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Chapter 9 Earth Science
VOLCANOES
Volcanic Eruptions
 Volcanoes – are areas of Earth’s surface
through which magma and volcanic gases
pass.
 Explosion of a volcanic eruption can turn an entire
mountain into a billowing cloud of ash and rock in
a matter of seconds.
 Help form fertile land.
 Create some of the largest mountains on
earth.
Volcanic Eruptions
 Magma – melted rock below the Earth’s
surface.
 Lava – Melted rock above the Earth’s surface.
Mt. Cleveland- Aleutian
Islands, Alaska
Stromboli Volcano - Sicily
Mount Redoubt - Alaska
Nonexplosive Eruptions
 Flows of lava are calm.
 Releases huge amounts of lava.
 Vast areas of the Earth’s surface, are covered with
lava from non-explosive eruptions such as Hawaii.
Types of Lava Flows
 Pahoehoe lava flows – sheet like lava flows
that result in continuous surfaces.
 Flow smoothly
 Move forwards in tongues or lobes and are
characterized by a glassy, plastic skin.
Pahoehoe Lava Flow
Types of Lava Flows
 Aa – lava pours out quickly and forms a brittle
crust.
 The crust is torn into jagged pieces as molten lava
continues to flow underneath.
 Got its name because of the painful
experience of walking barefoot across the
jagged surfaces.
 Associated with lava fountaining.
Aa Lava Flow
Lava Fountaining
 Lava Fountaining – spraying of lava into the
air pulsing with the pressure of escaping
gases.
Types of Lava Flows
 Pillow lava – forms when lava erupts
underwater.
 Lava forms rounded lumps that are the shape of
pillows.
Types of Lava
 Blocky Lava – cool, stiff lava that does not
travel far from the erupting vent.
 Normally forms jumbled heaps of sharp-edged
chunks.
 Get it’s shape from oozing from a volcano.
Blocky Lava
What is Inside a Volcano?
Water and Magma Are an
Explosive Combination
 If the water content of magma is high, an
explosive eruption is more likely.
 Underground the water and magma are under
extreme pressures.
 When water and magma rise to the top the gases
expand rapidly, an explosion can result.
 Sometime so rapid a water floating version of lava
called Pumice is formed.
Silica-Rich Magma Traps
Explosive Gases
 Magma that has a high silica content will tend
to cause explosive eruptions.
 Silica-rich magma has a stiff consistency.
 Flows slowly and hardens in a volcano’s vents
plugging the vents.
Gas Trapped in a Volcano
 The more that is trapped in a volcano the
more explosive it will be due to the rapid rate
in which the gas escapes from the volcano.
What erupts from a Volcano?
 Pyroclastic material - forms when magma is
blasted into the air and hardens.
 Volcano’s eruptions may alternate between lava
and pyroclastic eruptions.
Volcanic Bombs
 Volcanic Bombs – large blobs of magma that
harden in the air.
 The shape of the bomb was caused by the magma
spinning through the air as it cooled.
Lapilli
 Lapilli – pebblelike bits of magma that
hardened before they hit the ground.
 Means “little stones”
Volcanic Ash
 Volcanic Ash – forms when the gases in stiff
magma expand rapidly and the walls of the
gas bubbles explode into tiny, glasslike
silvers.
 Makes up most of the pyroclastic material in an
eruption.
Volcanic Ash
Pyroclastic Flows
 Pyroclastic Flow – produced when enormous
amounts of hot ash, dust, and gases are
ejected from a volcano.
 Can move downhill at 200km/h.
 Center of the flow can exceed 700 C.