Earth Science 10.2A The Nature of Volcanic Eruptions
The Nature
of Volcanic
Eruptions
Earth Science 10.2 The Nature of Volcanic Eruptions
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On May 18th, 1980, Mount St.
Helens erupted with tremendous
force. The blast blew out the entire
north side of the volcano.
The eruption ejected nearly a cubic
kilometer of ash and other debris.
Earth Science 10.2 The Nature of Volcanic Eruptions
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The primary factors that determine
whether a volcano erupts explosively
or quietly include characteristics of
the magma and the amount of
dissolved gases in the magma.
Magma that has reached the surface
is called lava. Lava cools and hardens
to form solid rock.
Earth Science 10.2 The Nature of Volcanic Eruptions
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Magma’s viscosity , the thickness of
the fluid, affects the type of
eruption that occurs.
Viscosity is a substance’s resistance
to flow.
For example; maple syrup is more
viscous than water; it flows more
slowly when you pour it.
or
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The thicker the fluid, the slower and
more viscous. The grade (5-30w) of a
motor oil measures the viscosity of
the oil, it’s thickness.
Earth Science 10.2 The Nature of Volcanic Eruptions
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Magma from an explosive eruption
may be thousands of times more
viscous than magma that erupts
quietly.
The temperature and chemical
composition determine the magma’s
viscosity.
The affect of temperature on
viscosity is easy to see. If you heat
maple syrup or honey it becomes
more fluid and less viscous.
In the same way, the viscosity of
lava is strongly affected by
temperature. As a lava flow cools, it
becomes more viscous; thickening
until it finally stops moving and
hardens into rock.
Earth Science 10.2 The Nature of Volcanic Eruptions
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The chemical composition of magma
has a more important effect on the
type of eruption.
In general, the more silica in magma,
the greater it’s viscosity (the
thicker it stays when heated)
Because of their high silica content;
rhyolitic lavas are very viscous and
erupt explosively.
Earth Science 10.2 The Nature of Volcanic Eruptions
Dissolved gases:
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During explosive eruptions, the
gasses trapped in magma provide the
force to propel molten rock out of
the vent, an opening to the surface.
These gases are mostly water vapor
and carbon-dioxide.
As magma moves nearer the surface,
the pressure in the upper part of
the magma is greatly reduced.
The reduced pressure allows
dissolved gases to be released
suddenly.
Earth Science 10.2 The Nature of Volcanic Eruptions
Dissolved gases:
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Very fluid basaltic magma allows the
expanding gases to bubble upward
and escape relatively easily.
Therefore, eruptions of fluid
basaltic magma , such as those that
occur in Hawaii, are relatively quiet.
Earth Science 10.2 The Nature of Volcanic Eruptions
Dissolved gases:
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But, highly viscous magma slows and
traps the upward movement of
expanding gases.
The gases are trapped, collect in
pockets that increase in size and
build pressure.
These pockets then explode when
the pressure gets to a critical level
and magma is ejected from the
volcano in an explosive display.
Earth Science 10.2 The Nature of Volcanic Eruptions
Volcanic Material:
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Lava may appear to be the main
material produced by a volcano but
this is not always the case.
Just as often, explosive eruptions
eject huge clouds of broken rock,
lava bombs, fine ash, and dust.
Depending on the type of eruption,
volcanoes may produce lava flows or
eject “pyroclastic” materials or both.
All volcanic eruptions also emit large
amounts of gas.
Earth Science 10.2 The Nature of Volcanic Eruptions
Volcanic Material:
Lava Flows:
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Silica content and temperature
affect the characteristics of lava
flows.
Hot basaltic lavas are usually very
fluid because of their low silica
content.
Flow rates of 10-300 meters per
hour are common.
In contrast, the movement of silicarich rhyolite lava is too slow to be
visible.
Earth Science 10.2 The Nature of Volcanic Eruptions
Lava Flows:
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Temperature differences produce
two types of basaltic lava:
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Pahoehoe lava
Pahoehoe
Aa
Pahoehoe is a hotter, faster-moving,
basaltic lava.
Aa is cooler, slower-moving basaltic lava.
Pahoehoe resembles braids in a twisted
rope where Aa forms a surface of
rough, jagged blocks of sharp, spiny
projections.
Aa lava
Earth Science 10.2 The Nature of Volcanic Eruptions
Gases:
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Magmas contain varied amounts of
dissolved gasses held under pressure in
the molten rock, just as carbon dioxide
is held in a soft drink.
Just like a soft drink, as soon as the
pressure is reduced, the gases begin to
escape.
The gaseous portion of most magmas is
only about 1 to 6 percent of the total
weight.
The percentage of gas may be small,
but the actual quantity of emitted gas
can exceed thousands of tons each
day.
Earth Science 10.2 The Nature of Volcanic Eruptions
Gases:
Samples taken during one Hawaiian
eruption consisted of about
 70 % water vapor
 15% carbon dioxide
 5 % nitrogen
 5% sulfur
 And smaller amounts of chlorine,
hydrogen, argon
Earth Science 10.2 The Nature of Volcanic Eruptions
Pyroclastic Materials:
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Particles produced in volcanic
eruptions are called pyroclastic
materials.
When basaltic lava is extruded,
dissolved gases propel chunks of lava
to great heights.
Some of this ejected material may
land near the vent and build a coneshaped structure.
The wind will carry particles great
distances. Viscous rhyolitic magmas
are highly charged with gases.
As the gases expand, pulverized rock
and lava fragments are blown from the
vent.
Earth Science 10.2 The Nature of Volcanic Eruptions
Pyroclastic Materials:
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The fragments ejected during
eruptions range in size from very
fine dust and volcanic ash to pieces
that weigh several tons.
Particles that range in size from
small beads to walnuts (2-64
millimeters) are called lapilli, or
cinders.
Particles larger than lapilli are called
blocks when they are made of
hardened lava and bombs when they
are ejected as glowing lava.
Computer Lab:
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Use the internet to
research the following
topic and write a
short 3 paragraphs
summarizing the topic.
DONOT COPY CUT
OR PASTE:
“The plaster casts at
right were taken from
the archeological dig
at Pompeii. How were
these made by a
volcanic eruption?”
Earth Science 10.2B Volcanic Eruptions
The Nature of
Volcanic
Eruptions
Part B
Types of Volcanoes:
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Volcanic landforms come in a wide
variety of shapes and sizes. The
three main types of volcanoes are
 Shield volcanoes
 Cinder cones
 Composite cones
(stratovolcano)
Types of Volcanoes:
Anatomy of a Volcano
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Volcanic activity begins when a
fissure, or crack, develops in the
crust as magma is forced toward
the surface.
The gas rich magma rises from the
magma chamber, travels through a
circular pipe, and reaches the
surface at a vent.
Repeated eruptions of lava or
pyroclastic material eventually
build a mountain called a volcano.
Located at the summit of many
steep walled volcanoes is a
depression called a crater.
Types of Volcanoes:
Anatomy of a Volcano:
 The form of the volcano is largely
determined by the composition of
the magma.
 Fluid lavas tend to build broad
structures with gentle slopes while
more viscous, silica-rich lavas
produce cones with steeper slopes.
Types of Volcanoes:
Shield Volcanoes:
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Shield volcanoes are produced
by the accumulation of fluid
basaltic lavas.
Shield volcanoes have the
shape of a broad , slightly
domed structure that
resembles a warrior’s shield.
Most shield volcanoes have
grown up from the deepocean floor to form islands.
Examples of shield volcanoes
include the Hawaiian Islands
and Iceland.
Types of Volcanoes:
Cinder Cones:
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Ejected lava fragments that
harden in the air build a
cinder cone.
The fragments range in size
from fine ash to bombs but
consist of lapilli, or cinders.
Cinder cones are usually a
product of relatively gas-rich
basaltic or rhyolitic magma.
Occasionally cinder cones
produce lava flows.
Types of Volcanoes:
Cinder Cones:
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Cinder cones are usually a
product of a single eruption
that sometimes lasts only a
few weeks and rarely more
than a few years.
Once the eruption ends, the
magma in the pipe connecting
the vent to the magma
chamber solidifies, and the
volcano never erupts again.
Compared to shield volcanoes,
cinder cones are relatively
small.
Types of Volcanoes:
Composite Cones:
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Earth’s most beautiful and
potentially most dangerous
volcanoes are composite cones
or stratovolcanoes.
A composite cone is a large,
nearly symmetrical volcanic
mountain composed of layers
of both lava and pyroclastic
deposits.
Composite cones are the
product of gas-rich magma
having an andesitic
composition.
Types of Volcanoes:
Composite Cones:
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The silica-rich magmas typical
of composite cones generate
thick viscous lava that can
travel only short distances.
Composite cones may
generate the most explosive
eruptions, ejecting huge
amounts of pyroclastic
materials.
Most composite cones are
located in a relatively narrow
zone that rims the Pacific
ocean that we call the “Ring
of Fire”.
Types of Volcanoes:
Composite Cones:
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The Ring of Fire includes the
large cones of the Andes in
South America and the
cascade range of the western
United States and Canada.
The Cascade Range includes
Mount St. Helens, Mount
Rainer, and Mount Shasta.
The most active regions in
the Ring of Fire are located
along volcanic island arcs next
to deep ocean trenches.
Mount Rainier
Types of Volcanoes:
Other Volcanic Landforms:
Volcanic mountains are not the
only landform to result from
volcanic activity
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calderas
volcanic necks
lava plateaus also are
byproducts.
Caldera
Lava plateau
Volcanic neck
Types of Volcanoes:
Calderas:
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A caldera is a depression in a
volcanic mountain.
Most calderas form in one of
two ways
 By the collapse of the top
of a composite volcano
after an explosive eruption
 From the collapse of the
top of a shield volcano
after the magma chamber
is drained
Caldera
Types of Volcanoes:
Calderas:
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Crater Lake in Oregon
occupies a caldera. This
caldera formed about 7000
years ago when a composite
cone, Mount Mazama, erupted
violently.
The eruption of Mount
Mazama partly emptied the
magma chamber. The roof of
the magma chamber then
collapsed, forming a huge
depression which today
contains Crater Lake.
A later eruption produced
Wizard Island, the small
cinder cone now an island in
the lake’s middle.
Crater Lake Oregon
Types of Volcanoes:
Volcanic Necks:
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Another volcanic landform
that provides evidence of
past volcanic activity is the
volcanic neck.
A volcanic neck is a landform
made of magma that
hardened in a volcano’s pipe
and later was exposed when
the volcano eroded away.
When a volcano’s activity
ends, the magma remaining in
the pipe hardens to form
igneous rock.
Devil’s Tower
Types of Volcanoes:
Volcanic Necks:
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Weathering and erosion act
constantly to wear away at
volcanoes.
Cinder cones are easily
eroded because they are
made of loose materials.
But the rock in the volcano’s
pipe is more resistant to
erosion, so it is left standing
above the surrounding land
after most of the cone has
been eroded away.
Devil’s Tower
Types of Volcanoes:
Lava Plateaus:
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If you visited the Colombia
River Gorge in Washington,
you would see huge cliffs
made up of layers of dark,
volcanic rock.
These layers of rock are part
of the Colombia Plateau, a
huge lava plateau that covers
much of Washington, Oregon
and Idaho.
A lava plateau is a volcanic
landform produced by
repeated eruptions of very
fluid, basaltic lava.
Types of Volcanoes:
Lava Plateaus:
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The lava that forms a lava
plateau erupts through long
cracks called fissures.
Instead of building a cone,
the lava spreads out over a
large area.
The Colombia Plateau is
almost 1.6 kilometers thick.
The plateau formed over
hundreds of thousands of
years as a series of lava
flows, some 50 meters thick,
buried the landscape.
Types of Volcanoes:
Volcanic Hazards:
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Throughout history, people
often have settled near
volcanoes because of the rich
volcanic soils surrounding it.
However, a dormant volcano
may become active with no
warning.
Volcano hazards include lava
flows, volcanic ash,
pyroclastic flows, and
mudflows.
Types of Volcanoes:
Volcanic Hazards:
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Lava flows are a major
volcanic hazard. Frequent lava
flows from Mount Kilauea in
Hawaii destroys anything in
it’s path.
Active composite volcanoes,
like those in the Cascade
Range, are among the most
dangerous volcanoes.
A composite volcano can eject
huge quantities of volcanic
ash, burying widespread areas
under thick ash deposits.
Types of Volcanoes:
Volcanic Hazards:
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An explosive eruption can also
release a pyroclastic flow; a
scorching mixture of glowing
volcanic particles and gases
that sweeps rapidly down the
sides of a volcano.
In 1902, a pyroclastic flow
from Mount Pelee on the
island of Martinique killed
29,000 people.
Composite volcanoes may also
produce mudflows called
lahars. A lahar occurs when
water-soaked volcanic ash and
rock slide rapidly downhill.
Ice and snow melted by an
eruption can trigger a lahar.
Computer Lab:
Use the internet to research one of the following
topics and write one paragraph on the subject.
DO NOT COPY CUT OR PASTE:
Choose one of the following and write a paragraph
on how it was formed or influenced by volcanic
activity
 Devils Tower
 Crater Lake
 Colombia River Gorge
 Pompeii Italy, the destruction of. . .