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Volcanoes
A Volcano is… • An opening in Earth’s crust through
which molten rock, gases, and ash erupt.
• Also, the landform that develops around this opening.
Kinds of Eruptions
• Geologists classify volcanic eruptions as quiet or explosive.
• How a volcano erupts depends on the magma.
• A quiet eruption moves slowly because it is low in silica and flows very easily.
• An explosive eruption has magma that is high in silica and does not flow easily. Magma builds up until it explodes out of the vent.
Magma Vs. Lava
• Magma – molten rock, and solids found
beneath the surface of Earth
• Lava – molten rock expelled by a volcano
during an eruption, found on the surface of
Earth.
Silica Content
Melting Temp.
Gas Content
Viscosity
Type of
Eruption
Location
Least (~50%)
Basaltic
Magma
Least
Least viscous
Rarely
explosive
Highest
Rifts, oceanic
hot spots
AndesiticMagma
Intermediate (~60%)
Intermediate
Intermediate
Sometimes explosive
Intermediate
Subduction
boundaries
Rhyolitic
Magma
Most (~70%)
Most
Most viscous
Usually
explosive
Lowest
Continental
hot spots
Magma and Erupted Materials
Stages of Volcanic Activity
• Geologists often use the term active, dormant, or, extinct to describe a volcano’s stage of activity.
• Active volcanoes are erupting or showing signs that it will erupt soon.
• Dormant volcanoes are no longer erupting, but can again in the future.
• Extinct volcanoes are not likely to erupt any longer.
Volcano Terms
• Ash – Fine particles of pulverized rock blown from an
explosion vent.
• Vent - The opening at the earth's surface through
which volcanic materials issue forth.
• Magma chamber - The cavity containing the gas-rich
liquid magma which feeds a volcano.
Volcano Terms • Crater- A steep-sided, usually circular
depression formed by either explosion or
collapse at a volcanic vent.
• Throat-Entrance of a volcano. The part of the
conduit that ejects lava and volcanic ash.
• Conduit (pipe)-A passage followed by magma
in a volcano.
• Caldera -a large, circular depression that
forms when the magma chamber below
a volcano partially empties and causes the
ground above to sink
Volcano Terms
• Base -the lowest part, bottom, or supporting
layer of the volcano.
• Sill - intrusive igneous rock, parallel to the
layering of the rocks into which it intrudes.
• Dike - intrusive body of magma that pushes its
way across layers of sediments.
When a Volcano erupts
• the force of the expanding gases pushes magma from the magma chamber through the pipe until it flows or explodes out of the vent.
• The volcano has a pocket of magma below the surface called a magma chamber. A long tube, called a pipe, connects the magma chamber to the surface.
• The opening at the top of the pipe is called the vent.
• When it leaves the volcano magma becomes lava.
Viscosity • Viscosity – measure of how thick (viscous) and
sticky a liquid is
• Lava Flow - A 'stream' of molten rock
• 'aa' lava flow - jagged,
rubbly, broken surface;
high viscosity
(pronounced 'ah-ah')
• 'pahoehoe' lava flow -
smooth, ropy surface;
low viscosity
(pronounced 'paw-hoey-
hoey")
Where do Most volcanoes Occur? • Latitude gives the position on Earth north or
south of the equator
– Positive north of the equator, negative south of the
equator
• Longitude gives the position on Earth east or
west of the Prime Meridian
– Positive east of the Prime Meridian, negative west
of the Prime Meridian
Why Volcanoes form
• Volcanic Eruptions occur when magma rises to the surface. This will happen when the asthenosphere melts enough to flow. There are three things that can cause this:
• 1- A decrease in pressure (like at a mid-ocean ridge or rift valley)
• 2- An increase in temperature (like at a hot spot)
• 3- An increase in the amount of water in the asthenosphere (like at subduction)
Where Volcanoes Form • Most volcanoes occur at:
1.DIVERGENT boundaries, and at
2.CONVERGENT boundaries that have SUBDUCTION.
3.Hot Spots
Volcanoes at Divergent Boundaries • Decrease in pressure as
plates pull apart lets magma rise.
• These fissure volcanoes are located at Mid-ocean ridges like Mid-Atlantic Ridge and the Great African Rift Valley
Divergent- Great Rift Valley
Volcanoes at Convergent Boundaries
(Subduction) • Subduction increases the amount of water in the asthenosphere, which lowers the melting temp.
• As the denser oceanic crust is pushed lower, it melts into magma.
• When continental and oceanic plates converge, a volcano forms on land.
• When 2 oceanic plates converge together, a volcano forms an island.
Mount Hood/ Portland
Volcano- Convergent
Sibayak, Indonesia
Mt St. Helens
Volcanic activity takes place primarily at subduction boundaries,
How Volcanoes Form
Oceanic
lithosphere
Continental
lithosphere
Asthenosphere
1. Water in the subducted rock is
released into the asthenosphere.
2. The water lowers the melting temperatures of
materials in the asthenosphere, leading to magma
formation.
3. The magma
is less dense
than its
surroundings,
so it rises.
4. Some of the magma reaches
Earth’s surface, and volcanoes
form on the overriding continental
plate.
Hot Spots • Sometimes, volcanoes occur at places that
aren’t plate boundaries.
• We call these HOT SPOTS.
–Hot spots are areas where hot magma rises from deep in Earth’s mantle.
–Magma escapes where the crust is the thinnest or weakest.
–It starts out solid then it melts when it reaches areas of lower pressure.
Types of Volcanoes
• There are 4 main types of Volcanoes:
–Shield Volcanoes
–Cinder Cone Volcanoes
–Composite Volcanoes
–Fissure Volcanoes
Volcano Categorization
• Type of volcano is based on:
– Eruption style
– Structure
– And primarily the composition of magma
• Tephra is fragmental material produced by a
volcanic eruption regardless of composition, or
fragment size. Volcanologists also refer to
airborne fragments as pyroclasts. Once
pyroclasts have reached the ground its tephra
Shield Volcanoes • Shield Volcanoes form from runny lava (low
viscosity) that tends to flow long distances
before hardening. They generally have quiet
eruptions.
– Volcano has a broad base and gently sloping
sides. Looks like a warrior’s shield hence the name
– Volcano has a less explosive eruption than other
types because the lava flows more easily
– Type of magma- Mafic Basalt: black, runny lava
– Made of layers of lava
– Example: Mauna Loa, Hawaii
Cinder Cones • Form when molten lava is thrown into the air
from a vent. As it falls, it breaks into fragments
called ash or tephra that harden before hitting
the ground.
– Most common type of volcano
– The ash and tephra make a cone-shaped mound.
– They are smaller than other types.
– Erupt explosively because magma is thick, which
allows pressure to build up. (medium viscosisty)
– Magma composition: Felsic Rhyolite (light colored)
Example:
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Cinder Cones
New Guinea
Iceland
Composite or Stratovolcano
• Form from alternating eruptions of quiet
lava and explosive ash. The layers build
up and make a moderate-sized volcano.
–High viscosity
–Made of layers of ash/tephra and lava
called strata.
–Magma composition- Andesite:
medium color
Example of
Composite/Stratovolocano
• Mt. Pinatubo-Philippines
Fissure Volcanoes • Form in long cracks where plates are pulled
apart and near other volcanoes where the crust
is weakened. Cinder Cone or Shield Volcanoes
may also be nearby.
Supervolcano • A volcano that ejects more than 1,000 cubic km
(240 cubic miles) of material,
• When one explodes, it can cover an entire
continent in ash.
• Form huge calderas after erupting
Supervolcano Effect
• Eruptions harm climate and living
organisms by blocking sunlight
– Affects photosynthesis (decreases) and
temperature (decreases)
– Thought to be behind several mass
extinctions in Earth’s history
Yellowstone
• Check out how large some of the lava flows
were from the Yellowstone Volcano!!!
How does a Caldera
Form?
• A) A Volcano’s
pressure starts to
build up
• B) The Volcano
releases large amounts
of Lava
• C) As the Lava is
released, the pressure
decreases and the
volcanic mountain
begins to collapse,
forming a concave
shape in the center of
the volcano.
• D) The center of the
volcanic mountain may
begin to fill with water
and form a lake.
Crater Lake in Oregon State should
actually be called Caldera Lake
Volcanic Explosivity Index
• This scale is
designed to
express the
severity of
volcanic
eruptions.
• Based on the
amount of
erupted
tephra
Label the parts of a volcano
Word Bank:
Main Vent/Crater
Secondary Vent
Magma Chamber
Lava
Ash & Dust Cloud
Tephra Bombs
Cone
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