Tectonic Activity

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• Tectonic Activity
• Volcanoes
• Cone Volcanoes
• Shield Volcanoes
• Composite Volcanoes
• Human effect of Volcanoes

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Volcanoes are formed along two types of plate
boundary destructive and constructive.   Volcanoe
s occur where molten rock (magma) comes to the
surface of the earth. The magma rises to the
surface through cracks in the crust called vents.
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Cone Volcanoes   When lava is thick and acidic it
doesnt flow far before cooling and solidifying,
this causes cone volcanoes (aka acid cone
volcanoes) to have steep sides.     Example
Mount Pelee on Martinique (an island in the
Caribbean)
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Shield Volcanoes   When lava is runny and thin it
can flow a long way before cooling and
solidifying, this causes shield volcanoes to have
gentle slopes and wide bases built almost
entirely of low viscosity basaltic lava
flows.   Shield volcanoes are the largest of the
three types.   The eruptions are generally non
explosive due to the low silica content and may
last for years     Example Mount Kilauea
in Hawaii (USA), Galapagos Islands, Snake River
Plain in Idaho, USA. Examples include the Wapi
lava field and Hells Half Acre and Iceland
(although Iceland also contains other types of
volcanoes.)
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Composite Volcanoes   Composite volcanoes are the
most deadly of volcano types. They are made of
alternate layers of ash and lava and have steep
sides built up by eruptions of intermediate
viscosity andesitic lava and explosive
tephra.   Often the lava cools creating a plug
which blocks the vent resulting in a huge
explosion blowing out the plug     Example
Mount St Helens in Alaska (USA), Mount Shasta in
California, Mount Rainier in Washington state,
and Mount Fuji in Japan.
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Human effects of Volcanic activity   The effects
of volcanoes can be divided into primary and
secondary effects.   Primary effects are produced
directly by the volcanic activity. Examples
include lava flows, ash-flows, lateral blasts,
ash-falls, and gases. Secondary effects are the
result of primary effects e.g. mud flows NB Refer
to specific examples Mt St Helens/Mt Vesuvius
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Lava Flows Lava flows are streams of molten rock.
Lava flows can erupt relatively non-explosively
and move very slowly (a few meters to a few
hundred meters per hour) or they can move rapidly
(typically down steep slopes).     Most
lava flows are slow enough that they are seldom a
threat to human life. Such flows generally follow
a predictable course. However, lava flows can
cause extensive damage or total destruction by
burning, crushing, or burying everything in their
paths. Whole villages have been known to
completely disappear beneath lava flows.   To
avoid such destruction, controlling a lava flow
has become important and successful deflection
has occurred in the past. photos of lava flows.
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Pyroclastic Hazards Volcanic explosions produce
volumes of tephra. Tephra is the material blown
out of the volcanic vent when an explosion
occurs. Ash-flows, lateral blasts, and ash-falls
are the types of pyroclastic activity that
produce tephra, with composite volcanoes and
large calderas the vent sources.
  Pyroclastic flows (also called
ash-flows) are high speed avalanches of hot ash,
rock fragments, and gas which move down the sides
of a volcano during explosive eruptions. These
flows occur when the vent area or ash column
collapses. Because pyroclastic flows can reach
1500 degrees F and travel at high speeds (160-250
kilometres per hour and up), they are extremely
destructive and deadly. Pyroclastic flows are
typical of composite volcano eruptions, but are
also associated with large caldera systems.
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Volcanic Gases All magmas contain dissolved gases
that are released during and between eruptive
episodes. These gases are predominately steam,
followed in abundance by carbon dioxide,
compounds of sulphur and chlorine, and lesser
amounts of other gases. While they rarely reach
populated areas in lethal concentrations, gases
can be injected to great heights in the
atmosphere by volcanic eruptions, in some cases
spreading throughout the globe.
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Mud flows (Lahars / Debris flows) are mixtures of
water, rock, ash, sand, and mud that originate
from the slopes of a volcano. They can travel
over 80 kilometres and commonly reach speeds of
35 to 65 kilometres per hour.   They contain a
high percentage of rock debris look like
fast-moving rivers of concrete. Close to a
volcano, they have the strength to rip huge
boulders, trees, and structures from the ground
and carry them for great distances. Farther
downstream the coarser debris settles to the
bottom of the flow, leaving mud to continue on to
cover everything it passes.   Mud Flows are
formed when masses of unconsolidated, wet debris
become mobilized, and are commonly start
by Large landslides of water-saturated
debris Heavy rainfall eroding volcanic
deposits Radiant heat emitted from a volcanic
vent suddenly melting snow and ice Pyroclastic
flows on the flanks of a volcano Breakout of
water from glaciers, crater lakes, or from lakes
dammed by volcanic eruptions   Historically, mud
flows have been one of the most deadly of the
volcanic hazards