Volcanoes 2
Cyamaba
Volcano,
Ecuador
Learning Goals
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Learn about the role of plate tectonics in causing volcanoes and
learn what causes eruptions to be gentle or highly explosive.

Identify the main types of volcanoes: shield volcanoes,
stratovolcanoes, and cinder cones.
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Learn about other forms of volcanic activity such as geysers, hot
springs, hydrothermal vents, and geothermal energy.
Vocabulary Terms
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caldera
cinder cone volcano
crater
geothermal energy
hydrothermal vent
lava
magma
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magma chamber
Ring of Fire
rock cycle
shield volcano
stratovolcano
vent
Volcanoes

The eruption of Mount St.
Helens in 1980 reduced
the height of this
mountain from 2,932
meters (9,677 feet) to
2,535 meters (8,364 feet).
 Early in the morning of
May 18, 1980, an
earthquake triggered a
landslide that caused the
bulge to eject magma,
water, and gases.
Lava Tubes, Santa Cruz Island Galapagos, Ecuador
Lava Tubes, Santa Cruz Island Galapagos, Ecuador
Lava Tubes, Isabella Island Galapagos, Ecuador
Volcanoes
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Solid rock melts and becomes magma under certain
conditions that lower the melting point of the material.
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At subduction zones, water is the key for solid rock to
melt and become magma.
Volcanoes
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Most volcanic
activity is found at
the edges of tectonic
plates, namely at
divergent and
convergent plate
boundaries, but
does not occur at
transform plate
boundaries.
Volcanoes
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High viscosity lava is
associated with
stratovolcanoes (also
called composite
volcanoes).
 These volcanoes
range in height from
500 to 10,000 meters
high.
Cotapoxi Volcano Summit, Ecuador
Cotapoxi Volcano, Ecuador
Laguna Cuicocha, Ecuador
Crater of the Extinct Volcano Puluahua, Ecuador
Tungurahua Volcano, Ecuador
Volcanoes
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Low viscosity, fastflowing lava is
associated with
shield volcanoes.
 Because this lava
easily flows down
hill, shield volcanoes
are gently sloped and
flattened.
Wolf Island, Galapagos, Ecuador
Bartolome Island and Pinnacle Rock, Ecuador
Sunken Crater Bartolome Island, Ecuador
Volcanoes
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Cinder cone
volcanoes are steep
stacks of loose
pyroclasts (clumps
and particles of lava).
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Cinder cones are
rarely higher than 300
meters.
Birth of Paricutin Volcano, Mexico 1943
Paricutin Volcano, Mexico
Paricutin Volcano, Mexico
Paricutin Volcano, Mexico 2000
Volcanoes
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Lava viscosity also determines how explosive an
eruption will be.
 Explosive eruptions occur when the lava has a lot of
water and dissolved gases.
 Gentle eruptions are associated with fast-flowing lava
from oceanic crust.
Volcanoes
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Volcanoes also form when an oceanic plate slides
under another oceanic plate.
Hydrothermal Vents

Hydrothermal vents are deep sea, chimney-like
structures that occur along midocean ridges.
Volcanoes
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Some gemstones are also associated with volcanic
activity.
 For example, diamonds form at high temperatures
deep underground when carbon crystallizes inside
rocks called kimberlites.
Volcanoes
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Geothermal energy is the
useful product of volcanic
activity.
 When steam from magma
collects below ground, it
can be tapped just like
water in a well.
 The pressurized steam can
be used to generate
electricity.
Volcanoes
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Volcanic activity results
in the formation of two
kinds of igneous rocks:
— extrusive
— intrusive
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A batholith is a large
underground rock that
formed when a mass of
magma cooled
underground.
Volcanoes
Key Question:
 Why
do some
volcanoes erupt
explosively?