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Chapter 5 Lecture Outline
Foundations of
Earth Science
Seventh Edition
Plate Tectonics:
A Scientific
Revolution Unfolds
Natalie Bursztyn
Utah State University
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From Continental Drift to Plate Tectonics
• Before 1960 geologists saw the positions of
ocean basins and continents as fixed
– Continental drift suggested but not agreeable
• A new model of tectonic processes
– A scientific revolution
– Tectonic processes deform crust and create major
structural features
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Focus Question 5.2
• What is the evidence in support of Wegener’s
continental drift hypothesis?
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Continental Drift: An Idea Before Its Time
• World maps in the 1600s suggested that South
America and Africa fit together
• In 1915, Alfred Wegener outlined the hypothesis
of continental drift
– Single supercontinent of all of Earth’s land: Pangaea
– Fragmented ~200 mya and smaller landmasses drifted to
their present positions
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Continental Drift: An Idea Before Its Time
• Similarity between coastlines on opposite sides
of the Atlantic
• Opponents argued that coastlines are modified
through time by erosion and deposition
• Continental shelf is a better approximation of the
boundary of a continent
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Continental Drift: An Idea Before Its Time
• Identical fossils found in South America and
Africa
• Paleontologists agree: land connection
necessary to explain fossil record
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Continental Drift: An Idea Before Its Time
• Mesosaurus
– Small Permian aquatic freshwater reptile
– Found in eastern S. America and western Africa
• Glossopteris
– Seed fern
– Africa, Australia, India, S. America, and Antarctica
• Opponents explain fossil patterns by rafting,
oceanic land bridges, and island stepping stones
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Continental Drift: An Idea Before Its Time
• Rock types and geologic features match up
– 2.2 billion-year-old igneous rocks in Brazil and Africa
– Mountain belts end at coastlines and reappear across
oceans
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Continental Drift: An Idea Before Its Time
• Evidence for glaciation on continents now at
tropical latitudes
– Can be explained by supercontinent located near the
South Pole
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Focus Question 5.2
• What is the evidence in support of Wegener’s
continental drift hypothesis?
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Jigsaw-puzzle fit of the continents
Fossils
Rock types and geologic structures
Evidence for glaciation
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Focus Question 5.3
• What are the main objections to the
continental drift hypothesis?
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The Great Debate
• Wegener’s hypothesis of continental drift was
met with criticism
• Objections were based on lack of mechanism for
continental drift
– Wegener proposed that tidal forces moved continents
and that sturdy continents broke through thin oceanic
crust
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Focus Question 5.3
• What are the main objections to the
continental drift hypothesis?
– No mechanism for moving the continents
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Focus Question 5.4
• What are the major differences between the
lithosphere and asthenosphere and how are
they each important to plate tectonics?
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The Theory of Plate Tectonics
• Oceanographic exploration increased
dramatically following World War II
– Discovery of global oceanic ridge system
– Earthquakes at great depths in western Pacific ocean
trenches
– No oceanic crust older than 180 million years
– Thin sediment accumulations in deep-ocean basins
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The Theory of Plate Tectonics
• Lithosphere is the crust and uppermost
(coolest) mantle
– Oceanic lithosphere varies in thickness
• Thin at ridges, up to 100 km thick
• Mafic composition
• More dense than continental lithosphere
– Continental lithosphere 150–200 km thick
• Felsic composition
– Responds to forces by bending or breaking
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The Theory of Plate Tectonics
• Asthenosphere is the hotter, weaker mantle
below the lithosphere
– Rocks are nearly melted at this temperature and
pressure
– Responds to forces by flowing
– Moves independently from lithosphere
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The Theory of Plate Tectonics
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The Theory of Plate Tectonics
• Lithosphere is broken into irregular plates
– Plates move as rigid units relative to other plates
– 7 major plates make up 94% of Earth
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The Theory of Plate Tectonics
• Interaction between plates at plate boundaries
– Divergent boundaries (constructive margins)
• Two plates move apart
• Upwelling of hot material from mantle creates new seafloor
– Convergent boundaries (destructive margins)
• Two plates move together
• Oceanic lithosphere descends and is reabsorbed into mantle
• Two continental blocks create a mountain belt
– Transform plate boundaries (conservative margins)
• Two plates slide past each other
• No lithosphere is created or destroyed
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The Theory of Plate Tectonics
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Focus Question 5.4
• What are the major differences between the
lithosphere and asthenosphere and how are
they each important to plate tectonics?
– Lithosphere is brittle and rigid
– Asthenosphere flows
– Lithosphere is broken up into plates which move
around relative to one another
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Focus Question 5.5
• How is lithosphere formed at a divergent
plate boundary?
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Divergent Plate Boundaries and Seafloor
Spreading
• Most divergent plate boundaries are along
the crests of oceanic ridges
• New ocean floor is generated when mantle
fills narrow fractures in oceanic crust
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Divergent Plate Boundaries and Seafloor
Spreading
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Divergent Plate Boundaries and Seafloor
Spreading
• Most divergent plate boundaries are associated
with oceanic ridges
– Elevated seafloor with high heat flow and
volcanism
– Longest topographic feature on Earth’s surface
(covers 20% of surface)
– Crest is 2 to 3 km higher than adjacent basin and
can be 1000 to 4000 km wide
• Rift valley is a deep canyon along the crest of a
ridge resulting from tensional forces
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Divergent Plate Boundaries and Seafloor
Spreading
• Seafloor spreading is the process by which
new seafloor is created along the ocean ridge
system
– Average spreading rate is ~5 cm/year
• Up to 15 cm/year or as slow as 2 cm/year
– New lithosphere is hot (less dense) but cools
and subsides with age and distance from the
ridge system
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Divergent Plate Boundaries and Seafloor
Spreading
• Continental rifting occurs when divergent
boundaries develop within a continent
– Tensional forces stretch and thin the lithosphere
– Brittle crust breaks into large blocks
– Eventually become ocean basins
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Divergent Plate Boundaries and Seafloor
Spreading
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Focus Question 5.5
• How is lithosphere formed at a divergent
plate boundary?
– Adjacent plates move apart and create space where
hot mantle rocks upwell
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Focus Question 5.6
• What are the characteristics of each of the
three types of convergent boundaries?
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Convergent Plate Boundaries and
Subduction
• Convergent plate boundaries occur when two
plates move toward each other
– Convergence rate is equal to seafloor spreading
– Characteristics vary depending on subducting crust
– Subduction zones
• Lithosphere descends into the mantle
– Old oceanic crust is ~2% denser than asthenosphere
– Continental crust less dense than asthenosphere
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Convergent Plate Boundaries and
Subduction
• Deep ocean trenches
– Long, linear depressions
– Result of subduction
• Angle of subduction varies
– Nearly flat to nearly vertical
– Depends on density of crust
• Older crust is cooler and denser
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Convergent Plate Boundaries and
Subduction
• Characteristics of convergent plate boundaries
vary depending on type of crust being subducted
– Oceanic + continental
– Oceanic + oceanic
– Continental + continental
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Convergent Plate Boundaries and
Subduction
• Oceanic lithosphere + continental
lithosphere = subduction of oceanic
lithosphere
– Continental lithosphere is less dense
– Water from descending oceanic crust triggers
partial melting of asthenosphere at ~100 km
– Molten material is less dense and rises
• Continental volcanic arcs
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Convergent Plate Boundaries and
Subduction
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Convergent Plate Boundaries and
Subduction
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Convergent Plate Boundaries and
Subduction
• One slab subducts under another at oceanicoceanic convergent boundaries
– Volcanism because of partial melting
– Generates volcanic island arcs
• Volcanic cones underlain by oceanic crust
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Convergent Plate Boundaries and
Subduction
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Convergent Plate Boundaries and
Subduction
• Continental crust is buoyant
– Neither plate subducts during continent-continent
collisions
• Folding and deformation of rocks
• Mountain building
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Focus Question 5.6
• What are the characteristics of each of the
three types of convergent boundaries?
– Oceanic-continental collisions result in subduction
zones, deep ocean trenches, and continental volcanic
arcs
– Oceanic-oceanic collisions result in volcanic island
arcs and deep ocean trenches
– Contiental-continental collisions result in deformed
sedimentary rocks and mountain belts
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Focus Question 5.7
• What is the relative motion along a transform
fault?
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Transform Plate Boundaries
• Transform plate boundaries form when two
plates slide horizontally past one another
– Transform faults
– No lithosphere is produced or destroyed
– Connect spreading centers and offsets oceanic ridges
• Linear breaks in the seafloor are fracture zones
– Fracture zones are inactive
– Active faults occur between offset ridge segments
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Transform Plate Boundaries
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Transform Plate Boundaries
• Few transform faults cut through continental
crust
– San Andreas Fault (California) and Alpine Fault (New
Zealand) are exceptions
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Focus Question 5.7
• What is the relative motion along a transform
fault?
– horizontal
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Plate Tectonics: A Scientific Revolution
Unfolds
• What type of plate boundary shapes the
landscape nearby you?
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Focus Question 5.8
• What is the evidence in support of the plate
tectonic theory?
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Testing the Plate Tectonics Model
• Evidence from Deep Sea Drilling Project
– Collect sediment and oceanic crust
– Date fossils in sediment
• Sediment age increases with distance from
ridge
• Sediment is thicker with increased distance
from the ridge
• Oldest seafloor is 180 million years old
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Testing the Plate Tectonics Model
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Testing the Plate Tectonics Model
• Volcanoes in the Hawaiian Island-Emperor
Seamount Chain increase in age with
distance from Hawaii
– A cylinder of upwelling hot rock (mantle plume) is
beneath Hawaii
– A hot spot is an area of volcanism, high heat flow,
and crustal uplift above a mantle plume
– A hot-spot track formed as the Pacific Plate moved
over the hot spot
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Testing the Plate Tectonics Model
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Testing the Plate Tectonics Model
• Today North and South magnetic poles align
approximately with geographic North and South
poles
• Iron-rich minerals influenced by magnetic pole
– Basalt erupts above the curie temperature, so
magnetite grains are nonmagnetic
– Grains align to magnetic field during cooling
– Rocks preserve a record of the direction of magnetic
poles at the time of formation
• Paleomagnetism or fossil magnetism
• Position of paleomagnetic poles appears to
change through time because of continental drift
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Testing the Plate Tectonics Model
• Magnetic field reverses polarity during a
magnetic reversal
– Rocks with same magnetic field as today have
normal polarity
– Rocks with opposite magnetism have reverse
polarity
• Polarity of lava flows with radiometric ages was
used to generate a magnetic time scale
– Divided into chrons ~1 million years long
– Finer-scale reversals within each chron
• Vine and Matthews (1963) suggested stripes of
normal and reverse polarity are evidence of
seafloor spreading
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Testing the Plate Tectonics Model
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Testing the Plate Tectonics Model
© 2014 Pearson Education, Inc.
Testing the Plate Tectonics Model
© 2014 Pearson Education, Inc.
Focus Question 5.8
• What is the evidence in support of the plate
tectonic theory?
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Age and thickness of seafloor sediment
Hot-spot tracks
Apparent polar wander
Magnetic reversals and seafloor spreading
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Focus Questions 5.9
• What is meant by plate-mantle convection?
• What are the primary driving forces of plate
motion?
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What Drives Plate Motions?
• Mantle is solid, but hot and weak enough to flow
• Convection occurs as hot, less dense material
rises and surface material cools and sinks
• During slab pull, cold, dense oceanic crust
sinks because it is denser than the
asthenosphere
• During ridge push, gravity causes lithospheric
slabs to slide down the ridge
• Drag in the mantle also affects plate motion
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What Drives Plate Motions?
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What Drives Plate Motions?
• Mantle convective flow drives plate motion
• Subducting plates drive downward
component of convection
• Upwelling of hot rock at oceanic ridges drives
upward component of convection
• Convective flow is the heat transfer
mechanism from Earth’s interior
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What Drives Plate Motions?
• Multiple models for convective flow:
– Whole-mantle convection
• Cold oceanic lithosphere sinks and stirs entire mantle
• Subducting slabs sink to core-mantle boundary
• Balanced by buoyant mantle plumes
– Layer cake model
• Subducting slabs do not sink past 1000 km
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What Drives Plate Motions?
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Focus Questions 5.9
• What is meant by plate-mantle convection?
– Cool, dense oceanic lithosphere sinks and warm
mantle upwells at mid-ocean ridges and mantle
plumes
• What are the primary driving forces of plate
motion?
– Slab pull and ridge push
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Focus Question 5.10
• Why are some plates getting larger while
others are getting smaller?
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How Do Plates and Plate Boundaries
Change?
• Total surface area of Earth is constant
• Size and shape of individual plates changes
– African and Antarctic Plates are growing
• Surrounded by divergent boundaries
– Pacific Plate is being consumed
• Surrounded by convergent boundaries)
• Plate boundaries move and change through time
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How Do Plates and Plate Boundaries
Change?
• New ocean basins were created during the
breakup of Pangaea
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How Do Plates and Plate Boundaries
Change?
• Present plate motions can be used to predict
future continental positions
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Focus Question 5.10
• Why are some plates getting larger while
others are getting smaller?
– Earth’s surface area does not increase
– As new crust forms, old crust is destroyed
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