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Volcanoes & Plate
Tectonics
What is a Volcano?
• A volcano is a landform that is created by
lava and/or pyroclastics
– Pyroclastics are the solid materials that are
ejected from a volcano
• Magma is molten rock that is found
BENEATH the earth’s surface
• When magma reaches the surface it is
called lava
• When lava cools &
hardens it forms solid
rock called an
igneous rock
• Lava that is released
during a volcanic
eruption adds new
rock to existing land
and forms new
islands – constructive
force
Location of Volcanoes
• 600 active volcanoes on land, more underwater
• Volcanoes occur in belts
– Major volcanic belt in the Ring of Fire which rims the
Pacific Ocean
– Volcanic belts form along plate boundaries
Volcanoes at Divergent Boundaries
• Divergent boundaries are also known as
spreading centers
• Lithospheric plates along the ocean floor
spread apart and allow lava to move
upward
– This creates new oceanic crust
• Some of the lava creates a volcanic cone
that may rise above the water
– Islands off the coast of Iceland
• Volcanoes located
within oceans extrude
lava that is mafic
– Mafic lava is thin and
runny and is dark in color
– Main rock that is created
from mafic lava is basalt
Volcanoes at Convergent
Boundaries
• Volcanoes at convergent boundaries occur
at subduction zones
• 2 types of subduction zones
– Oceanic-continental convergence
• Oceanic crust subducts beneath continental crust
and begins to melt – creating magma
• Magma slowly moves upward because it is less
dense than the surrounding rock
• When magma reaches the surface, a volcano is
formed
• Oceanic-oceanic convergence
– Denser oceanic plate will be subducted
creating a deep-ocean trench
– Subducting plate will begin to melt creating
magma
– Magma moves upward and creates a chain of
volcanic islands
– This is known as a volcanic island arc
– These occur 200-300 km from the deepocean trench
• Example: Aleutian Islands
Hot Spot Volcanoes
• A hot spot is an area in the middle of a
continental or oceanic plate where volcanism
occurs
– Not near a plate boundary
• Created by mantle plumes
– Areas of magma originating in the mantle
– Magma moves its way up to the crust and reaches the
surface through cracks
• Hot spots do not move, plates move over them
– Examples: Hawaii and Yellowstone
http://www.nps.gov/yell/naturescience/tracking_hotspot.ht
m
Volcanic Activity
• How Magma Reaches Earth’s Surface
– Magma rises because it is less dense than the
surrounding rock
– It flows upward through any crack in the rock
• Volcanic Eruption
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–
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Gas is dissolved in the magma
Gas is under tremendous pressure
As magma rises to the surface, pressure decreases
The dissolved gases begin to separate and form
bubbles
• An eruption occurs
when an opening
develops in the crust
• Gases rush out and
carry magma with
them
Inside a Volcano
• Pocket beneath the volcano that holds the
magma is the magma chamber
• Tube that connects the magma chamber
to the Earth’s surface is called the pipe
• Magma and pyroclastics leave the volcano
through an opening called the vent
• The bowl-shaped area at the top of the
volcano is the crater
Viscosity
• Viscosity is a fluid’s resistance to flow
– Similar to thickness
• Low viscosity fluids flow more easily than
high viscosity fluids
• The hotter a liquid, the less viscous it
becomes
• http://www.seed.slb.com/en/scictr/lab/visc
o_exp/viscosity.htm
Type of Liquid
Water
Syrup
Shampoo
Viscosity
Characteristics of Magma
• Magmas differ in viscosity
• The hotter the magma, the more fluid the
magma is
• Silica Content
– Material formed from oxygen and silicon
– The more silica there is in magma, the thicker
the magma will be
Types of Magma
Type of Magma
Silica Content
Viscosity
Basaltic
50%
Least
Andesitic
60%
Intermediate
Granitic
70%
Greatest
Types of Volcanic Eruptions
• Silica content of magma helps determine
whether the eruption is quiet or explosive
Quiet Eruptions
• Basaltic lava – thin and runny
• 2 types of lava in quiet eruptions
– Pahoehoe: fast-moving, hot lava which
resembles a braid or rope
– Aa: slow-moving, cooler lava that forms
jagged chunks
Explosive Eruptions
• If magma is thick and sticky, a volcano erupts
explosively
• Lava gets stuck in the volcano causing pressure
to build up
• Explosion breaks lava into pieces – pyroclastics
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Ash: smallest pieces
Cinders: pebble-sized particles
Bombs: range in size from a baseball to a car
Pyroclastic flow: highly heated mixture of pyroclastics
moving down a volcano
Stages of a Volcano
• Active – “live” volcano that is erupting or
has shown signs that it will in the near
future
• Dormant – “sleeping” volcano that
scientists expect to be active in the future
• Extinct – “dead” volcano, unlikely to erupt
again
Other Types of Volcanic Activity
• Hot spring – superheated pool of water
heated by a nearby body of magma
• Geyser – fountain of water and steam that
erupts from the ground
• Geothermal energy – energy created by
water heated by magma
– Used in Iceland
Monitoring Volcanoes
• Geologists can predict volcanic eruptions
more successfully than earthquakes
• Geologists monitor changes in elevation,
gases from the volcano, and earthquakes
Volcano Hazards
• Quiet Eruptions – lava pours out of the
volcano setting everything in its path on
fire
• Explosive Eruptions – emits hot, burning
clouds of ash and cinders/bombs
Volcanic Landforms
• 3 types of volcanoes
– Shield volcanoes
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Wide, gently sloping mountains
Formed by thin, runny basaltic magma with a low viscosity
Located at divergent boundaries and hot spots
Example: Mauna Loa
• Cinder cone volcano
– Steep, cone-shaped
hill or mountain
– Built from ejected lava
fragments
– Fragments are ejected
and begin piling up,
creating the cone
shape
– Example: Paricutin
Composite Volcano
• Tall, steep, symmetrical structure
• Composed of alternating layers of lava flows and
pyroclastic deposits
• Example: Mt. St. Helen’s
Lava Plateau
• Flat, level areas where lava flows have
traveled far and cooled
• Cooled lava flows pile up on one another
Caldera
• Roughly circular depression created by the
collapse of the empty vent and magma
chamber
• Greater than 1 km in diameter
Landforms from Magma
• Volcanic neck
– Forms when magma hardens in the volcano’s
pipe
– Softer rock wears away around the pipe
leaving the neck behind
Dike
• When magma forces its way across rock layers
and hardens
Sill
• When magma squeezes its way between
parallel rock layers
Batholith
• When a large body of magma cools and
hardens beneath the surface
Dome Mountain
• When small bodies of rising magma are
blocked by horizontal layers of rock
• Magma forces its way up and bends the
rock layers into a dome shape
Lahar
• Volcanic mudflow generated during a volcanic
eruption
Volcanoes in the Solar System
• Volcanoes on Venus
– Thousands of volcanoes
– Largest volcano is Theia Mons
– Mostly shield volcanoes
Volcanoes on Mars
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Shield volcanoes
Cone-shaped volcanoes
Lava flows
Olympus Mons
– Largest mountain in the solar system
– Shield volcano
– Covers an area as large as Ohio
Volcanoes on Distant Moons
• Io – moon of the planet Jupiter
• Triton – moon of the planet Neptune