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Magma
• The ash that 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.
http://science.howstuffworks.com/nature/natural-disasters/volcano3.htm
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.
How Magma Forms
Pressure
 Pressure (increases with depth) is one factor that
determines whether rocks will melt to form magma.
 As pressure increases,
the temperature
a substance
melts also increases.
 Most of the
rocks in Earth’s
lower crust and
upper mantle do not
melt to form magma.
Water
– Water also
influences
whether a
rock will melt.
– A wet mineral
or rock will
melt at a
lower
temperature
than the same
under dry
conditions.
Types of Magma
http://library.thinkquest.org/TQ0311
160/vollava.htm
– Basaltic magma - same composition as basalt and
fuels the volcanoes that make up the Hawaiian Islands
and Surtsey (Iceland).
– Andesitic magma - same composition as andesite
and fuels Mount St. Helens in Washington State and
Tambora in Indonesia.
– Rhyolitic magma - same composition as granite and
fueled the dormant volcanoes in Yellowstone National
Park.
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.
Basaltic Magma (magnetic)
– Basaltic magma forms when rocks in the upper
mantle melt.
– Most basaltic magma rises rapidly to Earth’s
surface and reacts very little because of its low
viscosity.
– The volcanoes erupt relatively quietly.
Types of Magma
Andesitic Magma
– Andesitic magma is found along continental
margins - oceanic subduction into Earth’s
mantle, and is formed from oceanic crust or
oceanic sediments.
– Andesitic magma contains about 60 percent
silica (intermediate viscosity).
– The volcanoes - intermediate eruptions.
Types of Magma
Rhyolitic Magma
– Rhyolitic magma - molten material rises
and mixes with silica- and water-rich continental crust.
– Rhyolitic magma - high viscosity and very explosive
volcanoes.
Types of 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 - high in silica have higher viscosities .
Magma
1. Match the magma types with their
characteristics.
A. intermediate viscosity
B basaltic
___
A andesitic
___
___
C rhyolitic
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
Section Assessment
Magma
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.
Section Assessment
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.
Intruding magma can affect the crust:
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 are (cooled) 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.
– Batholiths, the largest plutons, are irregularly
shaped masses of coarse-grained igneous rocks
and take millions of years to form.
– Stocks are irregularly shaped plutons that are
similar to batholiths but smaller in size.
– Both cut across older rocks and generally form 10–
30 km beneath Earth’s surface.
– 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; at most, up to 16 km wide.
Plutons
– 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.
Plutons
Batholiths – form from continental-continental and
oceanic-oceanic convergence.
The plutons that form deep beneath Earth’s surface
represent the majority of igneous activity on our
planet.
Plutons and Tectonics
Plutons are formed from mountain-building.
1. Match the following terms with their
definitions.
A. a pluton that forms when
D batholith
___
___
B stock
___
E laccolith
___
A sill
___
C dike
Section
Assessment
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
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.
Section Assessment
3. What surface feature are batholiths most
associated with?
Batholiths are found at the cores of many of
Earth’s mountain ranges.
Section Assessment
Anatomy of a Volcano
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 to form a
mountain known as a volcano.
A crater is a bowl-shaped
depression, connected to the
magma chamber by a vent,
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
Types of Volcanoes
Volcano depends on 2 factors:
– The type of material that forms the volcano
– The type of eruptions that occur
3 major types of volcanoes have been identified:
 Shield volcanoes
 Cinder-cone volcanoes
 Composite volcanoes
Volcanoes
Sizes and Shape
– Shield volcanoes – largest
but shield volcanoes have
the gentlest slopes.
– Cinder-cone – smallest, but steepest slopes
– Cinder-cone and composite
volcanoes are concave, and
shield volcanoes are straight.
Types of Volcanoes
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.
Types of Volcanoes
Volcanoes
– A cinder-cone volcano - small, steep-sided forms
when material ejected high into the air falls back to
Earth and piles up around the vent.
– The magma contains more water and silica than
shield volcanoes, more explosive in nature.
Types of Volcanoes
Volcanoes
– Composite volcanoes are large volcanoes that
form with layers of volcanic fragments alternate with
lava.
– The magma contains large amounts of silica, water,
and gases, making these volcanoes violently
explosive.
Types of Volcanoes
Tephra rock fragments from 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.
Volcanic Material

http://videos.howstuffworks.com/discovery/
7153-volcanoes-pyroclastic-flow-video.htm
– 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.
Volcanic Material
Where do volcanoes occur?
– About 80 % convergent boundaries.
– About 15 % divergent boundaries.
– Only about 5 % 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
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.
Where do volcanoes occur?
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,(sideways)
resulting in a trail of older volcanoes that
formed as a plate moved over a hot spot.
Volcanoes
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.
Where do
volcanoes occur?
Volcanoes
Hot Spots
– Hot spots can result in the formation of flood
basalts.
– Flood basalts erupt from fissures (cracks)
rather than a central vent and form flat plains
or plateaus.
– Volcanic activity is proof that Earth is a
dynamic planet.
Where do volcanoes occur?
Volcanoes
1. Match the following terms with their
definitions.
A. an opening in the crust from
A
___ vent
C
___ crater
D tephra
___
___
E pyroclastic flow
___
B caldera
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
Section Assessment
Volcanoes
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.
Section Assessment
Volcanoes
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.
Section Assessment