Magmatism is the formation of igneous rocks
Concept of Plate Tectonics is the idea that
The earth’s crust and upper mantle are
broken up into pieces into a series of rigid,
The plates move over a partly molten zone in
the mantle that is the source of most of the
magma that accounts for volcanic activity.
Melting is caused by reduction of pressure as
the magma goes up.
Magmas are typically generated in one of the
three plate tectonic settings:
at Divergent Plate Boundaries
Over Subduction Zones
Hot Spots , Intraplate Volcanism
At Divergent Plate Boundaries- where plates
split and move apart.
Most are seafloor ridges
Magma produced are mafic
This is due to the ultramafic composition of the upper
mantle beneath the seafloor.
Basaltic volcanism – dominant rock is basalt
There are also magmatism found in continental rifts but
they are less common. They also produce mafic magma
and they have basaltic volcanism like at the seafloor
At Subduction Zone – a type of plate
boundary at which two plates converge and
one plate is thrust beneath the other.
Andesitic volcanism- dominant rock is andesite.
This is due to the assimilation or melting of the
overriding plates that is continental (mostly silicic)
which is composed of granitic or granodioritic crustal
material by mafic magma often produces an
intermediate composition, andesitic melt.
Hot Spot, Intraplate Volcanism – isolated
areas of volcanic activity that are not
associated with plate boundaries.
Usually attributed to the presence of mantle
Mantle plumes are rising column of magma in the
upper mantle. What causes plumes is not known for
certain. Some geologists says that they formed over
regions of locally high concentrations of (heat
producing) radioactive element in the mantle. If the
overlying plate of a plume is sufficiently weak, the
magma breaks through to form a volcano
Expected to be basaltic volcanism.
The composition of the material erupted depends
on the composition of the overlyinng plate.
When magma rises up through an oceanic plate, it is
expected to be basaltic whether or not some seafloor
Hot spot volcanoes in the oceanic basins which
are commonly built of many thin layers of fluid
basaltic lavas, if the magma makes its way up
through a crust, there is more potential for
assimilation of granitic material and production
of a more silicic final magma.
Volcanoes and Fissure Eruptions
When most people think of volcanic activity, they
think in terms:
Volcanoes - individual mountains built
around discrete vents through which
magma can erupt at the surface.
Lava – is simply magma that reaches to the
surface Many volcanoes are built of layer upon layer of
lava. However, not all volcanoes erupt only lava. Differences
in the material that make up a volcano contribute to
differences in both form and eruptive style.
the eruption of a lava out of a long crack rather
than from a single pipe or vent.
Very flat and low in relation to its diameter.
Built by basaltic lavas (relatively low in silica
and high in iron and magnesium and
They form over mantle plumes from which the
magma comes from.
Are very rare that makes 10% of the active
volcanoes in the earth.
a compact, steep-sided volcanic structure from
a very viscous lava.
any steep-sided mound that is formed when
lava reaching the Earth’s surface is so
viscous that it cannot flow away readily and
accumulates around the vent.
built by andesitic & rhylolitic composed silicic
lavas. (tend to be more viscous and flow less
readily. They ooze out at the surface like thick
are volcanoes that are made primarily of
are generally not very tall, generally not
more than 500 m high.
are steep-sided, often symmetrical mountains that
match the popular expectation of what a volcano
should look like
Considered as monogenetic volcano because they
only erupt once and then they become extinct.
also known as stratovolcanoes.
Are the most common mountain volcano such as
Mt. Pinatubo, Mt. Fuji and Mt. St. Helens.
Are polygenetic volcano for the capability of
repeated eruptions and separated by dormancy
periods over hundreds of thousand years.
Built mostly by andesitic lava.
Are much more larger and taller than cinder cones
and they have more explosive eruptions.
A large, bowl-shaped summit depression in a
This is caused when much of the magma has
erupted or perhaps magma has drained back down
to deeper levels leaving the volcano partially
unsupported. The overlying rocks may collapse if
they are very weak.
They can be bigger then the original crater from
which the lava emerged.
This is the Crater Lake .This is actually a caldera collapse of the ancient volcano
Mt. Mazama in southern Oregon. At 600 m depth, it is the deepest freshwater
lake in the USA.
Direct Hazards: Materials and Eruptive Style
Primary volcanic hazards include:
Ash and dust
Most people have regarded lavas are the primary
hazard during volcano eruption but actually, lava
is not generally life-threatening . Most lava flows
advance at speed of a few kilometer an hour at
most, so one can evade the advancing lava readily
even a foot.
The lava will of course, destroy or bury any
property over it flows.
Lava temperatures are typically over 500°C over
950°F) and may be over 1400°C (2550°C).
Combustible materials like houses and even forests
are burn at such temperatures.
Other property are simply engulfed in lava, which
then solidifies into solid rock.
Lavas, like all liquids, flow downhill, so one way to
protect property is to simply live away from a
volcano. However, people still or build houses near
at a volcano for some reasons:
They simply think that a volcano will not erupt again
for a very long time.
Soil formed from the weathering of volcanic rock
forms slowly but is often very fertile.
Sometimes, a volcano is the only land available.
Some strategies do exist for reducing the property
damage from lava.
In Iceland, in 1973, flow-quenching operations saved a
crucial harbor when the Edjell Volcano in Heimaey
Heimaey Island is surrounded with plentiful
cooling water. Boats sprayed water on lava flows
encroaching on the harbor thus, saving the harbor.
As the lava cools, it becomes thicker, more viscous
and flows more slower. They used water to
fastened the cooling of the lava.
Some have tried diverting lava flow’s course away
from the properties by carefully placing explosives
to the newly solidified lavas (only the crustal part
of the lava have been solidified, the interior part is
still molten and would take several days before it
will fully solidify) which had been stopped flowing
due to the lessen output of the volcano or upon
encountering natural or artificial barrier.
In this way, the internal molten that have been
exploded would take another path. Careful placing
of the explosives would guide the flow to another
course. This strategy was used in Italy in 1983,
when Mt. Etna began another series of eruptions.
Unfortunately, the strategy was only brief
successful. Part of the flow deflected the course but
after few days the lava left the planned alternate
channel and resumed to its original path.
Lava flows may be hazardous, but they are at least
predictable. Like other fluids, they flow downhill.
Once they flowed on a relatively flat area, they tend
The Edjell Volcano in Heimaey, Iceland erupted.
Pyroclastics are the bits of magma and rocks that
are wildly going out from a volcano during
eruption. This is due to the sudden release and
forcefully explosion of the built up gas pressure
in a rising magma. There are also block-sized, still
molten lavas that are thrown out of the volcano
called, volcanic bombs.
These are more dangerous than lava flows. They
may erupt suddenly and explosively, and spread
faster and farther. The larger the blocks the more
danger it brings.
However, they usually fall quite close to the
volcanic vent, so they affect small area.
Ash and Dust
These are the severe problems every volcanic eruption.
They can be carried over to a larger area by air. They
cannot just be confined in a valley and low places but
they can also blanket a countryside. As what happened
on May 18, 1980 eruption of Mt. St. Helens was by no
means the largest eruption recorded in the USA, but
the ash blackened the midday skies more then 150
kilometers away, measurable ashfall was detected
halfway across the USA.
Volcanic ash can be a problem in transportation. They
will make the road slippery as they land on the ground
causing accidents. Volcanic dust can choke car engines
as they are in the air. Homes, cars and land were buried
the hot ash.
Volcanic ash is also a health hazard that makes
breathing both uncomfortable and difficult.
In the Philippines, when the 1991 eruption of Mt.
Pinatubo, the combination of thick ashfall and
soaking rains caused the widespread collapse of
homes under the weight of the sodden debris.
Lahar is the result when hot falling ashes melts
the snow on ice or even when falling ashes
combined with heavy rain producing a mudflow. In
the Philippines, the 1991 Mt. Pinatubo eruption
caused lahar when rain-soaked ash on the
mountain slopes suddenly slid downhill.
Nuée ardente- (French word for “glowing cloud”)
also known as pyroclastic flow is a special kind of
deadly pyroclastic outburst . It is a denser-than-air
mixture of hot gases and fine ash. Pyroclastic flow
has a temperature over 1000ºC in the interior and it
can rush down the slopes of the volcano at more
than 100 kilometers per hour, charring everything in
its path , flattening trees and weak buildings. The
most famous pyroclastic flow tragedy is the 1902
eruption of Mont Polée on the Caribbean in the
Island of Martinique which caused fatal injury, burn
to death and suffocation to approximately 25,000 to
40,000 people in the nearby town of St. Pierre and
its harbor. The single reported survivor in the town
was a convicted murder who was imprisoned
underground in the town dungeon.
the image of the victims of the
Nuée ardente in the eruption of
In the history, andesitic volcanoes have often
histories of explosive eruptions so do many of
them have a history of pyroclastic flow.
Volcano eruptions produce gases that could kill
humans either through suffocation and poisoning.
Some gases that are not considered poisonous but
cause suffocations include water vapor and carbon
dioxide. Gases like carbon monoxide, various
sulfur gases and hydrochloric and hydrofluoric
acids are all poisonous.
Some volcanoes are deadly because of their
location respectively. In case of an island
volcano, the volcano may have a phreatic
eruption, an eruption caused by large amount
of water that have seeped into the rocks and
went nearer to the hot magma below, turned
into steam and blow up the volcano. This will
produce an huge explosion and may cause for
a high sea wave that could wash up its
Phreatic eruption may also occur when any water
–groundwater, lake water, snowmelt and so on –
seeps in to the crust to a hot magma body. One
classic example of this is the eruption of the
Krakatoa, Indonesia (island volcano). The force of
explosion was compared to 100 million tons of
dynamite. The sound was heard over 3000
kilometers away in Australia. Some of the dust
was shot 80 kilometers into the air causing red
sunsets for years afterwards. The shock of
explosion generated a fast moving sea wave over
40 meters. Krakatoa is an uninhibited island yet its
1883 eruption killed estimated 30,000 people
mostly in the low-lying coastal regions.
There are also instances that a viscous
rhyolitic or andesitic lavas plug the vent, the
pressure of gases associated with the magma
may build until it rips the volcano apart.
When Mt. St. Helens erupted in May of
1980, the volcano had shown sighs of activity
for some time, and its north slope is bulging,
a sign of potential explosion. Authorities had
evacuated the area, leaving only scientific
personnel and a small number of commercial
loggers, turning away droves of sightseers.
But the moment of explosion was recognized
only seconds before, too late to rescue those
remaining from the searing blast that cropped
over 400 meters from the mountain’s
elevation , cost an estimated $ 1 billion in
damages, killed 25 people, and left another 37
people unaccounted for, presumed dead. The
suddenness of that event confirmed the
wisdom of evacuation.
Secondary Effects: Climate
Intense explosive eruptions put large
quantities of volcanic dust high into the
atmosphere and takes years to settle to the
ground. Due to this, it will cause a partial
blockage of incoming sunlight, thus causing
After the Krakatoa, Indonesia Eruption in 1883,
worldwide temperatures dropped nearly half a
degree centigrade, and the cooling effects
persisted for almost ten years.
The larger eruption in Indonesia happened at
Tambora on 1815, gave another cooling. 1816 was
the known year in the Northern Hemisphere as
“the years without a summer.”
Volcanic dusts are not all the cause of the
climatic impacts of volcanic eruptions to the
world. It is also caused by the gasses emitting
from the volcano during eruptions.
The 1982 eruption of the Mt. El Chichón in
Mexico did not produce large quantity of dust,
but it did shoot volumes of unusually sulfur gases
into the atmosphere. These gases produced clouds
of sulfuric droplets that spread around the earth.
Acid droplets do not just block the incoming
sunlight but they also become acid rain when they
settle back to the ground.
The 1991 eruption of the Mt. Pinatubo in the
Philippines became famous when it gave extensive
output of both ash, dust and sulfur gases. The
resultant sulfuric acid mist circled the globe.
The unusually cool summer of 1992 in the
Northern Hemisphere was attributed to the
eruption Mt. Pinatubo.
Prediction of Volcanic
Volcanoes are divided into 3 categories
according to their activity although there are
no precise rules for assigning volcano to a
Dormant or Sleeping volcano
Extinct or Dead volcano
Are those volcanoes that have erupted or shown signs of
activity in the past 600 years. Mt. Pinatubo, Mt. Mayon and
Mt. Taal are the most famous active volcano in the
There are about 220 volcanoes in the Philippines, 25 of
them are said to be active.
Dormant or Sleeping volcano
The volcano has not erupted but is fresh looking and not
too eroded or worn down.
Dormant volcanoes are inactive up to the present but have
the potentials to be come active again.
Mt. Apo, Mt. Arayat and Mt. Makiling are the example of
this kind of volcano.
Extinct or Dead volcano
A volcano that has not recent eruptive history but also
appears very much eroded.
As volcanologists learned, statistically, a
typical volcano erupts once every 220 years,
but 20% of all volcanoes erupt less than once
every 1000 years, and 2% erupt less than once
in 10,000 years.
As estimated, there are 300-500 volcanoes in
the world (the uncertainty arises from not
knowing whether some are truly active or
dormant). Most are located over subduction
The Ring of Fire is the collection of volcanoes
rimming around the Pacific Ocean, is a ring of
Monitoring all the volcano is a way of
predicting the volcanic eruptions but it is a
large task. Active volcanoes should be
monitored for any sudden eruptions. Dormant
volcanoes might become active at any time.
Extinct volcanoes can be ignored but that’s
assuming that they are long-term dormant
Volcanologists only uses information of the
recent eruption of a volcano as their guide
for the future eruption.
Advance Warnings for Volcanic Eruption
The rising magma and gas up through the
crust beneath the volcano puts stress on the
rocks, and the process may produce months
of small (and occasionally large)
Bulging, tilt, or uplift of the volcano’s
It often indicates the presence of a rising
magma mass, the build up of gas pressure
Uplift, tilt and seismic activity may indicate that
an eruption is approaching but geologists do not
know yet the exact timing.
Changes in the mix of gas coming out of the
Gas emissions may reflect the approach of
magma toward the surface as it rises associated
Surveys of ground-surface temperatures.
Warm areas where magma is particularly close
to the surface and are about to breakthrough.
There have been reports that animals can
anticipate volcanic eruptions because they
sensitive to the earth’s changes.
PHIVOLCS (Philippine Institute of
Volcanology and Seismology) is the agency
that observes volcanic activities in the
They set the Volcanic Danger Zone, the
boundary around a volcano covering about 6
There are observed simple cautionary action
when a volcano shows signs of eruption.
Store food, first aid kit, flashlights, water and other
necessities in case of eruption.
Listen to radio, TV and other media for further
informations and instructions regarding the
Evacuate immediately and do not wait for force
Do not go back to the houses without prior
instructions from the government.
Major Volcanic Eruptions Since 1900
MajorVolcanic Eruptions Since 1900
Volcano Location Year Deaths*
Santa María Guatemala 1902 1,500
Pelée Martinique 1902 29,000
Taal Philippines 1911 1,335
Kelut, Java Indonesia 1919 5,110
Merapi Indonesia 1930 1,369
Rabaul Caldera Papua New
Lamington Papua New
Hibok Hibok Philippines 1951 500
Volcano Location Year Deaths*
Agung Indonesia 1963 1,148
St. Helens United States 1980 57
El Chichón Mexico 1982 >2,000
Nevado del Ruiz Colombia 1985 23,000
Lake Nyos Cameroon 1986 1,700
1991- 1996 800
Unzen Japan 1991 39
Mayon Philippines 1993 70
*All death tolls are estimates.
Source: United States Geological Survey.
Active Volcanoes in the Philippines
Babuyan Claro Cagayan 0.843 4 1917
Banahaw Laguna, Quezon 2.169 3 1843
Biliran Biliran Island 1.340 1 1939 Sept. 26
Buddajo Sulu 0.62 2 1897
Bulusan Sorsogon 1.565 17 2010 Nov.-
Cagua Cagayan 1.160 2 1907
Cabalian Southern Leyte
Cagayan 0.712 1 1857
0.843 6 1978 Jan. 6-9
Province Elevation (Km) No. of
Hibok-hibok Camiguin 1.332 5 1948 Sept. 31-
Iraya Batanes 1.009 1 1454
Iriga Camarines Sur 1.143 2 1642 Jan. 4
Kanlaon NegrosOriental 2.435 26 2006 June
Davao del Norte 0.200 NO DATA 1800 years ago
Makaturing Lanao del Sur 1.960 10 1882
Matumtum Cotabato 2.286 1 1911 March 7
Mayon Albay 2.460 49 2009 Dec.
Musuan Bukidnon 0.646 2 1867
Parker Cotabato 1.784 1 1640 Jan. 4
Pinatubo Boundaries of
1.445 3 1992 July 9-
Ragang Cotabato 2.815 8 1916 July
Province Elevation (Km) No of Historical
0.688 5 1924
Taal Batangas 0.311 33 1977 Oct. 3
Geneveve I. Magpatoc