Download Volcano Lesson

V. Content:
Volcanoes
The term Volcano has two definitions:
1.An opening in the crust of the Earth in which molten rock called magma and gases can escape
to the surface.
2. The mountain that is formed from volcanic eruptions.
Volcanoes actually build themselves into a mountain with repeated eruptions. During the next
eight years the volcano did not grow much taller but the cone's base grew wider and wider (Light
gray). Paricutin stopped erupting in 1952 almost as fast as it started. The mountain has been
silent since.
Volcanoes can build themselves into high mountains one day and in the case of Mt. St. Helens
erupt violently blowing their top off the next day. Mt. St. Helens lost over 1300 feet of its
summit during the eruption and simultaneous landslide of 1980.
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.
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.
These mid-ocean or rift zone volcanoes are the world's longest continuous mountain chain. This
mountain chain encircles the entire Earth. It is more than 40,000 miles long.
The third way that volcanoes form occurs at a Hot Spot. Hot spots are usually found under
oceanic crust, but can be located under continental crust. You will learn more about Hot Spot
volcanoes in the lesson "Hot Spots-Yellowstone and Hawaii".
Geologists generally group volcanoes into four main kinds--cinder cones, composite volcanoes,
shield volcanoes, and lava domes.
Cinder 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. Most cinder cones have a bowl-shaped crater at the summit and rarely rise more
than a thousand feet or so above their surroundings. Cinder cones are numerous in western North
America as well as throughout other volcanic terrains of the world.
Some of the Earth's grandest mountains are composite volcanoes--sometimes called
stratovolcanoes. They are typically steep-sided, symmetrical cones of large dimension built of
alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs and may rise as much
as 8,000 feet above their bases. Some of the most conspicuous and beautiful mountains in the
world are composite volcanoes, including Mount Fuji in Japan, Mount Cotopaxi in Ecuador,
Mount Shasta in California, Mount Hood in Oregon, and Mount St. Helens and Mount Rainier in
Washington.
Most composite volcanoes have a crater at the summit, which contains a central vent or a
clustered group of vents. Lavas either flow through breaks in the crater wall or issue from
fissures on the flanks of the cone. Lava, solidified within the fissures, forms dikes that act as ribs
which greatly strengthen the cone.
The essential feature of a composite volcano is a conduit system through which magma from a
reservoir deep in the Earth's crust rises to the surface. The volcano is built up by the
accumulation of material erupted through the conduit and increases in size as lava, cinders, ash,
etc., are added to its slopes.
Shield volcanoes, the third type of volcano, are built almost entirely of fluid lava flows. Flow
after flow pours out in all directions from a central summit vent, or group of vents, building a
broad, gently sloping cone of flat, domical shape, with a profile much like that of a warrior's
shield. They are built up slowly by the accretion of thousands of highly fluid lava flows called
basalt lava that spread widely over great distances, and then cool as thin, gently dipping sheets.
Some of the largest volcanoes in the world are shield volcanoes. In northern California and
Oregon, many shield volcanoes have diameters of 3 or 4 miles and heights of 1,500 to 2,000 feet.
The Hawaiian Islands are composed of linear chains of these volcanoes. Two of these are the
world's most active volcanoes.
Earthquakes:
The biggest scientific breakthrough in the history of seismology -- the study of
earthquakes -- came in the middle of the 20th century, with the development of the theory
of plate tectonics. Scientists proposed the idea of plate tectonics to explain a number of
peculiar phenomenons on earth, such as the apparent movement of continents over time,
the clustering of volcanic activity in certain areas and the presence of huge ridges at the
bottom of the ocean.
The basic theory is that the surface layer of the earth -- the lithosphere -- is comprised of
many plates that slide over the lubricating athenosphere layer. At the boundaries
between these huge plates of soil and rock, three different things can happen:
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Plates can move apart - If two plates are moving apart from each other, hot,
molten rock flows up from the layers of mantle below the lithosphere. This magma
comes out on the surface (mostly at the bottom of the ocean), where it is called
lava. As the lava cools, it hardens to form new lithosphere material, filling in the gap.
This is called a divergent plate boundary.
Plates can push together - If the two plates are moving toward each other, one
plate typically pushes under the other one. This subducting plate sinks into the
lower mantle layers, where it melts. At some boundaries where two plates meet,
neither plate is in a position to subduct under the other, so they both push against
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each other to form mountains. The lines where plates push toward each other are
called convergent plate boundaries.
Plates slide against each other - At other boundaries, plates simply slide by each
other -- one moves north and one moves south, for example. While the plates don't
drift directly into each other at these transform boundaries, they are pushed
tightly together. A great deal of tension builds at the boundary.
Where these plates meet, you'll find faults -- breaks in the earth's crust where the blocks of
rock on each side are moving in different directions. Earthquakes are much more common
along fault lines than they are anywhere else on the planet.