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Volcanoes
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What are volcanoes?
Volcanoes are named after Vulcan, the Roman god of fire. The Romans
believed he lived under an island called Vulcano in the
Mediterranean Sea. This island is a volcano. They thought he was a
blacksmith and made weapons for the other gods. When Vulcan made
these weapons, the earth would shake and the island would erupt.
The earth is made of
four layers. We live on the earth’s crust made of rocks broken into
small pieces. The continents, ocean floors, mountains, and valleys
of the earth’s surface are in the earth’s crust. Below the crust
there are pockets of melted rock. Under the melted rock is the
mantle. The mantle is made of solid hot rock with temperatures from
900 to 3,000 degrees Celsius. The outer core is the third layer. It
is liquid metal melted from temperatures between 3,000 and 4,000
degrees Celsius. The inner core at the center of the earth is solid
metal and the temperature is between 3,000 and 6,500 degrees
Celsius. |
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Many scientists
believe that millions of years ago all the earth formed one big
continent called Pangaea. They think the hot liquid mantle, called
magma, under the earth’s crust bubbles like water boiling on a
stove. This process is called convection. The magma tosses and turns
and breaks apart the earth’s crust. Scientists have found 12 to 15
large pieces called plates in the earth’s crust. The earth’s crust
looks like an eggshell that has been broken by a spoon.
The plates are like
floating rafts, moving on layers of soft rocks under them. The huge
plates of the earth’s crust move all the time, but do not move more
than an inch or so every year. The continents and the ocean floors
are on these plates. The earth’s crust is weaker at the edges
where two plates meet and this is where most volcanoes are found.
Volcanoes are formed in different ways. The simplest to understand
is when the hot magma breaks through a weak spot in the crust. As
the magma shoots out of the crust, the cooling magma called lava
becomes hard. After a while, this hard lava forms a volcanic
mountain. The second way is more complicated. The convection process
in the magma causes the earth’s plates to shift and move. The plates
collide into each other. Sometimes one plate is pushed down into the
mantel below the crust and melts. This melted material with the
magma can then create a volcano.
There are also different kinds of volcanoes. The differences are the
way they are made, the type of lava and the kind of volcanic
material.
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Andesitic Volcano
Andesitic volcanoes were first noted in the Andes
mountains and hence the name. Andestic volcanoes can be
extremely violent. Eruptions can occur anytime and can send
clouds of hot ash and dust over great distances.
An andesitic volcano has a structure similar to a steep sided
cone. The volcanic vent is shaped like a funnel and is partly
filled with ash from previous eruptions. The volcano is
gradually built up due to the slow moving lava and ash layers.
The thick lava produced by such volcanoes is called Andesite.
On reaching the
surface, this lava naturally comes in contact with cooler
atmosphere and solidifies. Coming from such a great distance
below the earth's surface, it forms an opening and the
solidified lava piles around this. The suffocating cloud of
gases, rock fragments called the nuee ardente is a major
problem.
When an
andesitic volcano erupts, great clouds of ash and dust are
blasted into the atmosphere. This glowing cloud causes an
avalanche consisting of pieces of rock, dust and white hot ash
to roll down the hill side. This mixture is known as a nuee
ardente. |
Andesitic Volcano |
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Basaltic Volcano
These volcanoes normally form over areas such as hot
spots, which are areas of massive heat and turbulence in the
Earth's mantle. Continuous rock movements can cause a
series of volcanoes. The hot spots cause the molten
material to rise up from the mantle and if they break through
the surface, they create dark runny channels of lava called
basalt. Unlike the andesitic lava, basaltic
lava can travel for long distances before solidifying. These
volcanoes usually occur below the sea and therefore do not
cause much damage. The land ones do of course have a
strong destroying nature. This is because the molten
basaltic lava is sprayed into the air. This lava may solidify
mid air and form volcanic bombs which come crashing down to
earth.
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Basaltic Volcano |
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A volcano can continue to erupt for any amount of time
- days or years - until enough gas has been released from the
subterranean regions. It is then that the volcano becomes
dormant and inactive. Fujiyama, Japan's sacred
mountain is an example of dormant volcano and it has
been so since 107 A.D. Japan has nearly 200 volcanoes, but
only 70 are still active. One of the biggest tragedies caused
by volcanoes was seen in 79 A.D. when the Vesuvius
volcano erupted and brought about the death of the
inhabitants of the Roman city of Pompeii. The bodies of the
victims and their pets were covered with hot ash. When the
bodies decayed hollows were created in the ground, these
hollows are filled with plaster to make models of the victims
which can be seen today.
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Models of plaster of the
victims of Pompeii. |
Volcanoes, other than
what we would normally assume, do not just cause havoc and loss.
Some of the eruptions have provided benefit over the past
centuries:
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The eroded volcanic
matter was found to be very fertile due to the presence of mineral
content and has been used for agricultural purposes in Japan and
Italy.
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The gases released
by the eruption tend to mix with water under the Earth's surface
forming mineral water springs. These springs at Vichy (France) and
Bath (England) have given pure, mineral water to many people.
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New land masses
have been formed, for example the islands of Hawaii have been
formed as a result of volcanic eruptions.
Volcanoes are
and will remain a unique phenomenon of nature. |
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