Chapter 5: Volcanoes
• How does a volcano
erupt?
BrainPop: http://www.brainpop.com/science/earthsystem/volcanoes
/
1: Volcanoes and Plate Tectonics
– Where Are
Volcanoes Found
on Earth’s
Surface?
Volcano
• A mountain that forms in
Earth’s crust when molten
material, or magma,
reaches the surface.
– Magma: Molten mixture of
rock-forming substances,
gases, and water from the
mantle. (beneath the
surface)
– Lava: Magma that reaches
Earth’s surface. (on the
surface)
Volcanoes and Plate Boundaries
• Volcanic belts form along the boundaries of
Earth’s plates.
– Divergent boundaries
– Convergent boundaries
Divergent Boundaries
• Move apart
– Mid-ocean ridges
– Rift valleys
Converging Boundaries
• Convergent (come together)
– Volcanoes can form where two oceanic plates collide:
• Older, denser plate sinks into the mantle
creating a deep-ocean trench
• Due to the water entering the mantle, the
mantle partially melts
• Magma rises up and breaks through the ocean
floor creating a volcano
Ring of Fire
• Major belt of volcanoes
• Around the border of the Pacific Ocean
Island Arc
• String of islands (from the collision of two
oceanic plates)
• Examples: Japan, New Zealand, Aleutians, and
Caribbean islands
http://esminfo.prenhall.com/science/geoanimations/animations/35_VolcanicAct.html
Hot Spots
• Area where material from deep inside the
Earth’s mantle rises through the crust and
melts to form magma.
• Plates move over hot spots
• Example: Yellowstone National Park
– What Happens When a Volcano Erupts?
– What Are the Stages of Volcanic Activity?
INSIDE A VOLCANO
Open to page 139 in your textbook.
Fill in Figure 1 as we go! 
• Magma chamber:
– The pocket beneath a volcano where magma
collects.
Magma chamber
Inside a Volcano
• Pipe:
– A long tube through which magma moves from
the magma chamber to Earth’s surface.
Pipe
Inside a Volcano
• Vent:
– The opening through which molten rock and gas
leave a volcano.
Central vent
Side vent
Inside a Volcano
• Lava flow:
– The area covered by lava as it pours out of a
volcano’s vent.
Lava flow
Inside a Volcano
• Crater:
– A bowl-shaped area that forms around a volcano’s
central opening.
Crater
Inside a Volcano
Central vent
Crater
Lava flow
Side vent
Pipe
Magma chamber
– What Happens When a Volcano Erupts?
– What Are the Stages of Volcanic Activity?
A Volcanic Eruption
• 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.
– Quiet
– Explosive
Quiet Eruptions
• Magma is hot
• Low in silica
(forms from
elements oxygen
and silicon)
• Thin magma
• Runny magma
• Example:
Hawaiian Islands
Quiet Eruptions = Different Types of
Lava
• Pahoehoe: from fast-moving, hot lava that is thin
and runny; looks like ropelike coils
• Aa: from lava that is cooler and thicker (slower);
rough surface consisting of jagged lava chunks
•
•
•
•
•
Explosive Eruptions
High in silica
Thick
Sticky
Gases build up pressure (explosive)
Example: Mount St. Helens
Chunks Thrown in the Air
• Smallest: ash (dust-sized)
• Medium: cinders (pebble-sized)
• Large: bombs (size of golf ball to size of car)
Volcano Hazards
• Lava: set fire and bury
• Pyroclastic flow: mixture
of hot gases, ash (small
specks of dust), cinders,
and bombs (large chunks)
that flow down the sides
of a volcano (occur
during an explosive
eruption)
• Landslide: mud, melted
snow and rock
Stages of Volcanic Activity
• Active: Live; is
erupting or has shown
signs that it may erupt
in the near future
• Dormant: Sleeping;
expected to erupt in
the future
• Extinct: Dead;
Unlikely to ever erupt
again
Warnings of a Possible Eruption
• Tiltmeter: detects slight surface changes in
elevation and tilt caused by magma moving
underground
• Monitor gases escaping from the volcano
• Monitor the many small earthquakes that
occur around a volcano before an eruption
• Rising temperatures in underground water
1. Which has more silica – an explosive or a
quiet volcano?
-Explosive
2. What are golf-ball to car-sized particles
shooting out of a volcano called?
-Bombs
3. A volcano that may erupt sometime in the
future is known as _______.
-Dormant
3: Volcanic Landforms
– What Landforms Do Lava and Ash Create?
– What Landforms Does Magma Create?
What Landforms Do Lava and Ash
Create?
•
•
•
•
•
Calderas
Shield Volcanoes
Cinder Cone Volcanoes
Composite Volcanoes
Lava Plateaus
Caldera
• The hole left when a volcano collapses (a lake
can form, filling the hole)
http://dli.taftcollege.edu/streams/Geography/Animations/Caldera.html
Cinder Cone Volcanoes
•
•
•
•
Steep, cone-shaped hill or small mountain
High silica (thick, sticky)
Explosive
Example: Paricutin (Mexico, 1943)
Composite Volcanoes
• Tall, cone-shaped mountains in which layers of
lava alternate with layers of ash
• Silica content varies (lava flows and explosive
eruptions)
• Example: Mt. Fuji (Japan) and Mount St.
Helens (Washington)
Shield Volcanoes
• Gentle sloping mountain from lava flow
• Hot spot volcanoes are usually shield
volcanoes
• Example: Mauna Loa (Hawaii)
Lava Plateaus
• Lava flows out of long cracks
• Over time runny lava builds up a
high, level area (called a plateau)
• Example: Columbia Plateau
(Washington, Oregon, and Idaho)
Landforms
Cinder Cone
Shield
Composite
(Stratovolcano)
Lava Plateau
What Landforms Does Magma Create?
•
•
•
•
•
Volcanic necks
Dikes
Sills
Dome mountains
Batholiths
Volcanic Neck
• Formed when
magma hardened in
an ancient volcano’s
pipe.
• Later, soft rock
around the pipe
wore away, exposing
the harder rock
inside
Dikes and Sills
• Dike: Magma that forces itself vertically
across rock layers and hardens
• Sill: Magma that squeezes between
horizontal rock layers hardens to form a sill.
Dome Mountains
• Forms when uplift pushes a large body of
hardened magma toward the surface
• Example: Black Hills (South Dakota)
Batholiths
• A mass of rock formed when a large body of
magma cools inside the crust.
Half-Dome in
Yosemite Natl. Park
The Big Question
• How does a volcano erupt?