Lecture 3:
Dated: Feb., 18,2022
Volcanoes:
Relationship between earthquake and Volcanoes
re about the
relationship between these two natural processes.
Earthquakes and volcanoes are natural phenomena resulting from the plate
tectonics. In general, a volcanic eruption is accompanied by earthquakes.
Tariq Mahmood Bajwa, PhD, Feb., 18, 2023/CE-121
Earthquakes refer to the shaking or trembling of the Earth’s crust as a result of
abrupt release of energy. Fundamentally, they are seismic waves, generated
by the natural phenomena or at times due to man-made events. Volcanoes, on
the other hand, are openings in the Earth’s crust from which hot gases and
molten rock materials are ejected onto the surface of the Earth.
Earthquakes and volcanic activity are closely related to each other. In fact,
earthquakes usually accompany a volcanic eruption. Similarly, unusual
earthquakes can also lead to volcanic eruptions. Before discussing the
relationship between earthquakes and volcanoes, let’s take a brief look at each
of them individually.
Earthquakes
Earthquakes, as mentioned earlier, are caused due to a sudden release of
pressure that has been accumulated over a period of time. The generated
seismic waves are measured with the help of a seismometer, in order to
indicate the intensity or size of the earthquake.
The size of an earthquake is represented by moment magnitude scale (MMS);
a magnitude of 3 or lower is undetectable, whereas a magnitude equal to or
Tariq Mahmood Bajwa, PhD, Feb., 18, 2023/CE-121
greater than 7 causes maximum damage to life and property. The underground
point where this process originates is called the hypocenter or focus. Epicenter
refers to the point on the Earth’s surface, which is exactly above the
hypocenter.
Volcanic Eruptions
Volcanic eruptions are processes that involve extrusion of magma. They
usually form mountains or mountain-like landscapes after the ejected
materials cool down. They can occur in any part of the Earth’s surface, either
in land or seas and oceans. Volcanoes are classified into active (eruptive),
dormant (presently not active), and extinct (not eruptive) types based on the
activeness of a particular volcano. They are further classified into six different
types – shield, cinder, submarine, subglacial, stratovolcano, and
supervolcano, depending upon the mode of ejection and other features.
How are Earthquakes and Volcanoes Related
The close relationship between temblors and volcanic outbursts is evident
from the maps depicting the locations prone to both these phenomena. If you
compare the maps that illustrate earthquake zones and volcanic zones, you
will find them matching each other. This is because the main theory behind
both these natural calamities lies in the plate tectonics.
The planet Earth comprises irregular-shaped and varying-sized plates, which
constantly move at different speeds. To be precise, the plates drift over the
mantle layer of the Earth. Consequently, magma is generated along the plate
boundaries.
When the plates collide, move apart, or slide each other, it leads to generation
and accumulation of pressure (strain), which when released causes
earthquakes. The strongest earthquakes are manifested during the plate
collision, while the slowest earthquakes are observed when plates move apart
from each other.
Similar to earthquakes, volcanic activity is observed when the plates are
divergent (move apart) or convergent (move towards each other). In such
Tariq Mahmood Bajwa, PhD, Feb., 18, 2023/CE-121
plate movements, the magma present in the plate boundaries may rise to the
Earth’s surface, leading to volcanic eruptions. Divergent plates may cause
long volcanic rifts, whereas convergent plates result in individual volcanic
eruptions.
In addition, both activities occur within a plate, which are referred to as
intraplate earthquakes and volcanoes, respectively. It is estimated that about
10 percent of earthquakes are of this type.
Tariq Mahmood Bajwa, PhD, Feb., 18, 2023/CE-121
Tariq Mahmood Bajwa, PhD, Feb., 18, 2023/CE-121
Volcanoes
Figure: Volcanoes transport magma from Earth’s interior to its surface, where rocks are
formed and gases are injected into the atmosphere (or hydrosphere, in the case of an
underwater eruption).
Tariq Mahmood Bajwa, PhD, Feb., 18, 2023/CE-121
Eruptive style and landforms
Tariq Mahmood Bajwa, PhD, Feb., 18, 2023/CE-121
Types of Volcanoes
A volcano is a landform, a mountain, where molten rocks erupt through the surface
of the planet. The volcano mountain opens downwards to a pool of molten rocks
below the surface of the earth. When the pressure builds up in the earth’s crust,
eruptions occur. Gasses and rock shoot up through the opening and spill over or
fill the air with lava fragments. The volcano eruption can cause lateral blasts, hot
ash and lava flow, mudslides, and more.
Categories of Volcanoes
Volcanoes are categorised into three main categories:
•
•
•
Active
Dormant
Extinct
An active volcano is one which has recently erupted and there is a possibility
that it may erupt soon.
A dormant volcano is one that has not erupted in a long time but there is a
possibility it can erupt in the future.
An extinct volcano is one which has erupted thousands of years ago and there’s
no possibility of an eruption.
Tariq Mahmood Bajwa, PhD, Feb., 18, 2023/CE-121
Types of Volcanoes
Volcanoes are grouped into four types:
•
•
•
•
Cinder cones
Composite volcanoes
Shield volcanoes
Lava volcanoes
Cinder Cones
Cinder cones are circular or oval cones made up of small fragments of lava from a
single vent that have been blown up. Cinder cones result from eruptions of
mostly small pieces of scoria and pyroclastics that build up around the vent.
Most cinder cones erupt only once. Cinder cones may form as flank vents on
larger volcanoes, or occur on their own.
Composite Volcano
Composite volcanoes are steep-sided volcanoes composed of many layers of
volcanic rocks, usually made from high-viscosity lava, ash and rock debris. These
types of volcanoes are tall conical mountains composed of lava flows and other
ejecta in alternate layers, the strata that give rise to the name.
Tariq Mahmood Bajwa, PhD, Feb., 18, 2023/CE-121
Shield Volcano
Shield volcanoes are volcanoes shaped like a bowl or shield in the middle with
long gentle slopes made by basaltic lava flows. These are formed by the eruption
of low-viscosity lava that can flow a great distance from a vent.
They generally do not explode catastrophically. Since low-viscosity magma is
typically low in silica, shield volcanoes are more common in oceanic than
continental settings. The Hawaiian volcanic chain is a series of shield cones, and
they are common in Iceland, as well.
Tariq Mahmood Bajwa, PhD, Feb., 18, 2023/CE-121
Lava Domes
Lava domes are formed when erupting lava is too thick to flow and makes a
steep-sided mound as the lava piles up near the volcanic vent. They are built by
slow eruptions of highly viscous lava.
They are sometimes formed within the crater of a previous volcanic eruption.
Like a composite volcano, they can produce violent, explosive eruptions, but their
lava generally does not flow far from the originating vent.
Types of Volcanic Eruptions
Types of volcanic eruptions depend on various factors such as the chemistry of
magma, temperature, viscosity, volume, presence of groundwater, and water and
gas content.
Following are the different types of volcanic eruptions:
•
•
•
Hydrothermal eruption: These eruptions include ash and not magma. They
are driven by the heat caused by hydrothermal systems.
Phreatic eruption: This is driven when the heat of the magma interacts
with the water. These eruptions do not include magma and only ash.
Phreatomagmatic eruption: This eruption takes place when there is an
interaction between the newly formed magma and water.
Tariq Mahmood Bajwa, PhD, Feb., 18, 2023/CE-121
•
•
•
Strombolian and Hawaiian eruption: Hawaiian eruption has fire fountains
while the Strombolian eruption has explosions due to lava fragments.
Vulcanian eruption: These eruptions last for a short period of time and can
reach up to a height of 20 km.
Subplinian and Phinian eruptions: Subplinian eruptions reach up to 20 km
in height, while Plinian eruptions reach up to 20-35 km.
Where are the most active volcanoes?
The most active volcano in the world is found in the Kilauea volcano in Hawaii.
Other most active volcanoes are Etna in Italy and Piton de la Fournaise on La
Reunion island.
What are volcanic rocks?
Volcanic rocks are the igneous rocks that are found in volcanic regions. They are
fine-grained, glassy textured rocks. These rocks are also vesicular in texture which
Tariq Mahmood Bajwa, PhD, Feb., 18, 2023/CE-121
is in the form of voids that are created by the volatiles that try to escape from the
molten lava. The volcanic rocks are named based on their chemical composition.
The most common volcanic rock is basalt. The silica content in the basalt rock is
very low. Rhyolite is a volcanic rock that has the highest silica content.
What is the relationship between volcanoes and geysers?
The relationship between volcanoes and geysers is that both of them are
dependent on the strong heat source which is present in underground. The
mechanism of a geyser is based on the surface phenomenon. When the
groundwater beneath the shallow surface gets heated up, the surface explodes
resulting in the boiling and steaming of the water. These surfaces get refilled again
and the cycle continues. The difference between geysers and volcanoes is that
geysers usually occur in volcanic regions while volcanoes don’t have geysers
around them.
How volcanoes are formed?
When the magma from the earth’s upper mantle erupts upwards, volcanoes are
formed. When the volcano erupts there is a formation of lava and ashes. The lava
flows down depositing the ashes. This cycle continues making the site bigger and
bigger.
What is plate tectonics?
Plate tectonics describes the way in which the continents drift from one another
and form a new place. When the plates move, they collide with each other and
grind each other. The tectonic movement is related to the earth’s crust and upper
mantle. Earthquakes are the result of tectonic movements.
Tariq Mahmood Bajwa, PhD, Feb., 18, 2023/CE-121