Volcanoes and Volcanism

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Volcanoes and Volcanism
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Introduction
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The word Volcano comes from the little island of
Vulcano in the Mediterranean Sea off the coast of
Sicily. Centuries ago, the people living in this
area believed that Vulcano was the chimney of the
forge of Vulcan - the blacksmith of the Roman
gods. Volcanoes are mountains, but they are very
different from other mountains. They are not
created by folding or faulting or erosion.
Instead volcanoes are built by the accumulations
of their own eruptive products - lava, gas,
ashflows and tephra (airborne ash and dust). A
volcano is most commonly a conical hill or
mountain built around a vent that connects with
reservoirs of molten rock below the surface of
the earth. Driven by buoyancy and gas pressure,
the molten rock (called magma), which is lighter
than the surrounding solid rock, forces its way
upward and may ultimately break through zones of
weaknesses in the Earths crust and erupts onto
the earths surface.
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As the rising magma nears the Earths surface,
pressure decreases, which causes the gases in the
magma to expand. Once magma is erupted onto the
Earths surface it is called lava. If the ground
breaks open an eruption occurs, and the molten
rock may pour from the vent as non-explosive lava
flows, or it may shoot violently into the air as
dense clouds of lava fragments. Larger fragments
fall back around the vent, and accumulations
begin and the mountain takes its shape. Some of
the finer ash particles may be injected miles
into the atmosphere and are carried many times
around the world by stratospheric winds before
settling down.
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Types of Volcanoes
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Volcanoes can be categorized based on what sorts
of features they are or how they are created. The
following list is a combination of these two.
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1. Calderas The largest and most explosive
volcanic eruptions eject tens to hundreds of
cubic kilometers of magma onto the Earths
surface. When such a large volume of magma is
removed for beneath a volcano, the ground
subsides or collapses into the emptied space, to
form a huge depression called a caldera.
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http//www.btinternet.com/sa_sa/amsterdam/images/
caldera.jpg
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Amsterdam and St Paul Islands are located in the
southernmost Indian Ocean at 3750' South and
7735' East, and are amongst the most isolated in
the world.
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2. Cinder Cones 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.
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ercone.gif
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A Cinder Cone Volcano
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0424305-084_large.JPG
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3. Shield Volcanoes Shield volcanoes are the
largest on Earth. The Hawaiian volcanoes are
shield volcanoes. They are made up of the build
up of success lava flows. Because of this they
are not steep, their slopes are very gradual.
There eruptions are not explosive
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http//volcanoes.usgs.gov/Products/Pglossary/Shiel
dVolcano.html
Shield Volcano - Mauna Loa Volcano, Hawaii
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g
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4. Composite Volcanoes Composite
(stratovolcanoes) volcanoes comprise the largest
percent (60) of the Earths volcanoes. They are
typically symmetrical with steep sided and are
built from alternating layers of lava and
cinders. These volcanoes can also be very
explosive. Some of the worlds most majestic and
beautiful mountains are this type of volcano.
Most have a crater at the top which contains a
central vent that the magma flows up. Lavas
either flow through breaks in the crater wall or
issue from fissures on the flanks of the cone.
Mount Fuji in Japan and Mount St. Helens are
composite volcanoes.
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A cross section of a composite volcano
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5. Lava Plateaus - Not all eruptions have to form
mountains. If the flow of lava is very slow and
quiet, or it pours out quietly from long fissures
instead of central vents, it can flood the
surrounding countryside with lava flow upon lava
flow, forming broad plateaus. Iceland is a good
example.
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/Maps/map_columbia_river_flood_basalts.gif
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6. Hot Spot Volcanoes Like in Hawaii, when a
series of volcanoes are created due to the
constant eruption of a hot spot, an area in the
Earth's mantle (a magma chamber) where hot
material from the Earth's interior is moving
upward. If we can assume that such a hot spot is
stationary, then we can calculate the absolute
velocity of the Pacific Plate as it has moved
over the hot spot.
http//www.tulane.edu/sanelson/images/hotspot.gif
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Although most volcanic rocks are generated at
plate boundaries, there are a few exceptionally
active sites of volcanism within the plate
interiors. These intraplate regions of volcanism
are called hotspots. Most hotspots are thought
to be over top of a large plume of exceptionally
hot mantle. These mantle plumes appear to be
generated in the lower mantle and rise slowly
through the mantle by convection.
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Mantle plumes appear to be largely unaffected by
plate motions. As lithospheric plates move across
stationary hotspots, volcanism will generate
volcanic islands that are active above the mantle
plume, but become inactive and progressively
older as they move away from the mantle plume in
the direction of plate movement. Thus, a linear
belt of inactive volcanic islands and seamounts
will be produced. A classic example of this
mechanism is demonstrated by the Hawaiian and
Emperor seamount chains.
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The "Big Island" of Hawaii lies above the mantle
plume. It is the only island that is currently
volcanically active. The seven Hawaiian Islands
become progressively older to the northwest. The
main phase of volcanism on Oahu ceased about 3
million years ago, and on Kauai about 5 million
years ago. This trend continues beyond the
Hawaiian Islands, as demonstrated by a string of
seamounts (the Hawaiian chain) that becomes
progressively older toward Midway Island. Midway
is composed of lavas that are approximately 27
million years old.
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Northwest of Midway, the volcanic belt bends to
the north-northwest to form the Emperor seamount
chain. Here, the seamounts become progressively
older until they terminate against the Aleutian
trench. The oldest of these seamounts near the
trench is gt70 million years old. This implies
that the mantle plume currently generating
basaltic lavas on the Big Island has been in
existence for at least 70 million years!
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How does a chain of islands over a hot spot get
formed?
Ocean
Oceanic crust
Magma chamber
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Volcanic mountain/island is created.
Magma chamber fills magma flows to the surface,
erupts.
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Magma chamber empties eruption stops
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Volcanic island moves away from the vent of the
magma chamber. It is now an extinct volcano.
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A new volcanic/mountain island is created.
Magma chamber fills again and erupts.
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Magma chamber empties eruptions stops.
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The new island moves away as the plate keeps
moving.
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And so on . This creates an arc of islands.
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Other Volcanic Features
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If a volcano is formed under water it is called a
submarine volcano or a seamounts. Volcanoes that
are made under a ice or a glacier are called
tuyas. If the lava is very sticky and does not
flow far from its vent, it creates steep mounds
called lava domes.
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a.jpg
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A crater lake formed in a caldera
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Volcanic Arcs - A range of volcanoes created when
an oceanic plate subsides under a continental
plate. The old volcanoes on the west coast of
North America are of this type. The Pacific Ring
of Fire is made of these volcanoes.
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Volcanic Explosivity
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There is a great range in the explosivity of
volcanic eruptions. Many eruptions are
relatively quiet and are characterized by the
calm, nonviolent extrusion of lava flows on the
earth's surface. Other eruptions, however, are
highly explosive and are characterized by the
violent ejection of fragmented volcanic debris,
called tephra, which can extend tens of
kilometers into the atmosphere above the
volcano. The type of volcanic eruption depends
on a variety of factors, which are ultimately
linked to the composition of the magma (molten
rock) underlying the volcano.
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The controlling factors on explosivity are
viscosity, temperature, and the amount of
dissolved gases in the magma.
Non-explosive eruption Eruption of effusive
basalt from the Pu'u O'o volcano on the east rift
zone of the larger Kilauea volcano, Hawaii.
http//www.geology.sdsu.edu/how_volcanoes_work/Thu
mblinks/Puuoo84_page.html
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Explosive Eruption Eruption of a voluminous
plume of tephra is typical of explosive (also
called Plinian) eruptions, as demonstrated in the
1980 eruption of Mt. St. Helens. These eruptive
tephra plumes can travel tens of kilometers into
the stratosphere.
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mblinks/msh5_page.html
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How does the amount of dissolved gases affect the
explosivity of volcanoes? The dissolved gases in
the magma provides the force for explosive
eruptions. As magma rises toward the surface,
dissolved gases in the liquid rock begin to come
out of solution (called exsolution), and bubbles
begin to form in the magma. The magma gets
frothy. Not all magma has a lot of dissolved
gases.
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Explosive eruptions, which get started by
exsolution of gases, can be made more dramatic by
sudden decompression, which lowers the confining
pressure on the magma. Decompression is similar
to what can happen to a bottle of pop when the
cap is removed the sudden release of pressure
can cause the CO2 to come out of solution
explosively and rapidly, resulting in the pop to
spray out of the bottle. This happens when a
volcanic mountain suddenly breaks open or the
magma plug from a previous eruption gets pushed
out of the way. Mt St. Helens is an example.
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How does viscosity affect explosivity? Viscosity
is defined as the ability of a substance to
resist flow. In a sense, viscosity is the inverse
of fluidity. The higher the temperature, the more
fluid a substance becomes, thus lowering its
viscosity. The lava in this case is thin and
runny and gases escape easily. Magma's resistance
to flow is a function of its "internal friction"
due to the nature of the chemical bonds (called
polymerization) within the liquid.. Generally
speaking, magma with a higher silica
concentration has a higher viscosity. It resists
flow up through the vent. The lava here is thick
and sticky and gases cannot escape. Silica rich
rocks are typically found in granitic rocks.
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As magma cools and silica minerals begin to
crystallize, the magma becomes increasingly
viscous. The residual liquid becomes
increasingly enriched in gas because of a higher
vapor pressure. This higher vapour pressure can
lead to an explosive volcanic eruption if
confining pressure of the surrounding rock is
suddenly released. An earthquake or tremour can
open up the ground releasing this pressure and a
violent eruption occurs.
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The rapid eruption of expanding gases results in
the obliteration and fragmentation of magma and
rock. The greater the explosivity, the greater
the amount of fragmentation. Individual eruptive
fragments are called pyroclasts ("fire
fragments"). Tephra (Greek, for ash) is a term
for any airborne pyroclastic accumulation. If the
explosivity of the magma is low, the liquid rock
tends to flow out onto the surface as lava.
Basaltic rock tends to be low in silica and is
not explosive.
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Volcanoes and Plate Tectonics
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What is the relationship of volcanic eruptions to
plate tectonics? Volcanos tend to be highly
concentrated near convergent plate boundaries
(also called subduction zones). As one oceanic
crustal plate descends under the other plate, it
gets very hot as it enters the asthenosphere in
the upper mantle and the rock melts. Most of this
rock was formed from the asthenosphere millions
of years before at the divergent plate
boundaries. However, a (comparatively) small
amount of sedimentary rock accumulates on top of
the oceanic crust over the millions of years. The
sediments are from organic life in the ocean,
run-off into the oceans, and wind deposited
material, all of which settle to the bottom.
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This sedimentary rock has a different mineral
composition (it is mainly granitic) it is less
dense than the surrounding liquid rock (mainly
basaltic). Therefore, after it melts in the
mantle, it rises slowly toward the surface,
breaking through the crust and erupting onto the
surface.
Island arc formed by oceanic-oceanic subduction.
Volcanic arc formed by oceanic-continental
subduction
http//www.geology.sdsu.edu/how_volcanoes_work/sub
ducvolc_page.html
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This type of granitic magma has a high silica
concentration and is therefore more viscous. It
also tends to have high level of dissolved
gases. This accounts, therefore, for the more
violent eruptions found near subduction zones,
like the so-called Pacific Ring of Fire. Mt. St.
Helens, Krakatoa, Pompeii are examples of this
type of volcanic activity.
http//www.geology.sdsu.edu/how_volcanoes_work/Thu
mblinks/ring_page.html
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Products of volcanic eruption
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A pyroclastic flow is a fluidized mixture of
solid to semi-solid fragments and hot, expanding
gases that flows down the flank of a volcano.
These awesome features are heavier-than-air
emulsions (mixtures) that move much like a snow
avalanche, except that they are fiercely hot,
contain toxic gases, and move at phenomenal,
hurricane-force speeds, often over 100 km/hour.
They are the most deadly of all volcanic
phenomena. Sometimes the hot magma and
pyroclastic flow melts the ice on high mountains.
This creates a lahar, which is categorized as a
mudflow. This is also extremely dangerous because
a lahar not only carries with it the lava from
the eruptions but with the added water, part of
the mountain goes with it as well. http//www.ge
ology.sdsu.edu/how_volcanoes_work/Pyroflows.html
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THE END!