Fascinating facts about volcanoes

In the Philippines, a volcano is considered active if it has erupted within the last 600 years or historic times. However, this definition varies among countries. Most volcanoes in the Philippines are composite or stratovolcanoes, which means they are made up of both lava (magma that has been extruded into the surface) and fragmented materials (also called tephra or pyroclast), which range from fine ash to large boulders).

The unusual symmetry of Mayon is partly due to its eruptions that have been localized from a central vent and the webbing of its crater lip which allowed symmetrical distribution of the eruption products. Most stratovolcanoes are asymmetrical partly because of the flank eruptions that give rise to ‘parasitic’ lateral cones which destroy the edifice outline.

Extrusion of voluminous lava from the ocean floor has generated the broadest and largest volcanoes in the world. These are shield volcanoes. The Hawaiian volcanoes like Kilauea and Mauna Loa are examples. They rise from the ocean floor and come out of the sea as volcano.

Volcanoes of the other extreme end of the spectrum consist of dominant fragmented materials. These are the pyroclastic cones. They include the smallest volcanoes in the world like the scoria or cinder cones, tuff cones and tuff rings or maars. Example of Scoria cones can be seen around Makiling and Taal Volcano island. Tuff rings and maars are usually associated with lakes that may have crater floors that lie close or below the former ground level. The seven lakes in the vicinity of San Pablo City are such volcanoes.

Calderas are large depressions that mark sites of removal and explosive eruption or large amounts of magma. This depression may be water-filled to form lakes (such as Lake Taal). If the drainage is good, ephemeral lakes may form as in the Bulusan caldera in Sorsogon. Other potential calderas include Lake Lanao and Laguna de Bay.

Volcanic eruptions vary from mild to outpouring of lava to explosive paroxysmal explosions that send voluminous ash into the atmosphere and generate hot pyroclastic flows that can travel hundreds of kilometers away from the eruption site. For example, very sticky, gas-rich magma triggered the explosive eruption of Pinatubo in June 1991.

Pinatubo unloaded enormous amounts of sulfur dioxide and hydrogen sulfides into the atmosphere up to 40 kilometers above sea level. Through photochemical reactions, these were converted into clouds of sulphate aerosols, tiny droplets of sulphuric acid, that may stay suspended in the atmosphere for years.

These altered global radiation by back-scattering and absorbing incoming solar radiation. The result – worldwide drop in temperature by a few tenths of a degree.

Explosive eruptions can cause towns and settlements to relocate. For example, the 1754 eruption of Taal Volcano caused the abandonment of the old Taal town (now known as San Nicholas) to its present site near Lemery, Batangas. This exodus also happened in Tanauan, Lipa, Bauan. Historical records show that they were once lakeshore settlements that had to be relocated after the eruption of Taal.

Airfall deposits from the 1991 Pinatubo eruption generated ashfall that reached Singapore and Vietnam thousands of kilometers away, and caused damage to aircraft.

Lahars, relatively fastmoving slurry of volcanic debris, are capable of destructive burial erosion and loss of life. For example, the hotpumice-rich Pinatubo lahars caused significant economic dislocation and deaths. Deposition of pumiceous materials and associated burial of towns and cities remain unprecedented in the history of modern volcanology. Most of Pinatubo lahars are induced by rains, others were generated by lake breakouts.

Collapse of the volcano edifice may be triggered by earthquake and/or injection of magma in the form of domes. The resulting instabilities produce large avalanches which literally tear volcanoes down. The debris avalanche at Iriga volcano in the 17th century caused the damming of the river and thus formed Lake Buhi, home to the “sinarapan” or “tabios”, the smallest fish in the country.

The avalanche also led to the formation of hills in the area between Iriga City and Buhi town from the chunks and blocks of the volcano edifice that broke apart. Some of the depressions between these chunks of volcano edifice eventually became water-filled and thus formed the numerous lakes in Iriga City.

The towering edifice of Mayon volcano is likely candidate for structural failure in the future. (PNA)

[ Mayon Volcano Wiki | Taal Lake and Volcano Wiki ]