Why do volcanoes erupt?
The imminent eruption of Mayon Volcano and the recent acting up of Mt. Bulusan
and Canlaon and other volcanoes around the world pose questions on why volcanoes
erupt. It is imperative that the common man, particularly those who live near
volcanoes understand why they do erupt and learn basic facts about this phenomenon.
"An eruption occurs when magma rises from its source or from a storage
reservoir and finally reaches the Earth's surface. As it rises, the magma fractures
overlying rocks, which causes earthquakes, and parts of the volcano deform as
magma approaching the surface makes room for itself." - From: Brantley
and Topinko, 1984, Earthquake Information Bulletin, v.16 no. 2.
The Philippine Institute of Volcanology and Seismology on the other hand listed
the different kinds of eruption of our local volcanoes. Phreatic - explosion
driven by steam produced by heating and expansion of groundwater due to an underlying
hot surface. This type involves only water, steam and ash with other rock fragments
derived from pre-existing rocks, without ejection of fresh magmatic materials.
Examples of this kind of eruption is the 1993 and 1996 eruption of Mt. Canlaon
and the 1988 eruption of Bulusan volcano.
Phreatomagmatic - eruption resulting from the ejection of magmatic gases and
steam produced by the conversion of groundwater to steam by ascending magma,
mixed with water fine ash with or without accretionary lapilli and variably-sized
volcanic bombs fragmented from the pre-existing rock formations, and fresh magmatic
ejects. The eruption forms a high eruption column with a radially spreading
ring-shape horizontal cloud at the base due to peeling of the crater lip or
deflection in the rise of later ejections caused by the pressure of falling
pyroclastic materials. This type of eruption was exhibited by Taal Volcano in
its 1965 and 1967 eruptions.
Strombolian - weak to violent eruption characterized by lava fountaining and
effusion of molten lava. Typical ejects are ovoid and fusiform bombs and scoria
(scinders). Ash is relatively minor in amount and eruption cloud is generally
yellowish to white in color. This type of eruption was exhibited by Taal and
Mayon Volcanoes.
Vulcanian - eruption resulting from the release of large quantities of accumulated
gas which lefts fine ashes and blocks coming from the magma with great force
high in the air forming voluminous eruption clouds. Mayon Volcano showed this
type of eruption.
Peleean - eruption caused by the release of large quantities of gas from an
extremely viscous magma that hurls out ash and other pyroclastic materials and
is characterized by the presence of nuce ardente or glowing avalanche consisting
of hot gases made dense by a suspended load of pyroclastic material. Mt. Hibok-Hibok
exhibited this type of eruption on 1953.
Plinian - eruption of great violence characterized by voluminous explosive
ejections of pumice and pyroclastic flows. The copious extrusion of gas-rich
silicious magma is commonly accompanied by collapse of the top of the volcanic
cone forming a caldera. Example: Pinatubo Volcano 1991 eruption.
It is said that deep within the Earth, it is so hot that some rocks slowly
melt and become thick flowing substance called magma. Because it is lighter
than the solid rocks around it, magma rises and collects in magma chambers.
Eventually some of the magma pushes through vents and fissures in the Earth's
surface. A volcanic eruption occurs! Magma that has erupted is called lava.
Most volcanoes provide various types of warnings before eruptions begin. Although
an explosive eruption could occur without warning, some premonitory events more
likely will precede the next eruption. Steam-blast eruptions could occur with
little or no warning as superheated water flashes to steam; magmatic eruptions,
however involve rise of magma toward the surface. Such an upward movement of
magma normally will generate detectable earthquakes, may deform the ground surface,
and may cause anomalous heat flow or changes in the temperature and chemistry
of the ground and spring waters.
The word "volcano" comes from the little island of Vulcano in the
Mediterranean Sea off Sicily. Centuries ago, the people living in this area
believed that Vulcano was the chimney of the forge of Vulcan - the blacksmith
of the Roman gods. They thought that the hot lava fragments and clouds of dust
erupting from. Vulcano came from Vulcan's forge as he beat out thunderbolts
for Jupiter, king of the gods, and weapons for Mars, the god of war. In Polynesia,
the people attributed eruptive activity to the beautiful but wrathful Pele,
the Goddess of Volcanoes, whenever she was angry or spiteful. Today, we know
that volcanic eruptions are not super-natural but can be studied and interpreted
by scientists.
Source: Manila Bulletin, Tuesday, July 25, 2006, B-12
