Chapter 9 Notes
I.
Volcanic Eruptions
a. Magma= molten rock underneath the Earth’s surface
Lava= magma that flows onto the Earth’s surface
b. Volcanoes= areas of Earth’s surface through which
magma and volcanic gasses pass
i. Eruptions occur on the ocean floor and on land
c. Nonexplosive Eruptions
i. Are the most common type of eruption
ii. Produce relatively calm flows of lava
d. Explosive Eruptions
i. Their effects can be very destructive
ii. Don’t usually produce lava flows- instead they
cause molten rock to be blown into tiny particles
called ash
1. Ash can reach the upper atmosphere and
circle the Earth for years
e. Magma Chamber= a body of molten rock deep
underground that feeds a volcano
i. Vent= openings in Earth’s surface that magma
flows from
f. The composition of magma affects how explosive a
volcanic eruption is. The main factors are the amount
of silica, water and gas present,
i. If the water or silica content is high, an
explosive eruption is more likely
1. Water turns to a gas w
hen heated;
gases can expand rapidly creating explosive
eruptions
2. Silica rich magma is sticky and tends to
trap gas bubbles; magma with less silica is
II.
more fluid, enabling gages to escape,
leading to less explosive eruptions.
g. Pyroclastic Material= forms when magma is blasted
into the air and hardens.
i. Produced by explosive eruptions
ii. Volcanic eruption can alternated between lava
and pyroclastic eruptions.
h. The viscosity of lava= how it flows
i. Lava with a high viscosity is stiff, lava with a low
viscosity is more fluid.
ii. The viscosity of lava affects the surface or
appearance of a lava
i. The size of pyroclastic material ranges from boulders
the size of houses to tiny particles that can remain
suspended in the atmosphere for years.
j. Pyroclastic Flows= produced when large amounts of hot
ash, dust and gases are ejected from a volcano.
i. Can move at speeds over 200 km/hr- faster than
most hurricane winds
ii. Temperatures at the center can be over 700C
Effects of Volcanic Eruptions
a. During large scale, large amounts of volcanic ash and
gas are ejected into the upper atmosphere.
i. In some cases, this can block enough sunlight to
cause global temperatures to drop.
b. There are 3 basic types of volcanoes
i. Shield= built of layers of lava released from
repeated nonexplosive eruptions
1. have gently sloping sides
ii. Cinder Cone= made of pyroclastic material
released from moderately explosive eruptions.
1. Have steep sides and usually erode quickly
iii. Composite= form from explosive eruptions of
pyroclastic material followed by quieter flows of
lava.
1. Sometimes called stratovolcanoes
2. the most common types of volcanoes
3. Have broad bases and sides that get
steeper towards the top
c. Other types of volcanic landforms
i. Crater= a funnel shaped pit near the top of the
central vent at the top of a volcano
ii. Caldera= large, semicircular depression that
forms when the roof of a magma chamber
collapses
1. appear similar to a crater, but are much
larger
iii. Lava Plateau= results from repeated eruptions of
lava spread over a large area
III. Causes of Volcanic Eruptions
a. Scientists study the gases coming from active
volcanoes and look for changes in the volcanoes shaped
that could indicated that an eruption is near
b. Changes in pressure and temperature cause magma to
form.
i. Rock melts when temperature increases and
pressure decreases
c. Magma is less dense than the surrounding rock, so it
tends to slowly rise to the surface
d. About 80% of active volcanoes on land form plates
collide. Another 15% form where plates separate.
e. New crust forms where plates pulling apart
f. When an oceanic plate collides with a continental plate,
the oceanic crust is subducted, because it is denser
and thinner than continental crust.
i. When a oceanic plate subducts, it becomes
heated and releases water. The water lowers the
melting point of the rock in the mantle, causing a
body of magma to form
g. Hot Spots= volcanically active places on Earth’s
surface that are not near tectonic plate boundaries
i. They often produce long chains of volcanoes; for
example, the Hawaiian Islands
h. Volcanoes are classified into 3 categories
i. Extinct= have not erupted in recorded history
ii. Dormant= are currently not erupting, but have in
the past and may someday erupt again.
iii. Active= currently erupting or showing signs it will
erupt in the near future
i. Monitoring earthquakes is one of the best ways to
predict an eruption, because most volcanoes produce
small earthquakes if magma is moving around inside
them
j. Scientists use satellite technology such as GPS to
detect changes in a volcano’s slope that may signal an
eruption.

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