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Volcanism and Volcanoes...
Volcanism involves the activities of Magma,
molten
material..
below the surface of the Earth and lava magma,
that has reached
the Earth’s surface through cracks and
fissures. Volcanism is responsible for the presence of all
igneous rocks,
the primary rocks
of the Earth.. and for certain unique types
of rock structures such
as: Batholiths, dikes, and sills.
The
word "Volcano" originates from the mysterious Island Vulcan, situated in
the Mediterranean Sea. It was once
believed that the
hot lava fragments erupting from Vulcan.. flew from the forge of the
Ancient Roman Fire God, "Vulcan", as he
fashioned thunderbolts for Jupiter and arrows for Diana.
 
Lava
Scientists have made great advances,
in their understanding
of volcanoes.. and the facts they have
assembled, are steadily
replacing supernatural explanations.
Lava is magma, that does
not solidify below ground…
reaching the surface still fluid and
molten, but minus a large
portion of its gases.. mostly steam,
and its liquids… mostly
water.
Lava is red-hot when it pours from
the vent, but
turns
dark red, gray or black as it cools to extrusive igneous rock.
Pyroclastic debris... which are pellets of molten lava and
fragments
of older rocks…
form another group of extrusive igneous rocks. The finest particles... are called volcanic ash and
dust. Intermediate size
pieces are called cinders
or lapilli. larger fragments are called
volcanic blocks, if they were angular
solids at the time of ejection, volcanic bombs,
if they were ejected in a liquid state and subsequently
solidified in flight... into rounded or spindled
shapes.
The gases include a high proportion
of
steam, as well as carbon
dioxide, and compounds
of sulfur,
chlorine, fluorine and boron.
Gases are
responsible for the vehemence of explosive volcanic eruptions. Expanding
gases escape easily from the fluid basaltic
lavas, but are trapped for a longer
time, in the highly viscous lavas... such as Rhyolite. When they finally burst
forth, they
do
so with
great explosive force.
The Distribution
of Volcanoes
About 600 active volcanoes… those that have
erupted
at least once in
recorded history, exist
on Earth. Many of the
world’s tallest peaks are
large volcanoes. Many well-known
islands are
the
summits of volcanoes,
rising from the floor of
the sea.
Volcanoes do not appear as a
random scattering,
but are
concentrated within
certain well-defined
bands.. which roughly
resemble the distribution of Earthquakes.
Most volcanoes seem
to be located where
actively
growing mountain ranges are
concentrated. Most are
near the sea.
And most volcanoes
appear where there is
fracturing...
in the Earth’s crust,
fractures along
which magma might travel to reach the
surface.
The most striking volcanic band,
includes
Costa Rica’s Voluptuous Volcano Lands and it is called
the Pacific
"Ring of Fire", which
borders the Pacific Ocean.
From Tierra del Fuego, the belt extends through
the Andes
region, along
the western coast of
Central
America and into
Mexico.
The belt continues from Mexico through the Cascade
Ranges to Alaska,
then through the Aleutian Islands to Kamchatka.
From
there…
it goes to Japan, the Philippines,
the East Indies and New Zealand.
There is no comparable volcanic belt
along the margins
of the Atlantic. The lesser Antilles (West Indies), however, is a volcanic
Arc System… a curved line of small islands separating the Caribbean Sea from the
Atlantic
Ocean. A famous volcano of this
area is Mount Pelee.
Another volcanic belt stretches from the
Mediterranean through
Asia Minor to the
East Indies Archipelago.
Famous Mediterranean
Volcanoes include: Vulcano, Vesuvius,
Etna and Stromboli.
Volcanoes also rise
from the floors of the Pacific, Indian and
Atlantic
Oceans.
Those in the Atlantic, occur along
the
mid-Atlantic range, forming the islands of
the Azores, Cape Verde Islands, St. Paul’s Rocks and Iceland.
The Hawaiian Islands are Mid-Pacific volcanoes.
In the Indian
Ocean is the notorious Indonesian Volcano, "Krakatoa".
A few volcanoes are located in the interior
of the continents,
particularly in Central and
Eastern Africa, the most notable is the
snow-crested "Kilimanjaro".
Classification of Volcanoes
Volcanic mountains are built
by the
accumulation of
their own
eruptive products. Volcanoes are classified according to shape
and composition of their cones. "Cinder Cones" are heaped up piles
of pyroclastic materials
blown out in explosive eruptions.
These
steep sided cones may have 25-30
degree slopes,
but rarely rise
more than 1000
feet above their surroundings. They are very
numerous in Western North
America, and include Sunset Crater of Arizona, and Mexico’s Paricutin.
Shield volcanoes are built up gradually
by successive overlapping
and coalescing lava
flows. They have a broad,
near circular outline
and a gently rounded profile, with slopes
that
rarely exceed 10 degrees.
Some of our biggest volcanoes are shield volcanoes…
the Hawaiian Islands are composed
of clusters of them,
including
the world’s largest
active volcano, "Mauna Loa",
which stands
13,680 feet above sea level, and more than 15,000
feet below,
thus achieving a total height of
almost
30,000 feet!
Lava domes may form steep-sided
craggy knobs or spines over
the volcanic
vent or short, steep-sided
lava flows known
as "Coulees". Such volcanoes are composed
of very viscous or pasty lava.
An example is Mount Pelee in Martinique,
whose domes developed
in the course of the most destructive eruption known to
mankind. California’s Lassen Peak is another
lava dome.
Composite Cones also called strato-volcanoes,
have alternating
layers of lava and pyroclastic materials, because eruptions
in the
life history of such volcanoes...
include both the relatively quiet outpourings
of lava
and the violent explosions of ash,
cinders,
bombs and blocks. Such volcanic
mountains are steeper than
Shield Volcanoes, but not as steep as Cinder
Cones. Many are
6,000 - 8,000 feet high.
Composite Cones include snow-capped Fugiyama, in Japan, Vesuvius... in Italy,
Mount Hood in Oregon, Cotopaxi in Ecuador, and Arenal... in Costa Rica.
A volcano’s central vent
occupies
a funnel-shaped depression,
called a crater. Sometimes, as
a result of the collapse or
explosion
of a cone, this
crater enlarges,
becoming a basin...
known as
caldera.
Oregon’s splendid Crater Lake, nearly 2,000 feet
deep
in places.. fills a caldera 6
miles in diameter.
Secondary Volcanic Phenomena
Geysers and hot springs, are usually found
in regions
of volcanic activity. Also associated with volcanoes are fumaroles... openings
in
the Earth’s crust, from which steam
and other gases of
magmatic origin escape.
But none of these can perform with the flamboyant style, of an erupting volcano with its great,
sinister
clouds of stem and
ash, boiling upward,
its
fleet incandescent bombs, streaking through
the sky and its molten lava streaming implacably... down the cone,
it is
a powerful reminder of the forces within our Mother Earth.
Volcano Eruptions Deplete Ozone
Wednesday, 6 March 2002
A major volcanic eruption
during the next 30
years could result
in a ozone hole over
the North Pole... similar to the one already
over the South Pole, according to measurements, taken by
American scientists.
The researchers from NASA, San Jose
State University, in
Costa Rica and the University
of Colorado,
predict the
hole will appear if
volcanic activity coincides with cold
Arctic
winters, during a time... when there are still
high levels of ozone-depleting CFCs... in the atmosphere.
Their prediction is published in the
Proceedings of the
National Academy of
Sciences. "People have speculated about
this before,"
explained Dr Paul
Fraser, Chief
Research Scientist
at CSIRO Atmospheric
Research. "Whilst you have volcanic aerosols
present in the stratosphere,
ozone depletion
is more efficient."
The researchers used simulations
of the eruption
of Mt
Pinatubo,
in 1991, which showed that
the volcanic plume...
spread as far
as the North Pole
in the lowest
part of the stratosphere... within
a
few months of the eruption.
Volcanic eruptions that are big
enough to
send material into
the
stratosphere are rare.
The last
one was Mt Pinatubo.
While reduction in the use of CFCs has been
successful, ozone recovery
is not guaranteed, because it can be jeopardized
by volcanic
eruption.
Ozone depletion is caused by the release of CFCs...
into the atmosphere.
When the CFC molecules
reach the stratosphere,
they give off chlorine,
and react
with it catalytically — meaning
they
cause a reaction... but don't
participate in it. For every
one CFC molecule that
breaks down, theoretically
up to 10,000
ozone molecules... can
be destroyed. The
destruction of ozone,
doesn't destroy the chemical
that is
causing it."
In the mid-latitudes... between the tropics and
polar regions,
not all of
the chlorine released
into the atmosphere, reacts
with
ozone...
because other molecules, such as methane,
compete for
the highly
reactive atomic chlorine. The chlorine
is converted into stable
compounds,
such as hydrochloric acid.
According to Dr Fraser, there is
a 60 per cent loss
of ozone in
the springtime in Antarctica, at the
height of the ozone depletion.
In the mid-latitudes, equivalent to Perth or
Melbourne, ozone
depletion peaks at ten per cent, during the summer.
It is a very different story... at the polar regions.
"CFC's become virtually
100 per cent effective,"
he said. The
extremely cold temperatures,
mean there are ice nuclei in
the
stratosphere.
These reverse the formation of hydrochloric
acid and
convert the chlorine back to
highly
reactive atomic chlorine.
Danny Kingsley — ABC Science Online
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