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Sections 1 and 3
• A volcano is a weak spot in the crust where molten
material, or magma, comes to the surface.
• Magma is a molten mixture of rock-forming substances,
gases, and water from the mantle.
• When magma reaches the surface, it is called lava.
• Lava cools and forms solid rock, which builds up on
the Earth’s surface.
• There are about 600 active volcanoes on land.
• Volcanic Belts form on the edges of Earth’s plates.
• A major volcanic belt is the Ring of Fire in the Pacific
Volcanoes and Plate
• Volcanoes form along the mid-ocean ridges which are
where divergent plate boundaries are.
• Lava released from the cracks in the ocean floor build
new mountains.
• Ex: Great Rift Valley in East Africa
Divergent Boundaries
• Volcanoes also form at convergent plate boundaries
where plates meet, subducting one crust deep back into
the mantle.
• The older, more dense plate sinks beneath the deep ocean
trench and as melting occurs, magma, which is less
dense, rises through the ocean floor.
• Island arcs- strings of islands created by convergent
plate boundaries.
• Ex: Japan, New Zealand, Indonesia, the Philippines,
Caribbean Islands
Convergent Boundaries
• Hot Spot- an area where material from deep within the
mantle, rises then melts, forming magma.
• A volcano forms above a hot spot when magma erupts
through the crust and reaches the surface.
• ex: Hawaiian Islands, Yellowstone National Park in
Hot Spot Volcanoes
Where are volcanoes found?
Most volcanoes are found along
plate boundaries.
What are hot spots?
A hot spot is an area where
material from deep within the
mantle rises and then melts,
forming magma.
Section 3- Volcanic
• Where does lava come from?
• Lava begins as magma which forms in the
• Where is the asthenosphere located?
• The Mantle!!
• Dissolved gases that are trapped in the magma is what
causes magma to reach the surface.
• Think of a can of soda
• As magma rises up to the surface, pressure of the
surrounding rock decreases on the magma.
• Dissolved gases form bubbles as they expand.
• These expanding gasses exert a huge amount of force.
Process of a Volcanic
• All volcanoes have a pocket of magma deep beneath the
surface and one or many cracks that allow for magma to
force its way through.
• Magma chamber: pocket under the surface where
magma collects
• Magma moves through the volcano from the magma
chamber via a pipe- a long tube in the ground that
connects the magma chamber to the Earth’s surface
Inside a volcano
• When the magma and gasses travel up the pipe, it leaves
out of an opening called a VENT.
• Vents are usually at the top of a volcano but can also be
on the sides
• As magma reaches the surface, lava pours out of the vent
and covers the sides of the volcano as a lava flow.
• A crater, which is bowl-shaped, may form at the top of
the volcano at the top vent.
• When a volcano erupts, the force of the expanding gases
pushes magma up from the magma chamber through
the pipe until it flows gently or explodes out of the
• Volcanoes are classified by geologists into two types of
• Quiet Eruptions: occurs when magma has a low silica
content. Magma has a low viscosity. Lava gently oozes
out of the volcano.
• Ex: Hawaiian Islands are formed from quiet eruptions.
QE build up land over thousands of years.
Kinds of
• Explosive Eruptions: occurs when magma has high
silica content. Magma has a high viscosity. It is thick
and sticky and can build up pressure because it does not
flow easily.
• Trapped gases build pressure until they explode!!
• Lava breaks into several size fragments as it cools
quickly and hardens: volcanic ash (small as dust),
Cinders (pebble sized), Bombs (size of a baseball to
the size of a car)
• Pyroclastic Flow: when an explosive eruption hurls out a
mixture of gases, ash, cinders, and bombs.
• Geologist use terms to describe a volcano’s stage of
activity: active, dormant, extinct.
• Active: is currently erupting or shows signs it may erupt
in the future.
• Dormant: is not erupting but may in the future
• Extinct: dead, unlikely to erupt again
• Time between volcanic eruptions can be hundreds to
thousands of years.
Stages of Volcanic
• Within the last 150 years, major volcanic eruptions have
greatly affected the land and people around them.
• Geologists today can more successfully predict a volcanic
eruption than an earthquake.
• Tiltmeters and other tools monitor surface changes in
• Geologists monitor escaping gases, temperature increases
in the ground or water, and small earthquakes around the
volcano. Magma moving toward the surface triggers
these quakes.