Chapter 2 plate tectonics
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Chapter 2: Plate Tectonics. Tectonic Plates of Earth. Pangaea. Pangaea: Pan = all Gaea = Earth Panthalassa Thalassa = sea AKA Tethys Sea. Continental Drift. Progressive breakup of Pangaea into modern continents Similar to sea ice. Wegner’s Evidence.

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Chapter 2: Plate Tectonics

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Chapter 2: Plate Tectonics


Tectonic Plates of Earth


Pangaea

  • Pangaea:

    • Pan = all

    • Gaea = Earth

  • Panthalassa

    • Thalassa = sea

    • AKA Tethys Sea


Continental Drift

  • Progressive breakup of Pangaea into modern continents

  • Similar to sea ice


Wegner’s Evidence

  • Present-day shorelines fit like puzzle pieces

  • Better fit from continental shelves


Fossil Evidence

  • Identical fossils present in S. Amer. & Africa

  • e.g. Mesosaurus

  • e.g. Glossopteris

  • e.g. Marsupials


Geologic Evidence

  • Identical rocks on different continents

    • e.g. 2.2Ga igneous rocks in Brazil & Africa

  • Similar Mountain Ranges

    • e.g. Appalachian Mts ~ Caledonian Mts


Identical package of rocks & fossils found in S.Amer., Africa, Australia & Antarctica

Mesozoic Supergroup


Paleoclimatic Evidence

  • Paleo = ancient

  • Climate = weather conditions

  • Glacially transported sediments

  • Glacial striations


Rejection of Continental Drift Hypothesis

  • No evidence of continents “breaking through” oceanic crust

  • Tidal forces necessary would halt Earth’s rotation

  • Danish scientists found no astronomical evidence of drift from 1927-1948


Earth’s Magnetic Field

  • Similar to bar magnet

  • Magnetic materials align themselves to magnetic field


Magnetic orientation has 2 dimensions

North-South

Dip angle (Inclination)

Curie point (T)

Fossil Magnetism / Paleomagnetism

N-S Orientation & Dip


Magnetization = degrees from N pole

Magnetization + Latitude = 90°

e.g. Lavas from Puerto Rico show 75 ° from N pole & Puerto Rico = 15 ° from equator

Magnetic Inclination


Apparent Polar Wander


Seafloor Spreading


Paleomagnetism


Progression of Seafloor Spreading


Plate Boundaries


Plate Boundaries

  • Corresponds to Earthquakes & Volcanoes


Plate Boundaries

  • Three Types

    • Divergent

      • AKA Rift

    • Convergent

      • AKA Subduction

    • Transform

      • AKA Strike-slip


Divergent Boundaries

  • AKA Spreading Centers

  • AKA Rifts

  • Largest mountain chains

  • Plates move apart due to eruption of lava

    • New lava = new oceanic crust

  • Oldest oceanic crust 180Ma


Pillow Basalts

  • Form when lava extruded under water

  • Immediately outer layer freezes

  • New material pushes through like toothpaste


Rifts also can form in continental settings

Linear depressions

Lakes, valleys, etc.

Asthenosphere thins due to tension

e.g. East Africa Rift Zone, Mt. Kilimanjaro

Continental Rifting


Continental Rifting (con’t)

  • If tension continues, eventually continental rift develops into oceanic spreading

  • e.g. Red Sea, Sea of Cortez


Convergent Boundaries

  • Old oceanic crust dense & heavy

  • Heavy vs. light => subduction

  • AKA destructive margins

  • Large earthquake & explosive volcanoes

  • Melting triggered at ~100km depth


Oceanic-Oceanic Convergence

e.g. Virgin Islands, Japan, Philippines


Oceanic-Continental

e.g. Andes, MesoAmerica, Italy


Continental-Continental

e.g. India


Hot Spots

  • Caused by mantle plumes

  • Plumes do not move, plates do

  • Bend at 40Ma

    • Major change in plate motion


Relative Plate Motions

  • Relative to Hot Spots & other plates

  • Measure motions with Paleo-magnetism


Forces Driving Plate Motions

  • Convection of Mantle

  • Upwelling Mantle

    • Ridge-push

  • Slab Suction

    • AKA Slab-pull


Layer-Cake Model

  • Two zones of convection, above & below ~660km

  • Explains why mid-ocean ridge basalt different than hot spot basalt


Whole Mantle Convection

  • Cold oceanic crust descend to bottom of mantle, “stirring” it

  • Hot plumes rise from core-mantle boundary

    • Bring “primitive” mantle to surface

  • Not popular b/c complete mixing in 100s Ma


Deep Layer Model

  • Heat from Earth’s interior causes two layers to shrink & swell

    • Similar to lava lamp

  • Small amt of material rises to surface to create hot spots

  • Little seismic evidence to support this model


Importance of Plate Tectonics

  • First theory to provide comprehensive view & explain:

    • Earth’s major surficial processes

    • Geologic distribution of earthquakes, volcanoes & mountain ranges

    • Distribution of mineral resources & ancient organisms


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