MODULE 1: TYPES OF VOLCANOES AND VOLCANIC ERUPTIONS
Volcanic eruptions are one of Earth’s most dramatic and violent agents of change. Not only
powerful explosive eruptions drastically alter land and water for tens of kilometers around a volcano, but
tiny liquid droplets of sulfuric acid erupted into the stratosphere can change our planet’s climate
temporarily. Eruptions often force people living near volcanoes to abandon their land and homes,
sometimes forever.
So, if the volcanologist knows the type of volcano based on the type of eruption and materials it
emits, the communities at risk will be forewarned and there will be sufficient time to implement response
and mitigation.
What is a volcano and a volcanic eruption?
A typical volcano usually has a summit, slope, and base. At the summit, there is an opening which
may either be a crater or a 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 after an explosive eruption.Volcanoes are described
according to its type of eruption and cone shape.
Volcanic eruption happens when magma, gases and other molten materials are discharged from
the opening. Since volcanic eruptions are caused by magma expelled onto the earth’s surface, we must
first identify the factors that affect the viscosity of the magma.
Factors Affecting the Viscosity of the Magma
Viscosity is the property of the material’s resistance to flow. The more viscous and thicker the
magma is, the greater is its resistance to flow.
1.Temperature
The higher the temperature of magma, the
lower is its viscosity. As lava flows, it travels far before
it begins to harden. On the other hand, if the
temperature of magma is lower, the higher is its viscosity so the lava does not travel far.
2.Chemical composition
Magma with high silica content is more viscous
than those with low silica content. It is too viscous so it
does not travel far, and tends tobreak up
as it flows.
The magma that contains less
silica is relatively fluid, so it travels far
before solidifying.
3. Amount of dissolved gases it contains.
Whenmagma has high amount of
gas (mainly water vapor), lava flow is less
viscous and increases its ability to flow.
Magmahas high viscosity when it has low
amount of gas as it rises. The ability of the
lava to flow decreases, soit piles up at a vent resulting into a columnar or a dome.
Types of Volcanoes
After a volcanic eruption, a cone-shaped structure may be produced. Its slope also changes.The
following are the three (3) general types of volcanoes according to the shape of their cones:
Shield volcanoes are formed by the pileup of nonviscous lava that oozes out from the volcano. Since lava can
flow freely, a broad, slightly domed structure is formed that
resembles like a warrior’s shield. Mauna Loa Volcano in Hawaii
is an example of this type.
Composite volcanoes are formed due to alternating
solidification of both lava and pyroclastic deposits which makes
it nearly a perfect sloped structure. Pyroclastic flows occur when
these volcanoes erupt. Mount Saint Helens in Washington, Mt.
Vesuvius in Italy and Mt. Mayon in the Philippines are examples
of this type.
Cinder cones are formed from alternating ejected lava
fragments andash. They have a steep slope, wide crater on top
and are the most abundant of the three major volcano types. Although small, they are also destructive
since they release high amounts of pyroclastic materials. Mt. Paricutin in
Mexico which has erupted in 1943, Taal Volcano in Batangas are
examples of this type.
Types of Volcanic Eruptions
Volcanoes erupt differently and not all of them are violent. Magma
can explode from the vent, or it can flow out of the volcano. In addition to
the top vent, magma and gas may escape through cracks and weak
areas on the sides of the volcano. Aside from lava, pyroclastic flows, ash
clouds and mudflows(lahar) can be extremely dangerous. The levels of
silica and dissolved gases in the magma determine whether a volcano
erupts explosively or not.
a. Phreatic or hydrothermal – is a steam-blast small eruption as hot rocks heat the ground water or
surface water. This eruptiontears down surrounding rocks and can produce ash columns, but do not
include
new magma. Taal Volcano which erupted last January 12, 2020 is an example of this
type.
b. Phreatomagmatic - a violent and explosive eruption resulting from
the interaction of new magma or lava with water. It results to a large
column of very fine ash, high-speed and sideway emission of
pyroclastics.
c. Strombolian- least
violent
type
of
explosive
eruption
characterized by a fountain of lava fragments.
Eruptions are weak
to a little more violent which has thicker lava flows and small explosions.
d. Vulcanian - small to moderate explosive eruptions characterized by
tall columns that reach up to 20km high with pyroclastic flow and
ashfall tephra. They have larger explosions of rocks and ashes which
rise in dark clouds lasting for seconds to minutes. Paricutin Volcano
eruption in Mexico is an example of this type.
e. Plinian - excessively explosive type of eruption of
gas and pyroclastics like that of Mount Pinatubo
eruption in
1991. The explosive eruptions,
lasting for seconds to minutes have 20 to 35 km tall
gas, rock particles and ash columns.
MODULE 2: VOLCANIC ERUPTION
What Happen When Volcanoes Erupt
Volcanic eruption refers to the release of magma from a volcano. The term “magma” refers to the
molten materials that are still inside the volcano. When these molten materials are being ejected or reached
into the surface, it is then called “lava”.
When volcanoes erupt it ejects different materials into the Earth’s surface;
 It releases lava, which is made of molten materials that reach the surface of the earth.
 Lava contains different elements and compounds. One of the compounds it contains is Silica or
chemically known as silicon dioxide which greatly influences the lava’s viscosity. Thesilica content
of the lava determines the volcano’s explosiveness.
 Lava also contains useful elements like potassium and phosphorus which has a positive effect in
farming because it promotes soil fertility.
 In any type volcanic eruption, may it be violent or not, volcanoes release smoke. The smoke is a
mix of different gases like water vapor, carbon dioxide, sulfur dioxide, and hydrogen sulphide.
These gases that are ejected during volcanic eruptions have influences on climate. sulfur dioxide
can cause global cooling and carbon dioxide can cause an increase in the atmospheric
temperature contributing to global warming.
 Aside from smoke, volcanoes also release volcanic ashreferred to as “ tephra” which are made of
tiny fragments of rock and minerals. In some huge volcanic eruptions, volcanic ash fall may cause
the decrease in the Earth’s average temperature for a few years.
How do volcanic eruptions affect the society? It can have a devastating effect on people and the
environment. However, unlike earthquakes, volcanoes can also have a positive impact on an area. These
positive impacts can help to explain why people choose to live near volcanoes.
Advantages and Disadvantages of Volcanic Eruption
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
Raw material
Volcanic material can be made in to blocks for construction purposes. People became creative
also by making pots out of the ashfall from the volcanic eruption.