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Meaning
The word Volcano comes from the little island of
Vulcano in the Mediterranean Sea off the coast of
Sicily (Italy). 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.
As we have seen in the unit on Plate
Tectonics, the inside of the earth is
not all solid (magma) and is under
pressure and the outside of the earth
is a thin solid layer called the crust. In
very simple terms, when the crust is
broken or cracked, pressure is
released and at times a volcano is
formed.
A gross example would be to compare
the earth’s crust to your skin and the
molten magma can be compared to
your flesh and blood. Whenever the
skin is broken out comes the blood
and flesh. The resulting scab or build
up of dried blood is the volcano.
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, cinders, ash and gas.
Molten rock inside the earth is called Magma. As
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 and releases pressure, an
eruption occurs, and the molten rock may pour from
the opening (vent) as non-explosive lava flows, or it
may shoot violently into the air.
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 winds before settling
down.
Kilauea in Hawaii.
Non – Violent - lava.
Eyjafjallajökull
Volcano, Iceland – this
eruption actually
halted airlines from
flying for a few days.
Violent - ash.
Yes, Lightning!
TYPES OF VOLCANOES
1. 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.
A Cinder Volcano – Nicaragua, Crescent 2009
2. 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.
Shield Volcano
3. Composite Volcanoes (Strato-volcano)- 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.
Composite 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 volcano.
A Crater Lake formed in a Caldera
There are actually three calderas in this picture. You can see the wall
of a very large one in the background and two smaller more recent
ones to the right and to the left of the picture. (Nicaragua, Crescent
Trip, 2009)
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.
.
Hawaii – Hot Spot
Hot Spot Volcanoes
- 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.
A volcanic landscape can look very mysterious, barren and even
alien.
There are many more kinds, for example if a volcano is
formed under water it is called a Submarine Volcano. When
the lava breaks the surface it creates steam and very hot
water – this is called a Lava Delta.
If the lava is very sticky
and does not flow far
from its vent, it creates
steep mounds called
Lava Domes.
Lava Dome in Hawaii.
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.
Small horizontal creations are called Sills.
Sometimes a volcano tries to form and just doesn’t break
the surface, leaving behind a vertical pipe or tube called a
Dike. When these cool, they form intrusive igneous rock
and are only discovered through erosion or mining.
If lava flows beneath the surface, the hot lava can create a
long tunnel called a Lava Tube.
There is a great range in the severity 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 Pyroclastic 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
ash, 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.
Pyroclastic Flow - Hawaii
Sometimes the combination of ash and
gas can be fatal for any inhabitants
leaving near a Volcano – Vesuvius,
Italy,
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
Mount St. Helens – 1980.
Before
After
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
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 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 – remember
for the next few slides
http://www.geology.sdsu.edu/how_volcanoes_work/subducvolc_page.html
Above are two images illustrating the Volcanic Arc
created on the West side of Central America, by the
subduction of plates.
The Ring of Fire is a subduction zone that circles the
Pacific Ocean – the world’s most famous volcanoes are
found along the ring of fire.
So… who wants to be a
volcanologist?