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Volcanic Eruptions
Notes
By: Mrs. Hudson
Objectives:
• Distinguish between non-explosive and
explosive volcanic eruptions.
• Identify the features of a volcano.
• Explain how the composition of magma
affects the type of volcanic eruption that will
occur.
• Describe four types of lava and four types of
pryoclastic material.
Vocabulary Words:
1. Volcano: are areas of Earth’s surface through
which magma and volcanic gases pass.
2. Magma: molten rock
3. Lava: molten rock that flows onto the Earth’s
surface
4. Vent: an opening at the surface of the Earth
through which volcanic material passes.
5. Magma chamber: the body of molten rock that
feeds a volcano.
6. Ash : dust sized particles
Volcanic Eruptions
• Explosive pressure of a volcanic eruption can
turn an entire mountain into a billowing cloud
of ash and rock in a matter of seconds.
• Eruptions are also creative forces – they help
form fertile farmland
• During an eruption, molten rock, or magma, is
forced to the Earth’s surface. Magma that
flows onto the Earth’s surface is called lava.
Non-explosive Eruptions
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Most common type of eruption
Produces relatively calm flows of lava
Can release huge amounts of lava
Huge amounts of the sea floor and the
Northwest region of the United States, are
covered with lava from these types of
explosions.
Explosive Eruptions
• Much rarer than non-explosive eruptions
• Incredibly destructive
• Clouds of hot debris, ash, and gas rapidly
shoot out from a volcano
• Causes molten rock to be blown into tiny
particles that harden in the air
• Blast millions of tons of lava and rock
• Can demolish an entire mountainside
Diagram of a Volcano
What is inside a Volcano?
• Magma chamber is a body of molten rock
deep underground that feeds a volcano.
• Vents is where magma rises from the magma
chamber through cracks in the Earth’s crust to
openings
What Makes up Magma?
• The composition of the magma affects how
explosive a volcanic eruption is
• Key to whether an eruption will be explosive
lies in the silica, water, and gas content of
magma.
Water and Magma are an Explosive
Combination
• Water content of magma is high, an explosive
eruption is more likely
• Magma underground, it is under intense pressure
and water stays dissolved in the magma
• Magma quickly moves to the surface, the
pressure suddenly decreases and the water and
other compounds, such as carbon dioxide,
become gases. Gases expand rapidly and
explosion can result.
Silica-Rich Magma Traps Explosive
Gases
• Magma that has a high silica content also
tends to cause explosive eruptions
• Has a stiff consistency
• Flows slowly and tends to harden in a
volcano’s vents (plugs vents)
• The more the magma pushes up from below
the more pressure increases and an explosive
eruption takes place
• Stiff magma also prevents water vapor and
other gases from easily escaping
• Magma that contains less silica has a more
fluid, runnier consistency. Because gases
escape this type of magma more easily,
explosive eruptions are less likely to occur.
What Erupts from a Volcano?
• Magama erupts as either lava or pyroclastic
material
• Pyroclastic material forms when magma is
blasted into the air and hardents
• Non-explosive eruptions produce mostly lava
• Explosive eruptions produce mostly
pyroclastic
• Volcano’s eruptions may alternate between
lava and pyroclastic eruptions
Types of lava
• The viscosity of lava, or how lava flows, varies
qreatly
• Lava that have high viscosity is stiff
• Lava that have low viscosity is more fluid
• Four types: Aa (ah ah), Pahoehoe (puh Hoy
Hoy) Pilow lava, Blocky lava
• Aa: is so named because of the painful
experience of walking barefoot across its
jagged surface. This 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 surface has rounded
wrinkles.
• Pillow lava: forms when lava erupts
underwater. This lava forms rounded lumps
that are shape of pillows.
Blocky lava: is cool, stiff lava that does not
travel far from the erupting vent. Usually
oozes from a volcano and forms jumbled
heaps of sharp-edged chunks
Types of Pyroclastic Material
• Forms when magma explodes from a volcano
and solidifies in the air
• Also forms when powerful eruptions shatter
existing rock
• The size of the material ranges from boulders
that are the size of houses to tiny particles
that can remain suspend in the atmosphere
for years
Pyroclastic Flows
• Dangerous type of volcanic flow
• Produced when enormous amounts of hot
ash, dust, and gases are ejected from a
volcano
• Can race downhill at speeds of more than
200km/h – faster than most hurricane-forces
winds
• Temperature in the center can exceed 700
degrees Celsius
Four types of Pyroclastic Material
Volcanic bombs: are large blobs of magma that
harden in the air. The shape of this bomb was
caused by the magma spinning through the air
as it cooled.
• Volcanic blocks: the largest pieces of
pyroclastic material, are pieces of solid rock
erupted from a volcano
• Lapilli: which means “little stones” in Italian,
are pebble-like 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 silvers.
Makes up most of the pyroclastic material in
an eruption