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Volcanoes
Forces Inside the Earth
What is a volcano?
• A volcano is a weak spot in
the crust where molten
material, or magma, comes
to the surface.
owhen magma reaches the
surface, it is called lava
oAfter lava has cooled, it
forms solid rock, building
up Earth’s surface
Plate
Boundaries
• The Ring of Fire is
formed by many
volcanoes that rim the
Pacific Ocean.
• Why do volcanic
belts form along
Earth’s boundaries?
• Huge pieces of crust are diverging or converging, often
fractures in the crust allow magma to reach the
surface.
Divergent and Convergent
• form along mid-ocean
ridges.
• form along rift valley
• Form where oceanic and
continental crust meet or
where two oceanic plate
collide.
– An island arc is formed by
volcanoes along a deep ocean
trench
– Can also
form under
continents
Hot Spots
• An area where material from deep within the
mantle rises and then melts, forming magma.
Properties of Magma
Physical Properties
• Any characteristic of a
substance that can be
observed or measured
without changing the
composition of the
substance.
• Density, hardness,
melting point, boiling
point, whether it is
magnetic
Chemical Properties
• Any property that
produces a change in
the composition of
matter.
• Ability to burn,
combine or react with
other substances
Viscosity of Magma
• Viscosity is the resistance of a liquid to flow.
– The greater the viscosity, the slower the flow
• ex. honey
– Liquids have different viscosities because of the
particles that make them up.
• Viscosity of magma depends on the silica content
and temperature
– Silica is made of silicon and oxygen
• more silica, the higher viscosity, slow moving, cools to
rhyolite
• less silica, the lower the viscosity, faster moving, cools
to basalt
TemperatureRanges from 750° C to 1175° C
pahoehoe
•
•
•
•
Fast moving
Hot
Low viscosity
Solid mass of wrinkles
aa
•
•
•
•
Slow moving
Cooler
High viscosity
Rough surface, jagged lava
chunks
Magma Reaches the Surface
• Inside a volcano
– Magma chamber- a pocket where the magma collects
under the volcano
– Pipe- a long tube in the ground that connects the
magma chamber to the Earth’s surface
– Vent- the opening where molten rock and gas leave the
volcano
– Lava flow- area covered by lava as it pours out of the
vent
– Crater- bowl-shaped area that may form at the top of a
volcano around the central vent
A Volcanic Eruption
• What pushes magma to the surface?
– The force of the expanding gases push magma from the
magma chamber through the pipe until it flows or
explodes out of the vent.
• Quiet eruptions
– low-silica, magma bubbles out gently, oozes quietly from the
vent
• Explosive Eruptions
– high-silica, builds up pressure and explodes
– breaks lava into fragments that cool and harden into pieces of
different sizes; from volcanic ash to pebble sized to larger pieces
called bombs
– Pyroclastic flow occurs when an explosive eruption hurls out a
mixture of hot gases, ash, cinders, and bombs
» Obsidian and pumice forms
Mount St Helens
Life Cycle of a Volcano
• Geologist use the terms active, dormant,
or extinct
– Active or live volcano is erupting or showing
signs of erupting in the near future
– Dormant volcano is like a sleeping bear, to be
awakened in the future to become active
– Extinct is a dead volcano unlikely to erupt again
Volcanic eruptions create landforms
made of lava, ash, and other materials.
• Shield volcano- form when
thin layers of lava pour out
of a vent and harden on
top of previous layers
building a wide, gently
sloping mountain.
• Cinder cone volcano- high
viscosity lava with ash,
cinders, and bombs build
up around the vent in a
steep, cone-shaped hill or
small mountain.
• Composite volcano- tall,
cone shaped mountains in
which layers of lava
alternate with layers of ash
formed by lava flow
alternating with explosive
eruptions of ash, cinder,
and bombs.
• Lava plateaus-instead of
forming mountains, some
eruptions of lava form high
level area.
– Thin, runny lava travels far
before it cools and solidifies.
– This happens again and
again and after millions of
years, plateaus form.
Caldera- a huge hole left
by a volcanic mountain
Which is Which?
Why would anyone want to live near
an active volcano?
• To take advantage of the fertile volcanic soil.
– When the volcanic ash breaks down, it releases
potassium, phosphorus, and other substances
plants need to grow.
• This creates some of the richest soils in the world.
Features formed by magma include
volcanic necks, dikes, sill, batholiths,
and dome mountains.
• A volcanic neck forms when • Magma that forces itself into
magma hardens around a
rock layer and hardens
volcanic pipe.
becomes a
– Dike, slanted though bedrock
– Sill, horizontal through bedrock
Batholiths-a mass of rock
formed when a large body
of magma cools inside the
crust.
Dome Mountains- forms
when uplift pushes a
batholith toward the
surface.
Geothermal Activity
• “geo” means EARTH
• “therme” means HEAT
• hot spring- a natural pool of water heated by
a nearby body of magma
• geysers- a fountain of water and steam that
erupts from the ground as pressure builds