Earth Science, 10e
Edward J. Tarbuck &
Frederick K. Lutgens
Plate Tectonics
Chapter 7
Earth Science, 10e
Stan Hatfield and Ken Pinzke
Southwestern Illinois College
Continental drift: an
idea before its time
Alfred Wegener
• First proposed hypothesis, 1915
• Published The Origin of Continents and Oceans
Continental drift hypothesis
• Supercontinent called Pangaea began breaking
apart about 200 million years ago
• Continents "drifted" to present positions
• Continents "broke" through the ocean crust
Pangaea approximately 200
million years ago
Continental drift: an
idea before its time
Wegener's continental drift hypothesis
• Evidence used by Wegener
•
•
•
•
Fit of South America and Africa
Fossils match across the seas
Rock types and structures match
Ancient climates
• Main objection to Wegener's proposal was its
inability to provide a mechanism
Wegener’s matching of mountain
ranges on different continents
Paleoclimatic evidence for
Continental Drift
Plate tectonics: the
new paradigm
More encompassing than continental drift
Associated with Earth's rigid outer shell
• Called the lithosphere
• Consists of several plates
• Plates are moving slowly
• Largest plate is the Pacific plate
• Plates are mostly beneath the ocean
Plate tectonics: the
new paradigm
Asthenosphere
• Exists beneath the lithosphere
• Hotter and weaker than lithosphere
• Allows for motion of lithosphere
Plate boundaries
• All major interactions among plates occur along
their boundaries
Convergent: Continental to
Continental
Convergent plate boundaries (destructive
margins)
• Definition: When subducting plates contain
continental material, two continents collide
• How they move: two continental plates push
each other up
• Landform Results: new mountain ranges
• Example: Himalayas
• Density: Same density so they both push
each other up
A continental-continental
convergent plate boundary
The collision of India and Asia
produced the Himalayas (before)
The collision of India and Asia
produced the Himalayas (after)
Convergent Plate Boundaries:
Oceanic to Continental
Convergent plate boundaries (destructive margins)
• Oceanic-continental convergence
• Definition: an oceanic plate and continental plate
colliding
• How they move: oceanic plate slides underneath
continental plate Denser oceanic slab sinks into the
asthenosphere
• Land formations: Pockets of magma develop and
rise, continental volcanic arcs form, chain of volcanic
islands
• Examples include the Andes, Cascades, and the
Sierra Nevadan system
An oceanic-continental
convergent plate boundary
Convergent Plate Boundaries:
Oceanic to Oceanic
• Oceanic-oceanic convergence
• Definition: Two oceanic slabs converge and
one descends beneath the other
• How they move: Often forms volcanoes on
the ocean floor
• Land formation results: Volcanic island arcs
forms as volcanoes emerge from the sea
• Examples: include the Aleutian, Mariana,
and Tonga islands,
• Density: same
An oceanic-oceanic convergent
plate boundary
Divergent Plate Boundaries
Divergent plate boundaries (constructive margins)
• Definition: Two plates move apart
• How they move: Mantle material upwells to create new
seafloor
• Landform Results: Ocean ridges and seafloor spreading
• Oceanic ridges develop along well-developed
boundaries
• Along ridges, seafloor spreading creates new
seafloor
• Examples: Iceland, and East Africa
• Density: Same density
Divergent boundaries are located
mainly along oceanic ridges
The East African rift – a
divergent boundary on land
Transform Boundaries
• Definition: Plates slide past one another,
horizontally (side to side)
• No new crust is created
• No crust is destroyed
• How they move: At the time of formation, they
roughly parallel the direction of plate movement
• Aid the movement of oceanic crustal
material
• Examples: San Andres fault in California
• Density: Plates have the same density
Plate tectonics: the
new paradigm
Evidence for the plate tectonics model
• Paleomagnetism
• Probably the most persuasive evidence
• Ancient magnetism preserved in rocks
• Paleomagnetic records show
• Polar wandering (evidence that continents
moved)
• Earth's magnetic field reversals
• Recorded in rocks as they form at oceanic
ridges
Apparent polar-wandering paths
for Eurasia and North America
Paleomagnetic reversals recorded
by basalt flows at mid-ocean ridges
Plate tectonics: the
new paradigm
Evidence for the plate tectonics model
• Earthquake patterns
• Associated with plate boundaries
• Deep-focus earthquakes along trenches provide a
method for tracking the plate's descent
• Ocean drilling
• Deep Sea Drilling Project (ship: Glomar
Challenger)
Distribution of earthquake
foci at plate boundaries
Earthquake foci in the vicinity
of the Japan trench
Plate tectonics: the
new paradigm
Evidence for the plate tectonics model
• Hot spots
• Rising plumes of mantle material
• Volcanoes can form over them
• e.g., Hawaiian Island chain
• Chains of volcanoes mark plate movement
Plate tectonics: the
new paradigm
Evidence for the plate tectonics model
• Ocean drilling
• Age of deepest sediments
• Youngest are near the ridges
• Older are at a distance from the ridge
• Ocean basins are geologically young
The Hawaiian Islands have
formed over a stationary hot spot
Plate tectonics: the
new paradigm
Measuring plate motion
• By using hot spot “tracks” like those of the
Hawaiian Island - Emperor Seamount chain
• Using space-age technology to directly measure
the relative motion of plates
• Very Long Baseline Interferometry (VLBI)
• Global Positioning System (GPS)
Directions and rates
of plate motions
Plate tectonics: the
new paradigm
Driving mechanism of plate tectonics
• No one model explains all facets of plate
tectonics
• Earth's heat is the driving force
• Several models have been proposed
• Slab-pull and slab-push model
• Descending oceanic crust pulls the plate
• Elevated ridge system pushes the plate
Plate tectonics: the
new paradigm
Driving mechanism of plate tectonics
• Several models have been proposed
• Plate-mantle convection
• Mantle plumes extend from mantle-core
boundary and cause convection within the
mantle
• Models
• Layering at 660 kilometers
• Whole-mantle convection
• Deep-layer model
Layering at 660 kilometers
Whole-mantle convection
Deep-layer model
End of Chapter 7