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Types of Magma
1.Low Viscosity
2.About 50% silica
3. Least explosive
4. Source – upper
mantle material
5. Location – both
oceanic and continental
1 Intermediate
2. About 60% silica
3. Intermediate
4. Source – oceanic
sediments and
continental crust
5. Location –
continental margins at
subduction zones
1. High Viscosity
2. About 70% silica
3. Extremely explosive
4. Source – continental
5. Location Continental
Introduction to volcanoes
an opening in the earth’s surface
through which lava, hot gases, and
rock fragments erupt
Origin of Volcanoes
1. Magma 50-100 miles below the
earth’s surface slowly begins to rise
to the surface
2. As the magma rises it melts gaps in
the surrounding rock
3. As more magma rises a large
reservoir forms as close as 2 miles
below the surface (magma chamber)
Origin of Volcanoes
4. Pressure from the surrounding
rock causes the magma to blast
or melt a conduit (channel) to the
surface where magma erupts onto
the surface through a vent
Origin of Volcanoes
5. The magma, now called lava, builds
up at the vent forming a volcano
Origin of Volcanoes
6. Often the volcano sides will be
higher than the vent forming a
depression called a crater
an unusually large crater or the
remains when the cone collapses
into its own magma chamber
Anatomy of a Volcano
the above ground structure
built from lava and/or tephra
the path that magma takes from
the magma chamber to the vent
Magma Chamber:
the reservoir located under the
volcano where magma collects and
becomes the supply of magma/lava
to build the volcano
Parasitic Cone:
a smaller secondary volcano built
on the side of or near the main
volcano, but sharing the same
conduit to the magma chamber
a secondary vent that emits
only gases
a crack from which lava flows
Vent: opening of the volcano, through which lava,
ash and gases flow
Take a minute to
label the parts on
the diagram (not
all parts are
Ash Cloud/Gases
Parasitic Cone
Lava Flow
Magma chamber
Lava— There are 3 kinds:
(Magma is renamed Lava when it
flows or is ejected out onto the
surface of the earth)
Pahoehoe lava:
Hot, thin, fast
harden with a
relatively smooth
Often has a ropy or
wrinkled appearance
Pahoehoe lava:
Aa lava:
• Cooler, thicker,
slow moving
• Hardens with a
rough, jagged,
sharp edge
Pillow Lava:
 Lava suddenly cooled by
 shows sack-like
segments (stuffed
Can you identify the kinds
of lava from the pictures?
Circle your choice.
Tephra (pyroclastic, rock fragments)
Volcanic Dust: Smallest particles and
carried by atmosphere circulation
Volcanic Ash:
0.25-0.5 cm diameter
Generally settles out within miles
of the cone but can be carried
greater distances by stronger
Forms a mudflow when mixed with
Smaller bombs (gravel, pea size) are called
Walnut size bombs are called lapilli.
Larger fragments up to 4+ feet in diameter
are called bombs.
Lahar (mudflow):
mixture of ash, eroded land, and
water flowing down river valleys
Lahar (mudflow):
water vapor, carbon dioxide,
nitrogen, sulfur dioxide,
hydrogen sulfide, chlorine
Locations of Volcanoes
Divergent Boundaries:
 As the plate move long cracks
(rifts) form and lava builds up
If the boundary is on the ocean
floor, volcanoes can grow tall
enough to break the surface of
the ocean and become islands
Convergent Boundaries:
 Places where plates are moving
toward each other forming a
subduction zone.
 One plate melts under the other
and the magma moves upward to
form volcanoes.
Example: Pacific Ring of Fire
Example: Cascade Volcanoes
Hot Spots
Magma that may originate in the
mantle or outer core will move
upward, breaking the surface and
forming a volcano, they are
independent of plate boundaries and
a chain of volcanoes may form as the
plate moves across a hot spot.
Hot Spots
(Examples: Hawaiian Islands and
Yellowstone National Park)
Types of Volcanic Eruptions
Two factors determine the type of
Amount of water vapor &
other gases in the magma
The chemical composition of
the magma
Explosive Eruptions
Trapped gases under high
pressure will violently
explode when the magma
reaches the lower pressure
of the surface.
Has granitic (high SILICA
content) magma is very
thick and plugs the vent
causing the pressure to
build until it blows violently
out the vent
The high water content of
the magma produces more
water vapor which when
mixed in granitic magma
produces explosive
Explosive Eruptions
Mt. Pinatubo
Mont serrat
Mt. St. Helens
Quiet Eruptions
Low pressure gas
Has basaltic magma (is more
fluid and will flow instead of
And has low water content
Types of Volcano Mountains
Cinder Cones:
Small base, steep-sided, loosely
Up to 1000 feet tall
Life span of a few years
Commonly built from gravel size lava
rock fragments call cinders
Has violent eruptions, dangerous when
Types of Volcano Mountains
Cinder Cones:
 High pressure gas bubbles causes thick lava
to explode into the air, lava begins to cool as
it rises and falls becoming very sticky
 When lava hits the ground it sticks rather
than flows
 This builds a steep cone with a small base
Types of Volcano Mountains
Shield Volcanoes:
Large base, gentle slope, lava rock
A few miles wide
Life span of a million years or more
The lava is hot, thin, very fluid,
often basaltic.
Example: Hawaiian Islands
Take a look at these examples:
The Mauna Loa volcano in Hawaii—
the largest volcano on Earth—has
the broad expanse characteristic of
shield volcanoes. It spreads across
half the island of Hawaii.
Shield volcano on Mars;
Taken from space
Shield Volcanoes
Mauna Kea
Types of Volcano Mountains
Composite (strato) Volcanoes:
Large mountain volcano often snow capped, a
few miles high
Life span of million years or more
Have alternating eruptions of tephra (airborne) and lava. The tephra adds height to
the volcano and the lava cements the tephra
together and adds to the base.
Found mostly in subduction zones and have
violent eruptions.
Examples: Mt Rainier, Mt Fuji, Mt Kilimanjaro
Composite (strato) Volcanoes:
Mt. Fuji
Mt. Rainier
Mt. Kilimanjaro
Volcano Activity Levels (Stages)
Active (awake):
 Has erupted within recent time and
can erupt again at any time.
Pre-eruption activities:
 Increase in earthquake activity
under the cone
 increase in temperature of cone,
 melting of ice/snow in the crater
 swelling of the cone
 steam eruptions
 minor ash eruptions
Mt St. Helens
Dormant (sleeping):
No eruption within recent times,
but there is record of past
Can become active and erupt again
after a “wake up” period
Example: Mt. Rainier
 No eruption within recorded
 Not expected to ever erupt again
Example: Mount Mazama (Crater Lake)
Crater Lake
Mount Rainier
• The most dangerous volcano in the US
• The danger is mostly from lahars traveling down
river valleys at a speed of 25mph and destroying
everything in its path
• 100,000 people live on the solidified mudflows of
previous eruptions
Mount Rainier
• The mountain is dangerously unstable, a tall,
steep heap of loose rock held together by the
force of gravity and a cubic mile of glacier ice
that could be melted or shaken loose
• Lahar flows average every 500 years and have
gone as far as the Puget Sound lowlands (1 in 7
chance of it happening during your lifetime)
• Mount Rainier has erupted 4 times in the last
4000 years with the last eruption 200 years ago