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Lecture Presentation
Chapter 2
Internal Structure of
Earth and Plate
Tectonics
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Learning Objectives
 Understand the basic internal structure and
processes of Earth
 Know the basic ideas behind and evidence for
the theory of plate tectonics
 Understand the mechanisms of plate tectonics
 Understand the relationship of plate tectonics to
natural hazards
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2.1 Internal Structure of Earth
 Earth is layered and dynamic
 Internal structure of Earth
 By composition and density
 By physical properties
Figure 2.2
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Structure of Earth
 Inner core
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Solid
1,300 km (808 mi.) in thickness
High temperature
Composed of iron (90 percent by weight) and other
elements (sulfur, oxygen, and nickel)
 Outer core
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Liquid
2,000 km (1,243 mi.) in thickness
Composition similar to inner core
Density (10.7 g/cm3)
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Structure of Earth
 Mantle
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Solid
3,000 km (1,864 km) in thickness
Composed of iron- and magnesium-rich silicate rocks
Density 4.5 g/cm3
 Crust
 Outer rock layer of Earth
 Density 2.8 g/cm3
 Moho discontinuity
 Separates lighter crustal rocks from more dense mantle
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Lithosphere
 Cool, strong outermost layer of Earth
 Asthenosphere
 Below lithosphere
 Hot, slowly flowing layer of weak rock
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Continents versus Ocean Basins
 Crust is embedded on top of lithosphere
 Ocean crust is less dense than continental crust
 Ocean crust is also thinner
 Ocean crust is young (< 200 million years old)
 Continental crust is older (several billion years old)
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Convection
 Earth’s internal heat causes magma to heat up
and become less dense
 Less dense magma rises
 Cool magma falls
back downward
 Similar to pan of
boiling water
Figure 2.3
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How We Know About Internal Structure
of Earth
 Most of our knowledge of Earth’s structure
comes from seismology
 Study of earthquakes
 Earthquakes cause seismic energy to move
through Earth
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Some waves move through solid, but not liquids
Some waves are reflected
Some waves are refraction
Information on wave movement gives a picture of
inside of Earth
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Figure 2.4
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What We Have Learned About Earth from
Earthquakes
 Where magma is generated in the
asthenosphere
 The existence of slabs of lithosphere that have
apparently sunk deep into the mantle
 The extreme variability of lithospheric thickness,
reflecting its age and history
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Plate Tectonics
 Large-scale geologic processes that deform
Earth’s lithosphere
 Produce landforms such as ocean basins,
continents, and mountains.
 Processes are driven by forces within Earth
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What Is Plate Tectonics?
 Lithosphere is broken into pieces
 Lithospheric plates
 Plates move relative to one another
 Plates are created and destroyed
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Figure 2.5a
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Location of Earthquakes and Volcanoes
Define Plate Boundaries
 Boundaries between lithospheric plates are
geologically active areas
 Plate boundaries are defined by areas of seismic
activity
 Earthquakes and volcanoes are associated with
plate boundaries
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Figure 2.5b
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Seafloor Spreading Is the Mechanism for
Plate Tectonics
 At mid-ocean ridges new crust is added to edges
of lithospheric plates
 Continents are carried along plates
 Crust is destroyed along other plate edges
 Subduction zones
 Earth remains constant, never growing or
shrinking
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Sinking Plates Generate Earthquakes
 Sinking ocean plates are wet and cold
 Plates come in contact with hot asthenosphere
 Plates melt to generate magma
 Magma rises to produce volcanoes
 Earthquakes occur along the path of the
descending plate
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Figure 2.6
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Plate Tectonics Is a Unifying Theory
 Explains a variety of phenomena
 Convection likely drives plate tectonics
Figure 2.8
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Table 2.1
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Figure 2.9
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Rates of Plate Motion
 Plate motion is fast (geologically)
 Plates move of few centimeters per year
 Movement may not be smooth or steady
 Plates can displace by
several meters during
a great earthquake
Figure 2.12
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Figure 2.11
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A Detailed Look at Seafloor Spreading
 Mid-ocean ridges discovered by Harry H. Hess
 Validity of seafloor spreading established by:
 Identification and mapping of oceanic ridges
 Dating of volcanic rocks on the floor of the ocean
 Understanding and mapping of the paleomagnetic
history of ocean basins
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Paleomagnetism
 Earth’s magnetic field can be represented by
dipole
 Forces extend from North to South Poles
 Caused by convection in the outer core
 Magnetic field has permanently magnetized
some surface rocks at the time of their formation
 Iron-bearing minerals orient themselves parallel to
the magnetic field at the critical temperature known
as Curie Point
 Paleomagnetism is the study of magnetism of
such rocks
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Magnetic Reversals
 Volcanic rocks show magnetism in opposite
direction as today
 Earth’s magnetic field has reversed
 Cause is not well known
 Reversals are random
 Occur on average every few thousand years
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Figure 2.13
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Magnetic Stripes
 Geologists towed magnetometers along ocean
floor
 Instruments that measure magnetic properties of
rocks
 When mapped, the ocean floor had stripes
 Areas of “regular” and “irregular” magnetic fields
 Stripes were parallel to oceanic ridges
 Sequences of stripe width patterns matched the
sequences established by geologists on land
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Figure 2.14
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Seafloor Age
 Using the magnetic anomalies, geologists can
infer ages for the ocean rocks
 Seafloor is no older than 200 million years old
 Spreading at the mid-ocean ridges can explain
stripe patterns
 Rising magma at ridge is extruded
 Cooling rocks are normally magnetized
 Field is reversed with new rocks that push old rocks
away
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Figure 2.15
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Figure 2.16
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Hot Spots
 Volcanic centers resulting from hot materials from
deep in the mantle
 Materials move up through mantle and overlying
plates
 Found under both oceanic and continental crust
 Plates move over hot spots creating a chain of
island volcanoes
 Seamounts are submarine volcanoes
 Example: Hawaiian Island Chain
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Figure 2.17
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Plate Tectonics, Continental Shape and
Mountain Ranges
 Movement of plates is responsible for present
shapes and locations of continents
 180 million years ago there was the break-up of
Pangaea
 Supercontinent extending from pole to pole and
halfway around Earth
 Seafloor spreading 200 million years ago separated
Eurasia and North America from southern
continents; Eurasian from North America; southern
continents from each other
 50 Million years ago India crashed into China
creating the Himalayas
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Figure 2.18a, b
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Figure 2.18c, d
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Understanding Plate Tectonics Solves
Geologic Problems
 Reconstruction of Pangaea and recent
continental drift clears up:
 Fossil data difficult to explain with separated
continents
 Evidence of glaciation on several continents
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Figure 2.19
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Figure 2.20
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Driving Mechanism
 Two possible driving mechanisms for plate
tectonics
 Ridge Push and slab pull
 Ridge push is a gravitational push away from
crest of mid-ocean ridges
 Slab pull occurs when cool, dense ocean plates
sinks into the hotter, less dense asthenosphere
 Weight of the plate pulls the plate along
 Evidence suggests that slab pull is the more
important process
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Figure 2.21
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Plate Tectonics and Hazards
 Divergent plate boundaries (Mid-Atlantic Ridge)
exhibit earthquakes and volcanic eruptions
 Boundaries that slide past each other (San
Andreas Fault) have great earthquake hazards
 Convergent plate boundaries where one plate
sinks (subduction zones) are home to explosive
volcanoes and earthquake hazards
 Convergent plate boundaries where continents
collide (Himalayas) have high topography and
earthquakes
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End
Internal Structure of
Earth and Plate Tectonics
Chapter 2
© 2012 Pearson Education, Inc.