THE CHANGING EARTH
Deep History Roots of Modern Distributions

• Deep History of Earth
  • Atmospheric History [+]
    
  • Extinctions [+]

Geological Time Scale

• Based on Index (guide) Fossils
  • Paleozoic Age of Fishes (570 - 248 Ma)
    • (first hard fossils) extinct trilobites
  • Mesozoic Age of Reptiles (248 - 65 Ma)
    • marine ammonites
  • Cenozoic Age of Mammals (65 - 0 Ma)
  • Subdivisions of Geologic Time Scale [.]
    • eons eras periods epochs
    

Geological Time Scale

• Stratigraphic Principles
  • Original Horizontality
  • Superposition
• Annual Dating
• Radiometric Dating
  • Ar⁴⁰ / Ar³⁹ (K / Ar) Dating
  • Radiocarbon Dating
• Paleomagnetic Correlation

Theory of Continental Drift

• Continental Drift: Movement of rigid continental crust over plastic asthenosphere
• Plate Tectonics: geophysical processes producing and destroying oceanic crust
• Basic Observation: Fit of continental margins (Snider-Pelligrine, 1858)

• Primary Observation: fit of continental margins
• 1915 Book
  
  • Geology: trans-Atlantic alignment of mountains, alignment of rock strata
  • Geophysics: change in elevation implies crustal plasticity
  • Paleontology: Glosopteris flora distribution in S. Hemisphere
  • Paleoclimatology: tropical coal beds in Northern Europe, tillites in tropical Africa
  • Biogeography: marsupials and Nothofagus in South America and Australia

• 1929 Book (revision)
  • Match of continental margins (as in 1915 book)
  • Crust: Continental (felsic) rocks vs. oceanic (mafic) rocks
  • Proposed Tertiary Breakup of Pangea & Laurasia (no)
  • Breakup began at midocean ridge,
    spreading causes earthquakes (~)
  • Rates of movement 30-360 cm/yr (no)
  • Geothermal heat causes motion (~)
    died Nov. 1930

Scientific Opposition to Continental Drift

• Unsubstantiated assumptions (not proof)
  • Alternative explanations for paleontological patterns
• Errors in Fact
  • Continent movement rates too fast
  • Earth much older than Wegener believed
• Geological and paleontological data too scanty for rigorous test
• No Geophysical Mechanism to force the continents to move

Twentieth Century Evidence for Continental Drift

• Marine Geography: better maps
  
  • Continental Margins show closer match than do shorelines (Bullard et al. 1965)
• Better Stratigraphic Evidence
  • Precambrian Shield rocks match (Hurley and Rand, 1969)
  • Flood basalts match (Storey 1995)
  • Similar stratigraphic sequences (Allard & Hurst 1969)
    • Radiometric dates volcanic rocks associated with
      tillites, coal beds, and sand dunes
  

Twentieth Century Evidence for Continental Drift

• More Detailed Paleoclimatic evidence
  • extent & flow of Permian glaciers (Windley, 1977)
  
  
  
  

Twentieth Century Paleontological evidence

• Glossopteris (Permian plant) distribution matches across "fits." It was seasonal tree equatorward of continental ice (Schopf 1976).
• Lystrosaurus in Antarctica, Africa, and S. India (Elliott, 1970)

More Biogeographic Evidence

• Carabidae Beetle Family
• Galaxiidea Fish Superfamily (freshwater)
• Proteaceae Plant Family
• Pipidae Frog Family

Twentieth Century Evidence for Continental Drift

• Better Marine Geography from Sonar Data (electronics), seafloor cores, maps
  • Distinctive paleomagnetic pattern of Midocean ridges

Age: Sea floors are young

Composition: Sea floors composed of lava (basalt)

Ocean trenches
• very low heat flow
• very low gravitational force
• Island arcs (Murdock)

Ocean Ridges


• Guyots

Marine Geography and Geophysics
The hypothesis of Seafloor Spreading [.]

• Geological theory to explain new marine information
• The Ocean Floor is created at the mid-ocean ridge and subducted into the ocean trenches.
  • Midocean ridges (spreading centers)
  • Ocean trenches (oceanic plate subduction zones)
  • Island arcs (oceanic plate margins).
  • Guyots (cooling and increased density with age)
  • Age (ocean floors constantly created & subducted)
  • Composition (being created by volcanism)

Confirmation of Seafloor Spreading

• Earth's magnetic field varies with time
  Rock Remnant Magnetism records magnetic field
  • thermoremnant magnetism - Curie Point
  • depositional remnant magnetism
• sensitive electronic instruments can detect remnant magnetism (Blackett 1950)
  • Reversals of Polarity form the Paleomagnetic Time Scale
  • Magnetic orientation of midocean rocks "stripes" (Cox, 1973)
  • basalt at center normally magnetized
  • on either side, older rock with reverse magnetic polarity
  • symmetrical alternating normal - reversed stripes

• Earth's crust consists of two types of rocks
  • thick buoyant continental plates
  • thin dense oceanic plates
    
• Oceanic crust is formed at midocean ridge
• It spreads laterally, cooling and sinking
• Oceanic crust is subducted beneath lighter plates

The Current Model

• Three Forces Driving Plate Tectonics USGS
  • Ridge Push
  • Mantle Convection
  • Slab Pull
    

1. Passive Margin (N. American Atlantic Coast)
2. Active Margins
   • Divergent Plate Boundaries
     • Midocean ridges (formation of oceanic plates)
     • Rift Valleys (Gulf of California, Rio Grande Valley, Red Sea, East Africa)
       

• Convergent Plate Boundaries
  • Ocean-Continent Boundaries
    • oceanic plate subducted (more dense)
    • ocean trench formed
    • coastal mountains formed (e.g., Andes) Odsen
      
    

• Convergent Plate Boundaries
  • Continent-Continent Boundaries
    • neither plate subducted
    • mountain building by compressional tectonics
      (Alp-Himalayan Mts.)
      
  • Crust fragments accreted to continents (terranes) () ()

• Convergent Plate Boundaries
  • Ocean-Ocean collision
    • oldest (coldest, densest) plate subducted
    • ocean trench formed
    • island arc formed (e.g., Aleutians)
      

• Gondwana
  • unified from Precambrian (650 Ma) - Ordovician (475 Ma)
  • Southern Hemisphere fragments: (today's Southern Hemisphere)
  • Northern Hemisphere fragments: Arabia and India
• Laurasia
  • formed during Devonian (400 Ma)
• Gondwana & Laurasia collide during Carboniferous (306 Ma)
• Asia collides with Laurasia in Permian (270 Ma) (forms Urals)
  • all land masses connected (Pangea)
  • no divisions of ocean (Pantalassia)

Earth's Tectonic History

• Breakup of Pangea Triassic
  • rifting between Asia and North America forms Turgai Sea Jurassic (180 Ma)
  • rifting between North America and Europe Cretaceous (100 Ma)
  
• Breakup of Laurasia
  • rifting between Pangea and Laurasia opens circum-equatorial sea (Tethys) (100 Ma)
  • Beringia connects Asia and North America - produces "Holarctic" biota

Earth's Tectonic History

• Breakup of Gondwana [diagram]
  • rifting between eastern (Africa & S.A.) and western Pangea Jurassic (180 Ma)
  • rifting between Africa and South America Jurassic (160 Ma)
  • rifting between Madagascar-India and Antarctica-Australia Jurassic (130 Ma)
  • rifting between Antarctica and Australia Cretaceous (100 Ma)
  • rifting between Madagascar and India [Pliocene] (90 Ma)
  • collision of India with Asia Eocene/Oligocene (40-36 Ma *)

Earth's Tectonic History

• Formation of Central America and the Antilles [.]
  • rifting between North and South America Jurassic (150 Ma)
  • island arcs of Antilles and Central American Archipelago drift eastward (120 - 58 Ma)
  • proto Antilles form land bridge Cretaceous (65 Ma)
  • collision of oceanic plates creates Panama Land Bridge Pliocene (3.5 Ma)

Effects of Plate Tectonics

• Cretaceous Eperic (Epicontinental) Seas
  
  • few continental glaciers
  • shallow seafloor (small ocean basins)
    • Australia divided into 3 small continents
    • North American divided in half
• More biogeographic barriers for terrestrial organisms
• Distribution corridor for aquatic organisms

Effects of Plate Tectonics

• Mediterranean and Red Sea
  • Collision of Africa and Europe Eocene (35 Ma)
  • Closure of Gibraltar Miocene (24 Ma) salt deposits [.] (desert)
  • Rifting of Mediterranean Basin Miocene (5 Ma)
• Barrier (terrestrial) / Corridor (marine)

Effects of Plate Tectonics

• Oceanic Islands (Unique Biogeographic Entities)
  • Hot Spots
    • Hawaiian Island Emperor Seamount Chain [+] [+]
    • Pacific Plate moving northward into Aleutian Trench
    • Hotspot produces chain of islands
      • Hawaiian Chain 0 - 44 Ma
      • Emperor Chain 44 - 80 Ma
  • Triple Junctions
    • Galapagos Islands Juncture of Nazca, Cocos and S. American Plates

Effects of Plate Tectonics

• Paleo Ocean Circulation
  • Tethean circum-equatorial seaway (Cretaceous)
  • Rifting of Australia and Antarctica created circum-Antarctic circulation Eocene (40 Ma)
  • Rifting of Australia and South America
  • Collision of oceanic plates creates Panama Land Bridge Pliocene (3.5 Ma)
• Global-scale climatic and biogeographic effects

Climatic and Biogeographic Consequences of Plate Tectonics

• Formation of Pangea
  • Reduced area of shores continental shelves, globally
  • Produced cold and arid continental interior
  • Prevented circum-equatorial circulation (global cooling)
  • Formed land mass at south pole (global cooling)
• Ammonites vs. Area of Epicontinental seas (Mesozoic)
• Decreasing similarity of coastal faunas as Atlantic opens (Fallaw 1979)