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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 __________________.
• 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.
– Due to the effects of pressure, most of the
rocks in Earth’s lower crust and upper
mantle do not melt to form magma.
Water
– At any given pressure, a wet mineral or rock
will melt at a_________ temperature than the
same mineral or rock under dry conditions.
Magma
Types of Magma
• The three major types of magma are: _________
magma, __________ magma, and ____________
magma.
– ______________ is the internal resistance to flow;
the higher the viscosity, the thicker the magma.
Magma
Types of Magma
Magma
Viscosity
• The viscosity of magma and lava depends on both
_______________ and
– _____________________.
The hotter the magma or lava, the ______ the viscosity.
– Magmas and lavas high in silica have ____________
viscosities than magmas and lavas low in silica.
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.
B.
C. .
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.
• ________ 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
Batholiths and Stocks
– _____________, the largest plutons, are irregularly
shaped masses of coarse-grained igneous rocks
covering at least 100 km2 and take millions of years to
form.
– ________ are irregularly shaped plutons that are similar
to batholiths but smaller in size.
Intrusive Activity
Plutons
Laccoliths
– A __________ 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.
Intrusive Activity
Plutons
Sills and Dikes
– A ______ forms when magma intrudes parallel to
layers of rock.
– A ________ cuts across preexisting rocks.
Intrusive Activity
Plutons
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 ______ 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 ________ 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.
• __________ 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
Types of Volcanoes
• The appearance of a volcano depends on
two factors:
1.
2.
• Based on these two criteria, three major types of
volcanoes have been identified:
1.
2.
3.
Volcanoes
Types of Volcanoes
Shield Volcanoes
– A _____________________ is a mountain with broad,
gently sloping sides and a nearly circular base that
forms when layer upon layer of lava accumulates
during ________________________ eruptions.
Volcanoes
Types of Volcanoes
Cinder-Cone Volcanoes
– A __________________________is a generally small,
steep-sided volcano that forms when material ejected
high into the air, in a more explosive eruption, falls back
to Earth and piles up around the vent.
Volcanoes
Types of Volcanoes
Composite Volcanoes
– ___________________are large volcanoes that form
when layers of volcanic fragments alternate with lava.
The eruption of these volcanoes are violently explosive.
Volcanoes
Volcanic Material
• __________ are rock fragments thrown into the
air during a volcanic eruption.
• Tephra are classified by size:
1.
2.
3.
4.
5.
Volcanoes
Volcanic Material
Pyroclastic Flows
– A ______________________ 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 ________________
can exceed __________ .
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.
– Therefore, MOST volcanoes occur where?
______________________________________
Volcanoes
Where do volcanoes occur?
Convergent Volcanism
– The volcanoes associated with convergent plate
boundaries form two major belts:
• The larger belt is called the Pacific Ring of Fire.
• The smaller belt is called the Mediterranean Belt.
Volcanoes
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.
– ______________are unusually hot regions of Earth’s
mantle where high-temperature surges of mantle
material rise toward the surface.
– A surge 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 _______________
continue to rise above
the ocean floor as the
Pacific Plate moves
slowly over a hot spot.