Transcript No Slide Title

The word Volcano comes from the little island of Vulcano in the
Mediterranean Sea off the coast of Sicily. Centuries ago, the
people living in this area believed that Vulcano was the chimney
of the forge of Vulcan - the blacksmith of the Roman gods.
Volcanoes are mountains, but they are very different from other
mountains. They are not created by folding or faulting or
erosion. Instead volcanoes are built by the accumulations of their
own eruptive products - lava, gas, ashflows and tephra (airborne
ash and dust). A volcano is most commonly a conical hill or
mountain built around a vent that connects with reservoirs of
molten rock below the surface of the earth.
Driven by buoyancy and gas pressure, the molten rock (called
Magma), which is lighter than the surrounding solid rock, forces
its way upward and may ultimately break through zones of
weaknesses in the Earth’s crust.
As the rising magma nears the Earth’s surface, pressure decreases,
which causes the gases in the magma to expand. Once magma is
erupted, it is called Lava.
If the ground breaks open an eruption occurs, and the molten rock
may pour from the vent as non-explosive lava flows, or it may
shoot violently into the air as dense clouds of lava fragments.
Larger fragments fall back around the vent, and accumulations
begin and the mountain takes its shape.
Some of the finer ash particles may be injected miles into the
atmosphere and are carried many times around the world by
stratospheric winds before settling down.
TYPES OF VOLCANOES
Calderas - The largest and most explosive volcanic
eruptions eject tens to hundreds of cubic kilometers of
magma onto the Earth’s surface. When such a large volume
of magma is removed for beneath a volcano, the ground
subsides or collapses into the emptied space, to form a huge
depression called a Caldera. Usually found at the top of a
volcanoe.
Cinder Cones - Cinder cones (Scoria Cones) are the
simplest type of volcano. They are built from particles and
blobs of congealed lava ejected from a single vent. As the
gas-charged lava is blown violently into the air, it breaks into
small fragments that solidify and fall as cinders around the
vent to form a circular or oval cone. These are the most
explosive.
Shield Volcanoes - Shield volcanoes are the largest on Earth. The
Hawaiian volcanoes are shield volcanoes. They are made up of the
build up of success lava flows. Because of this they are not steep,
their slopes are very gradual. There eruptions are not explosive.
Composite Volcanoes - Composite (Strato-volcanoes) volcanoes
comprise the largest percent (60) of the Earth’s volcanoes. They are
typically steep sided and are built from alternating layers of lava
and cinders. These volcanoes can also be very explosive. Some of
the world’s most majestic and beautiful mountains are this type of
volcano. Mount Fuji in Japan and Mount St. Helens are composite
volcanoes.
Continental Volcanoes or Volcanic Arcs - A range of Volcanoes
created when an oceanic plate subsides under a continental plate.
The old volcanoes on the west coast of North America are of this
type.
Island Arc Volcanoes - These volcanoes form a range of
islands. They occur when two oceanic plates collide and one
subsides under the other. Many Pacific islands are Island Arc
volcanoes.
Hot Spot Volcanoes - Like in Hawaii, when a series of
volcanoes are created due to the constant eruption of a Hot Spot.
Lava Plateaus - Not all eruptions have to form mountains. If
the flow of lava is very slow and quiet the lava will literally
flood the surrounding area and create broad flat plateaus.
Iceland is a good example.
There are many more kinds, for example if a volcano is formed
under water it is called a Submarine Volcano or Seamounts.
Volcanoes that are made under a ice or a glacier are called
Tuyas. If the lava is very sticky and does not flow far from its
vent, it creates steep mounds called Lava Domes.
Shield Volcano
A Cinder Volcanoe
Composite Volcanoes
A Crater Lake formed in a Caldera
Products of
volcanic
eruption
http://www.geology.sdsu.edu/how
_volcanoes_work/Thumblinks/pro
ducts_page.html
In some instances the magma never reaches the surface or
breaks the ground above it. Huge reservoirs of magma are
created to form ‘Batholiths’ or smaller reservoirs called
‘Laccoliths’. Both these are horizontal in structure. Vertical
creations are called ‘Sills’. When these cool, they form
intrusive igneous rock and are only discovered through erosion
or mining.
There is a great range in the explosivity of volcanic eruptions.
Many eruptions are relatively quiet and are characterized by
the calm, nonviolent extrusion of lava flows on the earth's
surface. If the material is fluid in nature (solid and semi-solid)
this type of flow is called Pryoclastic Flow (the typical hot
lava that flows down a volcano).
Other eruptions, however, are highly explosive and are
characterized by the violent ejection of fragmented volcanic
debris, called tephra, which can extend tens of kilometers into
the atmosphere above the volcano.
The type of volcanic eruption depends on a variety of factors,
which are ultimately linked to the composition of the magma
(molten rock) underlying the volcano.
The controlling factors on explosivity are viscosity,
temperature, and the amount of dissolved gases in the magma.
Nonexplosive eruption
Eruption of the Pu'u O'o volcano
on the east zone of the larger
Kilauea volcano, Hawaii.
http://www.geology.sdsu.edu/how_volcanoes_
work/Thumblinks/Puuoo84_page.html
The amount of lava that is generated over a period of time is
called the effusion rate.
Explosive Eruption:
Eruption of a voluminous
plume of tephra is typical
of explosive eruptions, as
demonstrated in the 1980
eruption of Mt. St. Helens.
These eruptive tephra
plumes can travel tens of
kilometers into the
stratosphere.
http://www.geology.sdsu.edu/how_volcanoes_wo
rk/Thumblinks/msh5_page.html
Gas
How does the amount of dissolved gases affect the explosivity
of volcanoes?
The dissolved gases in the magma provides the force for
explosive eruptions.
As magma rises toward the surface, dissolved gases in the liquid
rock begin to come out of solution (called exsolution), and
bubbles begin to form in the magma. The magma gets “frothy”.
Not all magma has a lot of dissolved gases. The more the gas the
more explosive the volcano.
Explosive eruptions, which get started by exsolution of gases,
can be made more dramatic by sudden decompression, which
lowers the confining pressure on the magma. Decompression is
when the volcano blows its lid so to speak.
Decompression is similar to what can happen to a bottle of pop
when the cap is removed: the sudden release of pressure can
cause the CO2 to come out of solution explosively and rapidly,
resulting in the pop to spray out of the bottle.
This happens when a volcanic mountain suddenly breaks open
or the magma plug from a previous eruption gets pushed out of
the way. Mt St. Helen’s is an example. An earthquake or
tremour can open up the ground releasing this pressure and a
violent eruption occurs.
Viscosity
How does viscosity affect explosivity?
Viscosity is defined as the ability of a substance to resist flow. In a
sense, viscosity is the inverse of fluidity.
Magma may also contain a material that is crystal in structure
called silca. Generally speaking, magma with a higher silica
concentration has a higher viscosity. It resists flow up through the
vent. The lava here is thick and sticky and gases cannot escape.
Granitic rock is rich in silica.
Click here to read an excellent article on the study of why some
volcanoes erupt violently.
Temperature
The higher the temperature, the more fluid a substance
becomes, thus lowering its viscosity. The lava in this case is
thin and runny and gases escape easily. Think of engine oil!
Every year about 60 volcanoes erupt, but most of the activity
is pretty weak. How do volcanologists measure how big an
eruption is? There is not any single feature that determines
the "bigness", but the following eruption magnitude scale called the Volcanic Explosivity Index or VEI - is based on a
number of things that can be observed during an eruption.
According to this scale, really huge eruptions don't happen
very often, luckily!
Volcanic Eruption Index
VEI
Description
Plume Height
Volume
Classification
How often
Example
0
non-explosive
< 100 m
1000s m3
Hawaiian
daily
Kilauea
1
gentle
100-1000 m
10,000s m3
Haw/Strombolian
daily
Stromboli
2
explosive
1-5 km
1,000,000s m3
Strom/Vulcanian
weekly
Galeras, 1992
3
severe
3-15 km
10,000,000s m3
Vulcanian
yearly
Ruiz, 1985
4
cataclysmic
10-25 km
100,000,000s m3
Vulc/Plinian
10's of years
Galunggung, 1982
5
paroxysmal
>25 km
1 km3
Plinian
100's of years
St. Helens, 1981
6
colossal
>25 km
10s km3
Plin/Ultra-Plinian
100's of years
Krakatau, 1883
7
super-colossal
>25 km
100s km3
Ultra-Plinian
1000's of years
Tambora, 1815
8
mega-colossal
>25 km
1,000s km3
Ultra-Plinian
10,000's of years
Yellowstone, 2 Ma
Picture Break!
White Island, New Zealand
Pu’u in
Hawaii
What is the relationship of volcanic eruptions to plate
tectonics?
Volcanos tend to be highly concentrated near convergent plate
boundaries (also called subduction zones).
As one oceanic crustal plate descends under the other plate, it gets
very hot as it enters the asthenosphere in the upper mantle and the
rock melts.
Island arc formed by
oceanic-oceanic
subduction.
Volcanic arc formed by
oceanic-continental
subduction
http://www.geology.sdsu.edu/how_volcanoes_work/subducvolc_page.html
Volcanic Settings
http://www.geology.sdsu.edu/how_volcanoes_work/Thumblinks/ring_page.html
Volcanoes can in one of the following three states: Active, extinct
or dormant. You can figure these out!
Needless to say Volcanoes can be dangerous to human population.
They also bring excellent soil for farming. The advantageous and
disadvantages of living near a volcano are beyond this slide show.
Sounds like a good essay.
THE END!