in the United States, the Philippines, Indonesia, Ecuador, Democratic Republic of Congo, and Japan. His undergraduate study was in University of California (UC) Davis, with a year at University of Canterbury; his postgraduate studies were at UC Davis and Darthmouth (PhD 1980).
Most of his career was with the US Geological Survey at the same time that he worked as an affiliate professor at the University of Washington. Newhall had been a professor and volcano group leader at the Earth Observatory of Singapore, Nanyang Technological University.)