What causes a Volcano?
Volcanic activity often starts when a
crack develops in the crust. Magma is
then forced toward the surface.
 Repeated eruptions of lava or
pyroclastic materials eventually build a
mountain which is called a volcano

Factors Affecting Eruptions

The primary factors that determine
whether a volcano erupts violently or not
include:
 Magma Composition
 Magma Temperature
 Amount of dissolved gasses in the magma

Viscosity – is a substance’s resistance to flow
 Ex- Syrup is thicker than water
 Magma from a more explosive volcano can be 1000x
more viscous than that of a “quieter” volcano
 Higher Temperature = less viscous
 Lower Temperature = more viscous (hardens) =
mobility decreases
 More Silica in magma = higher viscosity
Dissolved Gases
 Gases are mostly water vapor and CO2
 As magma gets closer to the surface,
the pressure of the magma is reduced,
which allows for the dissolved gases to
be released

Volcanic Material

Lava Flows
 Depends on silica content
○ High silica = harder to flow
○ Low silica = more fluid like
Gases
 Pyroclastic Materials

 Particles that are produced in a volcanic
eruption
 Fragments ejected during an eruption range in
size from extremely fine dust & ash to pieces
that weigh several tons
Types of Volcanoes
Shield Volcanoes
 Cinder Cones
 Composite Cones

Shield Volcanoes
Produced by the build up of fluid basaltic
lava
 Shape = broad & slightly domed
 Have “grown up” and from the sea floor
and have formed islands
 Examples: Hawaiian Islands & Iceland

Cinder Cones
Cinder cones are built from ejected lava
fragments which harden in the air
 Fragments range in size
 Typically a result of gas rich basaltic
magma
 Composed mostly of Pyroclastic
material
 Occasionally releases lava

Shape: simple & smaller; determined by
the steep sided slopes
 Usually the result of a single eruption
that lasts only a few weeks and very
rarely more than a few years
 The eruption only occurs once due to
the fact that the pipe connecting the vent
to the magma chamber hardens

Composite Cones
Large & symmetrical
 Composed of layers of lava &
pyroclastic material
 Viscous lava (travel short distances)
 Beautiful & possibly the most dangerous
 Located in a narrow zone along the
Pacific Ocean “Ring of Fire”

Other Volcanic Landforms

Calderas – a large depression in a
volcano
 Form in 1 of 2 ways: collapse of the top of a
shield volcano OR collapse of the top of a
composite volcano

Necks & Pipes
 In most volcanoes, magma travels through
pipes that connect the magma chamber to
the surface

Lava Plateaus
 The greatest volume of material comes from
fissures
 Basaltic lava flows from the fissures (which
covers a large area)
Resources
http://video.nationalgeographic.com/vide
o/101-videos/volcanoes-101
 http://discoverykids.com/games/volcanoexplorer/

Sources
Google
 Prentice Hall Earth Science Book
 National Geographic
 Discovery
