Magma
Objectives
• Describe factors that affect the formation of magma.
• Compare and contrast the different types of magma.
Vocabulary
– viscosity
Magma
Magma
• The ash that spews from some volcanoes can
form billowy clouds that travel around the world
before raining back down to Earth.
• In the last 10 000 years, more than 1500
different volcanoes have erupted, providing
evidence that Earth is indeed geologically active.
Magma
How Magma Forms
• Magma is a mixture of molten rock, suspended
mineral grains, and dissolved gases that fuels all
volcanoes.
• Magma forms when temperatures are high
enough to melt the rocks involved, usually
between 800°C and 1200°C.
• Such temperatures exist at the base of the
lithosphere and in the asthenosphere.
Magma
How Magma Forms
Pressure
– Pressure, which increases with depth, is one factor that
determines whether rocks will melt to form magma.
– As pressure increases,
the temperature at
which a substance
melts also increases.
– Due to the effects of
pressure, most of the
rocks in Earth’s lower
crust and upper
mantle do not melt to
form magma.
Magma
How Magma Forms
Water
– The presence of water also influences whether a
rock will melt.
– At any given pressure,
a wet mineral or rock
will melt at a lower
temperature than the
same mineral or rock
under dry conditions.
Magma
Types of Magma
• The three major types of magma are: basaltic
magma, andesitic magma, and rhyolitic magma.
– Basaltic magma has the same composition as basalt
and fuels the volcanoes that make up the Hawaiian
Islands and Surtsey, which is south of Iceland.
– Andesitic magma has the same composition as
andesite and fuels Mount St. Helens in Washington
State and Tambora in Indonesia.
– Rhyolitic magma has the same composition as granite
and fueled the dormant volcanoes in Yellowstone
National Park.
Magma
Types of Magma
Magma
Types of Magma
Magma Composition
– A number of factors determine the composition
of magma.
– Viscosity is the internal resistance to flow; the higher
the viscosity, the thicker the magma.
Magma
Types of Magma
Basaltic Magma
– Basaltic magma typically forms when rocks in the
upper mantle melt.
– Most basaltic magma rises relatively rapidly to Earth’s
surface and reacts very little with crustal rocks because
of its low viscosity.
– The volcanoes fueled by basaltic magma erupt
relatively quietly.
Magma
Types of Magma
Andesitic Magma
– Andesitic magma is found along continental margins,
where oceanic crust is subducted into Earth’s mantle,
and is formed from oceanic crust or oceanic sediments.
– Andesitic magma contains about 60 percent silica,
resulting in an intermediate viscosity.
– The volcanoes it fuels are said to have intermediate
eruptions.
Magma
Types of Magma
Rhyolitic Magma
– Rhyolitic magma forms when molten material rises
and mixes with the overlying silica- and water-rich
continental crust.
– Rhyolitic magma has high viscosity and fuels very
explosive volcanoes.
Magma
Viscosity
• The viscosity of magma and lava depends on both
temperature and composition.
– The hotter the magma or lava, the lower the viscosity.
– Magmas and lavas high in silica have higher
viscosities than magmas and lavas low in silica.
Magma
Section Assessment
1. Match the magma types with their characteristics.
___
B basaltic
___
A andesitic
___
C rhyolitic
A. intermediate viscosity
content, forms from oceanic
crust and oceanic sediments
B. low viscosity and gas content,
forms from rocks in the upper
mantle
C. high viscosity, forms from
continental crust materials
Magma
Section Assessment
2. What would be the likely effect if the volcano at
Yellowstone National Park were to erupt? Why?
It would most likely be a devastating eruption
because it would be fueled by rhyolitic magma,
which has a very high viscosity and gas content.
Magma
Section Assessment
3. Identify whether the following statements are
true or false.
______
true It is unlikely that Mount Kilauea in Hawaii will
explosively erupt.
______
true Wet granite will melt at a lower temperature than
dry granite.
______
false A liquid with a high viscosity will also have a
high flow rate.
______
true Major eruptions of Mount St. Helens in
Washington state and Mount Fuji in Japan
would probably be similar in nature.
Intrusive Activity
Objectives
• Explain how magma affects overlying crustal rocks.
• Compare and contrast intrusive igneous rock bodies.
Vocabulary
– pluton
– batholith
– stock
– laccolith
– sill
– dike
Intrusive Activity
Intrusive Activity
• Magma, because it is molten, is less dense than
surrounding rocks.
• This density difference forces magma to move
upward and eventually come into contact with, or
intrude into, the overlying crust.
Intrusive Activity
Intrusive Activity
• Intruding magma can affect the crust in
several ways:
A. Magma can force the overlying rock apart and enter
the newly formed fissures.
B. Magma can also cause
blocks of rock to
break off and sink
into the magma, where
the rocks may
eventually melt.
C. Magma can melt the
rock into which it
intrudes.
Intrusive Activity
Plutons
• When magma cools, minerals form and over a
very long period of time; they will combine to form
intrusive igneous rock bodies.
• Plutons are intrusive igneous rock bodies that
can be exposed at Earth’s surface as a result of
uplift and erosion and are classified based on their
size, shape, and relationship to surrounding rocks.
Intrusive Activity
Plutons
Intrusive Activity
Plutons
Batholiths and Stocks
– Batholiths, the largest plutons, are irregularly shaped
masses of coarse-grained igneous rocks covering at
least 100 km2 and take millions of years to form.
– Stocks are irregularly shaped plutons that are similar to
batholiths but smaller in size.
– Both batholiths and stocks cut across older rocks and
generally form 10–30 km beneath Earth’s surface.
Intrusive Activity
Plutons
Laccoliths
– A laccolith is a mushroom-shaped pluton with a round
top and flat bottom resulting from a Magma intrusion
into parallel rock layers close to Earth’s surface.
– Compared to batholiths and stocks, laccoliths are
relatively small; they are, at most, up to 16 km wide.
Intrusive Activity
Plutons
Sills and Dikes
– A sill is a pluton, ranging from only a few centimeters
to hundreds of meters in thickness, that forms when
magma intrudes parallel to layers of rock.
– A dike is a pluton, ranging from a few centimeters to
several meters wide and up to tens of kilometers long,
that cuts across preexisting rocks.
– While the textures of sills and dikes vary, many are
coarse grained, which indicates they formed deep in
Earth’s crust and cooled slowly.
Intrusive Activity
Plutons and Tectonics
• Many plutons, especially batholiths, are formed as
the result of mountain-building processes.
• Batholiths are thought to have formed as a result
of continental-continental and oceanic-oceanic
convergence.
• The plutons that form deep beneath Earth’s
surface represent the majority of igneous activity
on our planet.
Intrusive Activity
Section Assessment
1. Match the following terms with their definitions.
___
D batholith
___
B stock
___
E laccolith
___
A sill
___
C dike
A. a pluton that forms when
magma intrudes parallel to
layers of rock
B. an irregularly shaped pluton
that is similar to a batholith but
smaller in size
C. a pluton that cuts across
preexisting rocks
D. an irregularly shaped pluton
that covers at least 100 km2
E. a mushroom-shaped pluton
with a round top and flat bottom
Intrusive Activity
Section Assessment
2. How do sills and laccoliths differ?
Sills and laccoliths both result from magma
intrusions that are parallel to existing rock.
Laccoliths push the overlying layers upward,
creating a distinct mushroom-shape when they
cool and solidify. Sills are generally thinner and
do not cause a noticeable bump in the surface.
Intrusive Activity
Section Assessment
3. What surface feature are batholiths most
associated with?
Batholiths are found at the cores of many of
Earth’s mountain ranges.
Volcanoes
Objectives
• Describe the major parts of a volcano.
• Compare and contrast shield, cinder-cone, and
composite volcanoes.
• Contrast the volcanism that occurs at plate boundaries.
• Explain the relationship between volcanism and hot spots.
Vocabulary
– cinder-cone volcano
– vent
– composite volcano
– crater
– tephra
– caldera
– pyroclastic flow
– shield volcano
– hot spot
Volcanoes
Anatomy of a Volcano
• When magma reaches Earth’s surface it is
called lava.
• A vent is where lava erupts through an opening
in the crust.
• As lava flows out onto the surface, it cools and
solidifies around the vent, eventually accumulating
to form a mountain known as a volcano.
• A crater is a bowl-shaped depression at the top
of a volcano that is connected to the magma
chamber by a vent.
Volcanoes
Anatomy of a Volcano
• Volcanic craters are usually less than 1 km
in diameter.
• Calderas are large depressions up to 50 km in
diameter that can form when the summit or the
side of a volcano collapses into the magma
chamber that once fueled the volcano.
Volcanoes
Anatomy of a Volcano
Volcanoes
Types of Volcanoes
• The appearance of a volcano depends on
two factors:
– The type of material that forms the volcano
– The type of eruptions that occur
• Based on these two criteria, three major types of
volcanoes have been identified:
– Shield volcanoes
– Cinder-cone volcanoes
– Composite volcanoes
Volcanoes
Types of Volcanoes
Shield Volcanoes
– A shield volcano is a mountain with broad, gently
sloping sides and a nearly circular base that forms
when layer upon layer of basaltic lava accumulates
during nonexplosive eruptions.
Volcanoes
Types of Volcanoes
Cinder-Cone Volcanoes
– A cinder-cone volcano is a generally small, steepsided volcano that forms when material ejected high into
the air falls back to Earth and piles up around the vent.
– The magma that fuels these volcanoes contains more
water and silica than shield volcanoes, which makes
them more explosive in nature.
Volcanoes
Types of Volcanoes
Composite Volcanoes
– Composite volcanoes are large volcanoes that form
when layers of volcanic fragments alternate with lava.
– The magma that forms composite volcanoes commonly
contains large amounts of silica, water, and gases,
making these volcanoes violently explosive.
Volcanoes
Types of Volcanoes
Sizes and Shape
– Shield volcanoes are by far
the largest, and cinder-cone
volcanoes are the smallest.
– Cinder-cone volcanoes have
the steepest slopes, while
shield volcanoes have the
gentlest slopes.
– The slopes of cinder-cone
and composite volcanoes
are concave, and the slopes
of shield volcanoes
are straight.
Volcanoes
Volcanic Material
• Tephra are rock fragments thrown into the air
during a volcanic eruption.
• Tephra are classified by size, the smallest being
dust (less than 0.25 mm) and ash (0.25–2 mm).
• Somewhat larger fragments are called
lapilli, or “little stones” (2–64 mm in diameter).
• The largest tephra thrown from a volcano include
angular volcanic blocks and rounded or
streamlined volcanic bombs, both of which can be
the size of a house or larger.
Volcanoes
Volcanic Material
Pyroclastic Flows
– Some tephra cause tremendous damage and kill
thousands of people.
– A pyroclastic flow is a cloud of volcanic gas, dust, and
other tephra traveling at speeds of nearly 200 km/h.
– The temperature at the center of a pyroclastic flow can
exceed 700°C.
Volcanoes
Where do volcanoes occur?
• Most volcanoes form at plate boundaries.
– About 80 percent of all volcanoes are found along
convergent boundaries.
– About 15 percent are found along divergent
boundaries.
– Only about 5 percent of extrusive igneous activity
occurs far from plate boundaries.
Volcanoes
Where do volcanoes occur?
Convergent Volcanism
– Convergence involving oceanic plates creates
subduction zones, and the magma generated is forced
upward through the overlying plate and forms volcanoes
when it reaches the surface.
– The volcanoes associated with convergent plate
boundaries form two major belts:
• The larger belt, the Circum-Pacific Belt, is also called
the Pacific Ring of Fire.
• The smaller belt is called the Mediterranean Belt.
Volcanoes
Where do volcanoes occur?
Volcanoes
Where do volcanoes occur?
Divergent Volcanism
– At divergent plate boundaries, magma is forced
upward into the fractures and faults that form as the
plates separate.
– These areas of major faults and fractures are called
rift zones.
– Most of the world’s rift volcanism occurs under water
along ocean ridges.
Volcanoes
Where do volcanoes occur?
Hot Spots
– Some volcanoes are located far from plate
boundaries and form as the result of hot spots.
– Hot spots are unusually hot regions of Earth’s mantle
where high-temperature plumes of mantle material
rise toward the surface.
– A plume does not move laterally, which results in a
trail of progressively older volcanoes that formed as a
plate moved over a hot spot.
Volcanoes
Where do volcanoes occur?
Hot Spots
– The Hawaiian Islands
continue to rise above
the ocean floor as the
Pacific Plate moves
slowly over a hot spot.
– The rate and direction
of plate motion can be
calculated from the
positions of volcanoes
in a chain that has
formed over a hot spot.
Volcanoes
Where do volcanoes occur?
Hot Spots
– Hot spots can result in the formation of flood basalts.
– Flood basalts erupt from fissures rather than a
central vent and form flat plains or plateaus.
– Volcanic activity is proof that Earth is a
dynamic planet.
Volcanoes
Section Assessment
1. Match the following terms with their definitions.
___
A vent
___
C crater
___
D tephra
___
E pyroclastic flow
___
B caldera
A. an opening in the crust from
which lava flows
B. depression caused by a
collapsed magma chamber
C. a bowl-shaped depression
around an opening in the crust
D. volcanic materials that are
thrown into the air during a
volcanic eruption
E. a cloud of rapidly moving,
extremely hot volcanic material
Volcanoes
Section Assessment
2. How can chains of volcanoes that form over a
hot spot track plate movement?
The hot spot is in a fixed location. All of the
volcanoes in the chain were over the hot spot
when they formed. The volcanoes’ movement
and the direction of the chain’s alignment
indicates the movement of the plate.
Volcanoes
Section Assessment
3. Identify whether the following statements are
true or false.
______
true Many cinder-cone volcanoes are less
than 500 m high.
______
false Volcanism is more common along divergent
boundaries as compared to convergent
boundaries.
______
false The Mediterranean Belt is also known as the
“Ring of Fire”.
______
true It can be 700ºC in the center of a pyroclastic flow.
Chapter Resources Menu
Study Guide
Section 18.1
Section 18.2
Section 18.3
Chapter Assessment
Image Bank
Section 18.1 Study Guide
Section 18.1 Main Ideas
• Temperature, pressure, and the presence of water are
factors that affect the formation of magma.
• As pressure increases, the temperature at which a
substance melts also increases. At any given pressure,
the presence of water will cause a substance to melt at a
lower temperature than the same substance under dry
conditions.
• There are three major types of magma: basaltic magma,
andesitic magma, and rhyolitic magma. These magmas
differ in the source rock from which they form, viscosity,
silica content, gas content, and explosiveness. Basaltic
magma is the least explosive magma; rhyolitic magma is
the most explosive.
Section 18.2 Study Guide
Section 18.2 Main Ideas
• Magmatic intrusions affect the crust in several ways.
Magma can force overlying rock apart and enter the newly
formed fissures. Magma can also cause blocks of rock to
break off and sink into the magma chamber. Magma can
melt the rock into which it intrudes.
• Batholiths, stocks, sills, dikes, and laccoliths are plutons
that are classified according to their size, shape, and
relationship to surrounding rocks. Batholiths are the
largest plutons and often form the cores of many of
Earth’s major mountain chains.
Section 18.3 Study Guide
Section 18.3 Main Ideas
• Lava flows onto Earth’s surface through a vent. Over time,
multiple lava flows may accumulate to form a volcano. A
crater is a depression that forms around the vent at the
summit of a volcano. A caldera is a large crater that forms
when a volcano collapses during or after an eruption.
• There are three types of volcanoes: shield volcanoes,
cinder-cone volcanoes, and composite volcanoes.
• Rock fragments ejected during eruptions are called tephra.
Section 18.3 Study Guide
Section 18.3 Main Ideas
• Most volcanoes form along convergent and divergent
plate boundaries. Volcanoes also form over hot spots,
which are unusually hot areas in the mantle that are
stationary for long periods of time.
• Flood basalts form when lava flows from fissures to form
flat plains or plateaus.
Chapter Assessment
Multiple Choice
1. Andesitic magma has a silica content of
about ___ percent.
a. 50
c. 70
b. 60
d. 80
Basaltic magma has a silica content of about 50%.
Rhyolitic magma has a silica content of about 70%.
Chapter Assessment
Multiple Choice
2. Which of the following is the largest type
of pluton?
a. stock
c. batholith
b. laccolith
d. sill
Batholith plutons are common in the interiors of many
mountain chains and are the result of the magma
generated during subduction at convergence zones.
Chapter Assessment
Multiple Choice
3. Shield cone volcanoes are fueled by what kind
of magma?
a. basaltic
c. tephra
b. andesitic
d. rhyolitic
Basaltic magma has a low viscosity and erupts with low
explosiveness. This allows the lava to flow down the
mountain’s slopes and continually add to the mountain.
Chapter Assessment
Multiple Choice
4. Where is divergent volcanism generally found?
a. ocean rifts
c. ocean trench
b. mid-plate
d. Abyssal plains
Divergent volcanism is almost entirely found underwater
(Iceland is the major exception) at ocean rifts. Through
divergent volcanism, new sea floor is created as the
plates spread apart.
Chapter Assessment
Multiple Choice
5. Where are most active volcanoes located?
a. divergent
boundaries
c. hot spots
b. convergent
boundaries
d. transform
boundaries
Most active volcanoes are in the either the Circum-Pacific
or Mediterranean Volcanic Belts, which are located on
convergent boundaries.
Chapter Assessment
Short Answer
6. How does the presence of water influence
whether a rock will melt?
At any given pressure, a wet mineral or rock
will melt at a lower temperature than the
same mineral or rock under dry conditions.
Chapter Assessment
Short Answer
7. What is the relationship between viscosity
and the temperature of magma or lava?
The hotter the magma or lava, the lower
the viscosity.
Chapter Assessment
True or False
8. Identify whether the following statements are
true or false.
______
false Andesitic magma has more explosive potential
than rhyolitic magma.
______
false A dike forms when magma intrudes parallel to
layers of rock.
______
true Cinder-cone volcanoes can form on the flanks of
a shield volcano.
______
true Many of the volcanoes in the Cascade range
are composite volcanoes.
______
true The hot spot under Hawaii has existed for at
least 75 million years.
Image Bank
Chapter 18 Images
Image Bank
Chapter 18 Images
Image Bank
Chapter 18 Images
To navigate within this Interactive Chalkboard product:
Click the Forward button to go to the next slide.
Click the Previous button to return to the previous slide.
Click the Chapter Resources button to go to the Chapter Resources
slide where you can access resources such as assessment questions
that are available for the chapter.
Click the Menu button to close the chapter presentation and return to
the Main Menu. If you opened the chapter presentation directly without
using the Main Menu this will exit the presentation. You also may press
the Escape key [Esc] to exit and return to the Main Menu.
Click the Help button to access this screen.
Click the Earth Science Online button to access the Web page
associated with the particular chapter with which you are working.
Click the Speaker button to hear the vocabulary term and definition
when available.
End of Custom Shows
This slide is intentionally blank.