Learning Material

2. VOLCANOES

3. • The Philippines is located along
the Pacific Ring of Fire.
• The most famous among our
volcanoes is the Mayon Volcano
that has erupted last May 7, 2013
while a group of hikers were
eploring its beauty.

4. • Who could forget the terrible eruption of the
Pinatubo Volcano in 1992 after 600 years of
inactivity?
• Based on statistics, in the first five years
following the eruption, lahars destroyed the
homes of more than 100,000 people.
• The eruption also affected other countries as its
emission in the atmosphere lowered the air
temperature.

5. • According to the Philippine Institute of
Volcanology and Seismology
(PHIVOLCS), our country is an ideal site
for any volcanic activity.
• It is therefore, important for us to know
how this natural phenomenon happens to
reduce the risks and effects it may cause.

6. • In Grade 8, you have learned about the
relationship between the occurence of
earthquakes and the location of the
Philippines.

7. CLASSIFICATION OF VOLCANOES
• There are several ways by which
volcanoes can be classified.
• PHIVOLCS have adapted a
system where the Philippine
volcanoes as active or inactive.

8. • ACTIVE VOLCANOES are those that have a record
of eruption within the last 600 years or those that
erupted 10,000 years ago based on analysis of their
materials.
• INACTIVE VOLCANO are those that have not
erupted for the last 10,000 years and their physical
form is being changed by agents of weathering and
erosion through formation of deep and long gullies.

10. • Magma inside the volcano has high
temperature.
• As the magma is continously heated, it goes
up.
• As it rises, gas bubbles are developed.
• The gas bubbles are trapped and expanded
causing the molten material to swell also,
resulting in a gradual increase in the
pressure within the volcano.

11. • When the pressure exceeds the strength
of the overlying rock, fracturing occurs.
• Lava may appear to be the primarily
material ejected from a volcano, but this is
not always the case.
• Aside from lava, broken rocks, lava
bombs, fine ash and dust are also ejected.

12. • Does this have any effect
on the volcanic shape?
• Let’s find out how!

13. VOLCANIC LANDFORMS AND
ERUPTIVE STYLES
• Volcanoes come in different
shapes and sizes, and each
structure has a unique history
of eruption.

14. • However, volcanologists have been
able to classify them according to their
landforms and eruptive patterns.
• We have considered the three general
volcanic types according to the shape
of their cones, namely: shield
volcanoes, cinder cones, and
composite cones.

16. • A volcano usually has a summit, slope,
and base.
• At the summit, there is an opening which
may either be a crater or caldera.
• A crater is a funnel-shaped opening at the
top of a volcano while a caldera is formed
when a part of the wall collapses following
an explosive eruption.

17. • Volcanic activity can be very fascinating.
After an explosive eruption, a scenic cone-
shaped sructure may be produced or
changes on its slope can be observed.
• The emissions of a volcano provide us
with clues on what materials can be found
inside the earth.
• Some eruptions are very explosive, while
many others are not.

18. • What determines the nature of eruption?
• There are primarily factors affecting the
volcanoes’ eruptive style, namely: the
magma’s temperature, its chemical
composition, and the amount of dissolved
gases it contains.
• These factors can affect the magma’s
viscosity in different ways.

19. • Viscosity is the property of material’s
resistance to flow.
• It is also described as the liquid’s
thickness and stickiness.
• The more viscous and thicker the
material is, the greater is its resistance
to flow.

20. • The viscosity of magma decreases with
temperature.
• The higher the temperature of magma
is, the lower its viscosity is.
• As lave flows, it cools and begins to
harden, its ability to flow decreases and
eventually stops.

21. • Magmas with high silica content are
more viscous than those with low silica
content.
• The magma that contains less silica is
relatively fluid and travels far before
solidifying.

22. TYPES OF VOLCANIC ERUPTION
• Phreatic or hydrothermal – is a stream-
driven eruption as the hot rocks come in
contact with water.
• It is short lived, characterized by ash
columns but may be onset of a larger
eruption.

23. • Phreatomagmatic – is a violent eruption
due to the contact between water and
magma.
• As a result, a large column of very fine
ash and high speed and sideway
emission of pyroclastics called base
surgers are observed.

24. • Strombolian – a periodic weak to violent
eruption characterized by fountain lava,
just like the Irazu Volcano in Costa Rica.

25. • Vulcanian – characterized by tall eruption
columns that reach up to 20 km high with
pyroclastic flow and ashfall tephra like that
of Paricutin Volcano in Mexico.

26. • Plinian – excessively explosive type of
eruption of gas and pyroclastics, just like
our Pinatubo Volcano in Zambales.

27. • Plinian – excessively explosive type of
eruption of gas and pyroclastics, just like
our Pinatubo Volcano in Zambales.

28. • Although volcanologists have
different basis for classifying
volcanoes by their cones.

29. • Shield volcanoes are formed by the
accumulation of lava that oozes out
from the volcano.
• Since non-viscous lava can flow
freely, a broad, slightly doomed
structure that resembles a warrior’s
shield is formed.

30. • Cinder cones, on the other hand,
are built from ejected lava fragments.
• They have a steep slope, wide crater
and are the most abundant of the
three major volcano types

31. • Composite cones or
stratovolcanoes are large, nearly
perfect sloped structure formed from
alternate solidification of both lava
and pyroclastic deposits.