Volcanoes

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Volcanoes
  Volcanologists - have classified volcanoes
into groups based on the shape of the volcano,
the materials they are built of, and the way the
volcano erupts.

• Types 1. Composite Volcanoes (also called
  strato volcanoes),
• 2. Shield Volcanoes (also called
  shields),
• 3. Cinder Cones,
• 4. Spatter Cones, and
• 5. Complex Volcanoes (also
  called compound volcanoes).

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Lets take a look at each type and describe
them. 1. Composite Volcanoes (also called strato
volcanoes), - are formed by alternating layers
of lava and rock fragments.  This is the reason
they are called composite. - Strato-volcanoes
often form impressive, snow-capped peaks which
are often exceeding 2500m in height, 1000km2 in
surface, and 400km3 in volume. - Between
eruptions they are often so quiet they seem
extinct.
- Composite volcanoes usually erupt in an
explosive way.  This is usually caused by viscous
magma.  - When very viscous magma rises to the
surface, it usually clogs the craterpipe, and
gas in the craterpipe gets locked up. -
Therefore, the pressure will increase resulting
in an explosive eruption.
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Composite Volcanoes (also called strato volcanoes)
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Although strato-volcanoes are usually large and
conical, we can distinguish different shapes of
them concave (like Agua), pyramidal (like
Stromboli), convex-concave (like Vesuvius),
helmet-shaped (like Mount Rainier), collapse
caldera (like Graciosa), nested (like El Piton in
Teide), multiple summits (like Shasta), elongated
along a fissure (like Hekla).
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Strato-volcanoes are constructed along subduction
zones.  Examples of composite volcanoes include
Mount Hood, Mount Rainier, Mount Shasta, Mount
Fugi, Mount Mayon, and Vesuvius.
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Shield volcanoes
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Shield volcanoes
Shield volcanoes are huge in size.  They are
built by many layers of runny lava flows. Lava
spills out of a central vent or group of vents.
A broad shaped, gently sloping cone is formed. 
This is caused by the very fluid, basaltic lava
which can't be piled up into steep mounds.
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Shield volcanoes may be produced by hot spots
which lay far away from the edges of tectonic
plates.   Shields also occur along the
mid-oceanic ridge, where sea-floor spreading
is in progress and along subduction related
volcanic arcs.
The eruptions of shield volcanoes are
characterized by low- explosivity
lava-fountaining that forms cinder cones and
spatter cones at the vent.  Famous shield
volcanoes can be found for example in Hawaii
(e.g. Mauna Loa and Kilauea).
Mauna Loa This volcano is the largest one on
earth. It began to form millions of years ago.
The summit crater, called Mokuaweoweo, has walls
that rise to 180 meters! Mauna Loa emits lava
around very 4 years. - It is an active shield
Volcano, 4170 meters tall
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Cinder cone
A cinder cone is a steep conical hill formed
above a vent.  Cinder cones are among the most
common volcanic landforms found in the world. 
They aren't famous as their eruptions usually
don't cause any loss of life.
Cinder cones are chiefly formed by Strombolian
eruptions.  The cones usually grow up in groups
and they often occur on the flanks of strato
volcanoes and shield volcanoes.
Strombolian - when the magma is viscous, the
locked up gas will escape intermittently when
rising to the surface.  Often this results in
blocks and bombs which will fall on the
surrounding environment
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Cinder cone
The cones usually grow up in groups and they
often occur on the flanks of strato volcanoes
and shield volcanoes.
Cinder cones are built from lava fragments
called cinders.  The lava fragments are ejected
from a single vent and accumulate around the
vent when they fall back to earth.
Cinder cones grow rapidly and soon approach
their maximum size.  They rarely exceed 250m in
height and 500m in diameter.
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A Few Volcanic Definitions to get you started
aa (pronounced "ah-ah") Hawaiian word used to
describe a lava flow whose surface
is broken into rough angular fragments composed
of broken lava blocks called clinkers.
A caldera is a volcanic feature formed by the
collapse of a volcano into itself, making it a
large, special form of volcanic crater. The word
'caldera' comes from a Spanish word meaning
"cauldron".
In volcanology, a lava dome is mound-shaped
growth resulting from the eruption of
high-silica lava (usually rhyolite and/or
dacite) from a volcano
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A lahar is a type of mudflow composed of
pyroclastic material and water that flows down
from a volcano, typically along a river valley.
The term 'lahar' originated in Indonesia.
Pahoehoe (Hawaiian English, from Hawaiian,
meaning "smooth, unbroken lava") is basaltic
lava that has a smooth, billowy, or ropy
surface. A pahoehoe flow typically advances as a
series of small lobes and toes that continually
break out from a cooled crust.
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Pillow lava is the rock type typically formed
when lava emerges from an underwater volcanic
vent or a lava flow enters the ocean. The
viscous lava gains a solid crust immediately upon
contact with the water, and this crust cracks
and oozes additional large blobs or "pillows" as
more lava emerges from the advancing flow.
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Pyroclastic rocks or tephra are deposits
comprising the entire range of fragmental
products deposited directly by explosive or
effusive (formed by a nonexplosive outpouring of
lava) volcanic eruptions Three modes of
transport can be distinguished pyroclastic
flow, pyroclastic surge, and pyroclastic fall
A spatter cone is formed of molten lava ejected
from a vent somewhat like taffy.
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Basalt Plateau extensive continental deposits
of basaltic volcanic rock.
Complex volcano means the "system" of those
volcanoes is not "simple".  Caldera complexes
for instance have often got a large caldera with
many subsidiary vents and deposits, some of
which could be considered "volcanoes" in their
own right.      A volcano that consists of a
complex of two or more vents is reckoned as a
compound or complex volcano.
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More Questions from text book (paper cover, 4th
ed.) 1) Where do you find pillow lava, and
describe what it looks like? 2) What is the
difference between aa lava and pahoehoe
lava? 3) What is Volcanism? 4) What are
mountains? 5) Why are volcanoes with lava domes
considered very dangerous? 6) How are calderas
formed? 7) How are pyroclastic sheet deposits
formed? 8) How do geologists evaluate/monitor
volcanoes? 9) Can we predict an eruption? And if
so how?