Volcanic Eruptions
Chapter 9
Section 1
Volcanic Eruptions

Volcanoes – areas of Earth’s surface
through which magma and volcanic gases
pass
– In seconds a volcanic eruption can turn an
entire mountain into a cloud of ash and rock
– Helps form fertile farmland
– Create some of the largest mountains on
earth
Volcanic Eruptions

During an eruption magma is forced to the
Earth’s surface
– Magma – molten rock below the Earth’s
surface
– Lava – magma that flows on the Earth’s
surface
Nonexplosive Eruptions
Most common type of eruptions
 Produce relatively calm flows of lava
 Can release huge amounts of lava
 Vast amounts of the Earth are covered
with lava from non-explosive eruptions

– Ex. The sea floor and the Northwest region of
the US
Nonexplosive Eruptions
Explosive Eruptions
Much rarer than non explosive eruptions
 Effects can be incredibly destructive
 Clouds of hot debris, ash and gas rapidly
shoot out from a volcano
 No lava flows
 Molten rock is blown into tiny particles
that harden in the air

Explosive Eruptions
Ash – dust sized particles of molten rock
that have hardened in the air
 Ash can reach the upper atmosphere and
circle the Earth for years
 Larger pieces fall closer to the volcano
 Explosive eruption can blas millions of
tons of lava and rock from a volcano
 In seconds an explosive eruption can
demolish a mountainside

Explosive Eruptions

In what resembles a
nuclear explosion,
volcanic ash rockets
skyward during the
1990 eruption of
Mount Redoubt in
Alaska
What is Inside a Volcano?



Magma chamber –
body of molten rock
deep underground that
feeds a volcano
Vents – an opening at
the surface of the Earth
through which volcanic
material passes
Magma is released from
vents during an
eruption
What Makes Up Magma?
The composition of magma affects how
explosive a volcanic eruption is.
 The key to whether an eruption will be
explosive lies in the silica, water, and gas
content of the magma

What Makes Up Magma?

Water and Magma are an Explosive
Combination
– If the water content is high an explosive
eruption is more likely
– Underground there is a lot of pressure that
keeps water and other compounds dissolved
in the magma
What Makes Up Magma
– When magma quickly rises the water and CO2
turn into gas which expand quickly
– When the gasses expand an explosion takes
place
– Pumice – some lava is so frothy with gas
when it reaches the surfaces that its solid
form, pumice, can float in water
Silica-Rich Magma Traps Explosive
Gases
Magma that has high silica content tends
to cause explosive eruptions
 Silica-rich magma has a stiff consistency
 Flows slowly and tends to harden in
volcano’s vents
 Plugs the vent
 As more magma pushes up from below
pressure increases

Silica-Rich Magma Traps Explosive
Gases
If enough pressure builds up an explosive
eruption takes place
 Stiff magma prevents water vapor and
other gasses from easily escaping
 Gas bubbles in the magma can expand
until they explode
 When they explode ash and pumice are
blasted from the vent

Silica-Rich Magma Traps Explosive
Gases
Magma with less silica has a more fluid
runnier consistency
 Because gas particles can escape more
easily explosive eruptions are less likely to
occur

What Erupts from a Volcano

Nonexplosive
Eruptions
– Lava – liquid magma
that flows from a
volcanic vent
Over

Explosive Eruptions
– Pyroclastic material –
forms when magma is
blasted into the air
and hardens
years or during the same eruption a volcano’s
eruption may alternate between lava and pyroclastic
eruptions
Types of Lava

High Viscosity
– Lava with high
viscosity is stiff
– Flows slowly
– Ex. Blocky lava and
Pahoehoe

Low Viscosity
– Lava with low viscosity
is more fluid
– Flow more quickly
– Ex. Aa and Pillow Lava
Types of Lava
Aa – lava pours out quickly and forms a
brittle crust. The crust is torn into jagged
pieces as molten lava continues to flow
underneath
 Pahoehoe – lava flows slowly, like wax
dripping from a candle. Its glassy surfaces
has rounded wrinkles

Types of Lava
Pillow Lava – forms when lava erupts
underwater. Forms rounded lumps that
are the shape of pillows
 Blocky Lava – cool, stiff lava that does not
travel far from the erupting vent. Blockly
lava usually oozes from a volcano and
forms jumbled heaps of sharp-edged
chunks

Types of Lava
Types of Pyroclastic Material
Forms when magma explodes from a
volcano and solidifies in the air
 Also forms when powerful eruptions
shatter existing rock
 Size ranges from boulders the size of
houses to tiny particles

Types of Pyroclastic Material

Volcanic Blocks – the largest pieces of
pyroclastic matieral, pieces of solid rock
erupted from a volcano

Volcanic Bombs – large blobs of magma
that harden in the air
Types of Pyroclastic Material

Lapilli – “little stones” pebblelike bits of
magma that hardened before they hit the
ground

Volcanic Ash – forms when the gases in
stiff magma expand rapidly and the walls
of the gas bubbles explode into tiny,
glasslike slivers. Makes up most of the
pyroclastic material in an eruption
Types of Pyroclastic Material
Pyroclastic Flows

Pyroclastic Flow
– Dangerous
– Produced when enormous amounts of hot
ash, dust and gases are ejected from a
volcano
– Can race downhill at more than 200 km/h
– Temperature at center can exceed 700°C
Pyroclastic Flows
1991 eruption of
Mount Pinatubo in the
Phillippines
 Scientists predicted
the eruption and
saved 250,000 people
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