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• How do volcanoes form? Where does
vocanism occur near?
Chapter 18: Volcanism
• The locations of volcanoes are mostly
determined by plate tectonics.
• Volcanism: al the processes associated
with the discharge of magma, hot fluids,
and gases.
Most volcanoes form at plate boundaries.
The majority form at convergent boundaries
and divergent boundaries.
Circum Pacific = Ring of Fire
• These
volcanoes
are
characteriz
ed by
explosive
eruptions
and Felsic
lavas (thick
and
chunky).
Convergent Volcanism
USA West Coast Volcanism
These volcanic mountains are the result of subduction between the Pacific
Plate and the North American Plate.
Divergent Volcanism
• Eruptions at divergent
boundaries tend to be
nonexplosive and
Mafic, or low in
silicon.
• Divergent boundaries
account for 2/3 of
earth’s volcanism.
• Ex- Iceland on the
Mid-Atlantic ridge.
Resume 3-8-77
A Hot Spot
• Some volcanoes form
far from plate
boundaries over hot
spots.
• A Hot Spot is an
unusually hot area in
Earth’s mantle where
high-temperature
plumes of mantle
material rise towards
the surface
Hawaiian Hot Spot
• The Hawaiian Islands are located over a
plume of magma.
• The hop spot formed by the magma plume
remained stationary while the Pacific Plate
slowly moved northwest.
• The world’s most active volcano, Kilauea,
is currently located over the hot spot.
Flood basalts
• Formed when lava flows out of long cracks
in Earth’s crust: Cracks called fissures.
• The Columbia River basalts, located in the
northwestern United States, were formed
this way.
• About 65 m.y.a in India, a huge flood of
lava created an enormous plateau called
the Deccan Traps.
• The volume of basalt in the Deccan Traps is
estimated to be about 512, 000 km3.
Resume 12 23 2010
Anatomy of a Volcano
• Lava reaches the
surface by traveling
through a tubelike
structure called a
conduit.
• The lava emerges
through an opening
called a vent
• A Magma chamber
Resume 3-10-11
• Over time, layers of solidified lava can
accumulate to form a mountain known as
a volcano.
• At the top of a volcano, around the vent, is
a bowl-shaped depression called a crater.
• Volcanic craters are usually less than 1 km
in diameter.
• Larger depressions are called calderas.
These can be up to 50 km in diameter or
more: Yellowstone.
Diamond Head: Crater
Types of Volcanoes
Shield Volcano
• A shield volcano is a
mountain with broad,
gently sloping sides
and a nearly circular
base.
• Form when layers of
lava accumulate
during nonexplosive
eruptions and are the
largest type of
volcano.
Cinder Cones
• When eruptions eject
small pieces of
magma into the air,
cinder cones form as
this material, called
tephra, falls back to
earth and piles up
around the vent: ash.
• Cinder comes have
steep sides and are
the smallest type of
volcano.
Composite Volcano or
Stratovolcano
• Formed of layers of
hardened chunks of
lava from violent
eruptions alternating
with layers of lava
that oozed downslope
before solidifying.
• Generally coneshaped with concave
slopes.
Two Major Belts
• The volcanoes associated with convergent plate
boundaries for two major belts.
• 1 the larger belt, the Circum – Pacific Belt, is
also called the Pacific Ring of Fire. The outline
of the belt corresponds to the outline of the
Pacific Plate.
• 2 the smaller is the Mediterranean Belt. Its
general outlines correspond to the boundaries
between the Eurasian, African and Arabian
plates.
Volcanic Dome
• Commonly occurs adjacent to craters of
composite volcanoes.
• Very thick lava, but small
• Can be very explosive: a plug
• Generally forms from side vents off the
main conduit.
A Caldera
• Formed after a major
eruption
• Magma chamber
releases most of its
pressure and magma
• Overhanging ceiling
cant support weight
• Ceiling collapses into
vacant chamber.
Do Now
• Where would you find volcanoes with
explosive eruptions?
• At what plate boundary would you find the
majority of the earth’s volcanism?
• What plate is Hawaii on?
• What is the scientific name for the ring of
fire?
• In your opinion, are flood basalts
volcanoes?
18.2
ERUPTIONS:
Thar she blows!
• OBJECTIVES
• Explain how magma
type influences
volcanic activity.
• Describe the role
pressure and
dissolved gases in
eruptions.
• Recognize
classifications of
material ejected by
eruptions.
COMPOSITION DETERMINES
CHARACTERISTICS
• The composition of magma determines the
characteristics of a volcanic eruption.
• Basaltic: also Mafic: relates to a group of rocks
rich in dark-colored minerals containing
magnesium and iron: generally low silicon
content.
• Felsic or Silicic: relates to a group of rocks low in
dark-colored minerals containing magnesium
and iron: generally High silicon content.
Making Magma
• The activity of the volcano depends on the
composition of the magma.
• Lava from an eruption can be thin and runny
(mafic) or thick and lumpy (felsic).
• The composition of magma determines a
volcano’s explosivity, which is how it erupts and
how its lava flows.
• Understanding the factors that determine the
behavior of magma can aid scientists in
predicting the explosivity of volcanic eruptions.
Temperature
• Depending on their composition, most
rocks begin to melt at temperatures
between 800°C and 1200°C.
• In addition to temperature, pressure and
the presence of water also affect the
formation of magma.
• Temperature keeps magma movable.
Pressure
• Pressure increases with depth because of
the weight of overlying rock.
• As pressure increases, the temperature at
which a substance melts also increases,
which explains why most of the rocks in
Earth’s lower crust and upper mantle do
not melt.
Dissolved Gases
• In general, as the amount of gases in
magma increases, the magma’s
explosivity also increases.
• Important gases in magma include water
vapor, carbon dioxide, sulfur dioxide, and
hydrogen sulfide.
• Minerals in the mantle, such as albite, melt
at high temperatures.
• The presence of dissolved water vapor
lowers the melting temperatures of
minerals, causing mantle material to melt
into magma.
• Hence, although pressure prevents
materials from melting, the presence of
water helps materials to melt.
Composition of Magma
• The physical property that describes a
material’s resistance to flow is called
viscosity.
• Temperature and silica content affect the
viscosity of magma.
• The silica content of magma determines
not only its explosivity and viscosity, but
also which type of volcanic rock it forms as
lava cools.
Types of Magma: Basaltic
Pahoehoe of Hawaii
• When rock in the upper
mantle melts, basaltic
magma typically forms.
• Basaltic magma contains
less than 50% silica
which produces lowviscosity magma.
• The resulting volcano is
characterized by quiet
eruptions (relatively
speaking).
Vesuvius and a Pahoehoe flow of
late 1800”s
Andesitic Magma
• 50% to 60% silica and is found along
oceanic – continental subduction zones.
• The source material for this magma can
be either oceanic crust of oceanic
sediments.
• The Andes Mountains of South America
• The higher silica content results in a
magma that has intermediate viscosity
• Intermediate explosivity.
Rhyolitic Magma
• When molten material rises and mixes
with the overlapping continental crust rich
in silica and water, it forms rhyolitic
magma.
• Contains more than 60% silica: Silicic
• High viscosity, along with the large volume
of gas trapped, makes the volcanoes
fueled by this magma explosive: makes
plugs.
The Eruption
• As magma rises due
to plate tectonics and
hot spots, it mixes
with Earth’s crust.
• This mixing caused
differences in
temperature, silica
content, and gas
content as it reaches
the Earth’s surface.
These
properties of
magma
determine how
volcanoes
erupt.
Explosive Eruptions
• When lava is too viscous to flow freely
from the vent, pressure builds up in the
lava until the volcano explodes, throwing
lava and rock into the air.
• The erupted materials are called tephra.
• This includes Hollywood’s ash.
• But this is not all.
Ejecta
Resume 3-16-11
• Tephra are classified by
size.
• The smallest fragments,
with diameters less than
2mm, are called ash.
• The largest tephra thrown
from a volcano are called
blocks.
• Large explosive eruptions
can disperse tephra over
much of the planet.
• In 1991, the eruption of
Mount Pinatubo in the
Philippines sent so much
ash into the stratosphere
that it lowered global
temperatures for two
years.
Pyroclastic Flows
• Violent volcanic
eruptions can send
clouds of ash and
other tephra down a
slope at speeds of
nearly 200 km/hr.
• Rapidly moving
clouds of tephra
mixed with hot,
suffocating gases are
called pyroclasitc
flows.
Eruption
Column
Eruption Column
• They occur when ash and material from
the volcano is light enough to be
suspended in the air like smoke.
• Once the column is either cooled or more
material is in the air than can be
suspended, the column will collapse.
• This results in the material flowing down
the sides of the volcano like water.
• In 1902, a pyroclastic flow from Mount
Pelee on the island of Martinique in the
Caribbean Sea was so powerful that it
destroyed the entire town of St. Pierre in
only a few Minutes.
• In 79 A.D. the towns of Pompeii and
Herculaneum were covered by many such
flows.
Pompeii revisited after 1800 years
18.3 Intrusive Activity
Resume 3-3-10
18.3 Intrusive Activity
Objectives
• Contrast and compare features formed
from magma the solidifies near the surface
with those that solidify deep underground.
• Classify the different types of intrusive
rock bodies.
• Describe how geologic processes result in
intrusive rock that appear at Earth’s
surface.
Plutons
• Magma that solidifies below ground forms
geologic features different from those formed by
magma that cools at the surface.
Plutons
• Most of Earth’s volcanism happens below the
surface because not all magma emerges at the
surface.
• Before it gets to the surface, rising magma can
interact with crust in several ways
• Magma can force the overlying rock apart
and enter the newly formed fissures
• Magma can cause blocks of rock to break
off and sink into the magma.
• It can melt its way through the rock into
which it intrudes
• Plutons are intrusive igneous rock bodies,
formed through mountain-building
processes and oceanic-oceanic collisions.
• They can be exposed due to uplift and
erosion and are classified based on their
size, shape, and relationship to
surrounding rocks.
Batholiths
• The largest plutons, are irregularly shaped
masses of coarse-grained igneous rocks
that cover at least 100 km2 and take
millions of years to form.
• They are common in the interior of
mountains.
Stock
• Irregularly shaped plutons that are similar
to batholiths but smaller in size are called
stocks.
• Both batholiths and stocks cut across
older rocks and generally form 5 to 30 km
beneath the Earth’s surface.
Laccoliths
• A lens-shaped pluton
with a rounded top
and a flat bottom.
• Compared to
batholiths and stocks,
laccoliths are
relatively small; at
most, they are 16 km
wide.
Sill and Dyke
• A sill forms when magma intrudes parallel
to the layers of rock.
• Because it takes great amounts of force to
lift entire layers of rock, most sills form
relatively close to the surface
• A dyke is a pluton that cuts across
preexisting rocks and often forms when
magma invades cracks is surrounding rock
bodies.
• A volcanic neck occurs when the magma
in a volcano conduit solidifies.
• Dykes are often associated with the
conduit but do not always form the neck.
• Coarse grained texture means they
formed deep: magma cooled slowly for
large grains.
• Fine grained texture formed closer to the
surface with fast crystallization.
Plutons and Tectonics
• Many plutons from as a result of mountain –
building processes.
• In fact, batholiths are found at the cores of many
of Earth’s mountain ranges.
• Scientists think that some of the collisions along
continental – continental convergent plate
boundaries might have forced continental crusts
down into the upper mantle where it melted,
intruding into the overlying rocks and eventurally
cooled to form batholiths
• Plutons are also thought to form as a
result of oceanic plate convergence.
• When an oceanic plate converges with
another plate, water from the subducted
plate causes the overlying mantel to melt.
• Plutons often form when the melted
material rises but does not erupt at the
surface.
Practice Questions
1. What is magma called when it reaches
Earth’s surface?
a. Pluton
b. Caldera
c. Tephra
d. Lava
2. Which type of volcano usually produces
the most violent eruptions?
a. Shield
b. Underwater
c. Cinder cone
d. Composite
3. Where are most of Earth’s active
volcanoes?
a. Circum-Pacific Belt
b. East African Rift
c. Mediterranean Belt
d. Cascade Range
4. Where do volcanic eruptions tend to be
nonexplosive?
a. Over hot spots
b. At divergent boundaries
c. In the upper mantle
d. At convergent boundaries
5. What is a pyroclastic flow?
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6.
a.
b.
c.
d.
Where do most volcanoes form?
Over hot spots
In the Pacific Ocean
At plate boundaries
In mountain ranges
7. Materials erupted explosively from
volcanoes is called…
a. Tephra
b. Rhyolite
c. Andesite
d. Basalt
8. What kind of volcanism occurs from a
subduction zone: oceanic-continental
boundary?
A. Divergent
B. Convergent
C. Hot Spot
D. Flood Basalt
9. What effect does the viscosity of a
volcano’s magma have on how it
erupts?
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10. Which structure connects a volacno’s
crater to its magma chamber?
A. cinder cone
B. Lava tube
C. Vent
D. Conduit
Questions
1.
2.
3.
4.
5.
6.
D. Lava
D. Composite
A. Circum-Pacific Belt
At divergent boundaries
A could of tephra mixed
with hot, suffocating
gases moving rapidly
down the side of a
volcano
C. At plate boundaries
7. A. Tephra
8. B. Convergent
9. Higher viscosity magmas
produce more explosive
eruptions.
10. D. Conduit