Volcanoes
Volcanic activity is the most powerful force in nature. Some volcanic
eruptions are much more powerful than the largest nuclear explosion. Volcanoes
have killed thousands of people and have created some of the most frightening
events in human history.
Volcanoes have been the basis for myths and legends the world over.
Volcanoes are also responsible for much of the land we live on, 90% of all
the continents and ocean basins are the product of volcanism. The air we
breathe, and the water we drink have been produced by millions of years of
eruptions of steam and other gases.
The word volcano is derived from the name of the ancient Roman island of
Vulcano which lies off the southwest coast of Italy. The Romans believed that
Vulcan, the god of fire and the maker of weapons, used the volcano on that
island to forge his weapons.
Volcanoes are not alive but scientists use human terms to talk about
volcanoes, such as active, alive, dormant, resting, sleeping, extinct, dead,
lifetime, and restless.
The term Volcano has two definitions;
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Volcanoes actually build themselves into a mountain with repeated
eruptions. In 1943 a farmer in Mexico noticed that some cracks (fissures) in
his corn field were growing wider and wider. The next day his field was
engulfed by a growing volcanic cone (Light Green). During the week the cone
grew 500 feet taller (Dark Green). Within a year (Dark Gray) Paricutin was
over 1200 feet higher than the surrounding landscape. During the next eight
years the volcano did not grow much taller but the cone's base grew wider
and wider (Light grey). Paricutin stopped erupting in 1952 almost as fast as
it started. The mountain has been silent since. |
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Volcanoes are classified as active, dormant, and extinct. Active volcanoes are either currently erupting or have erupted in recorded history. There are over 500 volcanoes on Earth that fit this category today. Dormant or resting volcanoes are not currently erupting but are considered likely to do so. Mt. St. Helens had been dormant for one hundred twenty-three years before it erupted in 1980. Extinct or dead volcanoes have not erupted in recorded history and are not expected to erupt again.
A lava dome is a steep mass of very thick and pasty lava that is
pushed up from the main vent. The lava is so viscous (thick and
pasty) that it does not flow but slowly rises higher with each movement of magma
in the conduit. Think of toothpaste that is slowly squeezed and then stopped and
then squeezed again from the tube. This is how the lava dome in Mt. St. Helen's
was formed.
The dome's exterior surface is very rough with chunks of lava that were
formed from small eruptions that broke the cooled and hardened surface into
blocks.
The dome slowly "grew" larger and larger over a seven year period. An
earlier dome started to form one month after the famous eruption when very thick
lava (dacitic lava) rose into the crater from the magma chamber below. This dome
was destroyed by an explosive eruption just a month later.
The large dome that is very visible today is over 900 feet tall (taller than
an 80 story building) and over 3000 feet wide (10 football fields). As large as
the lava dome is, it is still dwarfed by the huge crater that was the result of
the 1980 eruption. Steamy whisps of steam are still visible from the dome
telling us that the volcano's magma is filling the conduit, making the volcano
still active today.
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There are three ways that volcanoes form. Subduction Zone
volcanoes form at the boundaries of two plates, one overriding the
other. Subduction zone volcanoes are the most violent and destructive of the
volcanic types. Mt. St. Helens, Mt. Pinatubo, Krakatoa, and Mt. Vesuvius are
all famous explosive subduction zone volcanoes. Mid-ocean rift
volcanoes form where two oceanic plates are spreading apart. There
are more rift zone volcanoes than any other type. |
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Predicting exactly when a volcano will erupt is next to impossible. Today
geologists are becoming much more accurate in making the public aware that a
volcano is showing signs that it may erupt in the near future.
In the months before Mt. St. Helens erupted geologists knew the mountain was
getting restless. A magnitude 4.1 earthquake was recorded on March 20 (about 2
months before the large eruption). Many shallow earthquakes were recorded over
the next seven weeks. Magma moving higher and higher inside the mountain was
causing these earthquakes. As the magma rose it formed a large bulge on the
north flank. This bulge was growing daily and the geologists knew that an
eruption was soon to be.
What the authorities did was evacuate most of the people in and near the
mountain. Some decided to stay. Almost everyone that was near the eruption was
instantly killed. In all, 57 people died. Without the evacuation perhaps as many
as 30,000 deaths would have been attributed to Mt. St. Helens fury.
Scientists can not stop a volcano from erupting but with constant monitoring
they can warn and evacuate people and save lives.
Many volcanoes erupt in very consistent patterns, while other volcanoes have
no eruption pattern at all. This makes forecasting eruptions difficult.
What makes predicting eruptions even more difficult is the fact that many
volcanoes start with one type of eruption pattern and then change eruption
patterns as they grow older.
Some of the most powerful eruptions in recorded time have come from
volcanoes that have been dormant for hundreds and even thousands of years.
Answer the following:
1 At what type of plate boundaries do volcanoes form?
2 What are the two definitions for the term volcano.?
3 Write definitions in your own word for the following terms:
a) Active Volcano
b) Dormant Volcano
c) Extinct Volcano