Three Types of
Volcanoes

 1. List and describe the three types of
volcanoes.

EXPLOSIVE HAZARDS
VISCOUS LAVA (High Viscosity)
 Cool temperature
 Composition:
silica-rich (granitic)
 Thick & gooey → Erupt violently; scattering
ash and fragments
widely
 Does not flow very far;
builds steep-sides;
often destroys volcanoes
Ex. rhyolithic & andesitic
lava→ stratovolcanoes

NON-EXPLOSIVE HAZARD
FLUID LAVA (Low viscosity)
 Higher temperatures
 Composition: Low silica
(basaltic)
 Thin → Erupt “quietly”
 Great flows of lava that
build mountains
Ex. Basaltic lava→ shield volcanoes

MAGMA/LAVA →ERUPTIONS
Non-Explosive Eruptions:
Fluid lava flows easily
allows gases to bubble
away
Explosive Eruptions:
Viscous lava traps the gases
until large pressures build
up & the system explodes
Pyroclastic flow (ash, rock
fragments) flow out of vent

EXPLOSIVE HAZARDS
PYROCLASTIC FLOW
Travels over 200 mph
Tephra = all ash & rock
fragments
– ash: pieces smaller than 2 mm;
travel farthest
– lapelli: small pieces between 2-66
mm
– volcanic bomb: pieces larger than
64 mm
Burns EVERYTHING in its path

ERUPTION HAZARDS
LAHAR
Water, mud &
ash that flow
like a river

Shield Volcanoes
• The magma inside a shield volcano is rich in iron
and magnesium and is very fluid.
• Since the magma is very fluid, the lava coming
out of the volcano tends to flow great distances.
• When shield volcanoes erupt, the flowing lava
gives the volcano the shape of a gently sloping
mountain.

Shield Volcanoes
 Eruptions of shield volcanoes are mild and
can occur several times.
 Mauna Loa in Hawaii is an example of a
shield volcano.

Shield Volcanoes
 Low silica level
 Low viscosity Lava
 High or low levels of gas
 Low to medium explosivity
 Flattened mound
 Resembles a warrior’s shield

Mauna Loa, Hawaii

Piton de la Fournaise

Surtsey, Finland

Composite Volcanoes
• The magma inside a composite volcano is rich in
silica and much thicker than magma from a
shield volcano.
• Gases get trapped inside this thicker magma.
• Eruptions from composite volcanoes can be
flowing lava or explosions. The explosive
eruptions come from the trapped gases and
produce cinders and ash.

Composite Volcanoes
• These different types of eruptions are what give
composite volcanoes their alternating layers of
lava and cinders.
• Composite volcanoes have much steeper slopes
than shield volcanoes.
• Mount Fuji in Japan and Mount St. Helens in the
USA are examples of composite volcanoes.

Composite Volcanoes
 High in silica
 High viscosity magma
 High levels of gas
 Highly explosive
 Cone shaped
 Formed by layers of lava flow and ash
buildup

Mt. Rainier, Washington

Mt. Fuji, Japan

Mt. St. Helens, Washington

Cinder Cone Volcanoes
• The magma inside a cinder cone volcano has
large amounts of gas trapped in it.
• Eruptions from cinder cone volcanoes are
violent and explosive because of all the gas
trapped in the magma.
• The large amounts of hot ash and lava thrown
out of the vent fall to the ground forming the
cone shape that these volcanoes have.

Cinder Cone Volcanoes
• Cinder cone volcanoes are usually only
active for a short time and then become
dormant (inactive).
• Paricutin in Mexico is an example of a
cinder cone volcano.

Cinder cones
 Low silica lava
 High levels of gas
 “Fire-fountain” eruptions
 Commonly found on the flanks of shield
volcanoes
 Made from a pile of rock pieces
 Structurally weak

Pu'u ka Pele, Hawaii
(on the flanks of Mauna Loa)

Floreana Island, Galapagos

Puu OO, Hawaii

 2. Explain how volcanoes relate to plate
tectonics.

WHAT KIND OF LAVA FLOW?
Viscous lava
• thick (granitic – high silica
content)
• traps gas
• violent eruptions
• destroys mountains
Fluid lava
• thin ; flows far;
• allows gas to escape;
• quiet(non-violent) eruptions
• builds mountain
Fluid lava
Flows
great
distances

FORMATION →ERUPTIONS
Volcanoes are formed by
1.SUBDUCTION
explosive eruptions
2.Sea Floor Spreading
quiet eruptions
3.Hot Spots
usually quiet eruptions

VOLCANO FORMATION:
HOT SPOTS
 A fixed source of magma
rising beneath a plate
forming volcanic islands
 Magma can be basaltic or
granitic –so eruptions can
be explosive or “quiet”