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Chapter 8

The Theory of Plate Tectonics

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Guiding Questions

• What is the evidence for Gondwanaland?• How does paleomagnetism demonstrate that continents

have moved? • How was paleomagnetism used to show that lithosphere

forms at and migrates away from mid-ocean ridges?• How did features of the seafloor engender the concept of

plate tectonics?• Why do faulting and volcanism occur along oceanic plate

margins?• What causes lithospheric plates to move? • How can geologists measure rates of plate movement?

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Plate Tectonics

• tectonics– movement of Earth’s

crust

• plate tectonics– movement of discrete

segments of Earth’s cruise in relation to one another

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hypothesis

theory

law

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hypothesisA tentative explanation for an observation, phenomenon, or scientific

problem that can be tested by further investigation.

theory1. Common usage-a proposed explanation whose status is still

conjectural, in contrast to well-established propositions that are regarded as reporting matters of actual fact.

2. Scientific usage-a more or less verified or established explanation accounting for known facts or phenomena: the theory of relativity.

law

paradigm-the generally accepted perspective of a particular discipline at a given time 

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Theory-a Skinnerism

The best explanation we have (so far) that explains more of the facts than any other idea, and for which there is no serious evidence flaw.

Plate tectonics explains volcanoe and earthquake belts, mountain ranges, etc.

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Continental Drift

• the idea that continents move horizontally over Earth’s surface

• scoffed at in the 40s• revolutionary in the 60s• proposed and defended

by– Alfred Wegener– Alexander DuToit

Puzzle-like fit of continents using 500 fathom coastline depth

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Evidence for Continental Drift

Early scientists recognized the relationship between fossils on continents separated by sea

Glossopteris flora present only in southern hemisphere continents

– proposed land bridges– consistent with the distribution

pattern– so is the idea of united

continents

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Evidence for Continental Drift

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Early recognition that Mid-Atlantic Ridge was site of landmass rupture and Atlantic Ocean formation

Evidence for Continental Drift

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• Wegener’s Evidence– continents fit together

• Pangaea

– geologic similarities

– floral and faunal similarities

Evidence for Continental Drift

Wegener’s reconstruction of ancient world

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• DuToit’s evidence– expanded Wegener’s ideas

– Mesosaurus fossils• found on Gondwanaland

(southern hemisphere) continents

• a fresh/brackish water species

– could not swim across Atlantic

Evidence for Continental Drift

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• geologic similarities– Brazil and South

Africa have nearly identical geologic sequences

• similar in Antarctica and India

– Glacial sediments

– Coal

Evidence for Continental Drift

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Continuity of Geologic Structures

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• glaciers– orientation of

glacial markings on all continents suggest they were linked

Evidence for Continental Drift

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Additional Evidence

• Lystrosaurus – heavyset herbivore

– found in Africa and SE Asia

– recently (1969) found in Antarctica

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Paleomagnetism-the smoking gun

• magnetization of ancient rocks at the time of their formation

• declination– angle that a compass

needle makes with the line running to the geographic north pole

• rocks lock this orientation in at formation

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Paleomagnetism

• apparent polar wandering– first studies indicated

poles had moved

– instead, plates had moved

– North American and European paths met

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Rise of Plate Tectonics

• Harry Hess, 1962– Geopoetry

• continents didn’t plow through seafloor

• entire crust moved

– Crust must be created and destroyed

• sedimentary cover too thin for four b.y. of accumulation

– driven by convective cells

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Rise of Plate Tectonics

• ridges– site of crustal

formation

– hot rising mantle material rises to top of lithosphere, cools

– ocean crust is formed

– bends away from center to form ridge

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Rise of Plate Tectonics

• Guyots– had identified flat-

topped seamounts in Pacific

– realized they were volcanoes that had been eroded by waves at sea level

– postulated as crust moved away from ridge it cools and sinks

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Rise of Plate Tectonics

• crust is destroyed at subduction zones

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Test of Plate Tectonics

• Paleomagnetism– Vine and Matthews,

1963

– measured magnetization of rocks across the Indian Ocean central ridge

• found normal and reversed “stripes”

• mirror image

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Processes at Plate Boundaries

• Normal faults-extensional motion• Thrust faults-compressional motion• Strike-slip faults-e.g., San Andreas fault

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Processes at Plate Boundaries

• Mid-Ocean Ridges (MOR)– graben

• valley bounded by normal faults along which a central block has slipped downward

– pillow basalt

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Processes at Plate Boundaries

• transform faults– offset MOR’s

– enormous strike-slip faults

– seismically active

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Processes at Plate Boundaries

• Subduction– descending slab

undergoes partial melting

– molten material is less dense, rises

– common around Pacific Rim

– Ring of Fire• location of most of

the world’s trenches

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Processes at Plate Boundaries• associated with:

– volcanoes-island arc

– deep-focus earthquakes >300 km depth

– forearc basin• zone of intensely deformed rocks in belt between island arc and deep-sea

trench

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Processes at Plate Boundaries

• forearc Basin– deep-ocean dark muds and

graywackes with ocean crust mixed in

• mélange– chaotic deformed mixture

of rocks

• accretionary Wedge– body of rock that

accumulates as plate is subducted

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Plate Motion

• Why plates move: – drag on the base of the

plate

– elevation at ridge pushes plate ahead of it

– plate is pulled into subduction zone by preceding parent plate

– broken plate segments create additional forces

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Plate Motion• eight large plates

• several small plates

• not all plates move at the same rate– relative motion

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Plate Motion

• absolute plate motion– establish using fixed point

– hot spot• small geographic area where heating and igneous activity occur within the crust

• Yellowstone

• Hawaii

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Plate Motion

• Hawaiian hot spot– thermal plume creates

volcano

– plate moves away from plume

– stranded volcano cools, leaves a chain

– chain indicates direction and rate

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Plate Motion

• GPS-Global Positioning System– Earth-orbiting satellites identify motion

• transmitter on satellite with ground-based receiver

• Average rate of movement: 5 cm/year

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Effect on Global Climate

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