Frequency and Quantity
 There are ~ 550 historically active volcanoes
 About 60 erupt annually
 The majority are 1) at the edge of continents and 2)
underwater
Dormant volcanic
cone in Japan
What causes volcanoes?
Underwater Volcanoes
Rift valley
plate
plate
magma
Mid ocean ridges
Mid ocean ridge
Mid ocean ridges
 Are continuous underwater mountain ranges
 Are formed by rising magma
 Cause sea floor spreading
 Form a rift valley between spreading plates
Pillow lava at mid
ocean ridge
Hot Spots
 Hot spots create chains of islands like the Hawaiian
islands or the Aleutian chain.
Hawaiian Islands – hot
spot
Land Based Volcanoes
A volcano at the edge of a
continent
Mid ocean ridge
Sinking plate
?
Melted rock
forming magma
Three Types of Land Based
Volcanoes
Volcanoes take different shapes depending on the
amount of SILICA and DISSOLVED GASSES in the magma
 High Silica content…
 High Dissolved Gas content…
 More likely to be
explosive!!!
 More gas comes out, an
eruption can launch ash and
debris great distances
 Why?
 Silica has a thick, stiff
consistency
 Flows slowly
 Tends to Harden in the
volcano’s vent
 However, if lava is viscous and
flows quickly, these eruptions
may not be that dramatic
What Erupts from a
Volcano?
Blocky lava
Lava can be
thick or thin.
Pahoehoe
Aa
Pillow lava
What Erupts from a Volcano?
Pyroclastic material
• Rock fragments created by
eruptions
•magma explodes from volcano and solidifies in the air
•existing rock is shattered by powerful eruptions
Lapilli
Volcanic bombs
Volcanic blocks
Volcanic ash
Let’s see it in action
 http://environment.nationalgeographic.com/environ
ment/natural-disasters/forces-ofnature.html?section=t
Shield Volcanoes
 Volcanoes that build up from
many slow, steady, flows of hot lava, are called shield
volcanoes. This kind of volcano is low and broad
with gently sloping sides. They look like a warriors
shield.
 They have LOW silica content and LOW dissolved
gases.
 Unlikely to yield massive explostions
Shield Volcanoes
Cinder Cone Volcano
 Cinder cone volcanoes form when solid rock and ash
shoot up into the air and fall back around the
volcano opening. The cinder cone volcano has
steeply sloped sides.
 Cinder cone volcanoes have LOW silica content and
HIGH dissolved gas content
 Cinder cones can have violent eruptions, but are
unlikely to send ash or lava too far
Cinder Cone
Composite Volcano
 Composite volcanoes, erupt with molten lava, solid
rock, and ash. The layers pile up much like layers of
cake and frosting. The layers form into symmetrical
cones, and the slopes are steep.
 Composite volcanoes have HIGH silica content and
HIGH dissolved gas.
 Composite volcanoes erupt violently and can send
ash and lava great distances
Composite Volcano
Composite Volcano
Mt St Helens one day before eruption
Mt St Helens 4 months after
eruption
Another before…
…and 2 years later
Now, it is your turn. Pick a
type of volcano and build
it out of clay. Then, we are
going to make
TOPOGRAPHIC MAPS.