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Volcanoes
Chapter 7
Volcanoes and Plate Tectonics
• Volcano – weak spot in the crust where a
mountain forms when layers of lava and
volcanic ash erupt and build up and push
through
• Magma - less dense than the rock around it
so it is very slowly forced upward toward
Earth’s surface made up of gases, water
molten rock forming stuff
• Lava – magma that reaches the earths
surface
Where Volcanoes Occur?
• Volcanic Belts form along boundaries of Earth’s
crust, one major is the Pacific Ring of Fire
• 1. Divergent Boundary – plates moving apart –
seafloor spreading
• Example: N. Atlantic Ocean. The North American
plate is moving away from Eurasia and African
plates.
• Iceland formed by the separation of these plates at
the rift
• 2. Convergent Boundary – 2 plates collide, rumple up
and cause mountains instead of volcano
• Juan de Fuca Plate is converging with the N.
• Convergent Plates (cont)
• Magma that is forced upward forms volcanoes of the
Cascade Mountains
• Pacific Ring of Fire – area around the Pacific Plate
where there are earthquakes and volcano’s
• Hot Spots – middle of a plate, area’s in Earth’s
mantle that are hotter than neighboring areas
• Magma forms and rises towards the crust
• Example: Hawaiian islands, plates move and form
islands when it gets over the hot spot
Types of Eruptions – depend on the
pressure build up and material
• 2 factors:
• 1. Water Vapor and other gases trapped within the
magma,
• if they escape easily – quiet eruptions
• Gas trapped under high pressure eventually escapes –
explosive eruptions
• 2. Magma is either basaltic or granitic
• Basaltic magma – contains less silica, is very fluid and
produces quiet, non-explosive eruptions magma runs
down the side like in Hawaii
• Granitic magma – thick and contains lots of silica
and has a high water content, it gets trapped in
vents, causing pressure to build up so when it
erupts the gases expand rapidly carrying pieces of
lava
3 Forms of Volcanoes
• 1. Shield Volcano – Quiet eruptions that spread
out basaltic lava in broad, flat layers Hawaiian
Islands (pg 399)
• 2. Cinder Cone Volcano – Explosive eruptions
that throw lava into the air forms steep sided
consolidated volcano ( pg 400) Paricutin
• Tephra – various sizes of this lava after it has
cooled
• Ash – fine
• Bombs – large heavy rocks
• 3. Composite Volcano – formed at convergent
plate boundaries above subduction zone, this type
of volcano alternates between quiet and explosive
tephra layer alternating with a lava layer Mt. St.
Helens (pg 400)
• Subduction Zone – area where an ocean plate
descends into the upper mantle
Volcano Features
• Magma that cools underground forms igneus rock (most
magma does not reach the surface)
• Balholith – largest igneous rock bodies, hundreds of km
wide and thick Yosemete has exposed one
• Dike – magma squeezed into a vertical crack and
hardened
• Sill – magma squeezed into a horizontal crack
• Laccolith – dome of rock
• Volcanic Neck – cone of the volcano erodes and leaves
the igneous rock Ship Rock New Mexico
• Caldera – formed after eruption when the top collapses
down into magma chamber Crater Lake
Vent – opening in the surface which magma
reaches the earths surface through (1000
yrs)
Crater – opening at the top of a volcano’s
vent
Geothermal Energy from Volcanoes
• Geothermal energy – utilizing the heat from
magma and converting it to electricity by heating
water which produces steam that turns the turbines
that run the generators to make electricity (fig 15-5)
• Geothermal energy used in 20 countries and US
• Hawaii wants to heat by geothermal energy of
Kilauea
• Many people are against this because some of the
rain forest must be cleared for roads, drilling etc,
they say it will cut down cultural trees