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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

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Earth Science, 10e

Edward J. Tarbuck & Frederick K. Lutgens


Plate TectonicsChapter 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 plateDenser oceanic slab sinks into the asthenosphere

    • Land formations: Pockets of magma develop and rise, continental volcanic arcs form, chain of volcanic islands

    • Examplesinclude 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


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