Plate Tectonics: A Scientific Theory Unfolds Chet Sawyer Desoto High School.
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Transcript of Plate Tectonics: A Scientific Theory Unfolds Chet Sawyer Desoto High School.
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Plate Tectonics: A Scientific Theory Unfolds
Chet SawyerDesoto High School
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Objectives
• Briefly discuss the evidence used by Alfred Wegener to support his theory of continental drift.
• Briefly explain the theory of plate tectonics. • Compare and contrast the distribution and geologic
characteristics of tectonic plate boundaries, including divergent, convergent, and transform boundaries.
• Discuss the evidence used to test the plate tectonics model, including ocean drilling and hot spots.
• Describe how the continents were arranged in the past. • Discuss mantle convection and the various mechanisms proposed
to explain plate motion.
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Continental Drift: An idea before its time
• Alfred Wegener• First proposed his continental drift
hypothesis in 1915 • Published The Origin of Continents and
Oceans• Continental drift hypothesis
• Supercontinent called Pangaea began breaking apart about 200 million years ago
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Pangaea approximately 200 million years ago
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Continental Drift: An idea before its time
• Continental drift hypothesis• Continents “drifted” to present positions
• Evidence used in support of continental drift hypothesis
• Fit of the continents • Fossil evidence• Rock type and structural similarities • Paleoclimatic evidence
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Fossil evidence supporting continental drift
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Matching mountain ranges
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The great debate
• Objections to the continental drift hypothesis
• Lack of a mechanism for moving continents • Wegener incorrectly suggested that
continents broke through the ocean crust, much like ice breakers cut through ice • Strong opposition to the hypothesis from
the scientific community
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The great debate
• Continental drift and the scientific method• Wegener’s hypothesis was correct in
principle, but contained incorrect details • A few scientists considered Wegener’s ideas
plausible and continued the search
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Plate Tectonics: A modern version of an old idea
• Earth’s major plates • Associated with Earth’s strong, rigid outer
layer• Known as the lithosphere• Consists of uppermost mantle and overlying crust• Overlies a weaker region in the mantle called the
asthenosphere
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Plate Tectonics: A modern version of an old idea
• Earth’s major plates• Seven major lithospheric plates• Plates are in motion and continually changing
in shape and size• Largest plate is the Pacific plate• Several plates include an entire continent plus
a large area of seafloor
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Earth’s tectonic plates
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Plate Tectonics: A modern version of an old idea
• Earth’s major plates• Plates move relative to each other at a very
slow but continuous rate • About 5 centimeters (2 inches) per year• Cooler, denser slabs of oceanic lithosphere
descend into the mantle
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Plate Tectonics: A modern version of an old idea
• Plate boundaries• Interactions among individual plates occur
along their boundaries • Types of plate boundaries • Divergent plate boundaries
(constructive margins) • Convergent plate boundaries (destructive
margins)• Transform fault boundaries (conservative
margins)
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Divergent plate boundaries
• Most are located along the crests of oceanic ridges
• Oceanic ridges and seafloor spreading • Along well-developed divergent plate
boundaries, the seafloor is elevated forming oceanic ridges
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Divergent plate boundaries• Oceanic ridges and seafloor spreading
• Seafloor spreading occurs along the oceanic ridge system
• Spreading rates and ridge topography • Ridge systems exhibit topographic
differences • These differences are controlled by
spreading rates
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Divergent plate boundary
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Divergent plate boundaries
• Continental rifting• Splits landmasses into two or more smaller
segments along a continental rift• Examples include the East African rift
valleys and the Rhine Valley in northern Europe• Produced by extensional forces acting on
lithospheric plates
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Continentalrifting
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Convergent plate boundaries
• Older portions of oceanic plates are returned to the mantle in these destructive plate margins
• Surface expression of the descending plate is an ocean trench • Also called subduction zones • Average angle of subduction =
45 degrees
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World’s oceanic trenches and ridge system
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Convergent plate boundaries
• Types of convergent boundaries • Oceanic-continental convergence • Denser oceanic slab sinks into the asthenosphere • Along the descending plate partial melting of
mantle rock generates magma• Resulting volcanic mountain chain is called a
continental volcanic arc (Andes and Cascades)
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Oceanic-continental convergence
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Convergent plate boundaries
• Types of convergent boundaries• Oceanic-oceanic convergence• When two oceanic slabs converge, one descends
beneath the other • Often forms volcanoes on the ocean floor• If the volcanoes emerge as islands, a volcanic
island arc is formed (Japan, Aleutian islands, and Tonga islands)
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Oceanic-oceanic convergence
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Convergent plate boundaries
• Types of convergent boundaries• Continental-continental convergence • Less dense, buoyant continental lithosphere does
not subduct • Resulting collision between two continental blocks
produces mountains (Himalayas, Alps, and Appalachians)
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Continental-continental convergence
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Transform fault boundaries
• Plates slide past one another and no new lithosphere is created or destroyed
• Transform faults • Most join two segments of a mid-ocean ridge
along breaks in the oceanic crust known as fracture zones
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Transform fault boundaries
• Transform faults • A few (the San Andreas Fault and the Alpine
Fault of New Zealand) cut through continental crust
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Transform faults
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San Andreas Fault near Taft, California
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Testing the plate tectonics model
• Evidence from ocean drilling• Some of the most convincing evidence
confirming seafloor spreading has come from drilling directly into ocean-floor sediment • Age of deepest sediments • Thickness of ocean-floor sediments verifies
seafloor spreading
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Testing the plate tectonics model
• Hot spots and mantle plumes• Caused by rising plumes of mantle material • Volcanoes can form over them (Hawaiian
Island chain)• Mantle plumes • Long-lived structures• Some originate at great depth, perhaps at the
mantle-core boundary
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The Hawaiian Islands
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Testing the plate tectonics model
• Paleomagnetism• Iron-rich minerals become magnetized in the
existing magnetic field as they crystallize• Rocks that formed millions of years ago
contain a “record” of the direction of the magnetic poles at the time of their formation
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Testing the plate tectonics model
• Apparent polar wandering• Lava flows of different ages indicated several
different magnetic poles • Polar wandering paths are more readily
explained by the theory of plate tectonics
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Polar Wandering paths for Eurasia and North America
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Testing the plate tectonics model
• Geomagnetic reversals • Earth’s magnetic field periodically reverses
polarity—the north magnetic pole becomes the south magnetic pole, and vice versa • Dates when the polarity of Earth’s magnetism
changed were determined from lava flows
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Testing the plate tectonics model
• Geomagnetic reversals• Geomagnetic reversals are recorded in the
ocean crust • In 1963 Vine and Matthews tied the discovery
of magnetic stripes in the ocean crust near ridges to Hess’s concept of seafloor spreading
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Paleomagnetic reversals recorded in oceanic crust
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What drives plate motions?
• Researchers agree that convective flow in the mantle is the basic driving force of plate tectonics
• Forces that drive plate motion • Slab-pull• Ridge-push
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Some of the forces that act on plates
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What drives plate motions?
• Models of plate-mantle convection • Any model must be consistent with
observed physical and chemical properties of the mantle • Models • Layering at 660 kilometers • Whole-mantle convection