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Volcanoes
Magma Reaches the
Earth’s Surface
KEY CONCEPT: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.
Kinds of Eruptions
KEY CONCEPT: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.
Stages of Volcanic
Activity
KEY CONCEPT: 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.
Where Volcanoes
Form
KEY CONCEPT: Volcanoes form along the
boundaries of the Earth’s plates.
• A Volcano is a weak spot in the crust where
melted material comes to the surface.
• Most Volcanoes happen at plate boundaries.
• Lava can build up to form islands and
mountains.
• Some Volcanoes form above a hot spot
when magma erupts through the crust and
reaches the surface.