Download What Is a Volcano?

Section 1: Volcanoes and Plate Tectonics
What Is a Volcano?

Volcano – a weak spot in the earth’s crust
magma
where molten material (
), comes
to the surface
 Magma – molten mixture of rock-forming
substances, gases, and water from the
mantle

Lava – when magma reaches the surface it is
Lava
called
 Volcanic activity is a
constructive force that adds
new rock to existing land and forms new
islands
Where Are Volcanoes Located?
 There are about 600 active volcanoes
on land and more beneath the sea
Ring of fire – one major
volcanic belt where volcanoes rim the
pacific ocean
 Three major locations:
Diverging plate boundaries
Converging plate
boundaries
Hot spots
How Do Volcanoes Form at
Diverging Plate Boundaries?

away
The plates move
from each other and
Lava pours out of
cracks in the ocean floor
 Ex. mid-ocean ridge
In Iceland and the Azores
Islands the volcanoes of the
mid-ocean ridge rise above
the ocean’s surface
How Do Volcanoes Form at Hot
Spots?
Hot spot – an area where
magma from deep within the
mantle melts through the crust like
a blow torch
 These are often in the

middle of a plate
Locations: Hawaii,
Yellowstone national park in
Wyoming
How Do Volcanoes Form at
Convergent Plate Boundaries?

At
subduction zones:
Slabs of oceanic crust sink though a deep ocean
trench into the mantle
The crust melts forming magma
Magma rises back towards surface and erupts as lava

Many volcanoes occur on islands but can occur
on land where a continental plate collides with a
oceanic plate
Island arc – string of islands arcing
in the shape of the deep ocean trench
Major island arcs:
Japan
Indonesia
Section 2: Properties of Magma
Where does Magma come from?

Lava begins as liquid magma in
the mantle

Liquid magma is less dense than
the surrounding solid material, this
causes magma to flow upward into
any cracks in the rock above
 Magma will continue to travel upward
until it reaches the surface or is
trapped beneath layers of rocks
What is magma made up of?
 Elements and Compounds
 Each substance has a particular set of physical
and chemical properties that can be used to
identify a substance or to predict how it will
behave
Elements – substances that cannot be broken down
in other substances
Compound - a substance made of two or more
elements that have been chemically combined
What are physical and chemical
properties?
 Physical Property – any characteristic of a
substance that can be observed or measured
without changing the composition of the
substance Ex. Boiling point
 Chemical Properties – any property that
produces a change in the composition of matter.
Ex. Silver jewelry darkening when exposed to air
What causes some liquids to flow
more easily than others?
 Viscosity – the resistance of a liquid to flowing
 The greater the viscosity of a liquid, the slower
it flows. Ex. Honey
 The lower the viscosity of a liquid, the more
easily a liquid flows. Ex. water
Why do liquids differ in viscosity?
 In some liquids, there is a greater degree of
friction among the liquid’s particles – these
liquids have higher viscosity
What factors determine the
viscosity of magma?

Silica content
Silica – made up of particles of the elements
oxygen and silicon; most abundant
materials in Earth’s crust
Silica content of magma ranges from about
50% to 70%
The more silica magma contains the higher
its viscosity; ex. Rhyolite
The less silica magma contains the lower
its viscosity; ex. Basalt

Temperature
Viscosity increases as temperature
decreases; ex. Cold honey versus warm
honey
Pahoehoe – fast-moving hot lava that has a
low viscosity
Aa – slow-moving cooler lava that has a
higher viscosity
Section 3: Volcanic Eruptions
What happens as a volcano erupts?

As magma rises toward the
surface, the pressure
decreases allowing the
dissolved gases to separate out
forming bubbles

During a volcanic eruption, the
gases dissolved in magma
rush out, carrying the magma with
them
What does the inside of a volcano
look like?
 Magma chamber – the pocket beneath a volcano where
magma collects
 Pipe – a long tube through which magma moves from the
magma chamber to the Earth’s surface

Vent – an opening where magma and gas leave the volcano

Lava flow – area covered by lava as it pours out of a vent

Crater – a bowl-shaped area that may form as the top of a volcano
around the vent
Is all magma the same?
 No, some types of magma are thick and
flow very slowly. Other types are
fluid and flow almost as easily as
water.
 Temperature and the amount of
silica account for the differences

Silica – material that is formed from the
elements
and silicon
oxygen
 Obsidian – lava cools fast

Pumice – gas bubbles get trapped
leaving
spaces
What types of volcanic eruptions
are there?
 Quiet eruptions –
magma appears to
flow out easily
producing
pahoehoe and
aa lava
 Explosive
eruptions – pressure
inside the volcano
builds possibly
resulting in a
pyroclastic flow
Section 4: Volcanic Landforms
What type of landforms are created
from lava?
Shield Volcanoes
Cinder Cone Volcanoes
Composite Volcanoes
 Lava Plateaus
Calderas
What is a shield volcano?

A wide, gently-sloping
mountain made of layers of
lava and formed by quiet
eruptions
 Shield volcanoes created the
Hawaiian Islands
What is a cinder cone volcano?

A steep, cone-shaped
hill or mountain made of
volcanic ash, cinders,
and bombs piled up
around a volcano’s
opening.
What is a composite volcano?

A tall, cone shaped
mountain in which layers
of lava alternate with
layers of ash and other
volcanic materials.
What is a caldera?

The large hole at the
top of a volcano
formed when the roof
of a volcano’s magma
chamber collapses
Mount St. Helens