LESSON 5
At the end of this lesson, the students should be able to:
Recognize signs of
Explain various Interpret different
an impending
volcano-related volcano hazard
volcanic eruption;
hazards; maps
and
• The abundance of plates surrounding the
Philippines has led to the generation of faults
within the islands of the country. With it
comes to the generation of volcanoes as well.
• When subduction occurs, one of the major
effects is that pressure builds up close to the
surface and eventually creates a release of
energy such as earthquakes.
• As the earth’s tectonic plates move and collide with one another,
a tectonic plate subducts in an area called the subduction zone.
• The boundaries of plates are where majorities of volcanoes are
formed, as these are the areas where subduction continually
occurs.
• As the masses of land go deeper underground, the pressure and
temperature rise, thereby forcing water from the rocks to be
released. The water reduces the melting point of the rocks
underneath, which forms magma.
• The magma will eventually makes it way up the surface with the pressure
further building up. This will then give rise to the generation of volcanoes.
• Water is primarily present because the adjacent plate brings in parts of the
ocean floor. However, the formation does not always look like the photo in
Figure 5.1. It may start small, and then with repeated explosions, they stack
on top of each other to form a cone or they mar form through the continuous
lava flow.
• This mechanism of continuous flowing after the eruption contributed to the
formation of islands, archipelagos, and several other landmasses. It must be
recognized that not all volcanoes come from subduction—certain zones of
magmatic activity or hotspots also generate the rising of crusts in the middle
of a tectonic plate.
Volcano-related hazards can be summarized into six components:
1. Lahar;
2. Ashfall;
3. Pyroclastic flow;
4. Ballistic projectile;
5. Volcanic gas; and
6. Lava flow
The term lahar is of Indonesian
origin for “lava” or “lava flow”, which
volcanologists adapted to describe a wet
cement-like mixture of volcanic material
and water.
When lahar flows, it carries with it
fresh volcanic materials—pyroclastic
flows and tephra falls (ashfalls).
• Do not be fooled by its appearance; lahar is highly mobile,
spanning a movement speed of tens of meter per second,
which makes it impossible to outrun by people.
• With the thickness of volcanic
materials it brings as well as a
toxic chemicals, it is almost
impossible to survived being
submerged in this kind of
A house buried in lahar and volcanic debris
mixture.
• The formation of lahar is common in
areas with channels of water. It also
follows the trail of pyroclastic flows.
• Furthermore, lahar may also manifest
in the form of larger boulders as big as
houses scattered over the affected
areas. These are insightful indicators
that lahar has occurred in the area
before.
Lahar-affected areas from the 1991 Mount
Pinatubo volcanic eruption
• The formation of the wet cement-like structure of lahar does not
always come from rivers and water channels. Oftentimes, intense
rainfall may also bring about the formation of lahar.
• Volcanoes release various toxic gases and materials up the
atmosphere that affect the weather in the area. In turn, this leads to a
lot of rain, lightning and thunder. Materials spewed upward attract
water, which later falls due to gravity.
• The gray appearance of lahar can be attributed to
the presence of debris from ashfall.
• Tephra is another name that refers to ashfall;
however, there are notable differences.
• Ashfalls consist of fragmented volcanic particles
less than 2 mm in diameter in size while tephra
simply means fragmented volcanic particles in
general.
• After the eruption occurs, these minute particles get thrown into the
air and form a column of ash. They then settle down due to gravity,
cloaking affected areas with a blanket of tephra or ash.
• Taal Volcano, for example, showed intense volcanic activities in 2020
that affected various areas near the volcano. As a result of its
eruption, the ashfall posed a health hazard when inhaled as it can lead
to a variety of respiratory diseases.
• In some instances, it can also cause severe amounts of skin damage
due to its toxic content.
• In addition, once it reaches the reservoirs, it can induce significant
toxicity that will make these water sources contaminated.
• Likewise, it may also damaged infrastructure, residential houses,
natural environments, factories, facilities, and even aircraft
(PHIVOLCS, 2018).
A
B C
Figure 5.5 (A) Taal Volcano without volcanic activity; (B) Ash column from 2020 Taal Volcano eruption seen formation caused by the Taal
Volcano eruption in 2020; and (C) A vehicle affected by ashfall due to Taal Volcano eruption
• Apart from the ashes that trail down affected
areas, what comes from explosions are
pyroclastic flows.
• The materials released from the mouth of the
volcano are generally classified as pyroclastic
density currents (PDCs) made up of volcanic
particles such as pyroclastic, hot gases, and
ashes. All of this rush down from the mouth of
the volcano after its explosion.
• Depending on density, composition, and viscosity, PDCs may be
classified into two:
1. Pyroclastic Flow – it is a dense type of current that moves a bit
slower than a surge. This tends to be more attached to the
ground.
2. Pyroclastic Surge – It is a diluted type of current that has more
mobility, therefore posing more risk to affected communities.
As seen in Figure 5.7, PDCs
(pyroclastic flow or surge) may
originate from the mouth of the
volcano after a gravitational
collapse from the column is
manifested (PHIVOLCS, 2018).
In addition, an explosion of a
lava dome from a side vent or the
main vent may also cause
Figure 5.7 Volcanic eruption. Pyroclastic flow originates from
the main vent of a volcano and the lava dome from the side vent pyroclastic flow or surge.
of the volcano.
• Pyroclastic flows are the most harmful among all volcano-related
hazards because they cause fires in natural environments,
incineration of people and animals, and “asphyxiation, abrasion,
dynamic pressure impact and burial in hot volcanic material”.
• It truly pays to follow emergency evacuation protocols especially if
one’s area is within the danger zone.
• Apart from all the ashfall and pyroclastic flow that
comes out of the mouth of the volcano, some large
blocks or bombs are also formed called ballistic
projectiles. These do not go straight into the
volcanic column but are rather thrown into an
affected area.
• Their mechanism can be likened to a cannonball
usually landing within 2-5 km away from the vent.
It can also travel much further if the volcano is
more explosive.
• A small crater was formed by the landing of the block. Bombs also
present danger to people as they carry partially molten lava fragments
with high viscosity.
• Various shapes are formed throughout the projectile motion in the
air. If bombs encounter a home or a nearby community, the lava
inside would cause incineration or worst an explosion of partially
molten lava. These are just some of the many hazards that volcanoes
pose to humans.
• There are variety of gases released before and even
after an eruption. Before an eruption, gases have
already built up pressure inside the magma
waiting to be released.
• Magma consists of various dissolved gases that are
harmful to health, vegetation, and infrastructure.
• Most of the volcanic gases are released into the atmosphere through
the main vent; some of them being heavier than air, go down into
low-lying communities near the vicinity of the eruption.
• It can be seen in the picture that the
cattle were not safely removed from
the area, resulting in their death after
several days of exposure to the gases.
• One can only imagine what it can do
to humans.
Below are some of the gases released from an eruption and their effects:
• Water Vapor – colorless, odorless, and harmless; the most abundant
volcanic gas.
• Carbon Dioxide (CO2) – colorless and odorless; most dissipate into the
atmosphere; however, large concentrations released from a volcanic
eruption fall and get trapped in low-lying areas. According to the
USGS (2021): “Breathing Air with more than 3% CO2 can quickly lead
to headaches, dizziness, increased heart rate, and difficulty
breathing. At mixing ratios exceeding about 15%, carbon dioxide
quickly causes unconsciousness and death.
• Sulfur Dioxide (SO2) – colorless
gas with a pungent odor that
irritates the eyes, skin, and
respiratory system; high
concentrations of this gas may
create smog, which can cause
further damage to communities
directly within the path of the
wind. Figure 5.11 Volcanic smog (vog) is produced from
SO2 gas and is a hazard in Hawaii
• Hydrogen Sulfide (H2S) – colorless, flammable gas with a strong,
offensive odor and strongly associated with the smell of rotten eggs
or sewer systems. According to the USGS (2021): “Mixing ratios above
about 0.01%, H2S becomes odorless and very toxic, causing irritation
of the upper respiratory tract and pulmonary edema during
prolonged exposure. Exposure to 500 ppm can cause a human to fall
unconscious in 5 minutes and die in an hour or less”.
• Hydrogen Halides such as Halogens Flourine (HF), Chlorine (HCl) and
Bromine (HBr) – toxic acids that quickly dissolve in the atmosphere
becoming components of acid rain. It is poisonous to sources of
drinking water and vegetation.
• Lava flows emit a brought red-orange appearance,
signaling that the once enclosed magma has been
released in the form of lava.
• According to PHIVOLCS (2018), lava flows can be
described as rivers of incandescent molten rocks
flowing down a slope from an eruption vent.
Depending on the level of silica, lava flows have varying viscosities:
• Low silica magma = low viscosity lava flow; movement at high
speeds (kilometer per hour); and
• High silica magma – high viscosity lava flow; movement at low
speeds (kilometers per day)
❑ Note that the steepness of the slope adds to the movement speed of
the lava flow as it succumbs to gravity. Furthermore, lava flows carry
high temperatures capable of forest fires. It can also burn through
the soil, vegetation, residential houses, and infrastructure, creating a
river of lava.
According to the USGS (2021), there are several ways to tell if a volcano is
about to erupt. Some are related to seismic activity while others are related to the
release of gases. The signs include the following:
• The frequency and intensity of earthquakes felt within the area increase. As
pressure keeps building up, it releases more seismic energy that causes
ground shaking in the area.
• The release of steam and gases from the vents near the volcano increases.
From afar, it may be seen only as a cloud of air.
• The fumarolic activity increases. Fumaroles near the mouth of the volcano not
only emit gases but also show a noticeable increase in carbon dioxide
concentration in the air.
• Heat emission increases. The temperature within the area increases
dramatically. It is an indicator that magma is close to the surface.
The following are other common signs of impending volcanic eruption:
• Ground elevation (tilting) and ground fissuring (cracks) are visible. Rocks are
slowly being pushed upward from the mouth of the volcano affecting areas
adjacent to it.
• Flowing lava with red-orange colors is visible at the mouth of the volcano. The
magma enclosed underneath has started to release pressure.
• There is a change in the color of vegetation, an indicator of an increased
volume of volcanic gas emitted.
• Landslides are brought about by volcanic activity.
• Springs or bodies of water within the area are dying up.
• Hazard maps serve as a guide to people about the danger zones and safer
zones in the vicinity of the volcano.
• The maps may change from time to time depending on the activity of the
volcanoes.
• Manila Observatory (2005) also provided a list of provinces with the greatest
amount of risk for volcano-related damage:
1. Camiguin (or Camiguin Islands) 6. Sorsogon
2. Sulu 7. South Cotabato
3. Biliran 8. Laguna
4. Albay 9. Camarines Sur
5. Bataan 10. Batanes
• Camiguin has the highest risk because the land area is so small that a volcanic
eruption can affect the whole province.
• Sulu ranked second because it has the greatest number of active and
potentially active volcanoes (Manila Observatory, 2005).
• Aside from knowing which areas are at risk, another efficient way to prepare
for volcanic eruptions is knowing their distribution across the country as well
as their respective activity.
• The study of two maps---risk to volcano eruption hazard map and volcano
distribution map—can aid people to have a grasp of the possible hazards.
• These active volcanoes have
erupted within the last 600 years
based on the analysis of experts
after studying the materials
from volcanic deposits.
• Potentially active volcanoes are
classified as such due to their
young-looking morphology;
however, they have no historical
or analytical record of any
eruption.
Disaster Readiness and Risk Reduction
Joven Olaso Tataro
jovenolasotataro.shs2022@gmail.com
0938-601-8826
