Download Volcanoes - 6th Grade Science with Mrs. Harlow

VOLCANOES
THE BIG IDEA


Volcanoes are locations where molten rock
reaches Earth’s surface, and volcanoes can affect
landforms and societies.
WHAT YOU WILL LEARN
Distinguish between nonexplosive 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
pyroclastic material.

VOLCANOS
The explosive pressure of a volcanic eruption can
turn an entire mountain into a billowing cloud of
ash and rock in a matter of seconds.
 But eruptions are also creative forces—they help
form fertile farmland. They also create some of
the largest mountains on Earth.

VOLCANOS
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.
Volcanoes are areas of Earth’s
surface through which magma
and volcanic gases pass.
NONEXPLOSIVE ERUPTIONS
 At
this moment, volcanic eruptions
are occurring around the world—on
the ocean floor and on land.
Nonexplosive eruptions are the most
common type of eruption. These
eruptions produce relatively calm
flows of lava, such as those shown in
Figure 1. Nonexplosive eruptions
can release huge amounts of lava.
EXPLOSIVE ERUPTIONS
Explosive eruptions, such as the one shown in
Figure 2, are much rarer than nonexplosive
eruptions.
 However, the effects of explosive eruptions can be
incredibly destructive.
 During an explosive eruption, clouds of hot
debris, ash, and gas rapidly shoot out from a
volcano.

Instead of producing lava flows, explosive
eruptions cause molten rock to be blown into tiny
particles that harden in the air.
 The dust-sized particles, called ash, can reach the
upper atmosphere and can circle the Earth for
years.
 Larger pieces of debris fall closer to the volcano.

WHAT IS INSIDE A VOLCANO?
If you could look inside an erupting volcano, you
would see these features.
 A magma chamber is a body of molten rock
deep underground that feeds a volcano.
 Magma rises from the magma chamber through
cracks in the Earth’s crust to openings called
vents.
 Magma is released from the vents during an
eruption.

WHAT MAKES UP MAGMA?
By comparing the composition of magma from
different eruptions, scientists have made an
important discovery.
 The composition of the 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.

WATER AND MAGMA ARE AN EXPLOSIVE
COMBINATION
 If
the water content of magma is high,
an explosive eruption is more likely.
 Because magma is underground, it is
under intense pressure and water stays
dissolved in the magma.
 If the magma quickly moves to the
surface, the pressure suddenly decreases
and the water and other compounds,
such as carbon dioxide, become gases.
 As the gases expand rapidly, an
explosion can result.
WATER AND EXPLOSIVE VOLCANOES
 This process is similar to what happens
when you shake a can of soda and open it.
 When a can of soda is shaken, the CO2
dissolved in the soda is released and
pressure builds up.
 When the can is opened, the soda shoots
out, just as lava shoots out of a volcano
during an explosive eruption.
 In fact, some lava is so frothy with gas
when it reaches the surface it becomes
solid, called pumice, can float in water!
SILICA-RICH MAGMA TRAPS EXPLOSIVE
GASES
Magma that has a high silica content also tends
to cause explosive eruptions. Silica-rich magma
has a stiff consistency.
 It flows slowly and tends to harden in a volcano’s
vents.
 As a result, it plugs the vent. As more magma
pushes up from below, pressure increases.
 If enough pressure builds up, an explosive
eruption takes place.

SILICA-RICH MAGMA TRAPS EXPLOSIVE
GASES - CONTINUED
Stiff
magma also prevents water
vapor and other gases from
easily escaping.
Gas bubbles trapped in magma
can expand until they explode.
When they explode, the magma
shatters and ash and pumice are
blasted from the vent.
WHAT ERUPTS FROM A VOLCANO?
Magma erupts as either lava or pyroclastic (PIE
roh KLAS tik) material.
 Lava is liquid magma that flows from a volcanic
vent.
 Pyroclastic material forms when magma is
blasted into the air and hardens.
 Nonexplosive eruptions produce mostly lava.
Explosive eruptions produce mostly pyroclastic
material.

TYPES OF LAVA
The viscosity of lava, or how lava flows, varies
greatly.
 To understand viscosity, remember that a
milkshake has high viscosity and a glass of milk
has low viscosity.
 Lava that has high viscosity is stiff.
 Lava that has low viscosity is more fluid.

LAVA
5. Blocky lava and pahoehoe (puh HOY HOY)
have a high viscosity and flow slowly.
 Other types of lava flows, such as aa (AH AH)
and pillow lava, have lower viscosities and flow
more quickly.

TYPES OF PYROCLASTIC MATERIAL
Pyroclastic material forms when magma explodes
from a volcano and solidifies in the air.
 This material also forms when powerful
eruptions shatter existing rock.
 The size of pyroclastic material ranges from
boulders that are the size of houses to tiny
particles that can remain suspended in the
atmosphere for years.
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