What is Volcano?
Volcanoes are ruptures
in a planet’s crust that form because of upwelling magma or molten rock. The
magma collects in a magma chamber near the surface. Gas released from the magma
in the chamber creates pressure within the chamber which eventually creates a
breach in the rock, resulting in a volcanic eruption.
Some volcanoes produce
eruptions that are more explosive and produce more debris. Others produce
eruptions which result in more lava flows. Volcanoes are found on many
planetary bodies of the Solar System, including Earth, Mars, Io, and Venus.
There is also evidence of cryovolcanoes, volcanoes that erupt volatiles such as
water and ammonia that produce ice instead of rock, on icy bodies of the outer
Solar System such as Neptune’s moon Triton and Saturn’s moon Enceladus.
Classification of volcanoes
Volcanoes can be
classified in many ways. Two ways that volcanoes are often classified is by
eruption type and morphology. There are many different morphological types of
volcanoes, but three common types are shield volcanoes, stratovolcanoes, and
cinder-cone producing volcanoes. There is also a variety of different eruption
types. Some eruptions produce more explosions and debris. These are naturally
called explosive eruptions. Other eruptions produce more lava flows. These are
called effusive eruptions.
Classification by
Morphology
Cindercones
Cindercones are
cone-shaped vents of a large volcano made of piles of volcanic glass shards
such as scoria that quickly emerge out of the ground from continuous explosive
eruptions in which molten rock is “spit” out of a vent and quickly becomes
solidified. These volcanic features are common in rift basins where the crust
is thin, allowing magma to easily breach the surface.
Shield volcanoes
Shield volcanoes are
dome-shaped volcanoes that get their name from resembling a shield laid on its
side. They are usually composed of sequential lava flows stacked on top of each
other. Mauna Kea in Hawaii and the Tharsis volcanoes on Mars are examples of
this type of volcano.
Stratovolcanoes
These are volcanoes
that contain multiple layers of different types of volcanic material. They
contain large amounts of volcanic debris like cinder-cone producing volcanoes
and extensive lava flows like shield volcanoes. Famous stratovolcanoes include
Mount Fuji, Stromboli, and Mount Saint Helens.
Classification by
Eruption type
Volcanic eruptions vary
depending on rock composition, the amount of magma, gas content, and the
tectonic setting.
Hawaiian eruptions
Hawaiian eruptions consist
mainly of lava flows. These types of eruptions are common on volcanic islands
and at places where the magma has a particularly mafic, specifically basaltic,
composition such as oceanic island arcs and on ocean islands near hotspots. The
magmas associated with Hawaiian eruptions also have low gas content. Places on
Earth where Hawaiian type volcanic eruptions are common include Iceland,
Hawaii, and similar locations. The Martian volcanoes in Tharsis, Olympus Mons,
Tharsis Montes, Ascreaus Mons, and Arsia Mons, are also probably from Hawaiian
style eruptions which occurred on a much larger scale than their terrestrial
counterparts.
Strombolian eruptions
A strombolian eruption
occurs when the magma is less mafic, but still predominantly mafic, and the gas
content is higher. Strombolian eruptions consist of sequential bursts of lava
and volcanic debris followed by periods of quiescence lasting a few minutes to
a few hours. A very well-known volcano with strombolian style eruptions is the
volcano on the island of Stromboli which has been called the “Lighthouse of the
Mediterranean.”
Vulcanian Eruption
A vulcanian eruption is
similar to a strombolian eruption except that the eruptions are more explosive
and the periods of quiescence separating eruptions are longer. Magmas in
vulcanian eruptions are more felsic than strombolian or Hawaiian style
eruptions. Felsic magma, such as rhyolite, traps more gas than mafic magmas
and, as a result, volcanoes with felsic magma tend to be more explosive. This
makes vulcanian eruptions larger and more powerful than Strombolian eruptions.
Plinian Eruptions
The most powerful
common eruption which occurs on Earth is a Plinian eruption. Plinian eruptions
occur when the magma is even more felsic than in vulcanian eruptions and even
more gas is trapped. Plinian eruptions produce columns of volcanic debris that
can be as high as 45 kilometers. Columns that are higher than about 30
kilometers have long term effects on climate and thus these eruptions are
important for paleoclimate studies. Plinian eruptions were named for Pliny the
Younger who observed the Plinian eruption resulting from Mount Vesuvius which
destroyed Pompeii in A.D. 79. Other famous Plinian eruptions include Tambora
and Krakatoa.
Hazards of Volcanoes
Active volcanoes are
most common at active plate boundaries and hotspots. The plate boundaries where
volcanism is the most common are convergent plate boundaries such as subduction
zones where an oceanic plate is being subducted beneath either lighter oceanic
crust or continental crust since continental crust is always less dense than
oceanic crust. Volcanoes are also common in continental rifts where the crust
becomes thin enough that magma can easily breach the surface. These are the
areas where volcanic hazard is the greatest.
Eruptions can be very
destructive to local human communities. The hazards from volcanoes include mass
wasting, ashfalls, and falling debris.
Mass wasting associated
with volcanoes
Mudslides
Mudslides can occur
when a mass of muddy material becomes detached from the slope of a volcano and
slides in a coherent unit. Such mudslides can be very destructive to nearby
towns.
Mudflows
Mudflows can also be
triggered by volcanic eruptions and occur when the mud behaves as a fluid
creating a river of mud. Mud flows are very dense and can carry boulders at
high speeds.
Lahars
Lahars are mixtures of
mud, volcanic debris, and water. Their temperatures are hundreds of degrees
Celsius and they move at very high speeds. They are among the most destructive
forms of mass wasting associated with volcanic eruptions.
Ashfalls
Explosive volcanic
eruptions can produce copious amounts of ash sized particles which can be
carried great distances by wind. Ash can cover roofs and ground and is very
difficult to clean. Volcanic ash is also very sharp and jagged and can damage
car and plane engines as well as the lungs of animals and humans.
Falling debris
In explosive eruptions,
molten rock and mineral crystals that already solidified within the magma can
be ejected at high speeds. They range in size from ash-sized to pebble-sized in
the case of lapilli to a meter, or more, across in the case of blocks and
bombs. Flying volcanic debris is also dangerous as it can collide with
buildings and other objects as well as with humans.
Predicting Eruptions
There is no way of
predicting exactly when an eruption will occur but there are signs that show
that a volcanic eruption is imminent. These include, earthquake swarms and the
bulging of the slope of the volcano.
Earthquake swarms
When molten rock moves
through chambers beneath the surface, this can cause a cascade of earthquakes
as the molten rock moves against the walls of the chamber. This doesn’t
necessarily mean that an eruption will occur, but it does mean that molten rock
is moving and may be moving towards a volcanic vent.
Expansion of terrain
Because of the gas and
magma nearing the surface of a soon-to-erupt volcano, the slope of the volcano
may appear to bulge or deform as gas and magma push against the rock. This
bulging is usually only detectable by tiltmeters.
Alerting nearby
communities
Most volcanoes near
population centers have teams of volcanologists who monitor them and warn of
potentially dangerous activity. There is also a color-coded system used by
volcanologists to indicate the degree of danger of a volcanic eruption.
What is an Earthquake?
Earthquakes occur when
the surface is shaken or disturbed in some way due to interior processes within
the earth. Earthquakes are usually caused by slipping between two bodies of
rock along a fault. This slipping will result in seismic waves. Similar quakes
might also occur on other planets.
Earthquake waves
The two types of waves
involved in the cause of earthquakes are surface waves and body waves which
travel through Earth’s interior.
Body waves
The two types of body
waves are p-waves and s-waves.
P-waves
P-waves are
longitudinal waves, meaning that the oscillation caused by the wave is parallel
to the propagation of the wave through rock. They can travel through both solid
and liquid components of the earth or another planetary body. As p-waves move
through rock, the material will become compressed at the crests of the waves
and extended at the troughs.
S-waves
S-waves are transverse
waves, meaning that their oscillation is perpendicular to their propagation.
S-waves are slower than p-waves. In fact, the “s” in s-wave means “secondary”
while the “p” in p-wave means primary since s-waves will arrive after the
p-waves. Unlike p-waves, s-waves can only travel through solid material and
will not travel through liquid or air. One of the reasons that geophysicists
know that Earth has a liquid outer core is that there is a region within
Earth’s interior from which seismic detectors do not receive any s-waves, only
p-waves.
Surface waves
Surface waves can come
in a variety of forms. The two types of surface waves are waves which cause the
ground to move laterally and waves which also cause a vertical oscillation of
the ground. Surface waves that move the ground laterally are called love waves.
Surface waves that also cause a vertical oscillation of the surface are called
Rayleigh waves.
Geological settings of earthquakes
Earthquakes are caused
primarily by plate movements and movements along faults. Faults are essentially
cracks in Earth’s crust that are actively deforming as bodies of rock on either
side of the fault slide against each other. This movement of bodies of rock is
the basis of plate tectonics.
Earthquakes and faults
Earthquakes are
typically caused by movement of bodies of rock along faults. There are three
types of faults where earthquakes cluster. Normal faults, reverse faults, and
transform faults.
Normal faults
Normal faults are
faults where two tectonic blocks or bodies of rock are being pulled away from
each other. These faults occur in regions of extension such as rift basins and
at mid-oceanic ridges where tectonic plates are diverging from each other. These
faults are also apparent on other planetary bodies such as Mars in the Valles
Marineris region.
Reverse faults
Reverse faults occur
where two tectonic blocks are pushing against each other. This can cause one
block to be thrust upward and over another block. This type of fault is common
at subduction zones and at wrinkle ridges on planetary bodies such as Mercury,
the Moon, and Mars, where cooling of the planet has caused contraction of the
crust. Reverse faulting is, as a result, associated with compression.
Transform faults
Transform faults occur
where two tectonic blocks move laterally with respect to one another. A
well-known example of a transform fault is the San Andreas fault in the U.S.
state of California.
Oblique faults
Oblique faults exhibit
both reverse/normal and transform movement of the associated tectonic blocks.
Most major faults have segments which show varying degrees of obliquity.
How faults lead to earthquakes
As tectonic blocks move
along faults, they don’t move continuously. As blocks slide against each other,
they get caught on protrusions along the walls of the fault surface called
asperities. Once they get caught, pressure builds up on the asperities until
finally the asperities locking the two bodies of rock together break or melt,
causing the blocks to slide again. This breaking of the asperities and
subsequent sliding of the blocks produces an earthquake.
Predicting and measuring earthquakes
Because of the nature
of earthquakes, it is almost impossible to predict when an earthquake will
occur. The best that can be done in most cases is to avoid constructing
buildings where earthquakes are likely to occur such as along faults and to
design buildings in areas where earthquakes are common to withstand them.
Richter scale
The Richter scale is a
scale used to calculate the magnitude of an earthquake. The magnitude of an
earthquake is the energy released during the event. Most earthquakes are not
higher than magnitude 9. Very rarely there will be magnitude 9+ earthquakes
which are some of the most destructive earthquakes that have occurred in
Earth’s history. The magnitude of an earthquake is constrained by the length of
the associated fault. There is currently no fault on Earth large enough to
sustain a magnitude 10 earthquake.
Similarities between volcanoes and earthquakes
Volcanoes and
earthquakes are both related to a rupture that occurs in rock near or at the
surface of a planetary body.
Both are also phenomena
of geological origin that present serious hazards to humans. Volcanic eruptions
and earthquakes are also both difficult to predict.
Differences between volcanoes and earthquakes
Although there are
similarities between volcanoes and earthquakes, there are also significant
differences which include the following.
·
Volcanoes form at Earth’s surface
whereas earthquakes originate from deeper within the crust.
·
Volcanoes are also features of planetary
surfaces whereas earthquakes are just events though they are associated with
certain features such as faults.
·
Volcanoes are formed by release of gas
and magma. Earthquakes are caused by movement along a fault.
·
Volcanoes lead to the formation of new
rock whereas earthquakes simply cause waves which disturb the rock.
·
Volcanoes can produce significant debris
through ashfalls, mudslides, and the formation of features such as ignimbrites.
Earthquakes typically will not directly produce significant debris, but debris
will result from the disturbances caused by the earthquake.
·
It is possible to predict a volcanic
eruption a few weeks to a few days in advance, though the exact time of the
eruption can’t be predicted with any accuracy. The likelihood of an earthquake
can be predicted, but it is not possible to determine any timeframe of when the
earthquake will take place, just how likely it is to happen at some point in
the future.
Volcano vs. Earthquake:
Comparison Chart
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