Volcanoes
Lab 5
Concepts
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Location of active volcanoes
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Volcanism
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3 categories
Viscosity
5 types of extrusive volcanism
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Examples of each
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3 types of lava flows
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Dangers of volcanic eruptions
What do you notice about their locations?
Volcanism
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Definition: the origin and movement of magma
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Three categories
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Extrusive: magma is expelled onto surface
Intrusive: magma solidifies in the shallow crust near the
surface. Can be exposed after weathering.
Plutonic: magma solidifies deep inside the crust.
Viscosity: The ability for lava to flow. Reflective of the
amount of silica present in the magma.
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LOW silica = LOW viscosity = nonexplosive
HIGH silica = HIGH viscosity = explosive
Volcanism
Extrusive Examples
Mt. Shasta, CA
Mt. Fuji, Japan
Volcanism
Intrusive Examples
Right: Stone Mountain, GA
Below: Devil’s Tower, WY
Volcanism
Plutonic Examples
5 Types of Extrusive Volcanism
1. Shield Volcano
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Central vent allows
magma from chamber
to spill out, slowly
building the volcano in
layers.
Produces wide, gently
sloping mountains
Best example:
Hawai’ian volcanoes
(right: Mauna Loa)
5 Types of Extrusive Volcanism
2. Composite Volcano
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“Stratovolcano”
Central vent allows
magma to come up
from chamber below
Produces symmetrical,
steep-sloped
mountains
Most “explosive” type
of volcano
Examples: Mt Fuji,
Mt Shasta, Mt St Helens
5 Types of Extrusive Volcanism
2. Composite Volcano example: Mt St Helens in 1980
5 Types of Extrusive Volcanism
3. Caldera Volcano
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Formed when the upper part of a
volcano collapses
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The magma chamber below is (partially or
completely) emptied after an eruption
The emptied magma chamber can no
longer support the weight of the overlying
rock
The overlying rock collapses into itself,
forming a circular basin
Volcanic activity can continue if there
is still magma in the chamber below
Example: Crater Lake, OR
5 Types of Extrusive Volcanism
3. Caldera Volcano example: Crater Lake, OR
Crater Lake was formed after Mt Mazama (a stratovolcano) erupted
and collapsed back in on itself.
5 Types of Extrusive Volcanism
4. Cinder Cones
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Smallest mountain formation of all volcanic landform
types
Cone-shaped peaks build up from small, solid
particles, ejected into air by small volcanic vent
Generally <1,500’ high
Example: Wizard Island
5 Types of Extrusive Volcanism
4. Cinder Cone example: Wizard Island in Crater Lake, OR
5 Types of Extrusive Volcanism
5. Lava Dome
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“Plug Dome”
High-silica lava exits from
a vent. Due to its high
viscosity (doesn’t flow very
fast or far), it builds up into
a dome.
Effectively “plugs” the main
vent. Can still emit ash,
gases, and even lava.
Example: Lava Dome in
Mt St Helen’s crater.
3 Types of Lava Flows
’A’a
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“Clinky”
Blocky, choppy lava flows that does not flow well over land
Pahoehoe
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“Smooth, unbroken lava”
Ropy surface
Pillow Lava
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Lava emerges from underwater volcanic vents
Lava cools & crusts when it hits colder water
When crust fractures, lava oozes out and creates billowing
pillows
’A’a
Pahoehoe
Pillow Lava
Dangers of Volcanic Eruptions
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Burning Hot Lava
Earthquakes, Landslides and Tsunamis
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These are often related – an earthquake might jar
some land loose, resulting in a landslide. The
earthquake may set off the tsunami. Also, if the
landslide falls into the ocean, can trigger a
tsunami.
Mudslides (“Lahars”)
Ash Fallout
Pyroclastic Flow (“nuée ardente”)
Dangers of Volcanic Eruptions
Mudslide (Lahar) examples
Mt. Pinatubo, Philippines
1991 eruption
Mt. St. Helens, WA
1980 eruption
Dangers of Volcanic Eruptions
Ash fallout example: Mt. Pinatubo, Philippines 1991 eruption
Dangers of Volcanic Eruptions
Ash fallout example: Mt. St. Helens 1980 eruption
Dangers of Volcanic Eruptions
Pyroclastic flow example: Mt St Helens 1980 eruption
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Hot gases, ash, rock that become a “fluid cloud” that move down the side
of the volcano. Can travel great distances.
“nuée ardente”: French for “glowing cloud”

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