Volcanoes and Plate Boundaries
6th Grade
Do more volcanoes occur on land or in
the ocean?
Volcanoes and Plate Boundaries
• There are about 600 active volcanoes on land
• There are MANY MORE beneath the sea
• Volcanoes tend to occur in belts that extend
across continents and oceans
• Volcanic belts form along the boundaries of
Earth’s plates. WHY is this?
Volcanic belts form along the
boundaries of Earth’s plates because…
• At plate boundaries, huge pieces of the crust
diverge (pull apart) or converge (push together)
• These movements cause fractures in the crust
that allows magma to reach the surface
• Most volcanoes form along diverging plate
boundaries such as mid-ocean ridges and
converging plate boundaries where subduction
takes place
What is the Ring of Fire?
• A major volcanic belt that is formed by many
volcanoes that rim the Pacific Ocean
Ring of Fire
Ring of Fire
Volcano at a Converging Plate
Boundary
e.g the Ring of Fire
How can Subduction form volcanoes?
• When 2 plates collide, the older, denser plate sinks
back into the mantle
• The rock from subducting plates melts and forms
magma
• Because magma is less dense than surrounding rock, it
rises toward the surface
• Eventually the magma breaks through the crust and
creates a volcano
Subduction and Volcanoes
What is an island arc?
Island Arcs: a string of islands formed
by volcanoes along a deep-ocean
trench
Major island arcs include Japan, New
Zealand, Indonesia, the Philippines,
the Aleutians, the Caribbean islands
Volcano at a Diverging Plate Boundary
Note: Volcanoes do NOT occur along
transform boundaries, only
earthquakes occur there
Hot Spot Volcanoes
• Results from a “hot spot” in the Earth’s mantle
• Hot spot : an area where material from deep
within the mantle rises and then melts, forming
magma
• A volcano forms above a hot spot when magma
erupts through the crust and reaches the surface
Do hot spots have to occur along plate
boundaries?
• No! They CAN occur on or near plate
boundaries but they do not have to
• Hot spots can also form under oceanic floor or
continents
Hawaiian Islands formed one by one
over millions of years as the Pacific
plate drifted over a hot spot
Yellowstone National Park in Wyoming
marks a hot spot under the North American plate –
example of a hot spot that formed under the
continents
Lets look at the properties of
magma/lava. Why does lava exhibit
different characteristics at different
volcanoes?
Viscosity: the physical property of
liquids that determines the resistance
of flow.
• Because liquids differ in viscosity, some liquids
flow more easily than others
• The greater the viscosity, the more slowly lava
flows
• Which lava has a greater viscosity?
Types of Lava
• http://www.youtube.com/watch?v=Z9iW_oX
MBB8 (pahoehoe)
• http://www.youtube.com/watch?v=iyIV5fd1A
ww (aa)
• Which type of lava has a higher viscosity?
Aa has a higher viscosity
• Pahoehoe: fast-moving, hot lava with low
viscosity
– Looks like a solid mass of wrinkles, billows,
ropelike coils
• Aa: cooler, slow-moving lava with higher
viscosity
--when it hardens, it forms a rough surface
consisting of jagged lava chunks
Physical and Chemical Properties
• Physical properties- any property that can be
observed without changing the composition
of a substance. (temperature, hardness,
density, boiling point)
• Chemical Properties- property that causes a
change in composition. (the ability to burn or
to combine or react with another substance)
Why don’t all types of magma have
the same viscosity?
• The viscosity of magma depends upon its silica
content (chemical property) and temperature
(physical property).
• Silica = a compound that is made up of oxygen
and silicon
• Silica content of magma ranges from 50-70
percent
The more silica magma contains, the
higher its viscosity.
• Magma that is high is silica produces lightcolored lava that is too sticky to flow very far.
• When this type of lava cools, it forms rhyolite
The less silica magma contains, the
lower the viscosity.
• Low-silica magma flows readily and produces
dark-colored lava.
• When this kind of lava cools, it forms basalt.
Rhyolite
Basalt
Where does lava usually come from?
--lava begins as magma, which usually forms in
the asthenosphere
--the materials of the asthenosphere are under
great pressure
--liquid magma is less dense than solid material
around it, so magma flows upward into any
cracks in the rocks above
What pushes magma to the surface?
--Like the carbon dioxide in soda pop, dissolved
gases are trapped in magma
--The dissolved gases are under tremendous
pressure
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.
When is an eruption over?
• When the pressure eases after the gases
bubble out of the magma
What happens when the volcano stops
erupting?
The lava at the surface will have turned to rock,
magma will have solidified in the vent and pipe,
and some magma might remain in the chamber