Download Chapter 5: Volcanoes

5.1 Volcanoes and Plate Tectonics
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Volcano: mountain that forms in Earth’s
crust when molten material (magma)
reaches the surface.
Remember, magma is molten rock below
ground; lava is molten rock at the surface.
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Volcanoes form along plate boundaries.
 At divergent boundaries, the crust cracks and
allows molten rock to rise (mid-ocean ridge).
 At convergent boundaries, one plate can sink
beneath the other, which helps bring magma to
the surface (on ocean floor).
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An island arc is a string of islands formed
from volcanoes.
Some common ones are Japan, New
Zealand, the Aleutians, and the Caribbean
islands.
Some continental volcanoes include Mount
St. Helens and Mount Rainier.
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Some volcanoes form from hot spots: an
area where material from the mantle rises
through the crust.
Hot spots are stationary beneath the crust,
and the plates move over them.
5.2 Volcanic Eruptions
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Magma chamber: pocket of magma beneath
the volcano.
Pipe: connects the magma chamber to
Earth’s surface.
Vent: the area where molten rock and has
leave the volcano.
Lava flow: the spread of lava as it leaves the
vent
Crater: bowl-shaped area around the central
vent.
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Dissolved gases are built up in magma.
As they expand, pressure increases, and
eventually forces the magma through the
pipe and out of the vent.
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Eruptions can be quiet or explosive; it
depends on the magma’s silica content and
the thickness of the magma.
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If magma is very hot and low in silica, the
volcano erupts quietly.
 Lava can flow for long periods of time; built the
Hawaiian islands.
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Magma that is high in silica explodes from a
volcano.
 Mount St. Helens in Washington erupts this way.
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Volcanic Ash: Consists of particles ranging
from the size of dust to the size of rice grains
Volcanic Cinders
Volcanic Bombs
Volcanic Blocks: Solid pieces of rock, as big
as houses
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Quiet eruptions have large lava flows and
burn and bury almost everything in its path.
Explosive eruptions release dangerous
materials that form clouds around the
volcano.
 Pyroclastic flow: mixture of hot gases, ash,
cinders, and bombs that rush down a volcano.
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An active volcano is one currently erupting
or showing signs of erupting soon.
A dormant volcano is one that is “sleeping”
but expected to turn active in the future.
An extinct volcano is one that is unlikely to
ever erupt again.
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Geologists use tiltmeters to measure small
changes in the ground caused by magma
movement.
They also monitor earthquakes, which can
cause volcanic eruptions.
They measure gas from the volcano, and also
keep track of temperatures.
5.3 Volcanic Landforms
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A caldera is a hole left behind when a
volcano collapses.
Lakes may form in this hole over time.
Usually associated with shield and composite
volcanoes.
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A steep, cone-shaped hill or small mountain
Formed by ash, cinders, and bombs building
up around the volcano’s vent.
Usually magma high in silica
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Tall, cone-shaped mountains with layers of
lava and ash
Usually alternates between quiet and
explosive
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Wide, gently sloping mountain
Thin layers of lava pour out of a vent and cool
in layers.
Hot spot volcanoes are usually this type.
The largest volcano on Earth, Mauna Loa.
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Lava plateaus are made from thin, runny lava
floods that cool and solidify.
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Volcanic neck: forms when magma hardens
in a volcano’s pipe and the rock wears away.
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Dike: magma that moves across rock layers
Sill: magma that squeezes between rock
layers
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Batholith: mass of rock formed when a large
body of magma cools inside the crust.
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Dome Mountains: bodies of hardened
magma that are pushed upward toward the
surface.