Volcanoes & Japan
Throughout the ages volcanoes have
been a source of fear and intrigue. People have prospered from the
mineral rich volcanic soils that have provided abundant crops. Volcanic
provinces have also yielded precious metals and minerals used in industry
as well as jewelry. Volcanoes have proven that they are a valuable
resource. However, they are to be respected, and perhaps feared. Volcanic
activity has been responsible for thousands of human deaths. (Volcano
World) As you will find on this page, several factors influence
the behavior of volcanoes, and this will allow you to relate what you have
learned to Mount Fuji and Hakone.
Geologists believe
that Japan formed because of an unusual geological coincidence. Japan is
situated where a back arc spreading center meets subducting plates. To the
west of Japan is the spreading center. The ocean floor is spreading
at this area much like what is found at a mid-coean ridge. It is separating
Japan from the western continent. Japan is comprised of a series of
volcanic regions that have cropped up due to the denser crust from Pacific
Plate subducting beneath less dense crust to the west.
The country is
a series of semi-circular islands primarily comprised of five geologic components.
- Pre-Neogene
sedimentary and regional metamorphic rocks
- Late Mesozoic to Early Triassic granites and rhyolites
- Neogene sediments and associated volcanics
- Quaternary sediments
- Plio-Pleistocene volcanics (Hashimoto, 1991)
Japan was
formed by two events: subduction resulting
in volcanic activity and the opening of the Japan Sea. In the case of Japan,
the Pacific Plate and the Philippine Plate subduct the eastern border of
the Eurasian Plate. However, the eastern edge of the Eurasian Plate is not
the edge of a continent. Because of volcanic activity at such junctions,
island arcs are formed in the ocean. Japan is actually composed of four (or
five) such island arcs formed by volcanic activity where the Pacific Plate
and the Philippine Plate subduct the eastern border of the Eurasian Plate.
But, Japan was originally a part of the Asian continental mainland and was
separated off from it with the opening of the Japan Sea about 15 million
years ago. At this time, subduction of the oceanic plates under the continental
plates was already underway. It is thought that subduction can create a back
arc basin ahead of the subduction zone. The formation of this basin resulted
in the opening of the Japan Sea between Japan and the Asian continental mainland.
Subduction and its related volcanic activity continued (and still continues)
after the opening of the Japan Sea. (http://www.seinan-gu.ac.jp/~djohnson/natural/geology.html)
Many Japanese volcanoes are largely composed of silica-rich
rocks such as dacite and andesite and are prone to highly explosive eruptions
(e.g. Aso Volcano). Some Japanese volcanoes eject considerable quantities
of low-silica basaltic lava, such as Fuji Volcano. (http://www.aist.go.jp/GSJ/cGM/tour/tour10e.html)
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Volcanoes form because molten rock beneath the planet's surface,
called magma breaks through Earth's crust and erupts as lava. (picture from http://whyfiles.org/031volcano/)
Why
do some volcanoes erupt violently and others do not? The viscosity (thickness)
of magma determines how a volcanic eruption will occur. Volcanoes that are
highly explosive, and therefore dangerous to people, build-up as highly viscous
(very thick) magma forces its way to the surface. It often explodes
violently as large volumes of gases trapped within the magma try to escape.
Volcanoes that are less explosive and less of a threat to people are made
up of very fluid magmas. Fluid magma readily allows gases to escape, and
therefore create less explosive eruptions.
Three
factors determine the viscosity of magma:
- Chemical composition - variable silica content
- Temperature
- volume of dissolved gases
Chemical composition is a factor in viscosity because magmas vary in silica
content (amount of SiO2 -- silicon dioxide). Magma with a high
silica content is sticky highly viscous. Magma with lower silica content
are less sticky and more runny. Basaltic lava has a relatively low silica
content (< 55%), while rhyolitic magma has silica content up to 70%. As
the amount of silica increase, quartz and other high silica-content minerals
can form. These types of silicate minerals form from bonding together of silicon-oxygen
tetrahedrons. These silica bonds are what make the silica-rich magma more
viscous.
The
temperature of lava has been measured by a few brave scientists. Usually
lava is between 600 and 1200 degrees Celsius. Basaltic lavas have the highest
temperatures up to 1200 C. Rhyolitic and andesitic lavas are cooler when
they erupt.
|
SiO2 CONTENT
|
MAGMA TYPE
|
VISCOSITY
|
ERUPTION STYLE
|
TEMPERATURE
(centigrade)
|
VOLCANIC ROCK
|
|
~50%
|
Mafic
|
Low
|
nonexplosive
|
~1100
|
Basalt
|
|
~60%
|
Intermediate
|
Intermediate
|
intermediate
|
~1000
|
Andesite
|
|
~70%
|
Felsic (high Si)
|
high
|
explosive
|
~800
|
Rhyolite
|
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A Butter Analogy
From
our experience, we know that many materials become more runny as they are
heated. One example is butter. When its cold it is very viscous and it can
be manipulated with effort, but if left alone, it will remain solid.
As heat is applied to butter in a pan, the butter begins to flow, and as
it is heated more, it becomes liquid and runs. Low viscosity lava behaves
in a similar fashion.
What
a gas...
Gas
content also affects the behavior of magma. As gases escape from magma
in the vent as pressure is released, they propel material out in an explosive
way.
Soda Pop Scenario
What
happens when you open a can of pop? The sound made occurs because
the contents of the can are under pressure when the can is sealed.
When the can is opened, the gases expand rapidly and come out of the liquid.
If this occurs rapidly enough (as when the can is shaken prior to opening)
then you have a mess.
A
similar thing happens in a volcano as magma reaches the surface. As magma
moves to the surface through a vent, the confining pressure of the surrounding
environment decreases rapidly. This reduction of the confining pressure allows
the dissolved gases to be released suddenly. The escaping gases in the magma
expand and may occupy hundreds of times their original volume. If this occurs
rapidly or if large amounts of gas are released, the eruption can be explosive,
even catastrophic. (Colgan)
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Volcanic eruption styles
are assigned based on the eruption styles of well known volcanoes.
Criteria includes explosivity, lava type, volume, and eruption frequency.
Hawaiian - This is
one of the mildest types of eruptions. The flow of lava may begin after
a mild to moderate explosion and associated lava fountains. Often,
the vent is along linear fissures which may intersect a central caldera.
Examples other than Hawaii may be found in Iceland.
Peleean - Peleean
eruptions involve viscous magma and shares characteristics with Vulcanian
eruptions. The key characteristic of Peleean eruptions is the eruption of
glowing avalanches of hot ash. Some of these eruptions may produce domes
or short flows or ash and pumice cones. As the name indicates, this type of
eruption was first described at Mt. Pelee. Examples of Peleean eruptions
include the 1968 eruption of Mayon (Philippines), the 1956 eruption of Bezymianny
(Russia), the 1951 eruption of Lamington (Papua New Guinea), and the 1948-1951
eruption of Hibokhibok (Philippines). (Volcano World)
Plinian - Two
key characteristics are an exceptionally powerful, continuous gas blast
eruption and the ejection of large volumes of pumice (Walker and Crosdale,
1971). Plinian eruptions can last less than a day, such as the short-lived
explosions of gas-rich, siliceous magma prior to the eruption of fluid basaltic
lava flows in Iceland. Longer-lived, more voluminous Plinian eruptions can
last for weeks or months. The longer eruptions start with showers of ash
followed by glowing avalanches. In some cases, so much magma is erupted that
the summit of the volcano collapses to produce a caldera. Classic examples
of collapse to produce a caldera are Krakatau in 1883, Crater Lake about
7,000 years ago, and S santorini (Greece) in 1500 B.C. During Plinian eruptions
fine ash can be dispersed over very large areas. Total volume of tephra erupted
during the formation of Crater Lake was 18 cubic miles (75 cubic km). Plinian eruptions are named for the famous
Roman naturalist Pliny the Elder. He died during an eruption of Vesuvius
in A.D. 79. Pliny the Elder's nephew described the eruption, which is characteristic
of Plinian eruptions. (Colgan)
Strombolian
- This type of eruption is characterized by jetting of clots or "fountains"
of fluid basaltic lava from a central crater. In comparison to other styles
mentioned, this is a relatively mild eruption. However, frequent explosions
may be followed by lava flows. As indicated by the name, Stromboli
in Italy is a prime example as is Paricutin in Mexico. Strombolian eruptions
often produce cinder cones.
Vesuvian - These eruptions
are similar to plinian, but are not so enormous. They release large amounts
of ash and gas while the cloud typically forms a somewhat cauliflower shape.
Vesuvian eruptions are named after Mt. Vesuvius (Italy), which destroyed
the cities of Pompeii and Herculaneum. This eruptive style is associated with
large stratovolcanoes. (Colgan)
(Volcano World)
Vulcanian - This is
a violent eruption caused by the removal of a plug as gases and pressures
build up. The eruption interval can vary from tens to hundreds of years.
The main type of activity includes large pyroclastic flows, tuff and ash
falls. These eruptions are associated with andesitic volcanoes, and
they may produce lapilli and bombs. Vulcanian eruptions build large stratovolcanoes,
and are named for Vulcano, which is in the Lipari Islands west of Italy. (Volcano World)
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Volcano Explosivity Index (VEI)
| 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 |
(http://volcano.und.edu/vwdocs/eruption_scale.html) (Volcano
World)
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Shield volcanoes
are broad, gently sloping cones (2-10 degrees from horizontal), and they
are constructed of several solidified basaltic lava flows. During eruptions,
the lava spreads laterally a great distance, due to its low viscosity. The
name "shield" has been given to this type because the shape resembles a shield
laid down upon the ground.
(USGS)
Cinder cone volcanoes are constructed
of loose rock fragments ejected from a central vent. The slope of a cinder
cone is much steeper then that of a shield volcano. The slope may be as much
as 30 degrees. Most of the ejected material lands near the vent during eruption
building the cone up to a peak. The steepness of slopes of accumulation of
loose material is limited to approximately 33 degrees due to gravity. Cinder
cones tend to be much smaller then shield volcanoes. Few of them exceed
a height of 500 meters.
Composite
Volcanoes are also known as stratovolcanoes. The slope is in between that of a shield
volcano and a cinder cone. The pyroclastic layer builds up the steepness
of the slope by depositing debris at the vent, but then the lava flows will
flatten the profile of the cone and build up more at the base then the summit
area. Because of this protective lava flow layer the volcano is less vulnerable
to erosion then a cinder cone. (Skinner, 2004)
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Geologists believe
that Japan formed because of an unusual geological coincidence. Japan is
situated where a back arc spreading center meets subducting plates. To the
west of Japan is the spreading center. The ocean floor is spreading
at this area much like what is found at a mid-coean ridge. It is separating
Japan from the western continent. Japan is comprised of a series of
volcanic regions that have cropped up due to the denser crust from Pacific
Plate subducting beneath less dense crust to the west.