Table of Contents
Chapter: Earth’s Internal
Processes
Section 1: Evolution of Earth’s Crust
Section 2: Earthquakes
Section 3: Earth’s Interior
Section 4: Volcanoes
Evolution of Earth’s Crust
1
Continental Drift
• In 1915, Alfred Wegener (VEG nur)
proposed a hypothesis that suggested
that Earth’s continents once were part
of a large super-continent, Pangaea.
• About 200 million years ago, the supercontinent broke into pieces that drifted
over the surface of Earth like rafts on
water.
Evolution of Earth’s Crust
1
Matching Coastlines
• The most apparent match of continents is
the eastern coastline of South America
with the western coastline of Africa.
• Wegener argued that you could match
rock types, fossils, erosion features,
and mountain ranges.
• If you found similar formations and
structures on each continent then the
continents could have been joined
together in that place.
Evolution of Earth’s Crust
1
Matching Fossils
• Large land
animals
provided
better
evidence
because
they could
not have
crossed
oceans.
Evolution of Earth’s Crust
1
Matching Rocks and Mountains
• Mountain ranges were
shown to be continuous
in Pangaea.
• Once Pangaea broke
apart, the mountain
ranges became separated.
• Wegener was able to show
that continents that were
joined shared unique rocks
and minerals.
Evolution of Earth’s Crust
1
Matching Rocks and Mountains
• Wegener’s hypothesis was not accepted
by his contemporaries because he was
unable to conceive of a force or
mechanism that could drive continents
apart.
Evolution of Earth’s Crust
1
Seafloor Spreading Hypothesis
• Dr. Harry Hess used sonar, intended to
detect submarines, to obtain accurate maps
of the seafloor.
• A mid-ocean
ridge system,
or MOR, was
continuous and
wrapped
around Earth.
Evolution of Earth’s Crust
1
Seafloor Spreading Hypothesis
• Hess proposed a hypothesis of seafloor
spreading, or divergence.
• Magma from the mantle is forced upward
because of its low density.
• This causes the crust to crack (fault) and
move apart.
• The faulting causes twin mountain ranges
with a down-dropped rift valley between.
Evolution of Earth’s Crust
1
Ages of Sediment and Rocks
• When the ages
of rocks are
measured, the
continental rocks
are billions of
years old, while
seafloor rocks
are less than 200
million years of
age.
Evolution of Earth’s Crust
1
Magnetic Polarity of Rocks
• Studies show that Earth’s magnetic field
repeatedly reverses itself.
• Vine, Matthews, Wilson, et al discovered bands
of reversed polarity in the seafloor rocks.
• As magma crystals form, they take on the
polarity of Earth at the time they form.
• The pattern is identical on both sides of the
MOR.
Evolution of Earth’s Crust
1
Theory of Plate Tectonics
• This system consists of about a dozen
major plates and many minor ones.
• Plates are composed of a rigid layer of
uppermost mantle and a layer of either
oceanic or continental crust above.
Evolution of Earth’s Crust
1
Divergent Plate Boundaries
• At a mid-ocean ridge
(MOR), magma rises
along a faulted rift
valley, spreads, and
cools to form new
oceanic crust. This
spreading apart is what
happens at divergent
boundaries.
Evolution of Earth’s Crust
1
Divergent Plate Boundaries
• A MOR represents
divergence that is
well-developed.
• Divergent boundaries
exist as rift valleys,
where no mature
ocean basins exist
yet.
Evolution of Earth’s Crust
1
Convergent Plate Boundaries
• Where plates
collide, they
come together to
form convergent
boundaries.
• Less-dense, thick
continental
lithosphere moves
toward denser thin
oceanic lithosphere.
Evolution of Earth’s Crust
1
Convergent Plate Boundaries
• The ocean side is
forced downward
beneath the
continental slab in
a process called
subduction.
• The region of
collision also has a
deep-sea trench that
parallels the zone.
Evolution of Earth’s Crust
1
Convergent Plate Boundaries
• Convergent plate boundaries also exist
between two slabs of oceanic lithosphere.
In this case, the
oceanic lithosphere
that is colder, and
therefore denser,
subducts.
• Magma erupted here
produces chains of
volcanic islands
called island arcs.
Evolution of Earth’s Crust
1
Convergent Plate Boundaries
• Two continental slabs of low density collide
and tend not to subduct.
• The plates collide and
buckle upward to form
a high range of folded
mountains.
• Volcanic activity is
noticeably absent and
there is no trench.
Evolution of Earth’s Crust
1
Transform Plate Boundaries
• No new lithosphere is
forming, as along a
divergent boundary.
• Old lithosphere is not
being recycled, as along
a subduction zone.
• The main result of
transform boundaries
is horizontal motion of
lithosphere.
Evolution of Earth’s Crust
1
What drives the plates?
• Research indicates that plates are driven by
a combination of forces. One such force is
ridge push at a mid-ocean ridge.
• Divergent boundaries are higher at the center
of the ridge, gravity forces material down the
slopes of the MOR.
Evolution of Earth’s Crust
1
What drives the plates?
• When a plate subducts back into Earth at
some convergent boundaries, the process
of slab pull is thought to operate.
• Portions of descending plates are pulling
the rest of a plate down with them.
Evolution of Earth’s Crust
1
Thermal Energy
• Internal convection of mantle material is
the driving force for all mechanisms of
plate motion.
• The main source of thermal energy comes
from the decay of radioactive elements in
Earth.
Section Check
1
Question 1
__________ is the hypothesis that continents
have slowly moved to their current locations.
A. Continental drift
B. Mid-ocean shifting
C. Pangaea
D. Seafloor spreading
Section Check
1
Answer
The answer is A. Continental drift is the theory
that the continents have slowly moved.
Seafloor spreading is a process that would help
explain how the continental drift might occur.
Section Check
1
Question 2
Who proposed the hypothesis of continental
drift?
A. Esker
B. Gagarin
C. Hess
D. Wegener
Section Check
1
Answer
The answer is D. Wegener proposed the
hypothesis of continental drift. Hess theorized
that the seafloor is spreading.
Section Check
1
Question 3
What is Pangaea?
Section Check
1
Answer
Pangaea means “all land”
and is the name that
Wegener used to refer to
the one large landmass
that he believed existed
before it broke apart into
continents.
Earthquakes
2
Global Earthquake Distribution
• Earthquakes occur in well-defined zones.
• These zones coincide with the edges of
lithospheric plates.
Earthquakes
2
Depth of Focus
• Divergent boundaries are associated
plates that move in opposite directions.
• This faulting creates a narrow band of
numerous, shallow earthquakes.
Earthquakes
2
Depth of Focus
• Convergent boundaries have broad zones of
earthquakes with the shallowest foci near
the surface at the point of convergence, and
the deepest foci located under volcanoes or
mountains created in the collision area.
Earthquakes
2
Causes of Earthquakes
• An earthquake is any seismic vibration of
Earth caused by the rapid release of energy.
Deformation
• A strain is deformation in response
to a stress.
Earthquakes
2
Deformation
• Stress is the force per unit area that acts
on a material.
(1) compressive stress
(2) a tension stress
(3) a shear stress
(4) torsion stress
Earthquakes
2
Elastic Deformation
• Elastic deformation occurs when a material
deforms as a stress is applied, but returns to
its original shape when the stress is removed.
• Plastic deformation occurs when a material
deforms, or changes shape, as a stress is
applied and remains in the new shape when
the stress is released.
Earthquakes
2
Energy Release
• When this strain energy is released suddenly,
it causes rock to lurch to a new position.
• A fault is a crack along which movement
has taken place.
• The sudden energy release that goes with
fault movement is called elastic rebound.
Earthquakes
2
Earthquake Waves
• Earthquake waves
travel out in all
directions from a point
where strain energy is
released. This point is
the focus.
• The point on Earth’s
surface directly above
the focus is the
epicenter.
Earthquakes
2
Body Waves
• Primary waves, also called P-waves, cause
particles in a material to undergo a push-pull
type motion.
• The particles do not permanently change
location.
• Particles can bump into each other, then
primary waves can move through it.
• P-waves travel through all kinds of matter.
Earthquakes
2
Body Waves
• Secondary waves (S-waves) are sometimes
called shear waves, because of the relative
motion of particles as energy is transferred.
• S-waves cause particles to move
perpendicular to the direction of wave travel.
• S-waves can only travel through solids.
Earthquakes
2
Surface Waves
• Surface waves move in a more complex
manner.
• They can exhibit an up and down rolling
motion, and also a side-to-side motion that
parallels Earth’s surface.
Earthquakes
2
Surface Waves
Earthquakes
2
Earthquake Measurement
• The Modified
Mercalli scale ranks
earthquakes in a
range from I-XII,
XII being the worst
and uses eyewitness
observation and postearthquake
assessments to assign
an intensity value.
Earthquakes
2
Earthquake Measurement
• The Richter magnitude scale uses the
amplitude of the
largest
earthquake wave.
• Richter magnitude
is intended to give
a measure of the
energy released
during the
earthquake.
Earthquakes
2
Earthquake Measurement
• The table shows
the global
frequency of
different
magnitude
earthquakes.
Earthquakes
2
Levels of Destruction
• Research has shown that poor building
methods are the largest contributors to
earthquake damage and loss of life.
Earthquake Proofing
• Although no building can be made entirely
earthquake proof, scientists and engineers are
finding ways to reduce the damage to
structures during mild or moderate
earthquakes.
Section Check
2
Question 1
Which of the following is NOT a type of stress
in rock?
A. compression
B. epicenter
C. shearing
D. tension
Section Check
2
Answer
The answer is B. The epicenter is the point on
Earth’s surface located directly above the
earthquake’s center.
Section Check
2
Question 2
Where do P- and S-waves occur in relation to
surface waves?
Answer
Seismic waves travel away from the epicenter in
all directions. P-waves travel the fastest through
rock material. S-waves move through the rock
and cause particles to vibrate. Both P- and Swaves travel through the Earth’s interior while
surface waves move along Earth’s surface.
Section Check
2
Question 3
Why is it difficult to predict earthquakes?
Answer
Geologists can monitor changes in Earth that
are associated with earthquakes. Measuring
devices have been developed to assess changes
in groundwater level and rock layers; however,
no single change in Earth occurs for all
earthquakes.
Earth’s Interior
3
Earthquake Observations
• A boundary that marks a density change
between layers is called a discontinuity.
Earth’s Interior
3
Earthquake Observations
• One such discontinuity separates the crust
from uppermost mantle, and is known as the
Mohorovicic
(moh huh ROH
vee chihch)
discontinuity,
or Moho.
Earth’s Interior
3
Shadow Zones
• P-waves and S-waves travel through Earth
for 105 degrees of arc in all directions.
• Between 105 and 140 degrees from the
epicenter, nothing is recorded.
• This “dead zone” is termed the shadow
zone.
Earth’s Interior
3
Shadow Zones
Earth’s Interior
3
Solid Inner Core
• The fact that P-waves pass through the
core, but are refracted along the way,
indicates that the inner core is denser
than the outer core and solid.
• When pressure dominates, atoms are
squeezed together tightly and exist in
the solid state.
• If temperatures are high enough, atoms
move apart enough to exist in the liquid
state, even at extreme pressures.
Earth’s Interior
3
Composition of Earth’s Layers
• The crust and uppermost mantle, which
together form the lithosphere, are made
of rocky material—mostly silicates.
• The asthenosphere is a weaker, plasticlike
layer upon which Earth’s lithospheric
plates move.
• Mantle below the asthenosphere also
is composed of silicates.
• The cores are made mostly of metallic
material.
Section Check
3
Question 1
What is Earth’s core made of?
Answer
Earth’s core is primarily made of metallic
material such as iron and nickel.
Section Check
3
Question 2
Earth’s internal layers become _______
with depth.
A. cooler
B. darker
C. denser
D. lighter
Section Check
3
Answer
The answer is C.
Section Check
3
Question 3
What can’t S-waves penetrate the liquid
outer core?
Answer
S-wave only travel through solids. This
suggests that the outer core is in a liquid state.
Volcanoes
4
Origin of Magma
• Hot, nearly molten rock in Earth’s
asthenosphere can
change to a liquid
by decompression
melting.
• A buoyant force
acts on magma
that forms from
rock surrounding
it.
Volcanoes
4
Origin of Magma
• Rising magma
may reach
Earth’s surface
if pressure
conditions allow
and the rock has
conduits through
which it can
flow.
Volcanoes
4
Eruptive Products
Solids
• All solid materials expelled by a volcano
are collectively called pyroclasts.
Gases
• Volcanoes release a broad variety of
superheated gases, the most common
of which is water vapor.
• In addition carbon dioxide and gases
composed of sulfur compounds are
expelled.
Volcanoes
4
Liquids
• Lavas can vary considerably in
composition, which in turn affects
their physical properties.
• Viscosity is a measure of the resistance
of a fluid to flow.
Volcanoes
4
Eruptive Styles
• Eruptive style is strongly linked to
temperature and composition and can
be linked to the type of plate boundary
associated with it.
Volcanoes
4
Plate Boundary Setting
• Most of Earth’s volcanoes lie in subduction
zones where continental and oceanic materials
are being mixed and partially melted.
Volcanoes
4
Hot Spots
• Hot spots are volcanically active sites
that arise in places where large quantities
of magma move to the surface in large,
column-like plumes.
• A hot spot under an oceanic plate forms
volcanic island chains, such as the
Hawaiian islands.
• Yellowstone National Park is an example
of a hot spot under a continental plate.
Volcanoes
4
Types of Volcanoes
• Volcanoes are classified according to their
size, shape, and the materials that compose
them.
Cinder Cone Volcanoes
• When the primary eruptive products are
large fragments of solid material, cinder
cone volcanoes form.
Volcanoes
4
Shield Volcanoes
• Shield volcanoes erupt with abundant
lava flows that can move for kilometers
over Earth’s surface before stopping.
• Shield volcanoes are broad, flat structures
made up of layer upon layer of lava.
Volcanoes
4
Composite Volcanoes
• Volcanoes formed from alternating explosive
events that produce pyroclastic materials, and
lava flows are called composite volcanoes.
Section Check
4
Question 1
Where do most volcanoes occur?
Answer
Most volcanoes occur at plate boundaries where
huge pieces of the crust pull apart or push
together. As a result, the crust often fractures,
allowing magma to reach the surface.
Section Check
4
Question 2
What type of volcano is formed by an explosive
eruption followed by a quiet eruption?
A. cinder cone volcano
B. composite volcano
C. fissure eruption
D. shield volcano
Section Check
4
Answer
The correct answer is B. Composite volcanoes
erupt explosively releasing large quantities of
gas and ash. They are followed by quieter
eruptions that form a lava layer over the ash.
Section Check
4
Question 3
How does a hot spot volcano form?
Answer
A volcano forms above a hot spot when magma
erupts through the crust and reaches the surface.
Hot spot volcanoes may lie in the middle of
plates far from any plate boundaries or near or
on plate boundaries.
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